The National Dam 

Safety Program

Research Needs Workshop:

Outlet Works
Preface 

One of the activities authorized by the Dam Safety and Security Act of 2002 is research 

to enhance the Nation’s ability to assure that adequate dam safety programs and practices 

are in place throughout the United States. The Act of 2002 states that the Director of the 

Federal Emergency Management Agency (FEMA), in cooperation with the National Dam 

Safety Review Board (Review Board), shall carry out a program of technical and archival 

research to develop and support: 

• improved techniques, historical experience, and equipment for rapid and effective 

dam construction, rehabilitation, and inspection; 

• devices for continued monitoring of the safety of dams; 

• development and maintenance of information resources systems needed to 

support managing the safety of dams; and 

• initiatives to guide the formulation of effective policy and advance improvements 

in dam safety engineering, security, and management. 

With the funding authorized by the Congress, the goal of the Review Board and the Dam 

Safety Research Work Group (Work Group) is to encourage research in those areas 

expected to make significant contributions to improving the safety and security of dams 

throughout the United States. The Work Group (formerly the Research Subcommittee of 

the Interagency Committee on Dam Safety) met initially in February 1998. To identify 

and prioritize research needs, the Subcommittee sponsored a workshop on Research 

Needs in Dam Safety in Washington D.C. in April 1999. Representatives of state and 

federal agencies, academia, and private industry attended the workshop. Seventeen broad 

area topics related to the research needs of the dam safety community were identified. 

To more fully develop the research needs identified, the Research Subcommittee 

subsequently sponsored a series of nine workshops. Each workshop addressed a broad 

research topic (listed below) identified in the initial workshop. Experts attending the 

workshops included international representatives as well as representatives of state, 

federal, and private organizations within the United States. 

• Impacts of Plants and Animals on Earthen Dams 

• Risk Assessment for Dams 

• Spillway Gates 

• Seepage through Embankment Dams 

• Embankment Dam Failure Analysis 

• Hydrologic Issues for Dams 

• Dam Spillways 

• Seismic Issues for Dams 

• Dam Outlet Works 

In April 2003, the Work Group developed a 5-year Strategic Plan that prioritizes research 

needs based on the results of the research workshops. The 5-year Strategic Plan ensures 

that priority will be given to those projects that demonstrate a high degree of 
collaboration and expertise, and the likelihood of producing products that will contribute 

to the safety of dams in the United States. As part of the Strategic Plan, the Work Group 

developed criteria for evaluating the research needs identified in the research workshops. 

Scoring criteria was broken down into three broad evaluation areas: value, technical 

scope, and product. The framework adopted by the Work Group involved the use of a 

“decision quadrant” to enable the National Dam Safety Program to move research along 

to produce easily developed, timely, and useful products in the near-term and to develop 

more difficult, but useful, research over a 5-year timeframe. The decision quadrant 

format also makes it possible to revisit research each year and to revise research priorities 

based on current needs and knowledge gained from ongoing research and other 

developments. 

Based on the research workshops, research topics have been proposed and pursued. 

Several topics have progressed to products of use to the dam safety community, such as 

technical manuals and guidelines. For future research, it is the goal of the Work Group to 

expand dam safety research to other institutions and professionals performing research in 

this field. 

The proceedings from the research workshops present a comprehensive and detailed 

discussion and analysis of the research topics addressed by the experts participating in the 

workshops. The participants at all of the research workshops are to be commended for 

their diligent and highly professional efforts on behalf of the National Dam Safety 

Program. 
Acknowledgments 

The National Dam Safety Program research needs workshop on Outlet Works was held

on May 25-27, 2004, in Denver, Colorado. 

The Department of Homeland Security, Federal Emergency Management Agency, would 

like to acknowledge the contributions of the U.S. Army Corps of Engineers, Hydrologic 

Engineering Center, which was responsible for the development of the technical program, 

coordination of the workshop, and development of these workshop proceedings. A 

complete list of workshop facilitators, presenters, and participants is included in the 

proceedings. 
TABLE OF CONTENTS

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

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1. Section 1 ONE Introduction.................................................................................... 1-1

1.1 Introduction and scope of report ..............................................................1-1

2. Section 2 TWO Executive Summary ....................................................................... 2-1

2.1 State-of-the-practice.................................................................................2-2

2.1.1 Outlet Works Failure Modes........................................................2-2

2.1.2 Conduit Materials, Selection Criteria, and Construction 

Methods........................................................................................2-3

2.1.3 Gates, Valves and Controls..........................................................2-3

2.1.4 Energy Dissipators.......................................................................2-4

2.1.5 Rehabilitation of Conduits...........................................................2-5

2.1.6 Outlet Works Inspection ..............................................................2-6

2.2 Research and Development Ideas............................................................2-7

2.3 Closure .....................................................................................................2-9

3. Section 3 THREE Workshop Process ..................................................................... 3-1

3.1 Selection of Workshop Participants.........................................................3-1

3.2 Selection of Workshop Topics.................................................................3-1

3.3 Workshop Mechanics...............................................................................3-2

3.4 White Papers............................................................................................3-2

3.4.1 Discussion of White Papers .........................................................3-3

3.5 Identification and Prioritization of Research and Development 

Ideas.........................................................................................................3-3

3.6 Preliminary Implementation Plans...........................................................3-3

3.7 Overall Prioritization of Research and Development Ideas.....................3-4

4. Section 4 FOUR Workshop Results for Individual Topics ....................................... 4-1

4.1 Topic 1 – Outlet Works Failure Modes ...................................................4-1

4.1.1 State-of-the-Practice White Paper................................................4-1

4.1.2 State-of-the-Practice Refinement .................................................4-3

4.1.3 Prioritization of Research and Development Ideas......................4-4

4.1.4 Preliminary Research and Development Plans ............................4-4

4.2 Topic 2 – Conduit Materials, Selection Criteria, and Construction 

Methods....................................................................................................4-4

4.2.1 State-of-the-Practice White Paper................................................4-4

4.2.2 State-of-the-Practice Refinement .................................................4-6

4.2.3 Prioritization of Research and Development Ideas......................4-7

4.2.4 Preliminary Research and Development Plans ............................4-7

4.3 Topic 3 - Gates, Valves, and Controls.....................................................4-8

4.3.1 State-of-the-Practice White Paper................................................4-8

4.3.2 State-of-the-Practice Refinement ...............................................4-10

4.3.3 Prioritization of Research and Development Ideas....................4-10
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4.3.4 Preliminary Research and Development Plans ..........................4-11

4.4 Topic 4 – Energy Dissipators.................................................................4-11

4.4.1 State-of-the-Practice White Paper..............................................4-11

4.4.2 State-of-the-Practice Refinement ...............................................4-12

4.4.3 Prioritization of Research and Development Ideas....................4-13

4.4.4 Preliminary Research and Development Plans ..........................4-13

4.5 Topic 5 – Rehabilitation of Conduits.....................................................4-14

4.5.1 State-of-the-Practice White Paper..............................................4-14

4.5.2 State-of-the-Practice Refinement ...............................................4-15

4.5.3 Prioritization of Research and Development Ideas....................4-16

4.5.4 Preliminary Research and Development Plans ..........................4-16

4.6 Topic 6 – Outlet Works Inspection........................................................4-17

4.6.1 State-of-the-Practice White Paper..............................................4-17

4.6.2 State-of-the-Practice Refinement ...............................................4-20

4.6.2 Prioritization of Research and Development Ideas....................4-21

4.6.3 Preliminary Research and Development Plans ..........................4-21

5. Section 5 FIVE Overall Prioritization Of Research and Development Ideas............. 5-1

6. Section 6 SIX References ....................................................................................... 6-1
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EXHIBITS

Exhibit 3-1 Workshop Participants

Exhibit 3-2 Topics Selected for the Workshop

Exhibit 3-3 White Paper Authors and Titles

Exhibit 4-1 Prioritization of Research and Development Ideas Workshop Topic 1 -

Outlet Works Failure Modes

Exhibit 4-2 Research and Development Implementation Plan, R&D Topic 1A -

Guidelines for Venting Conduits

Exhibit 4-3 Research and Development Implementation Plan, R&D Topic 1B -

Best Practices Guide for Conduits through Embankment Dams.

Exhibit 4-4 Research and Development Implementation Plan, R&D Topic 1C - 

Best Practices for Mechanical and Electrical Design and Automated 

Control

Exhibit 4-5 Research and Development Implementation Plan, R&D Topic 1D -

Developing Lists of Failure Modes for Various Components of Outlet

Works Baseline, not “Cookbook.”

Exhibit 4-6 Prioritization of Research and Development Ideas Workshop Topic 2 –

Conduit Materials, Selection Criteria, and Construction Method

Exhibit 4-7 Research and Development Implementation Plan, R&D Topic 2A - 

Best Practices For Intake Towers – Location, Configuration, Access, 

Debris/Trash Control, Seismic Performance, and Selective Withdrawals.

Exhibit 4-8 Research and Development Implementation Plan, R&D Topic 2B -

Guidance on Application of Tunnels for Outlet Works – Including 

Microtunnelling in Soft and Hard Ground.

Exhibit 4-9 Research and Development Implementation Plan, R&D Topic 2C -

Compile Data on Performance of Conduits With Significant Settlement 

and Lateral Movement.

Exhibit 4-10 Research and Development Implementation Plan, R&D Topic 2D -

Guidance on Seismic Effects on Conduits and How to Analyze and Design 

for Them Including Embankment Spreading, Fault Offset, and the 

Propagation of earthquake waves.

Exhibit 4-11 Prioritization of Research and Development Ideas Workshop Topics 3 and 

4 – Gates, Valves, Controls and Energy Dissipators

Exhibit 4-12 Research and Development Implementation Plan, R&D Topic 3A – Best 

Practices for Selection of Gates and Valves
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Exhibit 4-13 Research and Development Implementation Plan, R&D Topic 4A – Best 

Practices for Energy Dissipators – Considering Heads, Velocities, 

Configurations, etc.

Exhibit 4-14 Research and Development Implementation Plan, R&D Topic 3B –

Guidelines for Maintenance and Exercise of Gates and Valves and 

Cavitation Repairs.

Exhibit 4-15 Research and Development Implementation Plan, R&D Topic 3C -

Develop Short Courses with Video/DVD Options on Design, Operation, 

and Maintenance of Gates and Valves.

Exhibit 4-16 Prioritization of Research and Development Ideas Workshop Topic 5 –

Rehabilitation of Conduits

Exhibit 4-17 Research and Development Implementation Plan, R&D Topic 5A –

Design guidance and Guide Specifications for Sliplining.

Exhibit 4-18 Research and Development Implementation Plan, R&D Topic 5B –

Compilation of Case Studies on Outlet Works Rehabilitation – Lessons 

Learned, Performance, and Comparative Costs.

Exhibit 4-19 Research and Development Implementation Plan, R&D Topic 5C –

Assessment of Microtunnelling and Boring Technologies for Application 

to Outlet Conduit Rehabilitation.

Exhibit 4-20 Research and Development Implementation Plan, R&D Topic 5D –

Best Practices Guide for Design and Construction Methods to Rebuild 

Embankment Sections for Conduit Removal and Replacement.

Exhibit 4-21 Prioritization of Research and Development Ideas Workshop Topic 6 –

Outlet Works Inspection

Exhibit 4-22 Research and Development Implementation Plan, R&D Topic 6A -

Applications of Non-Destructive Testing (NDT) for Inspection of Outlet 

Works.

Exhibit 4-23 Research and Development Implementation Plan, R&D Topic 6B –

Best Practices for Inspection of Gates, Valves, and Operators, Including 

Mechanical and Electrical Components.

Exhibit 4-24 Research and Development Implementation Plan, R&D Topic 6C –

Best Practices Guide for Inspection Frequency and Methods of Inspection 

for Typically Accessible Conduits.

Exhibit 4-25 Research and Development Implementation Plan, R&D Topic 6D –

Inspection of Normally Inundated Structures and Difficult to Access 

Areas.

Exhibit 5-1 Ranking of Research/Development Ideas Based on Individual Scores for 

Potential Benefit - Scores Given By Participants 

Exhibit 5-2 Ranking of Research/Development Ideas Based on Individual Scores for 

Cost/Ease of implementation - Scores Given By Participants 
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Exhibit 5-3 Ranking of Research/Development Ideas Based on Combination of 

Individual Scores for Potential Benefit and Cost/Ease of implementation

Exhibit 5-4 Cost-Benefit Comparison of Ranked Research and Development Topics
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ATTACHMENTS

Attachment 1 Workshop Agenda

Attachment 2 Research and Development Implementation Plan Form

Attachment 3 Ranking of Research and Development Ideas Form

Attachment 4 White Paper No. 1 – Outlet Works Failure Modes, Sal Todaro

Attachment 5 White Paper No. 2 – Design and Construction of Outlet Works Conduits 

for Embankment Dams, Sal Todaro

Attachment 6 White Paper No. 3 – Gates and Valves, Lee Gerbig

Attachment 7 White Paper No. 4 – Energy Dissipators, Henry Falvey

Attachment 8 White Paper No. 5 – Rehabilitation of Pipe Conduit Spillways Through 

Dams, James Crowder

Attachment 9 White Paper No. 6 – Outlet Works Inspection, Chuck Cooper
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LIST OF ABBREVIATIONS

Abbreviation Definition

ASCE American Society of Civil Engineers

ASDSO Association of State Dam safety Officials

AWWA American Water Works Association

CMP Corrugated metal pipe

EPRI Electric Power Research Institute

FEMA Federal Emergency Management Agency

FERC Federal Energy Regulatory Commission

FRS Flood Retarding Structure

HDPE High-density Polyethylene

ICOLD International Committee on Large Dams

NEHRP National Earthquake Hazard Reduction Program

NRCS National Resources Conservation Service

NSRB National Dam Safety Review Board 

REMR Repair, Evaluation, Maintenance, and Rehabilitation

RFP Request for proposal

USACE U.S. Army Corps of Engineers

USBR U.S. Bureau of Reclamation

USGS U.S. Geologic Survey

USSD United States Society on Dams

WRRI Water Resources Research Institute
SECTIONONE Introduction

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

1-1

1 . Section 1ONE Introduction

1.1 INTRODUCTION AND SCOPE OF REPORT

This workshop was sponsored by FEMA and was organized and facilitated by URS Corporation. 

The product of the workshop is this written report, produced by URS, documenting the results of 

the workshop. The report will be included in FEMA’s National Dam Safety Program Act Report 

Series and will be posted on FEMA’s website, http://www.fema.gov/fima/damsafe.

The workshop consisted of convening and facilitating a group of experts in the field of dam 

outlet works design and maintenance. The objectives of the workshop and the resulting written 

report were to document:

1. A state-of-practice concerning the cost effective techniques for maintenance and 

replacement of dam outlet works;

2. The short-term (immediate) and long-term research and development needs of the 

Federal and non-Federal dam safety communities with respect to dam outlet works; and 

3. A recommended course of action for the Federal and non-Federal dam safety 

communities to address these needs based on priorities relating to potential benefit and 

cost/ease of implementation.

The workshop was held in Denver, Colorado, on May 25, 26, and 27, 2004. The workshop was a 

successful undertaking that produced open communication among a wide range of experts in the 

field and identified research and development opportunities that could significantly improve the 

state-of-the-practice in the field.
SECTIONTWO Executive Summary

REPORT OF WORKSHOP ON ISSUES,

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RELATED TO DAM OUTLET WORKS

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2 . Section 2 TWO Executive Summary

A group of 22 individuals was assembled for a three-day workshop on Issues, Remedies, and 

Research Needs Related to Dam Outlet Works. Workshop participants were selected to provide 

a broad representation of individuals involved in the topic and included the authors of the six 

white papers presented. Participants included 7 representatives of five different U.S. federal 

agencies, 2 representatives from two different state dam safety and environmental agencies, 7 

representatives of six different consulting companies, 4 independent consultants, one university 

professor, and one representative of a hydropower organization. The group included individuals 

from 10 different U.S. states, the District of Columbia, and Canada.

During the three days, the workshop participants addressed the following six topics:

1. Outlet Works Failure Modes including consideration of failure by seepage and piping along 

the outlet works conduit.

2. Conduit Materials, Selection Criteria, and Construction Methods including pipe material 

types and their advantages, disadvantages, and appropriate applications; hard foundations; 

soft foundations; pressure conduits (high pressure and low pressure); and non-pressure 

conduits.

3. Gates, Valves, and Controls, including types of gates and valves and their applications; gate 

and valve locations; gate, valve, and control configurations; rehabilitation, and maintenance.

4. Energy Dissipators including stilling basins and energy dissipating valves.

5. Rehabilitation of Conduits including in-place rehabilitation and replacement; capacity –

reservoir evacuation criteria; and economic considerations.

6. Outlet Works Inspection including discussions about determination of appropriate frequency; 

systems, methods, and techniques; and consideration of design criteria to accommodate 

inspection.

These specific topics were selected to coincide with the six topics for which white papers were 

prepared and distributed to participants in advance of the workshop.

Each topic was treated in the following manner:

ß A “strawman” state-of-the-practice white paper was prepared and presented by one of the 

invited experts; a written copy of the white paper was distributed to the participants in 

advance of the workshop.

ß The entire group was then led in a facilitated discussion of refinements, modifications, and 

clarifications to the state-of-the-practice.

ß The group developed a list of possible research and development ideas for the topic being 

considered.

ß The possible research and development ideas were prioritized by the group considering 

potential benefit, probability of success, and cost/ease of implementation.

The top four or five research and development ideas for the topic were assigned to small work 

groups for development of preliminary implementation plans.

ß The small work groups reported back to the entire group on their preliminary implementation 

plans for each of the selected research and development ideas.
SECTIONTWO Executive Summary

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ß Each participant then ranked the identified research and development ideas according to 

considerations of potential benefit and cost/ease of implementation.

The “strawmen” state-of-the-practice white papers are presented in Attachments 4 through 9 of 

this report, and brief summaries of these papers are presented in Section 4. The discussions of 

the states-of-the-practice for the six topics are also summarized in Section 4 of this report, and 

some of the more significant points from those discussions are presented later in this Executive 

Summary. The research and development ideas generated by the group for all six workshop 

topics, and the preliminary implementation plans developed for the 20 leading ideas generated 

for these topics are also presented in Section 4.

In the closing session on the last day of the workshop the participants provided input for an 

overall ranking of the leading research and development ideas for all six topics. The overall 

rankings were also based on consideration of potential benefit, and cost/ease of implementation. 

The overall rankings of the leading research and development ideas are discussed in detail in 

Section 5 and are summarized later in this Executive Summary.

2.1 STATE-OF-THE-PRACTICE

During the state-of-the-practice white paper presentations and the ensuing discussions, it became 

apparent that there are several challenges to defining a single state-of-the-practice for any of the 

topics being considered. Much of this may be attributed to the absence of any single, consistent 

set of standards and guidelines covering the various topics discussed that could be used across 

various regions and organizations. The presented white papers provided a basis for further 

discussions of the state-of-the-practice for the various topics discussed. Consequently, much of 

the discussion centered on trying to develop consensus among the group of invited experts on 

what the state-of-the-practice should be for various aspects of outlet works design, maintenance, 

inspection and rehabilitation. All of the discussions on state-of-the-practice are documented in 

Section 4, but some of the more significant insights gleaned from the white papers and the 

ensuing discussions are presented below and are grouped according to the six topics addressed in 

the workshop. 

2.1.1 Outlet Works Failure Modes

1. The primary causes of outlet works failures are: foundation related failures, structural 

deterioration, the absence of timely rehabilitation and repair, structural failures, 

mechanical failures, failures related to hydraulics, failures related to ice and sediment 

loadings, and operator error. There is also an unwillingness of those involved to 

acknowledge the viability of different failure modes and to devise creative solutions for 

these problems. It has also been observed that many owners/operators will allow a 

known problem to persist without repair for long periods of time.

2. Methods that can be used to minimize the occurrence of failure of outlet works include: 

having an experienced mechanical engineer design components of the outlet works; using 

reliable standard designs that have been developed and tested over time; locating 

structures on a rock foundation to reduce settlement effects; and addressing air demand 

requirements, seismic loading (particularly of intake towers), ice loading, and vibration 

effects in the design.
SECTIONTWO Executive Summary

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3. The three primary ways of minimizing the failure potential of outlet conduits are: 1) 

Filtering the dam embankment in the vicinity of the outlet conduit to prevent internal 

erosion of the embankment along or near the conduit; 2) Constructing outlet conduits on 

rock foundations when possible; and 3) Designing conduits with redundant features to 

prevent water seepage from the conduit into the embankment resulting in the piping of 

embankment materials.

4. To minimize the occurrence of control structure failure the following steps are 

recommended: 1) An engineer experienced in the design of gates, valves and mechanical 

systems for dam outlet works should involved with both new design and rehabilitation 

activities; 2) Routinely test, operate and maintain components of control structures; and 

3) replace antiquated and unreliable components of control structures with newer, more 

reliable equipment.

5. To prevent the uncontrolled erosion of the downstream channel in cases of direct 

discharge, care should be taken in the design and maintenance of discharge mechanisms 

including the use of energy dissipation structures.

2.1.2 Conduit Materials, Selection Criteria, and Construction Methods

1. Due to the absence of any single recognized standard for designing dam outlet works 

there is currently great inconsistency in design rationale and review processes for this 

design aspect of small and medium sized embankment dam facilities. The need for a 

single design standard is further underscored by the fact that outle t works have been 

identified as a significant contributor to the occurrence of embankment dam failures.

2. A single, nationally recognized standard would lead to greater consistency between 

similar project designs, facilitate more effective and consistent review of proposed 

designs, and result in increased potential for safer more reliable facilities. The National 

Dam Safety Review Board (NSRB) is in the process of preparing a manual to be used for 

designing and maintaining outlet works for significant high-hazard dams. The manual 

covers topics for the design, construction, maintenance, and repair of conduits for 

embankment dams. It is recommended that in the interim the guidelines presented in this 

manual be used to form a design basis for smaller facilities.

3. The use of standards not intended for use in designing pressurized outlet systems for 

embankment dams is the most common practice that introduces additional risk to outlet 

works for smaller embankment dams.

4. Recommended approaches to the design of outlet conduits on soft foundations include: 1) 

Placement of freestanding pressurized outlet pipe inside of a cast-in-place reinforced 

concrete conduit; 2) Use of a non-pressurized outlet works system; 3) Use of welded steel 

outlet pipe encased in reinforced concrete; 4) Use of articulated joints for the outlet 

conduit; and 5) Protecting the foundation’s integrity during construction.

2.1.3 Gates, Valves and Controls

1. The types of gates commonly used at newer dam sites are: Slide gates, Knife gates, Jetflow gates, Clamshell gates, Radial gates, and Hinged Crest gates. Slide gates, also 

known as Sluice gates, are the most common type of gate used for dam outlet works. 
SECTIONTWO Executive Summary

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However, it should be noted that there have been frequent problems with leakage of slide 

gates mounted in the unseating direction relative to the head (i.e. the slide gates are 

placed downstream of the headwall).

2. For applications where the head does not exceed 100 feet, the two types of slide gates 

commonly used are the Cast Iron Slide gate and the Fabricated Slide gate. For 

applications where the head exceeds 100 feet, Bonneted Slide gates and Wheel gates are 

commonly used. Jet-flow gates are commonly used as freely discharging devices. The 

Clamshell gate which is specially designed by the USBR works well in submerged 

installations.

3. Due to susceptibility to corrosion and the build up of deposits that increase friction and 

affect the operation of its roller train, the use of Roller gates is increasingly avoided even 

though this was a type of slide gate that was also used for high head applications.

4. Valve types commonly used are: Butterfly valves, Gate valves, Fixed-Cone valves, and 

Sleeve valves. Butterfly valves are commonly used at power plants and water facilities. 

However, problems with the corrosion of the busing or stems of butterfly valves have 

been reported resulting in valves that did not fail in the intended position. A Gate valve 

is a variation of a slide gate, with a circular body and disk. They are often used for guard 

gates upstream of regulating valves, or used for throttling flow in low-head applications. 

A Fixed-Cone valve is commonly used for regulating the flow of water from a dam outlet 

works. Sleeve valves are used as energy dissipaters, both in-line and at the end of a 

conduit and are usually operated to provide precise flow control.

5. Due to high costs and problems with corrosion, seat erosion, cavitation damage, or 

general maintenance difficulties there has been a decreased use of Needle valves and 

Hollow-Jet valves.

6. The four basic types of gate operation systems are: manual operators, electric motoroperators, hydraulic operators, and pneumatic operators. Improved hydraulic fluids 

suitable for outdoor use, the ability to have submerged hydraulic operation, use of better 

seals, and automated operation features has inspired increased use of hydraulic 

components in the design of gates and valves. Hydraulic operation is most often used on 

large gates and valves operating at high head, but is becoming more common for small 

equipment at low heads for particular installations.

7. The knowledge and experience base in the design and fabrication of gates and valves in 

the United States is continually being eroded with decreased funding for research and 

development in control mechanisms and the increased occurrence of offshore fabrication 

and the disappearance of longstanding US-based designers and manufacturing firms.

2.1.4 Energy Dissipators

1. Energy dissipators that have been successfully used at outlet works include: stilling 

basins, baffled apron drops, stepped spillways, impact basins, stilling wells, various types 

of valves, sudden enlargements, in-line orifices, flip buckets, and plunge pools. Most 

outlet works incorporate a combination of different types of energy dissipators. The 

types of energy dissipators used depend on the design reservoir head, discharge 

requirements, and cost constraints of a particular design. However it should be noted that 
SECTIONTWO Executive Summary

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problems with backflows in USBR-type stilling basins have been reported. Backflows 

have drawn rock and soil materials into the basins, sometimes leading to ball-milling and 

related damage. Public safety is also a concern for these structures due to the high 

incidence of people using these structures for recreation activities like swimming even 

though egress from basins can be difficult.

2. USBR Engineering Monograph 25 provides a thorough discussion of the design elements 

for stilling basins, impact basins, flip buckets, and baffled aprons. General guidelines for 

designing outlet works are provided in both USACE Engineering Manual EM 1110-2-

1602 and in Design of Small Dams published by the USBR.

3. The combination of chute blocks, which create shear zones, and baffle blocks, which 

create additional turbulence, allows the length of stilling basins to be reduced. The use of 

baffle blocks also allows air content of discharge water to be regulated, and by releasing 

or entraining air in the water flowing through the baffles, water quality of the discharge 

flow is improved.

4. Although the USBR limits the dimensions and discharges of impact basins to relatively 

low energy levels, the basins have been used successfully at much higher energy levels 

by scaling the recommended dimensions to the larger sizes.

5. There is lack of widespread, definitive knowledge on reliable repair techniques for 

energy dissipators and lack of specific guidelines on the extent and size of riprap required 

downstream of energy dissipators.

2.1.5 Rehabilitation of Conduits

1. Corrugated metal pipe (CMP) commonly serves as the conduit found in small earthen 

embankment dams ranging in height from 10 to 50 feet constructed between 30 and 50 

years ago in the southeastern United States. Compared to conduit materials used in 

newer dams, CMP is highly susceptible to deterioration and becomes a problem when a 

spillway conduit has reached or exceeded its design service life, generally considered to 

be 25 to 30 years. Deterioration of these conduits generally consists of leaking joints, 

corrosion, and holes in the pipe.

2. The main risk associated with the deterioration of these conduits is piping of the 

surrounding embankment soils into the conduit, which can lead to the eventual failure of 

the earthen embankment.

3. The four primary remediation options most applicable to earthen embankment dams less 

than 50 feet in height are: 1) Replacement of the existing pipe conduit using cut and 

cover methods; 2) Sliplining the existing conduit; 3) Grouting along the exterior of the 

existing conduit; and 4) constructing either a siphon spillway or “short” riser and new 

outlet conduit after the existing conduit has been abandoned. Cured-in-place (CIP) 

linings like Insituform have also been used for outlet conduit rehabilitation.

4. Option 1 typically offers the most thorough rehabilitation for impaired conduits through 

dams, but can be cost-prohibitive. Option 2 offers the possibility of increasing the 

hydraulic capacity of the outlet pipe and increases the service life to the existing conduit 

spillway. Option 3 is classified as a temporary repair solution and does not extend the 
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design life of the existing conduit material. The depth of the conduit below the 

embankment surface also limits this method. Option 4 is generally limited to small 

drainage basins with relatively small peak inflows. The installation of the new siphon 

requires excavation of the embankment below normal pool. Siphon spillways do not 

typically exceed 12-inches in diameter and may be constructed of flexible conduit 

materials like PVC, HDPE, or ductile iron. Epoxy paints have become the preferred 

standard for paint used to coat outlet works conduits.

2.1.6 Outlet Works Inspection

1. Inspection of cut-and–cover outlet works typically includes review of features including: 

the entrance channel; intake structure; conduit(s); terminal structures; and the 

downstream channel. The advantages and disadvantages of each method should be 

assessed with due consideration given to the ease and efficiency with which inspection 

objectives can be met; compliance with health and safety guidelines, and cost.

2. The purpose of an outlet works inspection program is to ensure that conduits through 

dams are safely and efficiently operated and maintained. While inspection designations 

may differ among the various state and federal agencies/organizations, the types of 

inspections primarily vary according to frequency and scope.

3. Initial or formal inspections include in depth review of all pertinent data available for the 

outlet works to be inspected. Design and construction data are evaluated relative to 

current state-of-the-art guidelines to identify potential dam safety problems or areas 

requiring particular attention. Detailed crack survey mapping should be used on a 

repeated basis to monitor structural deterioration.

4. Scheduling periodic outlet works inspections may be influenced by the following factors: 

provision of sufficient notice to dam owners and operators allowing necessary 

arrangements and pre-inspection activities to be completed and needed clearances to be 

obtained; the ability to access most or all of the major components of the outlet works; 

and the opportunity to observe features operating under a wide range of conditions.

5. Inspections of features like intake structures and upstream conduits that are usually 

submerged require special coordination with the dam owners since the timing of such 

inspections has to be carefully planned.

6. Good preparation and planning is key to the success of an inspection. Factors to be 

considered include: selection of the inspection team; review of the existing project data 

for the dam to be inspected; and the preparation of a detailed inspection plan. Factors to 

be considered in planning conduit inspections should include: conduit diameter, the 

presence and angle of bends in the conduit, the inclination of the invert slope, and the 

distance between access entry locations.

7. Specialized inspections of inundated or hard to access features often utilize a combination 

of divers, remotely operated vehicles (ROVs), and closed circuit television (CCTV). 

Considerations that must be accounted for if divers are to be expected to conduct 

inspections include depth, altitude, access, leakage, currents, visibility, size of opening, 

and length of conduit requiring inspection. ROVs are good alternatives to dive 

inspections when conditions like depth, diameter, or length become prohibitive. CCTV 
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can be used to inspect submerged portions of intake structures, conduits, and terminal 

structures.

8. Permanent markers and thorough documentation should be used to facilitate easy location 

of intake structures that are usually submerged. GPS could also be used to aid location of 

inundated structures but a repeatable methodology incorporating reliable equipment 

needs to be developed.

2.2 RESEARCH AND DEVELOPMENT IDEAS

Before the research and development ideas were ranked, some 48 separate research and 

development (R&D) ideas were generated for the six different topics addressed in the workshop. 

During this phase, Topic 3 on Gates and Valves and Topic 4 on Energy Dissipators were 

combined and R&D ideas generated for the combined topic. In order to reduce the number of 

R&D ideas to 20, only four ideas from each topic receiving the highest number of votes from 

workshop participants was advanced to the preliminary planning phase.

Based on all of the input from participants and the combined ranking of R&D ideas assessing 

potential benefit and cost/ease of implementation, the five leading research and development 

ideas (including a two-way tie for 4th position) identified in the workshop are as follows:

1. Develop a best-practices guide for outlet works conduits through embankment dams, 

emphasizing small embankments. (R&D Topic 1B)

2. Develop a best-practices guide for selection of gates and valves for outlet works structures. 

(R&D Topic 3A)

3. Develop guidelines for maintenance and exercise of gates and valves and for cavitation 

repairs for outlet works. (R&D Topic 3B)

4. Develop a best-practices guide for energy dissipators for outlet works structures, considering 

heads, velocities, and configuration. (R&D Topic 4A)

5. Design guidance and guide specifications for sliplining outlet conduits. (R&D Topic 5A)

These five R&D topics were ranked in the top 10 in both of the overall ranking methods used to 

prioritize the research and development ideas assessing potential benefit and overall cost/ease of 

implementation. These R&D ideas were also ranked 1 though 4 when the ranking from both 

ranking methods were averaged as will be discussed in Section 5. Consequently, it is the 

authors’ opinion that these R&D topics are the highest priorities for implementation. The R&D 

topic designations given in parentheses after the research and development ideas are the 

designations assigned during the workshop and used in Sections 4 and 5 of this report. The 

preliminary implementation plans for the R&D topics are presented in Section 4 using those 

designations.

After the five R&D topics listed above, the remaining five ideas in the top 10, based on the 

average of the rankings from two ranking considerations were:

1. Develop a best-practices guide for mechanical and electrical design and for automated 

control for outlet works. (R&D Topic 1C)

2. Compile case studies on outlet works rehabilitation including lesson learned, performance 

and comparative costs. (R&D Topic 5B)
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3. Develop a best-practices guide for inspection of outlet works gates, valves, and operators, 

including mechanical and electrical components. (R&D Topic 6B)

4. Develop a list of failure modes for outlet works. (R&D Topic 1D)

5. Develop a best-practices guide for inspection frequency and method for typically accessible 

outlet works conduits. (R&D Topic 6C)

R&D topics 1C, 6B and 1D were ranked in the top 10 in both benefit and cost/ease of 

implementation rankings while R&D topic 5B was among the top 10 in the ranking of potential 

benefit, while R&D topic 6C was the second most favorable R&D idea from a cost/ease of 

implementation perspective. Consequently, it is the authors’ opinion that these five R&D topics 

should be considered high priority, but not as high as the top five ideas previously indicated.

Other R&D topics that received a top 10 ranking in at least one of the two overall ranking 

methods used to prioritize ideas were:

1. Develop a best-practices guide for outlet works intake towers, considering such factors as 

location, configuration, access, debris/trash control, seismic performance, and the practice of 

selective withdrawals. (R&D Topic 2A)

2. Develop a short course with the possibility of a companion video/DVD format on design, 

operation, and maintenance of outlet works conduits. (R&D Topic 3C)

3. Develop a best-practices guide for design and construction methods to rebuild embankment 

sections removed for outlet conduit removal/replacement. (R&D Topic 5D)

These R&D topics deserve some consideration for implementation, but at a lower priority than 

the 10 ideas previously indicated.

The remaining seven R&D topics received marginally less support in the overall rankings of the 

ideas with none of them ranked in the top 10 of either of the two assessment gauges used.

While a numerical ranking and average were calculated for the purposes of being able to rank all 

of the R&D ideas considered, the graph shown in Exhibit 5-4 indicates clustering of ranking 

data. According to this analysis tool, fifteen R&D ideas could be classified as high benefit with 

low cost/easy to implement, or as providing a high benefit with corresponding high cost/easy to 

implement.

In reviewing the leading R&D topics, it is interesting to note that none of them involve basic 

laboratory testing. Rather most of the R&D topics involve collecting or compiling available 

information and developing guidelines for dissemination to practitioners and to enable 

consistency in design, maintenance, and inspection activities throughout the field of practice. 

This suggests that the overall challenges associated with various aspects of outlet works design, 

maintenance and inspection result from the absence of documented information, inconsistencies 

among available information, misuse or misapplication of the available information by some 

practitioners, or lack of knowledge of available information by some practitioners. It also seem 

to reflect a feeling that the information on the overall topic is too dispersed for the profession to 

make the best use of lessons-learned from past performance, and that compilation of information 

into more readily available sources would be beneficial.
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2.3 CLOSURE

A review of the leading R&D topics indicates that none of them involve basic laboratory testing. 

Most of the R&D topics involve collecting or compiling available information and developing 

guidelines for dissemination to practitioners to enable consistency in design, maintenance, and 

inspection activities throughout the field of practice. This suggests that the overall challenges 

associated with various aspects of outlet works design, maintenance and inspection result from 

the absence of documented information, inconsistencies among available information, misuse or 

misapplication of the available information by some practitioners, or lack of knowledge of 

available information by some practitioners. It also seems to reflect a feeling that the 

information on the overall topic is too dispersed for the profession to make the best use of 

lessons-learned from past performance, and that compilation of information into more readily 

available sources would be beneficial.
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3 . Section 3 THREE Workshop Process

This discussion of the workshop process is divided into the following three topics:

- Selection of Workshop Participants

- Selection of Workshop Topics

- Workshop Mechanics

3.1 SELECTION OF WORKSHOP PARTICIPANTS

Workshop participants were selected to address the interest and knowledge base of Federal and 

Non-Federal dam owners, engineering practitioners, and academicians. Facilitators (white paper 

authors) were then selected from the participants based upon their recognized knowledge and 

expertise in the identified subject matter.

The workshop participants were a diverse group, including:

- 7 representatives of five different U.S. federal agencies;

- 1 representative from a state dam safety agency;

- 1 representative from a state environmental agency;

- 7 representatives of six different consulting companies;

- 4 independent consultants;

- 1 university professor;

- 1 representative of a hydropower organization; and

- Individuals from 10 different U.S. states, the District of Columbia, and Canada.

3.2 SELECTION OF WORKSHOP TOPICS

To accomplish as much as possible within the relatively short duration of the workshop, it was 

judged necessary to pre-select specific topics to be addressed. In pre-selecting the topics, the 

following three goals were established. Those goals were:

- To document the state-of-the-practice concerning cost-effective techniques for maintenance 

and replacement of components associated in dam outlet works.

- To outline a scope for both the short-term (immediate) and long-term research and 

development needs with respect to dam outlet works. Research and development were 

broadly defined to include “technology transfer.”

- To recommend a course of action to address the needs identified based on priorities factoring 

in potential benefit and cost/ease of implementation considerations.

Considering the three-day duration of the workshop, it was judged that six topics, each being 

addressed for about one-half day, was the maximum number of topics that could be addressed in 

any reasonable depth.

The six selected topics are presented in Exhibit 3-2. Furthermore, in order to focus discussion of 

the selected topics, white papers were prepared by invited experts and distributed to participants 
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in advance of the workshop for review. A summary of the white paper topics and their authors is 

presented in Exhibit 3-3.

3.3 WORKSHOP MECHANICS

The workshop was conducted over three full days divided into six half-day periods. By grouping 

the presentation and discussion of Gates, Valves, and Controls (Topic 3), and Energy Dissipators 

(Topic 4) into one half-day period on the second day, the six selected topics were covered in five 

half-day periods, allowing the last half-day period to be allocated to reviewing, discussing, and 

prioritizing the leading research and development needs. The generic agenda for each of the five 

half-day periods during which the selected topics were discussed was as follows:

- One-half hour – Presentation of a “State-of-the-Practice” white paper, prepared in advance 

by one or two (in the case of one paper) of the workshop participants.

- One-half hour – Facilitated discussion of the white paper by all workshop participants to 

identify revision, modifications, and refinements to the “State-of-the-Practice” presented by 

the white paper author(s).

- One-half hour – Facilitated brainstorming of possible research and development ideas that 

could advance the state-of-the practice.

- One-quarter hour – Prioritization of possible research and development ideas that could 

advance the state-of-the-practice.

- One hour – Small work groups established to develop preliminary implementation plans for 

the highest priority research and development ideas. (Typically 4 to 5 participants.)

- One-half hour – Oral reports from the Implementation Work Groups were given to 

participants on preliminary implementation plans developed.

Upon conclusion of the fifth half-day session on the third day, approximately 1 hour was devoted 

to reviewing, and discussing all of the leading research and development ideas for which 

preliminary implementations plans had been developed. Approximately 45 minutes were then 

allocated for each of the workshop participants to individually rank each of the research and 

development ideas according to both potential benefit and cost/ease of implementation.

The discussions, research and development ideas, and preliminary research and development 

plans were captured on flipcharts and comment forms used during the workshop, for compilation 

and inclusion in this report.

Some of the specific aspects of the workshop mechanics are discussed further below.

3.4 WHITE PAPERS

After the six topics were selected and prior to the workshop, some of the workshop participants 

were invited to prepare “white papers” to provide documents that would be “strawmen” for 

definitions of state-of-the-practice relative to the six topics. The white paper authors were also 

invited to put forward suggestions for research and development ideas related to their topics, if 

they so desired, but their primary responsibility was to develop “strawmen” to facilitate 

discussion of the state-of-the-practice.
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The authors all prepared their white papers for distribution to the participants in advance of the 

workshop. They are owed a debt of gratitude for their contribution to the success of the 

workshop. The white paper authors are all listed in Exhibit 3-3, and the individual white papers 

are presented in Section 4 of this report.

3.4.1 Discussion of White Papers

The revisions, modifications, and refinements to the white papers provided by the participants 

were captured on flipcharts during the workshop and are reported in Section 4 of this report. The 

state-of-the-practice discussions for each of the six selected topics identified key areas of

deficiency in both design and practice that need to be addressed either through the development 

of uniform standards for the various components of dam outlet works across the country or 

through further research and development initiatives. The discussions also indicated a 

widespread failure to routinely test components of dam outlet works to ensure their proper future 

functioning during the occurrence of an emergency condition. The adverse affects of human 

interference, tampering with operating equipment, and continued facility operations with known 

problems were also identified as pervasive and widespread problems that continued to be in need 

of redress.

3.5 IDENTIFICATION AND PRIORITIZATION OF RESEARCH AND DEVELOPMENT 

IDEAS

For each topic, the potential research and development ideas were compiled from a 

brainstorming process with the entire group. The ideas were listed on flipcharts visible to all 

participants. The research and development ideas for each topic were then prioritized in a simple 

voting process, in which each participant was given a fixed number of “stick-on-dots” that they 

could place next to the individual ideas. The number of dots (votes) given to each participant 

was typically about N/3, where N is the total number of research and development ideas being 

considered. The participants were allowed to cast their votes however they saw fit; there were 

no limits on the number of votes that a participant could cast for a particular idea. A participant 

could cast all of his votes for one research and development idea, if he thought it was a high 

enough priority. Before they cast their votes, the participants were instructed to balance the 

following three criteria in prioritizing the research and development ideas: 1) potential benefit, 2) 

probability of success, and 3) cost/ease of implementation.

All of the research and development ideas generated in the workshop as well as the results of the 

prioritizations for all six topics are presented in Section 4 of this report.

3.6 PRELIMINARY IMPLEMENTATION PLANS

After the prioritization was completed, the top four or five ideas for each topic were selected for 

development of preliminary implementation plans. The workshop participants were divided into 

small work groups (about 4 or 5 people per group), and each work group was assigned the 

responsibility to develop a preliminary implementation plan for one research and development 

idea. The small groups work independently for a period of time, and then all of the workshop 

participants reconvened to hear and discuss reports from all of the small work groups. The 

composition of the small work groups was shuffled for each topic, so that the same people were 
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not working together all of the time. Work group leadership was rotated among workshop 

attendees with responsibility rotated among most of the participants over the course of the three 

days.

The implementation plans developed by the work groups are presented in Section 4 of this 

report.

3.7 OVERALL PRIORITIZATION OF RESEARCH AND DEVELOPMENT IDEAS

The overall prioritization of the 20 leading research and development ideas resulting from 

consideration of the six different topics was completed at the workshop using the same voting 

technique used for prioritizations of preliminary ideas considered for each of the six different 

topics. Participants were then asked to make an assessment of each R&D idea’s potential benefit 

and associated implementation cost/ease of implementation by assigning a value between zero 

and ten as follows: high potential benefit would be indicated by assigning a value closer to 10, 

while an idea with low potential benefit would be indicated by assigning a value closer to 0; 

favorable low cost or ease of implementation would be indicated by assigning a value closer to 

10, while unfavorable high cost or difficulty to implement the R&D idea would be indicated by 

assigning a value closer to 0. The values for each of the research and development ideas 

assigned by each of the 16 participants were then averaged in order to determine overall rankings 

for each of the research and development ideas according to potential benefit, cost/ease of 

implementation, and on the combination of both factors.

The prioritization of the 20 leading research and development ideas is discussed in detail in 

Section 5 of this report.
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EXHIBIT 3-1

WORKSHOP PARTICIPANTS

ORGANIZATION NAME ADDRESS PHONE/FAX EMAIL

6911 Southpoint Drive P: 604-528-2418

Burnaby, BC V3N 4X8 F: 604-528-8133

2884 Saklan Indian Drive P: 925-933-5994

Walnut Creek, CA 94595 F: 925-287-0802

P.O. Box 4 P: 303-838-4920

Conifer, CO 80433 F: 303-825-4920

8181 East Tufts Avenue P: 303-740-3906

Denver, CO 80237 F: 303-694-3946

6505 West 31st Avenue

Denver, CO 80214

500 C Street, S.W., Room 416 P: 202-646-2802

Washington, D.C. 20472 F: 202-646-3990

San Francisco Regional Office

901 Market Street, Suite 350

San Francisco, CA 94103-1778 

4055 International Plaza, Suite 200 P: 817-735-7333

Fort Worth, TX F: 817-735-7490

PO Box 67100

Harrisburg, PA 17106

2353 130th Avenue NE, Suite 200

Bellevue, WA 98005

501 W. Felix, Bldg. 23

Fort Worth, TX 76115

MWH Americas, Inc.

NRCS-NDCSMC

P: 717-763-7211 

x. 2657 

F: 717-763-1140

BC Hydro

Consultant

Consultant

Consultant, URS

Consultant

Department of Homeland Security, 

EPR/FEMA

Federal Energy Regulatory Commission

Freese and Nicholls

Gannett Fleming

Charles Holder

Jack Cassidy

Hank Falvey

Lee Gerbig

Ed Rossilon

Gene Zeizel

Dave Ricketts

Ron Waters

Robert Kline, Jr.

Mark Hammer

Wade Anderson

charles.holder@bchydro.com

jcassidys@aol.com

h.falvey@worldnet.att.net

lee_gerbig@urscorp.com

rosie6617@juno.com

gene.zeizel@dhs.gov

dave.ricketts@ferc.gov

rhw@freese.com

rkline@gfnet.com

mark.a.hammer@mwhglobal.com

wade.anderson@ftw.nrcs.usda.gov

P: 303-233-2480

P: 415-369-3396 

F: 415-369-3322

P: 425-896-6961

P: 817-509-3764 

F: 817-509-3753
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EXHIBIT 3-1

WORKSHOP PARTICIPANTS

-CONTINUEDORGANIZATION NAME ADDRESS PHONE/FAX EMAIL

5975 Shiloh Road, Suite 114 P: 770-781-8008

Alphraretta, GA 30005 F: 770-781-8003

Dam Safety Branch

Colorado Division of Water Resources

1313 Sherman Street, Room 818

Denver, CO 80203

Department of Environmental Serv.

P.O. Box 95, 6 Hazen Drive

Concord, NH 03302-0095

8181 East Tufts Avenue P: 303-740-2685

Denver, CO 80237 F: 303-694-3946

8181 East Tufts Avenue P: 303-740-3812

Denver, CO 80237 F: 303-694-3946

8181 East Tufts Avenue P: 303-740-3812

Denver, CO 80237 F: 303-694-3946

P.O. Box 1159

Cincinnati, OH 45201-1159

P.O. Box 1159

Cincinnati, OH 45201-1159

P.O. Box 1159 P: 513-684-3804

Cincinnati, OH 45201-1159 F: 513-684-6361

P.O. Box 25007, Mail Code D8130

Building 67, Denver Federal Center

Denver, CO 80225-0007

USU Research Foundation P: 435-797-3176

F: 435-797-0296 mikejnsn@cc.usu.edu

P: 303-866-3585 

x. 8276 

F: 303-866-3589

P: 603-271-1961 

F: 603-271-2982

P: 303-445-3262 

F: 303-445-6490 ccooper@do.usbr.gov

salvatore_todaro@urscorp.com

john_france@urscorp.com

richard_millet@urscorp.com

jgallagher@des.state.nh.us

kenh@lrh.usace.army.mil 

eric.p.holand@saj02.usace.army.mil

robert.e.taylor@lrdor.usace.army.mil

jcrowder@schnabel-eng.com

mark.haynes@state.co.us

Michael Johnson

Chuck Cooper

Sal Todaro

John France

Richard Millet

Jim Gallagher

Ken Halstead

Eric Holand

Robert Taylor

Jimmy Crowder

Mark Haynes

Utah Water Research Laboratory

U.S. Bureau of Reclamation

URS Corporation

URS Corporation

URS Corporation

State of New Hampshire

U.S. Army Corps of Engineers

U.S. Army Corps of Engineers

U.S. Army Corps of Engineers

Schnabel Engineering Company

State of Colorado
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EXHIBIT 3-2

WORKSHOP TOPICS

1. Outlet works failure modes including consideration of failure by seepage and piping 

mechanisms along outlet works conduits.

2. Review of selection criteria, construction methods and the various conduit materials 

utilized in dam outlet works. Discussions included pipe material types and their 

advantages, disadvantages, and appropriate applications; hard foundations, soft 

foundations, and the operation of conduits under various pressure conditions.

3. Discussion about the role and types of gates, valves, and controls used in the design of 

dam outlet works. Gate, valve, and control configurations, rehabilitation, and 

maintenance considerations were topics that were also discussed.

4. Discussion about the role and various types of energy dissipators used in dam outlet 

works including stilling basins and energy dissipating valves.

5. Rehabilitation of pipe conduit spillways through dams including in-place rehabilitation, 

replacement, reservoir evacuation criteria, and economic consideration.

6. Review of the current state-of-the-practice regarding the frequency and scope of outlet 

works inspection. Systems, methods, techniques, and design criteria to accommodate 

inspection were also included in the discussion.
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EXHIBIT 3-3

WHITE PAPER AUTHORS AND TITLES

Author(s) White Paper Title/Focus

Sal Todaro Outlet Works Failure Modes

Sal Todaro Design and Construction of Outlet Works Conduits for 

Embankment Dams

Lee Gerbig Gates and Valves

Henry Falvey Energy Dissipators

James Crowder Rehabilitation of Pipe Conduit Spillways Through Dams

Chuck Cooper Outlet Works Inspection
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4 . Section 4 FOUR Work shop Results for Individual Topics

In this section of the report, the results of the workshop are presented and discussed separately 

for each of the six topics listed in Exhibit 3-2. For each topic, the following items are discussed:

1. The state-of-the-practice white paper.

2. Refinements to the state-of-the-practice white paper.

3. Identification and prioritization of research and development ideas.

4. Preliminary research and development implementation plans.

Consideration of each topic resulted in the selection of four or five research and development 

ideas for which preliminary implementation plans were developed. Preliminary implementation 

plans were developed for four research and development for the combined topic of gates valves, 

controls and energy dissipators. Section 5 of the report discusses overall prioritization of the 20 

potential research and development plans that were developed separately for the six topics 

considered.

4.1 TOPIC 1 – OUTLET WORKS FAILURE MODES

4.1.1 State-of-the-Practice White Paper

The strawman state-of-the-practice white paper for this topic was prepared by Rich Millet and 

Sal Todaro, and is presented in full in Attachment 4 of this report. A brief summary of the 

highlights of the paper is presented here.

The primary causes of outlet works failures are: foundation related failures, structural 

deterioration, structural failures, mechanical failures, failures related to hydraulics, failures 

related to ice and sediment loadings, and operator error. In addition to being one of the leading 

causes of embankment dam failures, outlet works failures can also result in less catastrophic 

consequences like: the inability to make required reservoir releases; structural or mechanical 

conditions requiring emergency draining of the reservoir; or the uncontrolled release of water 

from the reservoir. The components of outlet works systems generally include intake structures, 

conduits or tunnels, control structures and energy dissipation structures when they are 

incorporated into the overall system design. Therefore failure in one or more parts of these 

system could lead at best to failure of the outlet works, and at worst, result in catastrophic failure 

of the embankment dam.

Intake structures are those where water enters the outlet works and can be submerged structures, 

tower structures with multiple level inlet ports, or inclined structures located on a dam abutment 

with a single bottom inlet or with multiple level inlet ports. Intake gates, valves, and trash racks 

are the principle mechanical features on intake structures. These features are usually operated by 

mechanical control systems that include manual, motor driven, and hydraulic systems. The 

primary causes of gate and valve malfunction and failure include: the collection of debris, 

cavitation, ice, operator error, malfunctioning of gate operating mechanisms, lack of exercise, 

and deterioration from corrosion. Trash rack failure and malfunction usually result from 

plugging due to sediment, debris, or from plugging with frazil ice.

The most common failure modes for intake structures are as follows:
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• Poor foundation condition resulting in differential settlement between the intake structure 

and the outlet conduit. Care should be taken to locate intake structures on firm bedrock 

whenever possible;

• Premature deterioration of intake structures usually resulting from accelerated aging due 

to adverse environmental conditions including: damage from freeze-thaw processes; the 

use of poor concrete materials like alkaline aggregates; and exposure to water with either 

excessively high or low pH values which can accelerate the corrosion of reinforcement 

and cause concrete to deteriorate;

• Loading from expanding reservoir ice usually resulting in damage to trash racks, gate 

stems, intake towers and piers for intake tower access;

• Seismic loading from an earthquake resulting in the inoperability of components of the 

intake structures;

• Accumulation of sediments in intake structures resulting from improper reservoir 

operation, upstream construction, and other land clearing activities like forest fires;

• Hydraulic failures including cavitation of intake gates or areas at the intake structure or in 

the conduit downstream of the intake gate;

• Air/Water related failures including the inadequate supply of air to control gates causing

cavitation and vibration of the intake towers. Air blowback also creates operational 

problems and is caused when air and water are blown violently back into the intake 

structure in the reverse direction through the conduit; and

• The occurrence of structural fatigue caused by vibration or collapse that results from the 

progressive plugging of the intake structure. 

Methods that can be used to prevent the failure of intake structures include:

• Locating the intake structure on a rock foundation in order to reduce the magnitude of 

settlement effects;

• Designing these structures to ensure that air demand requirements to prevent blowback, 

pipe collapse, and cavitation of control gates are satisfied;

• Designing these structures to account for ice loading. Using bubbler systems to prevent 

ice formation is recommended;

• Using an experienced mechanical engineer to design the mechanical components of 

outlet works systems;

• Using reliable standard designs that have been developed and tested over time;

• Locating the inlets of the intake structure to minimize sediment collection; and

• Including consideration of vibration affects on trash racks. 

Conduit outlets are usually constructed through dams and can increase the risk of failure of 

embankment dams because of the potential for embankment piping near the conduit. Outlet 

conduits are usually made of cast-in-place concrete or constructed from manufactured pipe while 
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pre-cast concrete, plastic, steel and ductile iron are materials commonly used in pipe fabrication. 

The four most common failure modes of outlet conduit through embankment dams are:

1. Internal erosion of embankment into the conduit;

2. Pressurized defects in the conduit casing resulting in embankment piping;

3. Piping of the embankment along the outlet conduit; and 

4. Seepage through a fracture in the embankment near to the conduit.

The three primary ways of minimizing the failure potential of outlet conduits are: 1) Filtering the 

dam embankment in the vicinity of the outlet conduit to prevent internal erosion of the 

embankment along or near the conduit; 2) Constructing outlet conduits on rock foundations 

when possible; and 3) Designing conduits with redundant features to prevent water seepage from 

the conduit into the embankment resulting in the piping of embankment materials into the 

conduit. By contrast, tunnel outlet works are separated from the dam embankment and are 

generally considered to be a safer than conduits. These tunnels are usually lined with cast in 

place concrete or welded steel. However, even if cost considerations allow a tunnel outlet works 

system to be considered a viable design alternative, construction of the tunnel too close to the 

dam can result in internal erosion of a dam’s embankment, even though the occurrence is rare.

As the name implies, a control structure is one in which flow through the outlet works is 

controlled. Control structure location directly impacts the type of control devices used. 

Typically control valves may be located at intake structures, gate chambers or in a valve house at 

the downstream toe of the dam. The most common failure modes of these structures are 

associated with the failure or malfunction of gates, valves, or control systems used to operate 

these devices. To minimize the occurrence of failure therefore, it is recommended that the 

following be done: retain a mechanical engineer experienced in the design of gates, valves and 

mechanical systems for dam outlet works for both new design and rehabilitation activities; 

routinely test, operate and maintain components of control structures; and replace antiquated and 

unreliable gates, valves, and operators with newer, more reliable equipment.

Finally, in order to prevent the uncontrolled erosion of the downstream channel in cases of direct 

discharge, care should also be taken in the design and maintenance of discharge mechanisms and 

in the use of energy dissipation structures.

4.1.2 State-of-the-Practice Refinement

The discussion of the white paper resulted in the identification of the following items to further 

refine the understanding of the state-of-the-practice:

• Unwillingness of those involved to acknowledge viability of failure modes and resistance to 

thinking outside of the box.

• Misoperation or lack of operation of outworks components because of the failure of 

automatic control systems stemming from conflicting program instructions, and the 

expiration of these programs.

• Structural deterioration because of water chemistry and reaction with air and the deterioration 

of concrete because of the presence of hydrogen sulfide (H2S) in the water which can be 

evident in the presence of cavitation.
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• Cavitation because of inadequate air vents.

• Seismic loading on intake towers.

• Cracked concrete encasement resulting from earthquake loading and a discussion of 

allowable pressure on metal pipe, and the presence and role of buckled pipe in failure 

mechanisms.

• Lateral or longitudinal joint offsets also know as embankment spreading due to earthquake 

loading.

• Rupture of pressurized prestressed concrete cylinder pipe.

• Plugging of air vents by those annoyed with the sound (e.g. campers).

• The fact that entities will sometimes operate for a long time with a known problem.

• Vandalism of operating equipment.

4.1.3 Prioritization of Research and Development Ideas

The group brainstorming process resulted in the generation of 11 potential research and 

development ideas, which were then prioritized by the group. The 11 ideas and the results of the 

prioritization are presented in Exhibit 4-1.

4.1.4 Preliminary Research and Development Plans

The top five ideas listed in Exhibit 4-1 were selected for development of preliminary 

implementation plans, and those plans are presented in Exhibits 4-2 through 4-5. Note that in 

some cases, the individual work groups reworded the research and development idea, so the 

ideas listed in Exhibit 4-1 may not exactly match the ideas listed in Exhibits 4-2 through 4-5.

4.2 TOPIC 2 – CONDUIT MATERIALS, SELECTION CRITERIA, AND 

CONSTRUCTION METHODS

4.2.1 State-of-the-Practice White Paper

The strawman state-of-the-practice white paper for this topic was prepared by Sal Todaro, and is 

presented in full in Attachment 5 of this report. A brief summary of the highlights of the paper is 

presented here.

Due to the absence of any single recognized standard for designing dam outlet works there is 

currently great inconsistency in design rationale and review processes for this design aspect of 

small and medium sized embankment dam facilities. The need for a single design standard is 

further underscored by the fact that outlet works have been identified as a significant contributor 

to the occurrence of embankment dam failures. While the similar design of outlet works at all 

small and medium sized embankment dams is not proposed, consistency should exist in the 

guidelines and criteria used to prepare designs for these structures. Design guidelines could 

include recommendations for pipe encasement, the need for redundancy, seepage control, 
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methods for addressing compressible foundations, pipe selection, joint details, and criteria for 

reinforced concrete conduit design.

A single, nationally recognized standard would lead to greater consistency between similar 

project designs, facilitate more effective and consistent review of proposed designs, and result in 

increased potential for safer more reliable facilities. The National Dam Safety Review Board 

(NSRB) is in the process of preparing a manual to be used for designing and maintaining outlet 

works for significant high-hazard dams. The manual covers topics for the design, construction, 

maintenance, and repair of conduits for embankment dams. It is recommended that in the 

interim the guidelines presented in this manual be used to form a design basis for smaller 

facilities.

By determining best practices for designing and constructing outlet works, and by preparing 

nationally accepted standards to be used for engineering review, important improvements in 

outlet works design methods and construction procedures can be achieved. Furthermore, dam 

safety officials could then require and enforce minimum standards for all new non-federal 

designs, simplifying the task of design engineers by providing a consistent standard for design 

they know will be accepted by review agencies.

Presently, the use of standards not intended for use in designing pressurized outlet systems for 

embankment dams is the most common practice that introduces additional risk to outlet works 

for smaller embankment dams. Examples of the misapplication of design standards to design 

outlet works include the following:

• Use of State Highway Department standard plans for culverts and culvert structures 

even though culvert designs for highways were not intended for use in dams.

• Use of Natural Resources Conservation Service (NRCS) standards for the structural 

design of flood control outlets to design high-hazard pressurized outlet facilities even 

though these standards were not intended for such application, and 

• Use of the precast concrete outlet pipe detail with partial encasement shown in the 

USBR’s Design of Small Dams for pressurized outlet works for high-hazard dams. 

This detail allows embankments to be placed directly against the pipe surface which 

is an area where leakage from the pipe joint has direct access to the embankment.

The creation of an approved standard would therefore reduce the use of inappropriate design 

standards. The USBR and U.S. Army Corps of Engineers (USACE) design standards for outlet 

works combined with the forthcoming NSRB manual could form a basis for preparing a single 

design and construction standard for smaller dam outlet facilities.

Appropriate modifications that may be required due to variability in foundation conditions 

should also be included in the proposed design manual. Where possible, outlet works structures 

should be founded on firm non-erodable foundations. However, variable foundation conditions 

may result in the founding of outlet works structures upon compressible foundations that are 

subject to differential foundation settlement and movement. Soft foundation conditions incur 

greater risk of embankment piping and structural distress of the conduit. Foundation movements 

are especially dangerous for rigid outlet pipes with open gasketed joints. Therefore outlet 

conduit should be located on uniform foundations whenever possible to prevent abrupt changes 

in conduit settlement.
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Recommended approaches to the design of outlet conduits on soft foundations include: 1) 

Placement of freestanding pressurized outlet pipe inside of a cast-in-place reinforced concrete 

conduit; 2) Use of a non-pressurized outlet works system; 3) Use of welded steel outlet pipe 

encased in reinforced concrete; 4) Use of articulated joints for the outlet conduit; and 5) 

Protecting the foundation’s integrity during construction.

While various pipe materials have particular advantages and disadvantages, the ones most

commonly used for outlet works conduits are: cast-in-place reinforced concrete; welded steel 

pipe encased in reinforced concrete, high density polyethylene (HDPE) pipe, ductile iron pipe, 

and reinforced concrete pressure (RCP) pipe.

While not usually considered as a design alternative where soft ground conditions are 

encountered, tunnel outlet works present several distinct technical advantages when compared to 

cut-and-cover outlet conduit designs, especially for pressurized outlets. These advantages 

include:

• Elimination of embankment failure modes associated with conduit outlet works.

• Stream diversion capabilities around the dam site during construction.

• Construction of tunnel outlet works independent of embankment construction.

• Embankment placement unobstructed by the outlet conduit

• Elimination of special compaction requirements, and the need for special filter 

placement required for conduit systems.

However, tunnel outlet works systems are often more expensive than conduit systems and 

typically involve more cost risk for overruns. Thus, development of cost curves for tunnel 

outlets based on outlet diameter would be helpful allowing engineers to compare cost/ease of 

implementations for preliminary evaluations of tunnel and conduit outlets. The design standard 

would therefore provide a consistent design approach, and allow various design alternatives to be 

evaluated and considered in a systematic and rational manner. 

4.2.2 State-of-the-Practice Refinement

The discussion of the white paper resulted in the identification of the following items to further 

refine the understanding of the state-of-the-practice:

• Conduits need to be large enough for inspection and repair.

• Bends in conduit alignments can eliminate repair/rehabilitation options (e.g. slip-lining).

• Concrete-encased, welded steel pipe conduits need to be designed for full reservoir pressure 

on the outside of the pipe. Leaks in the encasement and associated buckling failures have 

been caused by pressure between the steel pipe and the encasement.

• Conduit sizing criteria (e.g. for evacuation) vary among different jurisdictions, and sizing 

criteria do not exist for some jurisdictions.

• There are a number of negative factors for conduit configurations through embankments in 

addition to those sited in the white paper:
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o Lack of sufficiently detailed geotechnical information along the alignment to calculate 

settlements (total and differential).

o Differential settlement at material contacts (embankment zones and different 

foundation materials) is expected.

o Many opportunities exist for improper construction to lead to problems.

• Diving technologies are available for maintaining, rehabilitating, and replacing deep, 

underwater, upstream gates; however, these technologies are very expensive.

• Outlet tower/intake configurations vary widely. Examples include freestanding, upstream 

towers; inclined, on-slope, upstream intakes structures; towers within the embankment 

between the upstream toe and the crest, with an upstream, underwater inlet.

• There has been an increasing desire from some owners and some material manufacturers to 

use plastic pipes (e.g. HDPE), but accepted standards for use of this type of pipe in dams do 

not exist. FEMA is sponsoring a Best-Practices Guide for use of plastic pipe in embankment 

dams, which should address this issue.

• Some engineers and owners still propose use of corrugated metal pipe (CMP) conduits in 

embankments, but it is unusual. Most regulators will not accept it for high and significant 

hazard dams.

• Analysis methodologies have improved dramatically: 3-D analysis now available instead of 

1-D; and soil structure interaction included in many programs like FLAC, and Plaxis.

• For design loads, we are still typically using old guidance (e.g. Marston equations), even with 

a sense that these loads are conservative (i.e. too high).

• AWWA guidelines and ASCE penstock guidelines are available for steel pipe design. ASCE 

guidelines are more appropriate for high pressure and transient flow.

• Pressurized downstream control conduits exist in some older facilities and in some retrofits 

for inoperative upstream gates. Proposals for new installations with downstream control are 

typically concrete-encased steel, but there are exceptions.

4.2.3 Prioritization of Research and Development Ideas

The group brainstorming process resulted in the generation of 7 potential research and 

development ideas, which were then prioritized by the group. The 7 ideas and the results of the 

prioritization are presented in Exhibit 4-6.

4.2.4 Preliminary Research and Development Plans

The top four ideas listed in Exhibit 4-6 were selected for development of preliminary 

implementation plans, and those plans are presented in Exhibits 4-7 through 4-10. In some 

cases, the individual work groups reworded the research and development idea, so the ideas 

listed in Exhibit 4-6 may not exactly match the ideas listed in Exhibits 4-7 through 4-10.
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4.3 TOPIC 3 - GATES, VALVES, AND CONTROLS

4.3.1 State-of-the-Practice White Paper

The strawman state-of-the-practice white paper for this topic was prepared by Lee Gerbig, and is 

presented in full in Attachment 6 of this report. A brief summary of the highlights of the paper is 

presented here.

Releases from outlet works are often controlled to maintain river flows, provide water for 

irrigation, to a water treatment facility, or to an industrial user, and to lower reservoir levels in 

the event of an emergency. Gates and valves are the two general names for the types of 

equipment used to control the release of water in outlet works systems. A gate is a mechanical 

device with a sliding flat member within a square or rectangular framework or structure that 

controls the flow of water. A valve is usually a circular structure with a sliding member, flat or 

circular, that controls the flow of water.

The types of gates commonly used at newer dam sites are: Slide gates, Knife gates, Jet-flow 

gates, Clamshell gates, Radial gates, and Hinged Crest gates. Slide gates, also known as Sluice 

gates, are the most common type of gate used for dam outlet works. These devices have a 

movable leaf or disk which slides against bearing surfaces in the frame. The frame is attached to 

a conduit or wall, which provides support against the water load of the gate. The gate leaf is 

moved by a gate stem, which is operated by a manual hand wheel or crank, electric motoroperator, or hydraulic cylinder.

For applications where the head does not exceed 100 feet, the two types of slide gates commonly 

used are the Cast Iron Slide gate and the Fabricated Slide gate. As the name implies, both the 

rectangular gate leaf and frame of a Cast Iron Slide gate is fabricated from cast iron. Gate 

leakage from this type of slide gate is usually very low, seldom exceeding 0.1 gal/minute/foot of 

the perimeter of the leaf gate. Fabricated Slide gates are made from a variety of materials 

including steel, stainless steel, and aluminum. The gate leaf uses the metal plate as a sliding 

surface against low friction material attached to the frame. Typical leakage from this type of 

slide gate is 0.05 gal/minute/foot of the perimeter of the leaf gate. 

For applications where the head exceeds 100 feet, Bonneted Slide gates and Wheel gates are 

commonly used. Bonneted slide gates are similar in design to Fabricated Slide gates, except that 

the body is fabricated to completely enclose the gate leaf. This type of slide gate is usually 

embedded in reinforced concrete to provide additional structural reinforcement for the 

pressurized gate body. In Wheel gates, the gate leaf is fitted with multiple wheels that ride on 

rails attached to the face of the structure. It should also be noted that Wheel gates could be 

designed to close by gravity in the event of an emergency. However, one of the main drawbacks 

with Wheel gates is difficult installation due to the critical alignment that is required to achieve 

smooth operation and effective sealing. Due to susceptibility to corrosion and the build up of 

deposits that increase friction and affect the operation of its roller train, the use of Roller gates is 

increasingly avoided even though this was a type of slide gate that was also used for high head 

applications. 

Knife gates and Jet-flow gates are two other variations of the slide gate. While Knife gates are 

not commonly used in the hydropower or dam industry, Jet-flow gates are commonly used as 

freely discharging devices. The gate has a circular opening with a sliding disk that moves over a 
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floating, sloping orifice shaped opening. The sloping orifice concentrates the flow to help 

introduce air to prevent cavitation. The Clamshell gate is a special design used by the U.S. 

Bureau of Reclamation consisting of two leafs operating in an arc over the end of a conduit, and 

machined to match the arc of the gate leafs. The gate leafs move in a symmetrical manner to 

provide uniform flow between the two leafs and allow this device to work very well in 

submerged installations. Radial gates have been used for many installations at spillways, canal 

check structures, and canal turnouts. Hinged Crest gates are used at the crest of a spillway, to 

allow flow overtopping the gate to regulate the reservoir or pool elevation. The gate consists of a 

flat or slightly curved plate structure, with a hinge system at the bottom to allow the gate to 

rotate from near vertical to horizontal. 

Valve types commonly used are: Butterfly valves, Gate valves, Fixed-Cone valves, and Sleeve 

valves. Butterfly valves are commonly used at power plants and water facilities and have a 

circular body with a circular disk in the fluid way that rotates around a shaft in the centerline of

the valve, perpendicular to the flow. The flow velocities through butterfly valves are usually 

limited to 25 to 30 feet per second, although high-performance butterfly valves can operate up 

top 50 feet per second. Butterfly valve operating forces are very low, with very little friction and 

hydraulic effects. They may be operated manually, by electric motor-operators, or by hydraulic 

operation. A Gate valve is a variation of a slide gate, with a circular body and disk. They are 

often used for guard gates upstream of regulating valves, or used for throttling flow in low-head 

applications. A Fixed-Cone valve is commonly used for regulating the flow of water from a dam 

outlet works. This cylindrical valve has a sliding sleeve that covers the opening at the 

downstream end of the valve. Sleeve valves are used as energy dissipaters, both in-line and at 

the end of a conduit and are usually operated to provide precise flow control. Due to high costs 

and problems with corrosion, seat erosion, cavitation damage, or general maintenance difficulties 

there has been a decreased incidence of Needle valves and Hollow-Jet valves.

The four basic types of gate operation systems are: manual operators, electric motor-operators, 

hydraulic operators, and pneumatic operators. Manual operation usually involves use of a crank 

or handwheel connected to the gate or valve operating mechanism with threaded stems or a 

gearbox arrangement. Electric motor-operators also provide either linear motion with threaded 

stems or rotary motion converted to linear motion. Electric motor-operators are usually designed 

for outdoor applications, with weatherproof enclosures. In the event of an interruption in power, 

hand wheels are usually provided on these units for back up manual operation. Hydraulic 

operation is most often used on large gates and valves operating at high head, but is becoming 

more common for small equipment at low heads for particular installations. Pneumatic operators 

are rarely used for operating gates and valves. The problem with pneumatic operation in typical 

gates and valves is that, with the air, being compressible, it is difficult to maintain a set position. 

Portable drills, portable hydraulic operators, and chain saw operators are the most common 

portable operators that are available to work in conjunction with manual operators.

With increased equipment reliability and performance, remote and hydraulic operation of gates 

and valves occurs more frequently. Remote sensing may be achieved using electric motor 

operators, hydraulic cylinders, and inclinometers. Improved hydraulic fluids suitable for outdoor 

use, submerged hydraulic operation, use of better seals, and automated operation features has 

inspired increased use of hydraulic components in the design of gates and valves.
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However, the design and fabrication of gates and vales has increasingly shifted overseas with 

major US manufacturers in the process emerging from bankruptcy proceedings. However the 

ultimate effect of offshore fabrication and the disappearance of longstanding US-based gate and 

valve manufactures is the significant decrease in the number of gate designers in the United 

States. Moreover, with government agencies like USBR, USACE, and TVA scaling back dam 

design and construction phases of work, there has also been a corresponding decrease in the 

amount of available research on new equipment and operation.

4.3.2 State-of-the-Practice Refinement

The discussion of the white paper resulted in the identification of the following items to further 

refine the understanding of the state-of-the-practice:

• There are two versions of hoods on fixed-cone valves: in one configuration the hood is 

connected to the body of the valve and moves with the valve, while in the other the hood is 

separate from the body of the valve and stays in a fixed position.

• A hood on the fixed cone valve limits the applicable head to about 175 feet.

• A bullnose bottom should be used if a slide gate will be used for regulating the flow. This 

works well up to heads of at least 75 feet, and possibly as much as 100 feet. 

• Although it is desirable to design outlet works with non-pressure conduits, it can be difficult 

to design upstream control for outlet works systems for high heads. There is a lack of 

confidence in the available gates and valves for these high head, flow control applications.

• Gates intended to close against head may not be as reliable as we would like.

• AWWA standards for design of stems and operators may not be adequate for dam outlet 

works. The stems can be too easily overloaded and buckle.

• There have been frequent problems with leakage of slide gates mounted in the unseating 

direction (relative to the head).

• There is a lack of industry-wide knowledge of the potential for catapulting of gates because 

of insufficient airflow.

• Butterfly valves and operators can be designed to fail in the open or closed position. Design 

needs to consider which is preferred for that particular installation. If the valve fails in the 

closed position, dampers may be needed to slow the rate of closing to prevent surge damage.

• There have been problems with the corrosion of butterfly valve bushings or stems, resulting 

in valves that did not close, as intended.

4.3.3 Prioritization of Research and Development Ideas

Since this topic had been combined with the topic on energy dissipators, the group brainstorming 

process resulted in the generation of 10 potential research and development ideas for the 

combined topics. The group then prioritized these topics and an overall ranking of research and 

development ideas for the combined topics was generated. The 10 ideas and the results of the 

prioritization are presented in Exhibit 4-11.
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4.3.4 Preliminary Research and Development Plans

The top four ideas listed in Exhibit 4-11 were selected for development of preliminary 

implementation plans. The ideas directly related to the topics of gates, valves, and controls are 

presented in Exhibit 4-12, Exhibit 4-14, and Exhibit 4-15. In some cases, the individual work 

groups reworded the research and development idea, so the ideas listed in Exhibit 4-11 may not 

exactly match the ideas listed in Exhibit 4-12 and Exhibits 4-14 and 4-15.

4.4 TOPIC 4 – ENERGY DISSIPATORS

4.4.1 State-of-the-Practice White Paper

The strawman state-of-the-practice white paper for this topic was prepared by Henry Falvey, and 

is presented in full in Attachment 7 of this report. A brief summary of the highlights of the paper 

is presented here.

Almost every outlet works requires an energy dissipator so that the high-energy flow from the 

reservoir does not damage the downstream channel or structures. While the laws of conservation 

of energy technically prohibit energy from being “dissipated,” energy dissipators enable energy 

conversion for high energy water flows by: creating fine grain turbulence; and creating a loss 

through a change in the flow’s momentum. Wide varieties of structures are available to 

designers to dissipate high head-high energy flows from outlet works. Many designs have been 

developed for site-specific installations that have not appeared in the literature and it should be 

also noted that some of these designs have also not been verified by either modeling or prototype 

tests. 

However, energy dissipators that have been successfully used at outlet works include: stilling 

basins, impact basins, stilling wells, various types of valves, sudden enlargements, in-line 

orifices, flip buckets, and plunge pools. Outlets works can incorporate a combination of 

different types of energy dissipators. The types of energy dissipators used depend on the design 

reservoir head, discharge requirements, and cost constraints of a particular design.

USBR Engineering Monograph 25 provides a thorough discussion of the design elements for 

stilling basins, impact basins, flip buckets, and baffled aprons. General guidelines for designing 

outlet works are provided in both USACE Engineering Manual EM 1110-2-1602 and in Design 

of Small Dams published by the USBR.

The USBR defines three types of hydraulic jump stilling basins. The Type I Basin is simply the 

combination of a hydraulic jump with a horizontal apron with energy dissipation accomplished 

by turbulence created within the jump. Type II Basins contain chute blocks located at the end of 

the basin and a dentated end sill. The chute blocks create shear zones that generate fine grain 

turbulence while the end sill prevents downstream erosion and does not contribute significantly 

to energy dissipation. The Type III Basin contains an additional set of baffle blocks within the 

horizontal apron to create additional turbulence. The combination of chute blocks and baffle 

blocks allows the length of the stilling basin to be reduced.
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Reclamation classifies impact basins as Type VI Basins. In these structures energy is dissipated 

by turbulence that is generated when a jet of water makes impact with a vertical wall. Although 

Reclamation limits the dimensions and discharges of impact basins to relatively low energy 

levels, the basins have been used successfully at much higher energy levels by scaling the 

recommended dimensions to the larger sizes.

Stilling wells are frequently used for low discharge flows into canals. Energy dissipation is 

achieved by the change in momentum between the water entering the well through the pipe and 

the flow up the square well. Flow is controlled by changing the size of the opening using a 

sleeve valve on the bottom of the down comer pipe.

For applications with high head or high flow requirements, fixed-cone valves are normally used 

to dissipate energy. These valves can use the principle of impinging jets and baffles downstream 

of the valve to provide energy dissipation. It should be noted however, that the distance between 

the end of the fixed cone valve and the hood is critical as severe blowback has been experienced 

in prototype installations where this distance was off by only an inch or two.

Sudden enlargements occurring within or at the end of a pipeline can be very effective energy 

dissipators. Precautions must be taken during the design of these features to ensure that the 

formation of cavitation within the structure is minimized even though some ranges of operating 

conditions will still generate cavitation. Incorporation of a series of in-line orifices into a 

pipeline also utilizes the energy dissipation function of sudden enlargements.

Flip buckets are often used at the end of an outlet chute to throw discharge flow away from the 

toe of the dam. Flip buckets are actually not energy dissipation devices although they are 

designated as such in the USBR Engineering Monograph No. 25. Dentates are frequently used 

to spread the compact jet and increase the area over which the jet enters the plunge pool thereby 

decreasing the energy per unit area of the inflow jet.

Plunge pools are commonly used in combination with flip buckets and are designed to minimize 

the erosion of the downstream channel that a plunging jet of water can cause. Care must be 

taken in the design of these structures since material excavated by a plunging jet of water can 

raise the tailwater in the pool such that it sufficiently drowns out the effects of the flip bucket. 

4.4.2 State-of-the-Practice Refinement

The discussion of the white paper resulted in the identification of the following items to further 

refine the understanding of the state-of-the-practice:

• There have been problems with backflows in USBR-type stilling basins. The backflows have 

drawn rock and soil materials into the basins, sometimes leading to ball-milling and related 

damage.

• There is a lack of available information on ultimate development of plunge pool basins in 

rock including ultimate size and depth. A Swiss engineer is doing some work in this area, as 

is George Annandale, with work to be published soon.
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• There have been cases where energy dissipation characteristics have been affected by valve 

changes. Examples offe red at the workshop include changing geometry/dimensions for 

fixed-cone valve installations and replacement of needle valves with fixed-cone valves.

• There is a lack of widespread, definitive knowledge about reliable repair techniques for 

energy dissipators – one available source is the USACE REMR documents.

• There is a lack of specific guidelines on the extent and size of riprap required downstream of 

energy dissipators. This can contribute to a ball-milling problem, if the riprap is small 

enough to be drawn into the stilling basin. Grouted riprap has sometimes been used to 

address this concern.

• The sleeve valve concept has been modified in some applications to consist of a fixed section 

of pipe with holes and an upstream control valve. This concept has been used for 

intermittent flows with a limited range of discharge.

• No guidelines are available on the appropriate frequency of inspection of stilling basins.

• With increased use of failure modes analysis, downstream erosion and undercutting of 

stilling basins or concrete dams is becoming an increasingly frequent concern. Methods for 

reliably evaluating this issue are not readily available.

• Vandalism leading to ball-milling is a problem which often results from people throwing 

rocks into stilling basins. This has lead to measures to make it more difficult to throw rocks 

into the structures.

• Public safety can be an issue for stilling basins. People will swim in the basins, and sloping 

downstream ends of the basins can make it difficult for people to get out of them. Steps in 

place of the sloped surface have been used to address this safety concern.

4.4.3 Prioritization of Research and Development Ideas

Since this topic had been combined with the topic on gates, valves and controls, the group 

brainstorming process resulted in the generation of 10 potential research and development ideas 

for the combined topics. The group then prioritized these topics and an overall ranking of 

research and development ideas for the combined topics was generated. The 10 ideas and the

results of the prioritization are presented in Exhibit 4-11.

4.4.4 Preliminary Research and Development Plans

The top four ideas listed in Exhibit 4-11 were selected for development of preliminary 

implementation plans. The idea directly related to the topic of energy dissipators is presented in 

Exhibit 4-13.
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4.5 TOPIC 5 – REHABILITATION OF CONDUITS

4.5.1 State-of-the-Practice White Paper

The strawman state-of-the-practice white paper for this topic was prepared by James Crowder, 

and is presented in full in Attachment 8 of this report. A brief summary of the highlights of the 

paper is presented here.

Corrugated metal pipe (CMP) commonly serves as the conduit found in small earthen 

embankment dams ranging in height from 10 to 50 feet constructed between 30 and 50 years ago

in the southeastern United States. Compared to conduit materials used in newer dams, CMP is 

highly susceptible to deterioration and becomes a problem when a spillway conduit has reached 

or exceeded its design service life, generally considered to be 25 to 30 years. Earthfill 

embankments of this vintage were often constructed with CMP spillway conduits passing 

through the base/foundation of the embankment. The deterioration of these conduits generally 

consists of leaking joints, corrosion, and holes in the pipe. 

The main risk associated with the deterioration of these conduits is piping of the surrounding 

embankment soils into the conduit, which can lead to the eventual failure of the earthen 

embankment. Owners of most old embankment dams exhibiting problems with CMP conduits 

usually have a number of concerns or issues that will usually involve several remediation or 

repair options. The four primary options discussed, including the advantages and disadvantages 

of each method, are based upon the experiences of the author in the Southeast United States and 

are mainly limited to CMPs in earthen dams having a maximum height of 50 feet. These options 

include:

• Replacement of the existing pipe conduit using cut and cover methods;

• Sliplining the existing conduit;

• Grouting along the exterior of the existing conduit; and

• Constructing either a siphon spillway or “short” riser and new outlet conduit after the 

existing conduit has been abandoned.

In order to replace the existing conduit, excavation of the earthen embankment, removal of the 

deteriorated conduit, installation or construction of a new conduit, and backfill of the excavation 

is required. The side slopes of the excavation through embankment dams are typically limited to 

2H: 1V or flatter, in order to facilitate compaction of the replacement backfill against the existing 

embankment. On relatively high embankments in narrow valleys, the excavation required to 

remove the existing low-level conduit may necessitate the removal of nearly the entire dam. The 

replacement option typically offers the greatest operational flexibility among the spillway 

rehabilitation options. This option typically offers the most thorough rehabilitation option for 

impaired conduits through dams, although this option can be cost-prohibitive.

Sliplining an existing pipe consists of placing a new pipe with a smaller diameter inside the 

existing deteriorated conduit and grouting the annular space between the two pipes. Depending 

on the pipe used, sliplining may increase the hydraulic capacity of the outlet pipe, even when 
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reducing the inside diameter of the original pipe by reducing the roughness of the pipe. When 

designed and installed properly, the slipline technique will also add additional years to the 

service life to the existing conduit spillway. A sand diaphragm drain is typically installed around 

the downstream portion of the existing conduit to limit the potential for soil piping as the CMP 

eventually corrodes and deteriorates.

Grouting along the exterior of an existing conduit is often used to rehabilitate conduits that 

exhibit seepage through joints or deteriorated portions of the existing pipe wall. Grouting fills 

voids that may have been created, and to plug openings in the pipe walls or joints. Cementitious 

or chemical grouts are typically utilized for the purpose. Injection of the materials can be 

performed either from the surface of the embankment or from the interior of the conduit. This 

approach requires the availability of working room for grouting equipment and personnel. Use 

of this method, however, is limited by the depth of the conduit below the embankment surface, 

and whether the remediation is performed with the lake full. Injection of the grout from the 

interior of the pipe typically offers a better opportunity to deliver the grout to specified areas. 

Drilling through the embankment to grout along the conduit is usually performed for small 

diameter conduits or when the pipe is very shallow within the embankment. The greater the 

depth of fill over the conduit, the less certainty the grouting procedures are impacting the 

targeted areas. Drilling through the embankment is not practical for situations in which the lake 

cannot be drained and portions of the conduit within the upstream slope of the dam are to be 

grouted. Moreover, this type of repair is generally classified as “temporary” and does not extend 

the design life of the existing conduit material. Grouting of the exterior of a conduit pipe also 

retains the stage/discharge relationship of the existing spillway.

Abandonment consists of completely grouting the interior of the existing CMP with either a 

sand/cement grout or a gravel/sand/cement grout. In either scenario, an expansive chemical 

admixture is typically included in the grout mix to control or reduce shrinkage during curing of 

the grout. Siphons have been used successfully in the southeast but are generally limited to 

small drainage basins with relatively small peak inflows. Siphons can be constructed of flexible 

conduit such as PVC, HDPE, or Ductile Iron. Siphon spillways do not typically exceed 12-

inches in diameter. Abandonment of an existing conduit and construction of a short riser and 

pipe spillway may be considered to be a modified version of the traditional pipe-and-riser 

spillway system or an economical hood inlet spillway system. Usually, the riser invert is set a 

vertical distance below normal pool equal to 1/5 to 1⁄4 of maximum water depth at the upstream 

toe of the dam. The conduit invert should be a minimum depth of 1.5 diameters below the pool 

elevation to avoid the capture of air due to surface water drawdown, and the associated 

“slugging” under full flow conditions. Installation of the siphon also requires excavation of the 

embankment below normal pool.

4.5.2 State-of-the-Practice Refinement

The discussion of the white paper resulted in the identification of the following items to further 

refine the understanding of the state-of-the-practice:

• There is limited guidance available on siphon outlet design. EPRI published a document a

number of years ago.
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• Lack of guidelines for design of sliplining rehabilitation, including issues such as design 

criteria, grout characteristics (shrinkage), construction issues, etc.

• Feasibility of sliplining can be limited by configuration of conduit and associated gates and 

valves – e.g. difficulty often exists in sealing the lining located next to the gate or valve.

• BC Hydro has done some replacement of underwater, upstream structures, including 

control/guard gates.

• Cured-in-place (CIP) linings (e.g. Insituform) have been used for outlet conduit 

rehabilitation.

• NRCS has done one installation in Texas replacing a conduit by augering through an 

embankment dam. NRCS is considering use of microtunneling for other projects.

• Tunneling and microtunneling through a rock (sometimes soft rock) abutment with a lake tap 

with a full reservoir has been used and example of which was at Standley Lake located in 

Westminster, Colorado.

• A case was cited where an outlet works in a concrete dam in Texas was rehabilitated to

replace the upstream gates and line the conduits. The upstream square-to-round transition 

was found to be badly deteriorated and was reformed using Belzona polymer.

• Epoxy paints have become the preferred standard for paint coatings of outlet works conduits.

• Repainting of conduits can be very difficult – hard to get moisture and temperature in a 

suitable range for application of a durable coating application; also access is difficult, and in 

some cases impractical, for small diameter conduits.

• Lead paint on existing structures can cause problems in repainting of conduits and other 

outlet components. There are a few products available that can be used to render the lead 

paint debris non-hazardous.

• Asbestos in some operating equipment can cause some environmental problems with 

rehabilitation. 

4.5.3 Prioritization of Research and Development Ideas

The group brainstorming process resulted in the generation of 13 potential research and 

development ideas, which were then prioritized by the group. The 13 ideas and the results of the 

prioritization are presented in Exhibit 4-16.

4.5.4 Preliminary Research and Development Plans

The top four ideas listed in Exhibit 4-16 were selected for development of preliminary 

implementation plans, and those plans are presented in Exhibits 4-17 through 4-20. In some 

cases, the individual work groups reworded the research and development idea, so the ideas 

listed in Exhibit 4-16 may not exactly match the ideas listed in Exhibits 4-17 through 4-20.
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4.6 TOPIC 6 – OUTLET WORKS INSPECTION

4.6.1 State-of-the-Practice White Paper

The strawman state-of-the-practice white paper for this topic was prepared by Chuck Cooper, 

and is presented in full in Attachment 9 of this report. A brief summary of the highlights of the 

paper is presented here.

Inspection of cut-and–cover outlet works typically includes review of features including: the 

entrance channel; intake structure; conduit(s); terminal structures; and the downstream channel. 

While there are several methodologies that may be employed in inspections, the advantages and 

disadvantages of each method should be assessed with due consideration given to the ease and 

efficiency with which inspection objectives can be met; compliance with health and safety 

guidelines, and cost.

The purpose of an outlet works inspection program is to ensure that conduits through dams are 

safely and efficiently operated and maintained. Factors to be considered in specifying inspection 

programs include inspection frequency; consideration of the systems, methods, and techniques 

used for inspection; and compliance with design criteria. Inspection intervals may vary 

depending on the overall conditions determined from previous inspections and the existence of 

any dam safety concerns. Inspections primarily vary according to scope, and frequency. The 

five types of inspections generally conducted are: initial or formal; periodic or intermediate; 

routine; special; and emergency.

Initial or formal inspections include in depth review of all pertinent data available for the outlet 

works to be inspected. Design and construction data are evaluated relative to current state-ofthe-art guidelines to identify potential dam safety problems or areas requiring particular 

attention. During these inspections, an attempt is made to operate all mechanical equipment 

through their full operating range, if possible. Periodic or intermediate inspections are conducted 

between formal inspections and all mechanical equipment may not be tested during a particular 

inspection. Field or operating personnel typically conduct routine inspections. The primary 

focus in these inspections is the current condition of the outlet works. These inspections may be 

scheduled on a regular basis or performed in conjunction with other routine tasks. Special 

inspections are conducted when a unique opportunity, like low water conditions, exist. 

Emergency inspections are performed when an immediate dam safety concern is present or in the 

event of an unusual or potentially adverse condition like those resulting from the occurrence of 

an earthquake.

Scheduling periodic outlet works inspections may be influenced by the following factors: 

provision of sufficient notice to dam owners and operators allowing necessary arrangements and 

pre-inspection activities to be completed and needed clearances to be obtained; the ability to 

access most or all of the major components of the outlet works; and the opportunity to observe 

features operating under a wide range of conditions. Inspections of features like intake structures 

and upstream conduits that are usually submerged require special coordination with the dam 

owners since the timing of such inspections has to be carefully orchestrated and planned for. 
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Factors that should be considered when determining the frequency of inspection for features that 

are usually inundated include:

• The critical function of the feature to overall dam operations;

• Operational history and performance record of the feature since its last inspection;

• Condition of the concrete;

• Age of the feature;

• Condition of the embankment and foundation and the presence of faults in the 

foundation;

• Observed seepage;

• Changes in the discharge capacity of the outlet works;

• Observed damage or deterioration to visible features in the vicinity of the submerged 

feature;

• Water quality detrimental to concrete;

• Consideration of the secondary inspection costs including costs associated with lost water 

and power revenues, to provide inspection access to a feature;

• Design and construction considerations;

• Changes in relevant standards or guidelines;

• Presence of unfavorable stresses conducive to arching;

• Lack of filters and drainage material around the conduit downstream from the impervious 

zone of the embankment to convey seepage safely; and the 

• Presence of any site conditions that may compromise the safety of the feature, and 

ultimately the safety of the dam.

The USBR has designated three inspection frequencies at which high hazard dams are inspected 

in order to detect potential dam safety deficiencies at a site. They are: annual, periodic, and 

comprehensive. Generalist engineers familiar with the dam and its operations, who can readily 

distinguish changes from year to year at the site, conduct annual inspections. Periodic 

inspections, also known as Periodic Facility Reviews (PFR) occur on a 6-year cycle and are 

conducted by a team from a regional office of the USBR that includes the regional examination 

specialist. Comprehensive inspections, also known as Comprehensive Facility Reviews (CFR) 

alternate on six-year cycles with the PFR and include examination and evaluation of the dam 

facility by a team of specialists from USBR’s Technical Service Center.
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The NRCS requires the sponsor/owner of a dam to be responsible for making inspections after 

the sponsor/owner assumes responsibility for a dam. Special, annual, and formal (once every 5 

years) inspections are performed by personnel trained in conducting the inspections. 

The U.S. Army Corps of Engineers (USACE) performs periodic, intermediate, and informal 

inspections on the basis of project size, importance, or potential hazard as follows:

• Initial periodic inspection – includes inspection and evaluation of a new earth or rockfill 

dam and is carried out immediately after topping out of the embankment prior to 

impoundment of the pool.

• Second periodic inspection – performed no later than one year after impoundment is 

initiated.

• Subsequent periodic inspections – performed at one-year intervals for the next two years 

of dam life. The next two inspections are performed at two-year intervals and then 

extended to a maximum interval of five years.

• Intermediate inspections – are performed for all or some of the features of dams on a 

five-year inspection cycle, if warranted. Selection is based on consequences of failure, 

age, degree of routine observation, a natural event (e.g. earthquake), performance record 

and history of remedial measures.

• Informal inspections – are performed by appropriate employees at the project at frequent 

time intervals to identify and report abnormal conditions and evidence of distress. 

Good preparation and planning is key to the success of an inspection. Factors to be considered 

include: selection of the inspection team; review of the existing project data for the dam to be 

inspected; and the preparation of a detailed inspection plan.

The inspection of outlet works conduits is directly affected by the size of conduit to be inspected. 

A minimum diameter of 36 inches is required if the feature is to be inspected using man entry, or 

a minimum diameter of 48 inches if it is to be inspected by a diver. Specialized inspections of 

inundated or hard to access features often utilize a combination of divers, remotely operated 

vehicles (ROVs), and closed circuit television (CCTV).

Considerations that must be accounted for if divers are to be expected to conduct inspections 

include depth, altitude, access, leakage, currents, visibility, size of opening, and length of conduit 

requiring inspection. ROVs are good alternatives to dive inspections when conditions like depth, 

diameter, or length become prohibitive. CCTV can be used to inspect submerged portions of 

intake structures, conduits, and terminal structures. They can also be used to inspect structures 

or conduits where confined space entry issues arise. CCTV inspection equipment consists of a 

video camera attached to a self-propelled transport vehicle known as a crawler. An operator 

remotely controls both the transport vehicle and camera and video images are recorded and 

stored for future evaluation and documentation. Modifications to the crawler and to the camera 

size enable CCTV technologies to be applied to a wide range of conduit sizes.
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It is further recommended that permanent markers and thorough documentation be used to 

facilitate easy location of intake structures that are usually submerged. Therefore, factors to be 

considered in planning conduit inspections should include: conduit diameter, the presence and 

angle of bends in the conduit, the inclination of the invert slope, and the distance between access 

entry locations.

4.6.2 State-of-the-Practice Refinement

The discussion of the white paper resulted in the identification of the following items to further 

refine the understanding of the state-of-the-practice:

• Inspection terminology, evident in the use of a term like periodic, differs among different 

agencies and organizations like the USACE, USBR, and FERC. Information in white paper 

on USACE methods, frequency, etc. is not what USACE is currently doing.

• Inspection activities should include required inspections of gates, valves, and operators. 

Input is needed from mechanical and electrical engineers concerning various elements that 

should be included in required inspections.

• Required/recommended frequencies of inspections vary among different organizations.

• GPS could be used to locate underwater structures, so they can be found more easily for 

future inspection. Could be done for new structures or when the structures are actually 

located underwater for existing structures.

• There is no clear consensus on need for periodic inspection of difficult/inaccessible features.

• Need to open valves that haven’t been operated for a long time, especially for low flow 

(small) valves. Owners are reluctant because they are afraid the valve will not close or they 

are afraid of sediment flushing downstream which could result in an environmental problem.

• Emergency gates are often (sometimes) not exercised and may not work when needed.

• Full emergency gate exercise under load is rare – USACE has an operating requirement to do 

this test if the gate was designed to operate under load.

• Hydro-acoustic surveys of stilling basins have been done successfully by the USACE.

• Dewatering/unwatering of outlet works for gate and conduit inspections can be difficult, 

because of leaky gates or bulkheads.

• Requirements differ between permit-required and non-permit-required confined spaces.

• Use of air monitoring devices is recommended for outlet works inspections.

• There has been some movement toward replacing bulkheads with upstream guard gates as 

has been done by Denver Water.

• Conduits are not typically surveyed on a periodic basis for elevation and alignment – this is 

typically done only if a problem is identified.

• Ultrasonic tests can be used to test for material deterioration, however, there can be problems 

distinguishing between good steel and bad steel (it may all look like thickness).
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• Detailed crack survey mapping (on a repeated basis) has been used to monitor structural 

deterioration.

4.6.2 Prioritization of Research and Development Ideas

The group brainstorming process resulted in the generation of 7 potential research and 

development ideas, which were then prioritized by the group. The 7 ideas and the results of the 

prioritization are presented in Exhibit 4-21.

4.6.3 Preliminary Research and Development Plans

The top four ideas listed in Exhibit 4-21 were selected for development of preliminary 

implementation plans, and those plans are presented in Exhibits 4-22 through 4-25. In some 

cases, the individual work groups reworded the research and development idea, so the ideas 

listed in Exhibit 4-21 may not exactly match the ideas listed in Exhibits 4-22 through 4-25.
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EXHIBIT 4-1

PRIORITIZATION OF RESEARCH AND DEVELOPMENT IDEAS

WORKSHOP TOPIC 1

OUTLET WORKS FAILURE MODES

* - Idea selected for development.

Note: Best practices for mechanical and electrical equipment was combined with best practices 

for automated control gates and valves for preliminary plan development.

Number of 

Votes Rank Research and Development Idea

13* 1 Guidance on air inlets and blow-back.

11* 2 (Tie) Best practices guide for conduits through small embankment 

dams.

11* 2 (Tie) Best practices for mechanical and electrical equipment.

9* 4 Best practices for automated control of gates and valves.

8* 5

Developing lists of failure modes for various components of 

outlet works – baseline, not “cookbook.”

6 6 Solutions to H2S issues for structure deterioration.

3 7 (tie) Develop recommended exercise schedule for outlet works 

equipment.

3 7 (tie) Development of requirements for dam and outlet works 

designers.

3 7(tie) Application of risk and reliability to assessment of failure 

modes.

1 10 Development of guidance for operator training requirements.

0 11 Development of a “no-fault” reporting system of incidents not 

leading to failure.
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EXHIBIT 4-2

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 1A

GUIDELINES FOR VENTING CONDUITS

1. Description

A. Why is this a priority research/development item?

Poor venting can cause operational problems that lead to catastrophic failure.

B. What is the expected outcome?

Better guidelines for design, remediation and operation.

2. Project Tasks and Needs

A. What tasks are to be done?

A literature search to produce a bibliography of existing information.

B. How is the problem to be solved?

Additional physical model studies at large and small embankment dams, additional 

analytical work, and additional prototype testing should be conducted.

3. Project Lead and Contact

A. Who is working in this area?

Possibly European specialist in this area.

B. Who might be able to lead the project?

Some universities and private laboratories located in the United States.

C. Who are good candidates to complete the work?

1. The University of Utah and Colorado State University. Both universities have large 

discharge and head capacities that are capable of physically modeling the airflow.

2. A private independent consultant or a university professor might be a good source for 

the literature review.
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EXHIBIT 4-3

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 1B

BEST PRACTICES GUIDE FOR CONDUITS

THROUGH EMBANKMENT DAMS

1. Description

A. Why is this a priority research/development item?

No established guidelines exist, especially for consultant use. Furthermore, established 

standards that can be enforced by dam safety officials currently do not exist.

B. What is the expected outcome?

Guidelines for use of design engineers, inspectors, and dam safety officials.

2. Project Tasks and Needs

A. What tasks are to be done?

Use the “Conduits through Embankment Dams” document and expand for use with 

smaller dams.

B. How is the problem to be solved?

1. Use the existing FEMA team and expand team with additional state members and 

consultants.

2. Conduct research into long term performance into conduit materials.

3. Conduct research into failure of preferred outlet conduit designs.

4. Compile and evaluate different outlet conduit design standards.

5. Clearing house for updating design references to be used in the guidelines. ASDSO 

would be an agency to consider.

6. Develop a brochure for the new document for distribution to the engineering 

community that can also be posted on the FEMA website. Data from the guidelines 

summarized in the brochure could be used to update documents published by 

organizations like the AWWA. The guidelines should be used during the design of 

conduits for embankment dams.

3. Project Lead and Contact

A. Who is working in this area?

It is currently not known if anyone is working in this area.

B. Who might be able to lead the project?
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Chuck Cooper currently of the U.S. Bureau of Reclamation.

C. Who are good candidates to complete the work?

Mr. Sal Todaro, currently of URS Corporation and independent consultant Ed Rossilson.
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EXHIBIT 4-4

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 1C

BEST PRACTICES FOR MECHANICAL AND

ELECTRICAL DESIGN AND AUTOMATED CONTROL

1. Description

A. Why is this a priority research/development item?

Lack of knowledge leads to failure modes, agencies and consultants could benefit, and 

mechanical/electrical design is often secondary to civil design.

B. What is the expected outcome?

Transfer of knowledge and technology.

2. Project Tasks and Needs

A. What tasks are to be done?

Find knowledgeable people and define a process to gather data and information which 

should be processed into a usable form.

B. How is the problem to be solved?

No answer provided.

3. Project Lead and Contact

A. Who is working in this area?

To the best knowledge of workshop team members no one is currently working in this 

area.

B. Who might be able to lead the project?

The USBR, USACE, and the USSD.

C. Who are good candidates to complete the work?

Retirees and experienced designers, operations and maintenance personnel, field 

technicians, gate designers, and manufactures of related components.
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EXHIBIT 4-5

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 1D

DEVELOP A LIST OF FAILURE MODES FOR OUTLET WORKS

1. Description

A. Why is this a priority research/development item?

Reduce future outlet works failures by making dam designers and owners aware of 

potential failure modes of outlet works components.

B. What is the expected outcome?

A compendium of potential failure modes for use in design, inspection, and operation of 

outlet works to ultimately reduce outlet works failures.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Identify forms of failure.

2. Basic research and literature review of outlet failure modes.

3. Solicit input from various federal dams, state, dam safety organizations in order to 

develop a list of failure modes

4. Convene workshop to refine list

5. Incorporate comments from federal and state agencies

6. Publish final list.

B. How is the problem to be solved?

No answer provided.

3. Project Lead and Contact

A. Who is working in this area?

FERC, the USACE, BC Hydro, USBR, ICOLD and Swede Power.

B. Who might be able to lead the project?

Dr. David Bowles (Utah State University), Marty McCann of the NPDP, Dr. Des 

Hartford, and Larry Von Thun, consultant.

C. Who are good candidates to complete the work?

No answer provided.
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EXHIBIT 4-6

PRIORITIZATION OF RESEARCH AND DEVELOPMENT IDEAS

WORKSHOP TOPIC 2

CONDUIT MATERIALS, SELECTION CRITERIA,

AND CONSTRUCTION METHODS

* - Idea selected for development.

Number of 

Votes Rank Research and Development Idea

15* 1

Best practices guidelines for intake towers – configuration, 

access, debris/trash control, seismic performance, selective 

withdrawal.

13* 2

Guidance on application of tunnels to outlet works – including 

consideration of microtunneling and of soft ground and hard 

ground tunneling alternatives.

12* 3

Compile data on performance of conduits through 

embankments for cases where the conduits have experienced 

significant settlement (such as some of those shown in the 

white paper presentation).

6* 4

Guidance on seismic effects on conduits and how conduits 

should be analyzed/designed for these effects – embankment 

spreading, fault offset, propagation of earthquake waves.

5 5

Measurement of loads on conduits for comparison to predicted 

loads,

1 6 (tie) Revision to the ASCE penstock guidelines.

1 6 (tie) Guidance on use and applicability of siphon outlet works –

update earlier EPRI document.
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EXHIBIT 4-7

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 2A

BEST PRACTICES FOR INTAKE TOWERS INCLUDING LOCATION, 

CONFIGURATION, ACCESS, DEBRIS/TRASH CONTROL, SEISMIC 

PERFORMANCE, AND SELECTIVE WITHDRAWALS

1. Description

A. Why is this a priority research/development item?

1. Changes in the state-of-the-practice in the application of seismic loads.

2. Changes in water quality requirements.

3. Past experience with poorly designed and positioned intake towers.

B. What is the expected outcome?

Development of a best practices guide.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Identify features of intake towers that merit study.

2. Survey existing owners and operators of intake towers via the ASDSO.

3. Gather available literature.

4. Compile information gathered into a summary document with recommendations.

B. How is the problem to be solved?

Development of a best practices guide.

3. Project Lead and Contact

A. Who is working in this area?

The USACE, independent consulting engineers, and the USSD.

B. Who might be able to lead the project?

FEMA.

C. Who are good candidates to complete the work?

The USBR and the USACE.
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EXHIBIT 4-8

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 2B

GUIDANCE ON APPLICATION OF TUNNELS FOR OUTLET WORKS

INCLUDING MICROTUNNELLING IN SOFT AND HARD GROUND

1. Description

A. Why is this a priority research/development item?

1. The high rate of failures due to piping accounting for approximately one quarter 

(25%) of embankment dam failures.

2. Penetration through dams needs to be eliminated.

B. What is the expected outcome?

1. A document or manual to provided guidance to help decide whether to use a tunnel or 

not, and to provide design guidance and costs.

2. Define appropriate geotechnical conditions for tunnel construction.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Literature search on the use of tunnels and on the rationale for the use of tunnels in 

dams. Case studies where tunnels were used or selected for design should be 

researched and compiled.

2. Develop pros and cons of the use of tunnels versus conduits in dam outlet works.

3. Develop selection criteria including relative cost.

4. Cost benefit analyses of the inclusion of tunnels should be performed.

B. How is the problem to be solved?

No answer provided.

3. Project Lead and Contact

A. Who is working in this area?

To the best knowledge of workgroup team members, no one is currently working in this 

area.

B. Who might be able to lead the project?

FEMA, ASDSO, USBR, and experienced designers.

C. Who are good candidates to complete the work?

Former USBR tunnel designers, non-government tunnel designers, experienced tunnel 

expert, and geotechnical designers with tunneling experience. The USACE also has 

experience with microtunneling under levees.
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EXHIBIT 4-9

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 2C

COMPILE DATA ON PERFORMANCE OF CONDUITS

WITH SIGNIFICANT SETTLEMENT AND LATERAL MOVEMENT

1. Description

A. Why is this a priority research/development item?

There are procedures to estimate settlement and lateral movement that have not been 

validated.

B. What is the expected outcome?

Verification of the existing procedures used to estimate settlement and lateral movement.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Gather existing data on USBR and USACE conduits along with site specific 

conditions.

2. Collect conduit data and site specific conditions on NRCS dams.

3. Compare actual settlements and lateral movements with estimates using current 

design procedures.

4. Look for site specific conditions or construction procedures that may lead to 

satisfactory performance.

5. If current analysis procedures prove incorrect, conduct a computer analysis to fit the 

data and develop new design guidance.

B. How is the problem to be solved?

No answer provided.

3. Project Lead and Contact

A. Who is working in this area?

The USACE and the USBR may be collecting some data.

B. Who might be able to lead the project?

FEMA could assist with the acquisition of data.

C. Who are good candidates to complete the work?

1. A representative from USBR, USACE, and NRCS is recommended.

2. State and city agencies that may have some data

3. Several consultants involved in design, inspection, and analysis of conduits.
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EXHIBIT 4-10

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 2D

GUIDANCE ON SEISMIC EFFECTS ON CONDUITS AND HOW

TO ANALYZE AND DESIGN FOR THEM INCLUDING EMBANKMENT SPREADING, 

FAULT OFFSET, AND THE PROPAGATION OF EARTHQUAKE WAVES

1. Description

A. Why is this a priority research/development item?

A priority because it is a dam safety concern with limited documented available for 

analysis and design.

B. What is the expected outcome?

A document or chapter or section in a larger document of reference material and design 

guidance.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Literature research of currently available documentation and design guidance.

2. Address various types of material i.e. soil, rock, etc., that will be encountered.

3. Establish a multi-disciplined team/committee to formulate the framework/outline of 

the document and to identify available information and areas of need or where gaps in 

information exist.

4. Distinguish the difference between analysis of existing conduits and the design and 

construction of new conduits.

B. How is the problem to be solved?

Solicit information from state and federal agencies by surveys/questionnaires dealing 

with earthquakes where their structures are located.

3. Project Lead and Contact

A. Who is working in this area?

The USGS, NEHRP, and the University of California at Berkeley.

B. Who might be able to lead the project?

EPRI, the ASDSO, and the dam safety groups of western states.

C. Who are good candidates to complete the work?

1. Blast research groups like those at the USACE, and the University of Florida.

2. The USBR and the USACE, particularly units located in the western states.
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3. Furthermore, since this is a multidiscipline issue, we propose that a professional 

society like ASCE produce a design manual, or solicit papers for a conference and 

publish the proceedings.
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EXHIBIT 4-11

PRIORITIZATION OF RESEARCH AND DEVELOPMENT IDEAS

WORKSHOP TOPIC 3 AND TOPIC 4

GATES, VALVES, CONTROLS AND ENERGY DISSIPATORS

* - Idea selected for development.

Number of 

Votes Rank Research and Development Idea

11* 1

Best practices for selection of gates, valves, and controls –

considering heads, velocities, configurations, etc.

10* 2

Best practices for energy dissipators – considering heads, 

velocities, configurations, etc.

9* 3

Guidelines for maintenance and exercise of gates and valves 

and for cavitation repair.

7* 4

Develop and present a short course on gates and valves for 

dams – design, operation, and maintenance – (consider 

videotaping the course?)

4 5

Develop expert videos (DVDs) as has been done for other dam 

safety topics (e.g. Peck, Idriss) for gates and valves and energy 

dissipators.

1 6 (tie) Research to develop guidance for variation of air demand for 

fixed-cone valves

1 6 (tie) Compile understanding of international efforts in gates and 

valves and energy dissipators.

1 6 (tie) Develop guidance for incorporation of seismic requirements 

into design of gates, valves, and operating systems.

0 9 (tie) Guidance document for environmentally-acceptable fluids for 

hydraulic controls.

0 9 (tie) Develop an information document on current gate and valve 

manufacturers – domestic and international.
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EXHIBIT 4-12

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 3A

BEST PRACTICES FOR SELECTION OF GATES AND VALVES

1. Description

A. Why is this a priority research/development item?

• It would aid in transferring information.

• To aid in preventing errors in selection, operation, and placement of gates and valves.

B. What is the expected outcome?

An up-to-date manual on selection, operation, and system layout.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Identify a sponsor like ASME or EPRI.

2. Identify a team.

B. How is the problem to be solved?

Team effort with perseverance.

3. Project Lead and Contact

A. Who is working in this area?

• Consulting firms.

• Chinese water resources.

B. Who might be able to lead the project?

An active consultant.

C. Who are good candidates to complete the work?

• Consultants.

• Technical personnel from manufacturers in both the United States and Europe.

• Western European engineers.
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EXHIBIT 4-13

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 4A

BEST PRACTICES FOR ENERGY DISSIPATORS –

CONSIDERING HEADS, VELOCITIES, CONFIGURATIONS, ETC.

1. Description

A. Why is this a priority research/development item?

Experienced engineers are rapidly becoming fewer and their knowledge and experience 

needs to be captured.

B. What is the expected outcome?

1. Individual publications on each energy dissipator (E.D.).

2. Report with overview of all energy dissipators with advantages, disadvantages, and 

applications based on variety of parameters, e.g. head (H) versus flow (Q).

2. Project Tasks and Needs

A. What tasks are to be done?

1. Develop theoretical model for each E.D.

2. Gather empirical data for each E.D.

3. Calibrate theoretical to empirical.

4. Characterize range of satisfactory behavior.

5. Identify gaps in range of application.

6. Identify environmental effects.

7. Summarize findings in overview report.

B. How is the problem to be solved?

• Criteria need to be examined and updated.

• Lessons learned and experience needs to be captured.

• Theories were developed years ago and applications need to be assessed.

3. Project Lead and Contact

A. Who is working in this area?

Many people might be working on individual aspects, but it all needs to be brought 

together.

B. Who might be able to lead the project?

The laboratories of the USACE (ERDC).
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C. Who are good candidates to complete the work?

Individuals like Jim Gordon, Will Hager, State and Federal Agencies, or Chinese 

researchers/specialists in this area.
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EXHIBIT 4-14

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 3B

GUIDELINES FOR MAINTENANCE AND EXERCISE OF

GATES AND VALVES AND CAVITATION REPAIRS

1. Description

A. Why is this a priority research/development item?

1. It is well documented that poor maintenance of outlet works is a problem and 

represents a dam safety concern.

2. Currently in cases of emergency, gates and valves are assumed to be operational 

without the exercise of these components on a routine basis.

B. What is the expected outcome?

1. Guideline documents for maintenance and exercise of gates and valves.

2. Training device for dam owners to show the importance of maintaining the facility in 

an operable condition.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Research on known cases of failure of outlet works gates and valves.

2. Research on corrosion effects of water chemistry on operation and/or maintenance.

3. Survey federal, state, and large private dam owners, consultants in the field, and dam 

regulators about maintenance concerns, programs, and past experiences.

4. Impress upon members of academia the importance of maintenance consideration 

along with the design of outlet gates and valves.

5. Standardize inspection protocol by preparing a review checklist.

B. How is the problem to be solved?

ASDSO needs to be the vehicle to develop a document for the procedures and means of 

maintenance and exercise of gates and valves.

3. Project Lead and Contact

A. Who is working in this area?

Currently no one is working in this area to the knowledge of the workgroup team.
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B. Who might be able to lead the project?

FEMA and ASDSO with contributions from USACE, FERC, and the USBR.

C. Who are good candidates to complete the work?

FEMA and ASDSO.
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EXHIBIT 4-15

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 3C

DEVELOP SHORT COURSES WITH VIDEO/DVD OPTIONS ON

DESIGN, OPERATION, AND MAINTENANCE OF GATES AND VALVES

1. Description

A. Why is this a priority research/development item?

We are losing knowledge of senior experts and technology in the United States is 

declining due to not building new dams while at the same time we have an aging 

infrastructure of existing dams that will need rehabilitation.

B. What is the expected outcome?

A transfer of knowledge and technology to current and future practitioners.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Develop scope of short course (including such topics as history, state of the practice 

discussions, world practices, lessons learned, and best practices).

2. Identify target audiences that would include civil engineers, mechanical engineers, 

electrical engineers, and other individuals to be identified whose professional practice 

would be benefited by such a knowledge transfer.

B. How is the problem to be solved?

1. Through a contract RFP. The selected contractor needs to bring recognized experts as 

instructors. A major factor in selection will be the knowledge and experience breadth 

of experts brought to the courses.

2. Evaluation of the video/DVD needs to be an option of the RFP and needs to be 

considered within the structure of the short course.

3. Project Lead and Contact

A. Who is working in this area?

No single entity is working in this area and as such a composite team would be required.

B. Who might be able to lead the project?

This project could be sponsored by FEMA, ASDSO, USSD, and EPRI.
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C. Who are good candidates to complete the work?

The composite team of members from the sponsoring agencies could be lead by an expert 

or practicing consulting firm.
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EXHIBIT 4-16

PRIORITIZATION OF RESEARCH AND DEVELOPMENT IDEAS

WORKSHOP TOPIC 5

REHABILITATION OF CONDUITS

* - Idea selected for development.

Number of 

Votes Rank Research and Development Idea

14* 1 (tie) Design guidance and guide specifications for sliplining of 

outlet conduits.

14* 1 (tie) Compilation of case studies on outlet works rehabilitation –

lessons learned, performance, comparative costs.

8* 3 (tie) Assessment of microtunneling and boring technologies for 

application to outlet conduit rehabilitation.

8* 3 (tie)

Best practices guide for design and construction methods to 

rebuild embankment sections removed for outlet conduit 

removal/replacement.

7 5

Design guidance and guide specifications for cured-in-place 

lining of outlet conduits – considering high velocity flows, end 

constraints, etc..

6 6 (tie) Research/guidance on methods for rehabilitation of conduit 

deterioration from chemical attack (e.g. H2S).

6 6 (tie) Best practices guide for abandonment of outlet conduits inplace.

2 8 (tie) Grout materials research on grout properties for sliplining 

applications.

2 8 (tie)

Assessment and development of paint technologies for 

application in damp, cool, confined spaces and for operation in 

high velocity environments.

1

10 

(tie)

Research on aging and thermal properties of HDPE pipe 

materials and installations.

1

10 

(tie)

Applicability and appropriateness of using downstream 

controls to replace difficult-access upstream gates and valves –

special requirements for converting non-pressure pipe to 

pressure pipe.

1

10 

(tie)

Best practices guide for design, construction, and operation of 

siphon outlet works/spillways.

1

10 

(tie)

Consideration of effects on seepage pattern in embankments 

after relining a leaky conduit.
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EXHIBIT 4-17

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 5A

DESIGN GUIDANCE AND GUIDE SPECIFICATIONS FOR SLIPLINING

1. Description

A. Why is this a priority research/development item?

1. Potential big market given the large number of projects in need, particularly those that 

have either reached or are fast approaching the end of their service life.

2. Need cost effective solutions given owner’s ability to pay i.e. cost sensitivity is 

critical.

3. Limited information is available, especially for material selection, and primarily from 

pipe suppliers.

B. What is the expected outcome?

1. Material selection (paint, pipe, grouts) guide specifications for grouting procedures.

2. Standard design details, especially for special arrangements.

3. Distinguish between pressure versus non-pressure conditions.

4. A list of design considerations including corrosion, erosion, head, flow and hydraulic 

capacity.

5. A logic tree to aid in the evaluations of response modes including abandonment, 

sliplining, or other options like excavation and replacement.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Data search

2. Develop case studies through discussions with designers, owners and contractors. 

Find out what experience exists.

3. Offer a symposium or conference potentially sponsored by the USSD, at which white 

papers are presented.

4. Form a committee to develop guidelines and provide examples.

5. Work with ASCE, FEMA, and USSD to publish results of all R&D activities 

previously indicated. 
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3. Project Lead and Contact

A. Who is working in this area?

USBR, NRCS, and consultants.

B. Who might be able to lead the project?

NRCS.

C. Who are good candidates to complete the work?

USBR, NRCS, pipe suppliers, grout manufacturers, contractors, and designers. 
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EXHIBIT 4-18

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 5B

COMPILATION OF CASE STUDIES ON OUTLET WORKS

REHABILITATION – LESSONS LEARNED, PERFORMANCE,

AND COMPARATIVE COSTS

1. Description

A. Why is this a priority research/development item?

1. It provides a forum for lessons learned to be discussed.

2. Allows for the transfer of experiences among practitioners.

B. What is the expected outcome?

1. A decision tool for selecting rehabilitation alternatives for a specific application.

2. Transfer experiences.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Distinguish between low versus high need application

2. Distinguish access versus pipe size methods.

3. Survey agencies, private industry, and consultants.

4. Have a conference or forum from which the proceedings can be published.

5. Summarize trends similar to USSD and ICOLD publications in a document that 

should be updated on a periodic basis.

6. An audit group or committee should be assembled to balance bias with 

information that is too generalized.

7. Review case studies to ensure that sensitive specifics about actual projects are 

removed to eliminate concerns about liability or disclosure of failures.

3. Project Lead and Contact

A. Who is working in this area?

ASCE has previously published lessons learned. 

B. Who might be able to lead the project?

A group committee comprised of engineers with a range of experience in both small and 

large dams should be comprised of members from professional organizations.
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C. Who are good candidates to complete the work?

Water Resources Research Institute (WRRI) for land-grant colleges, university professors 

(although a drawback is that this individual may have limited hands-on experience), Earl 

Eiker of USSD’s Hydraulics Committee, and ASCE’s Hydraulics Structures Committee. 

The knowledge and experience of FEMA’s incident response group should be tapped for 

larger-scale problems. The Department of Homeland Security may also fund incident 

investigations.
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EXHIBIT 4-19

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 5C

ASSESSMENT OF MICROTUNNELLING AND BORING TECHNOLOGIES FOR

APPLICATION TO OUTLET CONDUIT REHABILITATION

1. Description

A. Why is this a priority research/development item?

Because these techniques are being applied to low head dams in limited applications and 

there are no guidelines.

B. What is the expected outcome?

Guidelines for appropriate or inappropriate use of these techniques.

2. Project Tasks and Needs

A. What tasks are to be done?

A literature search fro application of micro-tunneling in embankment and foundation 

material including upstream seepage cutoffs backing filter systems and stilling systems 

for steep pipe applications.

B. How is the problem to be solved?

Explore contracts and manufacturer’s associations for funding this research and 

application. Back up by advisory board of dam consultants.

3. Project Lead and Contact

A. Who is working in this area?

NRCS applications in upgrade of existing FRS facilities about normal water pool as seen 

at Standley Dam’s outlet works in Westminster, Colorado where the abutment was 

rehabilitated. 

B. Who might be able to lead the project?

The dam groups of the NRCS, Fish and Wildlife, and the Bureau of Indian Affairs, and 

dam consultants.

C. Who are good candidates to complete the work?

NRCS’ national engineering group with the assistance of consulting engineers.
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EXHIBIT 4-20

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 5D

BEST PRACTICES GUIDE FOR DESIGN AND CONSTRUCTION

METHODS TO REBUILD EMBANKMENT SECTIONS

FOR CONDUIT REMOVAL AND REPLACEMENT

1. Description

A. Why is this a priority research/development item?

1. There are currently no design or construction guidelines for reconstruction of 

breaches through embankment dams for outlet works replacement.

2. No preferred method of replacement or rehabilitation of outlet works structures has 

been identified.

3. There is more of a need to replace outlet works than currently practiced.

4. If the maximum section of a dam is not constructed properly, failure could result.

5. There are currently unaddressed concerns with the treatment of soft foundations.

6. New embankments need to be tied into zoning of the existing embankment with 

draining systems an area of particular concern.

B. What is the expected outcome?

Guidelines to address the issues of the reconstruction of partial removal of dam 

embankments to be used by consultants and dam regulators.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Conduct literature research of existing guidelines like the Levee Design Manual 

developed by the USACE, and case histories. Information gathered from a 

compilation of case studies should be used to develop a set of guidelines for this 

R&D topic. 

2. Survey designers, contractors and designers to develop documentation of experience 

resources.

3. Develop guidance documents to be peer reviewed.

4. Produce the final product in the form of a published set of embankment rebuilding 

and conduit removal and replacement guidelines.
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3. Project Lead and Contact

A. Who is working in this area?

Currently no one is working in this area to the knowledge of the workgroup team. Most 

practitioners have their own independent in-house practices that they follow.

B. Who might be able to lead the project?

NRCS, ASDSO, and consultants.

C. Who are good candidates to complete the work?

Mr. David Hammer, formerly of the USACE who has experience with the construction of 

embankments around existing outlets, and who has also authored several papers. Mr. Art 

Walhs with Garnett-Flemming, and Mr. Clark Stanage, formerly of the USACE would 

also be good candidates to conduct this work.
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EXHIBIT 4-21

PRIORITIZATION OF RESEARCH AND DEVELOPMENT IDEAS

WORKSHOP TOPIC 6

OUTLET WORKS INSPECTION

* - Idea selected for development.

Number of 

Votes Rank Research and Development Idea

13* 1

Applications of nondestructive testing to inspection of outlet 

works.

11* 2 (tie) Best practices for inspection of gates, valves, and operators 

(mechanical and electrical components).

11* 2 (tie) Best practices guide for inspection frequency and methods for 

more-typically accessible outlet works features.

9* 4

Develop consensus recommendations for inspection of 

normally inundated structures and very-difficult-to-inspect 

structures.

2 5

Use of digital imaging or other technologies to improve the 

efficiency of crack mapping or defect mapping.

1 6 (tie) Updating the TADS modules to reflect newer methods, 

technologies, and standards.

1 6 (tie)

Establish practical and environmentally acceptable methods 

and materials to seal leaking gates and bulkheads to allow 

inspection of outlet works.
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EXHIBIT 4-22

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 6A

APPLICATIONS OF NON-DESTRUCTIVE TESTING (NDT)

FOR INSPECTION OF OUTLET WORKS

1. Description

A. Why is this a priority research/development item?

1. Non Destructive Testing provides a unique and consistent technique enabling the 

inspection of outlet works.

2. There is currently no clear description of the capabilities of NDT for application to 

outlet works inspections.

3. Consistent guidelines of how to interpret the results of non-destructive tests need to 

be developed.

B. What is the expected outcome?

Non-destruction tests being used to confirm the structural integrity of outlet works 

structures.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Conduct a survey of suppliers or manufacturers of what equipment is available on the 

market.

2. An inventory of what is needed that is currently unava ilable needs to be made.

3. The state of the practice today needs to be documented.

4. A best practices guide on the use of equipment including costs, capabilities, and 

applications needs to be developed to increase the understanding of non-expert 

practitioners.

5. Develop tests to confirm theoretical projections of performance with actual 

conditions.

3. Project Lead and Contact

A. Who is working in this area?

Mr. Chuck Copper of the USBR, the Vicksburg, MS or Champagne, IL divisions of the 

USACE, and Hydro Quebec.
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B. Who might be able to lead the project?

USBR.

C. Who are good candidates to complete the work?

USBR.
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EXHIBIT 4-23

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 6B

BEST PRACTICES FOR INSPECTION OF GATES, VALVES,

AND OPERATORS, INCLUDING MECHANICAL

AND ELECTRICAL COMPONENTS

1. Description

A. Why is this a priority research/development item?

1. To avoid the failure of these features.

2. These features are exposed elements and more sensitive than other components of the 

dam and need to be more closely monitored.

B. What is the expected outcome?

1. Guidelines for the inspection of mechanical and electrical equipment.

2. Increased confidence in the reliability of operation of the equipment.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Research existing practices of federal and non-federal dam owner organizations, 

including international experience e.g. ICOLD.

2. Consult with equipment manufacturers about recommended practices.

3. Investigate alternative testing procedures.

B. How is the problem to be solved?

1. Developing guidelines.

2. Consideration of additional state regulations.

3. Project Lead and Contact

A. Who is working in this area?

No one to the knowledge of the workgroup team.
SECTIONFOUR Workshop Results for Individual Topics

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

4-54

B. Who might be able to lead the project?

Dam Safety organizations like ASDSO, and USSD.

C. Who are good candidates to complete the work?

An interdisciplinary team consisting of regulators, dam owners, equipment 

manufacturers, and mechanical and electrical designers is believed best suited for this 

work. 
SECTIONFOUR Workshop Results for Individual Topics

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

4-55

EXHIBIT 4-24

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 6C

BEST PRACTICES GUIDE FOR INSPECTION FREQUENCY AND

METHODS OF INSPECTION FOR TYPICALLY ACCESSIBLE CONDUITS

1. Description

A. Why is this a priority research/development item?

1. No consistency on frequency and methods of inspection.

2. Problems may be uncovered that may not otherwise be noted where no inspections 

are done.

3. May provide guidelines that state or federal agencies may use to require dam owners 

to follow.

B. What is the expected outcome?

1. Inspection guidelines including frequency of inspection tied to hazard potential 

conduit materials and the age of the outlet.

2. May influence state or federal policy on outlet inspections.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Literature search on current practices. Develop table of practices and provide a list of 

references.

2. List and discuss various remote monitoring techniques and advantages and 

disadvantages.

B. How is the problem to be solved?

1. Solicit input from various dam safety agencies at both the state and federal level, 

private consultants for inspections versus the type of outlet.

2. Prepare a question and answer sheet to be answered by private consultants from both 

state and federal agencies.

3. Workshop to provide best recommendations.

3. Project Lead and Contact

A. Who is working in this area?

1. ASDSO, ASCE, USSD, USBR, and Mr. Larry Von Thun. This work might also 

include revisiting the inspection portion of the Model Dam Safety Law.
SECTIONFOUR Workshop Results for Individual Topics

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

4-56

B. Who might be able to lead the project?

FEMA through its internal R&D mechanisms.

C. Who are good candidates to complete the work?

The same team leading this project is a good candidate to complete work tasks.
SECTIONFOUR Workshop Results for Individual Topics

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

4-57

EXHIBIT 4-25

RESEARCH AND DEVELOPMENT

IMPLEMENTATION PLAN, R&D TOPIC 6D

INSPECTION OF NORMALLY INUNDATED STRUCTURES

AND DIFFICULT TO ACCESS AREAS

1. Description

A. Why is this a priority research/development item?

1. This subject is of high priority because this work is often deferred and does not get 

done.

2. No guidelines currently exist to determine the importance or necessity.

B. What is the expected outcome?

1. Develop guidelines for go/no-go procedures that might include consideration of 

outcomes, water quality, design margins, and operation history.

2. Develop guidelines for inspection frequency.

3. Develop methods and/or technologies for actually performing inspections. 

4. Possible recommendations or design considerations to facilitate future inspections.

2. Project Tasks and Needs

A. What tasks are to be done?

1. Identify type of structure first.

2. Data search and case studies to get a feel for the risks that are involved.

3. Probability and risk assessment of structure to rank priorities.

3. Project Lead and Contact

A. Who is working in this area?

No one is currently working in this area to the knowledge of the respondent.

B. Who might be able to lead the project?

The USBR.

C. Who are good candidates to complete the work?

Dam owners and diving and inspection agencies.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-1

5 . Section 5 FIVE Overall Priori tization Of Research and Development Ideas

As discussed in Section 4, the prioritization of the research and development ideas for each of 

the six topics (with Topic 3 and Topic 4 combined) considered in the workshop resulted in the 

identification of 20 leading ideas (4 ideas for each topic), for which preliminary implementation 

plans were developed.

The potential benefit and associated implementation cost of each of the 20 R&D topics were 

assigned a score between zero and ten by sixteen workshop participants as will be discussed 

below. The scores, which were based on consideration of the balance of benefit to practice and 

cost/ease of implementation, were then averaged in order to determine overall rankings for each 

of the research and development ideas.

Potential Benefit

In order to provide an assessment of potential benefit, workshop participants were asked to use 

the form shown in Attachment 3 to assign a score from 0 to 10 with 0 being least beneficial and 

10 most beneficial. to each of the 20 R&D ideas selected to develop preliminary plans. As may 

be seen on Exhibit 5-1, the top 11 R&D ideas (including a tie for the 10th position) generated by 

assessing this factor provided by 16 participating workshop respondents were as follows:

1. Best-practices guide for outlet works conduits through embankment dams, emphasizing small 

embankments. (R& D Topic 1B)

2. Best-practices guide for selection of gates and valves for outlet works structures. (R&D Topic 

3A)

3. Compilation of case studies on outlet works rehabilitation – lessons learned, performance, 

comparative costs. (R&D Topic 5B)

4. Best-practices guide for energy dissipators for outlet works structures, considering heads, 

velocities, configuration, etc. (R&D Topic 4A)

4. Design guidance and guide specifications for sliplining of outlet conduits. (R&D Topic 5A)

6. Guidelines for maintenance and exercise of gates and valves and for cavitation repairs for 

outlet works. (R&D Topic 3B)

7. Best-practices guide for mechanical and electrical design and for automated control for outlet 

works. (R&D Topic 1C)

8. Best-practices guide for inspection of outlet works gates, valves, and operators, including 

mechanical and electrical components. (R&D Topic 6B)

9. Best-practices guide for outlet works intake towers, considering such factors as location, 

configuration, access, debris/trash control, seismic performance, selective withdrawals, etc. 

(R&D Topic 2A)

10. Develop a list of failure modes for outlet works. (R&D Topic 1D)

10. Develop short course (with video/DVD option) on design, operation, and maintenance of 

outlet works gates and valves. (R&D Topic 3C)

Please note that this list includes ideas tied at ranks of 4 and 10.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-2

Cost/ Ease of Implementation

To provide an assessment of cost or perceived difficulty/ease of implementation associated with 

the same 20 R&D ideas selected for preliminary development, workshop participants were asked 

to assign a score for this factor from 0 to 10 with 0 indicating unfavorable high cost/difficulty of 

implementation and 10 indicating favorable low cost or perceived easy implementation. As may 

be seen on Exhibit 5-2, the top 10 R&D ideas as ranked by the same 16 participating workshop 

respondents were as follows:

1. Guidelines for maintenance and exercise of gates and valves and for cavitation repairs for 

outlet works. (R&D Topic 3B)

2. Best-practices guide for inspection frequency and methods of inspection for typically 

accessible outlet works conduits. (R&D Topic 6C)

3. Best-practices guide for selection of gates and valves for outlet works structures. (R&D Topic 

3A)

3. Best-practices guide for outlet works conduits through embankment dams, emphasizing small 

embankments. (R&D Topic 1B)

5. Develop a list of failure modes for outlet works. (R&D Topic 1D)

5. Best practices guide for design and construction methods to rebuild embankment sections 

removed for outlet conduit removal/replacement. (R&D Topic 5D)

5. Best-practices guide for inspection of outlet works gates, valves, and operators, including 

mechanical and electrical components. (R&D Topic 6B)

8. Best-practices guide for mechanical and electrical design and for automated control for outlet 

works. (R&D Topic 1C)

8. Best-practices guide for energy dissipators for outlet works structures, considering heads, 

velocities, configuration, etc. (R&D Topic 4A)

8. Design guidance and guide specifications for sliplining of outlet conduits. (R&D Topic 5A)

As seen above, ranking according to this measure resulted in ties at ranks 3, 5, and 8.

Overall Prioritization of Research and Development Ideas

In reviewing the results of the overall ranking according to potential benefit (Exhibit 5-1) and 

favorable cost/ease of implementation (Exhibit 5-2), it is seen that the priority order of R&D 

ideas varied somewhat between the two factors assessed. However, by averaging the scores 

from both assessments an overall average score and ranking of the 20 R&D ideas for which 

preliminary plans were developed could be obtained as seen in Exhibit 5-3. The resulting top 10 

ideas are:

1. Best-practices guide for outlet works conduits through embankment dams, emphasizing small 

embankments. (R&D Topic 1B)

2. Best-practices guide for selection of gates and valves for outlet works structures. (R&D Topic 3A)

3. Guidelines for maintenance and exercise of gates and valves and for cavitation repairs for outlet works. 

(R&D Topic 3B)
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-3

4. Best-practices guide for energy dissipators for outlet works structures, considering heads, velocities, 

configuration, etc. (R&D Topic 4A)

4. Design guidance and guide specifications for sliplining of outlet conduits. (R&D Topic 5A)

6. Best-practices guide for mechanical and electrical design and for automated control for outlet works. 

(R&D Topic 1C)

7. Compilation of case studies on outlet works rehabilitation – lessons learned, performance, comparative 

costs. (R&D Topic 5B)

7. Best-practices guide for inspection of outlet works gates, valves, and operators, including mechanical 

and electrical components. (R&D Topic 6B)

9. Develop a list of failure modes for outlet works. (R&D Topic 1D)

9. Best-practices guide for inspection frequency and methods of inspection for typically 

accessible outlet works conduits. (R&D Topic 6C)

This combined ranking generated ties at rank 4, 7, and 9 and is believed to indicate some degree 

of prioritization among R&D ideas. The cost-benefit comparison chart shown in Exhibit 5-4 

further indicates that the top 10 R&D ideas from the overall rankings would all generate high 

benefit and had a comparatively low associated implementation cost.

Based on the combination of the rankings, the following five topics should be considered highest 

priority for research and development:

• Best-practices guide for outlet works conduits through embankment dams, emphasizing small 

embankments. (R&D Topic 1B)

• Best-practices guide for selection of gates and valves for outlet works structures. (R&D Topic 3A)

• Guidelines for maintenance and exercise of gates and valves and for cavitation repairs for outlet 

works. (R&D Topic 3B)

• Best-practices guide for energy dissipators for outlet works structures, considering heads, velocities, 

configuration, etc. (R&D Topic 4A)

• Design guidance and guide specifications for sliplining of outlet conduits. (R&D Topic 5A)

All five of these ideas ranked in the top 10 in both assessments of potential benefit and cost/ease 

of implementation, and were ranked 1 through 4 (with a tie for 4th) in the average ranking.

The next five research and development ideas listed in Exhibit 5-3 should also be considered 

high priority R&D ideas, but not as high as the first four topics indicated above. These five 

topics are:

• Best-practices guide for mechanical and electrical design and for automated control for outlet works. 

(R&D Topic 1C)

• Compilation of case studies on outlet works rehabilitation – lessons learned, performance, 

comparative costs. (R&D Topic 5B)

• Best-practices guide for inspection of outlet works gates, valves, and operators, including mechanical 

and electrical components. (R&D Topic 6B)

• Develop a list of failure modes for outlet works. (R&D Topic 1D)
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-4

• Best-practices guide for inspection frequency and methods of inspection for typically 

accessible outlet works conduits. (R&D Topic 6C)

R&D topics 1C, 6B and 1D were ranked in the top 10 in both benefit and cost/ease of 

implementation rankings while R&D topic 5B was among the top 10 in the ranking of potential 

benefit, while R&D topic 6C was the second most favorable R&D idea from a cost/ease of 

implementation perspective. Consequently, it is the authors’ opinion that these five R&D topics 

should be considered high priority, but not as high as the top five ideas previously indicated.

Other R&D topics that received a top 10 ranking in at least one of the two overall ranking 

methods used to prioritize ideas were:

• Develop a best-practices guide for design and construction methods to rebuild embankment 

sections removed for outlet conduit removal/replacement. (R&D Topic 5D)

• Develop a best-practices guide for outlet works intake towers, considering such factors as 

location, configuration, access, debris/trash control, seismic performance, and the practice of 

selective withdrawals. (R&D Topic 2A)

• Develop a short course with the possibility of a companion video/DVD format on design, 

operation, and maintenance of outlet works conduits. (R&D Topic 3C)

These R&D topics deserve some consideration for implementation, but at a lower priority than 

the 10 ideas previously indicated.

The remaining seven R&D topics did not receive top 10 ratings in either of the two assessment 

gauges used, and therefore should be considered much lower on the priority scale for 

implementation.

Observations Concerning Research and Development Ideas

As discussed in Section 2, a review of the leading R&D topics indicates that none of them 

involve basic laboratory testing. Rather most of the R&D topics involve collecting or compiling 

available information and developing guidelines for dissemination to practitioners to enable 

consistency in design, maintenance, and inspection activities throughout the field of practice. 

This suggests that the overall challenges associated with various aspects of outlet works design, 

maintenance and inspection result from the absence of documented information, inconsistencies 

among available information, misuse or misapplication of the available information by some 

practitioners, or lack of knowledge of available information by some practitioners. It also seems 

to reflect a feeling that the information on the overall topic is too dispersed for the profession to 

make the best use of lessons-learned from past performance, and that compilation of information 

into more readily available sources would be beneficial.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-5

EXHIBIT 5-1

RANKING OF RESEARCH/DEVELOPMENT IDEAS

BASED ON INDIVIDUAL SCORES FOR POTENTIAL BENEFIT

TOPIC 

NUMBER RESEARCH/DEVELOPMENT TOPIC(S)

SCORE

0 to 10(1) RANK

1B

Best-practices guide for outlet works conduits 

through embankment dams, emphasizing small 

embankments.

8.1 1

3A Best-practices guide for selection of gates and valves 

for outlet works structures. 7.6 2

5B

Compilation of case studies on outlet works 

rehabilitation – lessons learned, performance, 

comparative costs.

7.5 3

4A

Best-practices guide for energy dissipators for outlet 

works structures, considering heads, velocities, 

configuration, etc.

7.4 4

5A Design guidance and guide specifications for 

sliplining of outlet conduits. 7.4 4

3B Guidelines for maintenance and exercise of gates and 

valves and for cavitation repairs for outlet works. 7.1 6

1C Best-practices guide for mechanical and electrical 

design and for automated control for outlet works. 7.0 7

6B

Best-practices guide for inspection of outlet works 

gates, valves, and operators, including mechanical 

and electrical components.

6.8 8

2A

Best-practices guide for outlet works intake towers, 

considering such factors as location, configuration, 

access, debris/trash control, seismic performance, 

selective withdrawals, etc.

6.7 9

1D Develop a list of failure modes for outlet works. 6.4 10

3C

Develop short course (with video/DVD option) on 

design, operation, and maintenance of outlet works 

gates and valves.

6.4 10

Note: (1) 0 is lowest benefit and 10 is highest benefit.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-6

EXHIBIT 5-1

RANKING OF RESEARCH/DEVELOPMENT IDEAS

BASED ON INDIVIDUAL SCORES FOR POTENTIAL BENEFIT

-CONTINUEDTOPIC 

NUMBER RESEARCH/DEVELOPMENT TOPIC(S)

SCORE

0 to 10(1) RANK

6C

Best-practices guide for inspection frequency and 

methods of inspection for typically accessible outlet 

works conduits.

6.2 12

1A Guidelines for venting outlet works. 6.1 13

2C Compile data on performance of conduits with 

significant settlement and lateral movement. 6.1 13

2D

Guidance on seismic effects on conduits and methods 

for analysis and design to address those effects (e.g. 

embankment spreading, fault offset, propagation of 

earthquake waves).

6.1 13

5D

Best practices guide for design and construction 

methods to rebuild embankment sections removed for 

outlet conduit removal/replacement.

5.9 16

6D Guidelines for inspection of normally inundated 

structures and other difficult-to-access features. 5.4 17

2B

Guidance on application of tunnels for outlet works –

including consideration of microtunneling and of both 

soft-ground and hard-ground tunneling.

5.3 18

5C

Assessment of microtunneling and boring 

technologies for application to outlet conduit 

rehabilitation.

5.3 18

6A Applications of non-destructive testing (NDT) for 

inspection of outlet works. 5.3 18

Notes: (1) 0 is lowest benefit and 10 is highest benefit.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-7

EXHIBIT 5-2

RANKING OF RESEARCH/DEVELOPMENT IDEAS

BASED ON INDIVIDUAL SCORES FOR COST/EASE OF IMPLEMENTATION

TOPIC 

NUMBER RESEARCH/DEVELOPMENT TOPIC(S)

SCORE

0 to 10(1) RANK

3B Guidelines for maintenance and exercise of gates and 

valves and for cavitation repairs for outlet works. 6.5 1

6C

Best-practices guide for inspection frequency and 

methods of inspection for typically accessible outlet 

works conduits.

6.4 2

3A Best-practices guide for selection of gates and valves 

for outlet works structures. 6.3 3

1B

Best-practices guide for outlet works conduits 

through embankment dams, emphasizing small 

embankments.

6.3 3

1D Develop a list of failure modes for outlet works. 6.2 5

5D

Best practices guide for design and construction 

methods to rebuild embankment sections removed for 

outlet conduit removal/replacement.

6.2 5

6B

Best-practices guide for inspection of outlet works 

gates, valves, and operators, including mechanical 

and electrical components.

6.2 5

1C Best-practices guide for mechanical and electrical 

design and for automated control for outlet works. 6.1 8

4A

Best-practices guide for energy dissipators for outlet 

works structures, considering heads, velocities, 

configuration, etc.

6.1 8

5A Design guidance and guide specifications for 

sliplining of outlet conduits. 6.1 8

Note: (1) 0 is unfavorable high cost/difficulty of implementation and 10 is 

favorable low cost/ease of implementation.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-8

EXHIBIT 5-2

RANKING OF RESEARCH/DEVELOPMENT IDEAS

BASED ON INDIVIDUAL SCORES FOR COST/EASE OF IMPLEMENTATION

-CONTINUEDTOPIC 

NUMBER RESEARCH/DEVELOPMENT TOPIC(S)

SCORE

0 to 10(1) RANK

6D Guidelines for inspection of normally inundated 

structures and other difficult-to-access features. 5.8 11

5B

Compilation of case studies on outlet works 

rehabilitation – lessons learned, performance, 

comparative costs.

5.5 12

1A Guidelines for venting outlet works. 5.4 13

6A Applications of non-destructive testing (NDT) for 

inspection of outlet works. 5.4 13

5C

Assessment of microtunneling and boring 

technologies for application to outlet conduit 

rehabilitation.

5.2 15

2A

Best-practices guide for outlet works intake towers, 

considering such factors as location, configuration, 

access, debris/trash control, seismic performance, 

selective withdrawals, etc.

4.9 16

2C Compile data on performance of conduits with 

significant settlement and lateral movement. 4.7 17

2D

Guidance on seismic effects on conduits and methods 

for analysis and design to address those effects (e.g. 

embankment spreading, fault offset, propagation of 

earthquake waves).

4.6 18

2B

Guidance on application of tunnels for outlet works –

including consideration of microtunneling and of both 

soft-ground and hard-ground tunneling.

4.4 19

3C

Develop short course (with video/DVD option) on 

design, operation, and maintenance of outlet works 

gates and valves.

3.6 20

Note: (1) 0 is unfavorable high cost/difficulty of implementation and 10 is 

favorable low cost/ease of implementation.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-9

EXHIBIT 5-3

RANKING OF RESEARCH/DEVELOPMENT IDEAS

BASED ON THE COMBINATION OF INDIVIDUAL SCORES

FOR POTENTIAL BENEFIT AND COST/EASE OF IMPLEMENTATION

INDIVIDUAL SCORES 

(1)

TOPIC 

NUMBE

R

RESEARCH/DEVELOPMENT 

TOPIC(S)

POTENTIAL 

BENEFIT

COST

SCORE

0 to 10(2) RANK

1B

Best-practices guide for outlet works 

conduits through embankment dams, 

emphasizing small embankments.

8.1 6.3 7.2 1

3A

Best-practices guide for selection of 

gates and valves for outlet works 

structures.

7.6 6.3 7.0 2

3B

Guidelines for maintenance and 

exercise of gates and valves and for 

cavitation repairs for outlet works.

7.1 6.5 6.8 3

4A

Best-practices guide for energy 

dissipators for outlet works structures, 

considering heads, velocities, 

configuration, etc.

7.4 6.1 6.8 4

5A

Design guidance and guide 

specifications for sliplining of outlet 

conduits.

7.4 6.1 6.8 4

1C

Best-practices guide for mechanical 

and electrical design and for 

automated control for outlet works.

7.0 6.1 6.6 6

5B

Compilation of case studies on outlet 

works rehabilitation – lessons learned, 

performance, comparative costs.

7.5 5.5 6.5 7

6B

Best-practices guide for inspection of 

outlet works gates, valves, and 

operators, including mechanical and 

electrical components.

6.8 6.2 6.5 7

Note: (1) From Exhibits 5-1, and 5-2.

(2) Arithmetic average of the two individual scores.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-10

EXHIBIT 5-3

RANKING OF RESEARCH/DEVELOPMENT IDEAS

BASED ON THE COMBINATION OF INDIVIDUAL SCORES

FOR POTENTIAL BENEFIT AND COST/EASE OF IMPLEMENTATION

-CONTINUEDINDIVIDUAL SCORES 

(1)

TOPIC 

NUMBER

RESEARCH/DEVELOPMENT 

TOPIC(S)

POTENTIAL 

BENEFIT COST

SCORE

0 to 10(2) RANK

1D Develop a list of failure modes 

for outlet works.

6.4 6.2 6.3 9

6C

Best-practices guide for 

inspection frequency and methods 

of inspection for typically 

accessible outlet works conduits.

6.2 6.4 6.3 9

5D

Best practices guide for design 

and construction methods to 

rebuild embankment sections 

removed for outlet conduit 

removal/replacement.

5.9 6.2 6.1 11

2A

Best-practices guide for outlet works 

intake towers, considering such 

factors as location, configuration, 

access, debris/trash control, seismic 

performance, selective withdrawals, 

etc.

6.7 4.9 5.8 12

1A Guidelines for venting outlet works. 6.1 5.4 5.8 13

6D

Guidelines for inspection of 

normally inundated structures and 

other difficult-to-access features.

5.4 5.8 5.6 14

2C

Compile data on performance of 

conduits with significant settlement 

and lateral movement.

6.1 4.7 5.4 15

Note: (1) From Exhibits 5-1, and 5-2.

(2) Arithmetic average of the two individual scores.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-11

EXHIBIT 5-3

RANKING OF RESEARCH/DEVELOPMENT IDEAS

BASED ON THE COMBINATION OF INDIVIDUAL SCORES

FOR POTENTIAL BENEFIT AND COST/EASE OF IMPLEMENTATION

-CONTINUEDINDIVIDUAL SCORES 

(1)

TOPIC 

NUMBER

RESEARCH/DEVELOPMENT 

TOPIC(S)

POTENTIAL 

BENEFIT COST

SCORE

0 to 10(2) RANK

2D

Guidance on seismic effects on 

conduits and methods for analysis 

and design to address those effects 

(e.g. embankment spreading, fault 

offset, propagation of earthquake 

waves).

6.1 4.6 5.4 16

6A

Applications of non-destructive 

testing (NDT) for inspection of 

outlet works.

5.3 5.4 5.4 16

5C

Assessment of microtunneling 

and boring technologies for 

application to outlet conduit 

rehabilitation.

5.3 5.2 5.3 18

3C

Develop short course (with 

video/DVD option) on design, 

operation, and maintenance of outlet 

works gates and valves.

6.4 3.6 5.0 19

2B

Guidance on application of tunnels 

for outlet works – including 

consideration of microtunneling and 

of both soft-ground and hard-ground 

tunneling.

5.3 4.4 4.9 20

Note: (1) From Exhibits 5-1, and 5-2.

(2) Arithmetic average of the two individual scores.
SECTIONFIVE Overall Prioritization of Research and Development Ideas

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

5-12

EXHIBIT 5-4

COST-BENEFIT COMPARISON OF RANKED R&D TOPICS

10 5 0

COST/EASE OF IMPLEMENTATION

0

5

10

P

O

T

E

N

TIA

L

B

E

N

E

FIT

1A

1C

1B

1D

6A

6C

6B

6D

R& D TOPICS:

3A

4A

3B

3C

2A

2C

2B

2D

5A

5B

5C

5D

REGION I:

HIGH BENEFIT AND

EASY/LOW COST

REGION III:

LOW BENEFIT AND

EASY/LOW COST

REGION IV:

LOW BENEFIT AND

DIFFICULT/HIGH COST

REGION II:

HIGH BENEFIT AND

DIFFICULT/HIGH COST
SECTIONSIX References

REPORT OF WORKSHOP ON ISSUES,

REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

6-1

6 . Section 6 SIX References

All of the white papers authors provided reference lists in cases where references were cited, 

which can be found in the footnotes or bibliographies provided at the back of the white papers 

presented in Attachments 4 through 9.

In addition, due to recommendations of several authors that standardized guidelines to be used in 

outlet works design and consistent guidelines for the various topics included in this workshop be 

developed, the following internet addresses may prove useful in locating the most recent outlet 

works publications.

www.usace.army.mil/publications/

www.usbr.gov/pmts/hydraulics_lab/

www.ferc.gov/industries/hydropower/safety.asp

www.info.usda.gov/CED/
Attachment 1 
Attachment 1_Workshop Agenda.doc 1 of 3

AGENDA 

FEMA WORKSHOP 

ISSUES, REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

May 25, 26, and 27, 2004 

Denver, Colorado 

Day 1 - Tuesday, May 25, 2004 

8:00 to 8:15 Self- Introductions 

8:15 to 8:30 Workshop Introduction 

8:30 to 9:00 White Paper Presentation – Failure Modes 

9:00 to 9:30 Discussion of State-of-Practice – Failure Modes 

9:30 to 10:00 Brainstorming of R & D Needs – Failure Modes 

10:00 to 10:15 Morning Break 

10:15 to 10:30 Prioritization of R&D Needs – Failure Modes 

10:30 to 11:30 R & D Implementation Work Groups – Failure Modes 

11:30 to 12:00 Implementation Work Group Reports – Failure Modes 

12:00 to 1:00 Lunch 

1:00 to 1:30 White Paper Presentation – Inspection 

1:30 to 2:00 Discussion of State-of-Practice – Inspection 

2:00 to 2:30 Brainstorming of R & D Needs – Inspection 

2:30 to 2:45 Prioritization of R&D Needs – Inspection 

2:45 to 3:00 Afternoon Break 

3:00 to 4:00 R & D Implementation Work Groups – Inspection 

4:00 to 4:30 Implementation Work Group Reports – Inspection 

4:30 to 5:00 Day 1 Closure 
Attachment 1_Workshop Agenda.doc 2 of 3

AGENDA 

FEMA WORKSHOP 

ISSUES, REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

May 25, 26, and 27, 2004 

Denver, Colorado 

Day 2 - Wednesday, May 26, 2004 

8:00 to 8:30 White Paper Presentation – Energy Dissipators 

8:30 to 9:00 White Paper Presentation – Gates and Valves 

9:00 to 9:30 Discussion of State-of-Practice – Energy Dissipators and 

Gates and Valves 

9:30 to 10:00 Brainstorming of R & D Needs – Energy Dissipators and 

Gates and Valves 

10:00 to 10:15 Morning Break 

10:15 to 10:30 Prioritization of R&D Needs – Energy Dissipators and 

Gates and Valves 

10:30 to 11:30 R & D Implementation Work Groups – Energy Dissipators 

and Gates and Valves 

11:30 to 12:00 Implementation Work Group Reports – Energy Dissipators 

and Gates and Valves 

12:00 to 1:00 Lunch 

1:00 to 1:30 White Paper Presentation – Conduits 

1:30 to 2:00 Discussion of State-of-Practice – Conduits 

2:00 to 2:30 Brainstorming of R & D Needs – Conduits 

2:30 to 2:45 Prioritization of R&D Needs – Conduits 

2:45 to 3:00 Afternoon Break 

3:00 to 4:00 R & D Implementation Work Groups – Conduits 

4:00 to 4:30 Implementation Work Group Reports – Conduits 

4:30 to 5:00 Day 2 Closure 
Attachment 1_Workshop Agenda.doc 3 of 3

AGENDA 

FEMA WORKSHOP 

ISSUES, REMEDIES, AND RESEARCH NEEDS 

RELATED TO DAM OUTLET WORKS

May 25, 26, and 27, 2004 

Denver, Colorado 

Day 3 - Thursday, May 27, 2004 

8:00 to 8:30 White Paper Presentation – Rehabilitation 

8:30 to 9:00 Discussion of State-of-Practice – Rehabilitation 

9:00 to 9:30 Brainstorming of R & D Needs – Rehabilitation 

9:30 to 9:45 Prioritization of R&D Needs – Rehabilitation 

10:00 to 10:15 Morning Break 

10:15 to 11:15 R & D Implementation Work Groups – Rehabilitation 

11:15 to 11:45 Implementation Work Group Reports – Rehabilitation 

11:45 to 12:45 Lunch 

12:45 to 1:45 Recap and Discussion of R&D Needs 

1:45 to 2:30 Prioritization of Leading R&D Needs 

2:30 to 3:00 Day 3 - Closure 
Attachment 2 
Attachment 2_RD Implementation Plan Form.doc 1 of 2

FEMA WORKSHOP ON ISSUES, SOLUTIONS 

AND RESEARCH NEEDS RELATED 

TO DAM OUTLET WORKS 

MAY 25, 26, AND 27, 2004 

DENVER, COLORADO 

1. Title/Description of Research/Development Item 

2. Description 

A. Why is this a priority research/development item? 

B. What is the expected outcome? 

3. Project Tasks and Needs 

A. What tasks are to be done? 

 B. How is the problem to be solved? 
Attachment 2_RD Implementation Plan Form.doc 2 of 2

3. Project Tasks and Needs - continued 

4. Project Lead and Contact 

A. Who is working in this area? 

B. Who might be able to lead the project? 

C. Who are good candidates to complete the work? 
Attachment 3 
Attachment 3_ Ranking of RD Ideas Form.doc 1 of 2

RANKING OF RESEARCH AND DEVELOPMENT IDEAS 

No. Research and Development Idea 

Benefit 

Ranking, 

0 to 10(1)

Difficulty/ 

Cost 

Ranking, 

0 to 10(2)

1 Guidelines for venting outlet works. 

2 Best-practices guide for mechanical and electrical 

design and for automated control for outlet works. 

 

3 Best-practices guide for outlet works conduits through 

embankment dams, emphasizing small embankments. 

 

4 Develop a list of failure modes for outlet works. 

5 Applications of non-destructive testing (NDT) for 

inspection of outlet works. 

 

6 Best-practices guide for inspection frequency and 

methods of inspection for typically accessible outlet 

works conduits. 

 

7 Best-practices guide for inspection of outlet works 

gates, valves, and operators, including mechanical and 

electrical components. 

 

8 Guidelines for inspection of normally inundated 

structures and other difficult-to-access features. 

 

9 Best-practices guide for selection of gates and valves 

for outlet works structures. 

 

10 Best-practices guide for energy dissipators for outlet 

works structures, considering heads, velocities, 

configuration, etc. 

 

11 Guidelines for maintenance and exercise of gates and 

valves and for cavitation repairs for outlet works. 

 

12 Develop short course (with video/DVD option) on 

design, operation, and maintenance of outlet works 

gates and valves. 

 

13 Best-practices guide for outlet works intake towers, 

considering such factors as location, configuration, 

access, debris/trash control, seismic performance, 

selective withdrawals, etc. 

 

Notes: (1) 0 is lowest benefit and 10 is highest benefit. 

 (2) 0 is unfavorable high cost/difficulty in implementation and 10 is favorable 

low cost/easy implementation. 
Attachment 3_ Ranking of RD Ideas Form.doc 2 of 2

RANKING OF RESEARCH AND DEVELOPMENT IDEAS 

Page 2 of 2 

No. Research and Development Idea 

Benefit 

Ranking, 

0 to 10(1)

Difficulty/ 

Cost 

Ranking, 

0 to 10(2)

14 Compile data on performance of conduits with 

significant settlement and lateral movement. 

 

15 Guidance on application of tunnels for outlet works – 

including consideration of microtunneling and of both 

soft-ground and hard-ground tunneling. 

 

16 Guidance on seismic effects on conduits and methods 

for analysis and design to address those effects (e.g. 

embankment spreading, fault offset, propagation of 

earthquake waves). 

 

17 Design guidance and guide specifications for 

sliplining of outlet conduits. 

 

18 Compilation of case studies on outlet works 

rehabilitation – lessons learned, performance, 

comparative costs. 

 

19 Assessment of microtunneling and boring 

technologies for application to outlet conduit 

rehabilitation. 

 

20 Best practices guide for design and construction 

methods to rebuild embankment sections removed for 

outlet conduit removal/replacement. 

 

Notes: (1) 0 is lowest benefit and 10 is highest benefit. 

 (2) 0 is unfavorable high cost/difficulty in implementation and 10 is favorable 

low cost/easy implementation. 
Attachment 4 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 1 of 12

OUTLET WORKS FAILURE MODES 

FEMA Outlet Works Workshop 

May 25- 27, 2004 Denver, Colorado 

By Sal Todaro 

1.0 INTRODUCTION 

This paper overviews the ways in which outlet works can fail and methods that can be used to 

prevent failures. Outlet works features are grouped into four basic structures (1) Intake 

Structures, (2) Conduits and Tunnels (3) Control Structures and (4) Energy Dissipators. Gates, 

valves and other mechanical features are discussed with the structures where they are located. 

Causes of outlet works failures include: foundation related failures, structural deterioration, 

structural failures, mechanical failures, failures related to hydraulics, failures related to ice and 

sediment loadings, and operator error. Methods are recommended to prevent failures in new 

design, by retrofitting existing facilities, and by facility operation. 

The outlet works is an important cause of dam failures, since approximately 25 percent of 

embankment dam failures are associated with the outlet works.1

 Outlet works failures can also 

result in less catastrophic consequences than dam failure such as: the inability to make required 

reservoir releases, structural or mechanical conditions requiring emergency draining of the 

reservoir, or the uncontrolled release of the reservoir. 

2.0 INTAKE STRUCTURES 

2.1 Intake Structures 

The intake structure is where water enters the outlet works. Intake structures can be either gated 

or ungated. 

2.2 Types of intake structures and their application 

Intake structures can be submerged structures, tower structures with multiple level inlet ports, 

inclined structures located on an abutment of the dam with a single bottom inlet or with multiple 

level inlet ports. 

2.3 Mechanical Features of Intake Structures 

The principal mechanical features of intake structures are the intake gates and the trash racks. 

Ungated inlets are often fitted for the installation of a removable bulkhead gate that could be 

installed using divers. 

 

1

 Foster, M., R. Fell, and M. Spannagle. 2000. The statistics of embankment dam failures and accidents. Canadian 

Geotechnical Journal. Volume 37, Number 5, pp. 1000-1024. October. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 2 of 12

• Intake Gates and Valves- Causes of gate and valve malfunction and failure include: 

debris, cavitation, ice, operator error, and malfunction of gate operating mechanisms, lack 

of exercise, deterioration and corrosion. Use of obsolete equipment is common because it 

is often difficult and costly to replace this equipment. Agencies that would not consider 

driving a Model T car commonly use similar vintage equipment to release water from 

major dams. 

• Control Systems to open and Close Gates and Valves-Mechanical control systems can 

include motor driven, hydraulic, and hand operators. Hydraulic control systems include 

hydraulic power units and hydraulic cylinders to open and close gates and valves. 

• Trash racks- Failure and malfunction of trash racks can result from: plugging due to 

sediment or debris or collapse of trash racks resulting for plugging from frazil ice.

2.4 Failure Modes of Intake Structures 

• Foundation Related Failures of Intake Structures - It is important for the designer to 

locate the intake structure on firm bedrock whenever possible. A firm foundation for the 

intake structure is important to reduce differential settlement between the intake structure 

and the outlet conduit. A rock foundation will not only reduce settlements but also 

provide rock mass for anchoring the structure to the rock foundation. Small or 

undetectable settlements at low or submerged inlet structures can be magnified several 

times at high intake towers. No engineer wants to be responsible for the design an intake 

tower that looks like the Leaning Tower of Pisa. Rock foundations can also be used for 

anchoring high intake towers to resist seismic loadings 

• Structural Deterioration - Intake structures can prematurely deteriorate over time because 

of the following: aging; freeze thaw damage which open intake towers can be especially 

susceptible to; poor concrete materials such as alkali aggregates; or poor water quality 

(excessively low or high pH can accelerate corrosion of reinforcement and cause 

deterioration of concrete). 

• Ice loading – Loading from expanding reservoir ice have caused damage to trash racks, 

gates stems, intake towers and piers for intake tower access. 

• Seismic Failure- It is important that intake structures are operable after earthquakes so 

that the outlet works can be used for emergency reservoir evacuation or to provide water 

to surrounding communities following a disaster. 

• Sedimentation of Intake Structures- Sedimentation of intake structures has caused serious 

operational problems at many dams. Sedimentation of intakes can be caused by: improper 

reservoir operation, upstream development, forest fires or design deficiencies. 

• Hydraulic Related Failures- Hydraulic related failures at intake structures could include 

cavitation of intake gates or at areas of the intake structure or in the conduit downstream 

of the intake gate. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 3 of 12

• Air/Water Related Failures - Inadequate air supply to control gates can cause cavitation 

and vibration at intake towers. Another air /water related operational problem is air 

blowback caused by air coming out solution in the conduit violently blowing air and 

water into intake structure. This type of air blowback resulted in serious damage at the 

Bureau of Reclamation’s Navajo Dam. Air blowback also caused the loss of the trash 

racks at Denver Water’s Dillon Dam by ejecting them into the reservoir. 

• Structural fatigue caused by vibration, or collapse as a result plugging such as at Yard’s 

Creek Dam as seen in Photo 1. 

Photo 1 – Collapse of Trash Rack at Yard’s Creek Dam. 

2.5 Methods to Prevent Failure of Intake Structures 

Methods that can be used to prevent failures of intake structures include the following actions: 

• Locate the intake structure on a rock foundation whenever possible to reduce settlement 

and to provide cohesion and rock mass for anchorage to the foundation rock to increased 

seismic stability 

• Design for air demand requirements to prevent blowback, pipe collapse, and cavitation 

of control gates. 

• Design for ice loading and consider the use of bubbler systems to prevent ice formation. 

Gate stems which will pass through a reservoir ice cover should either have a bubbler 

system to prevent ice formation or the gate stem should be located in an oil filled pipe. 

This is a standard detail that can be supplied by the gate manufacturers. 

• Use a mechanical engineer experienced in design of systems for dams to design the 

mechanical features of the outlet works. This engineer should be responsible for 

selection of intake gates, operators, and operating systems. This engineer should 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 4 of 12

conducting inspections of the installation and testing of gates and other critical operating 

systems. 

• Use reliable standard designs that have been developed and tested over time. 

Standardized reliable designs can be used to design most intake structures. Use of 

standardized designs will reduce design time and also help provide the owner with a 

tested design. 

• Use standardized design criteria, for loading condition and allowable stress. 

• Locate the intake structure to reduce sediment and debris accumulation. The design 

may require invert of the intake structure to be located at a higher elevation to reduce 

sediment accumulation. 

• Operate the reservoir to reduce movement of reservoir sediment to the intake

structure. At Twitchell and Strontia Dams, reservoir fluctuation is minimized since draw 

down lowering of the reservoir can transport sediment to the lower depths of the 

reservoir. 

• Consider the vibration in design of trash racks. 

3.0 CONDUITS AND TUNNELS 

3.1 Conduits and Tunnels for Dams 

Conduits and tunnels are used to convey outlet works discharges from the reservoir through the 

dam. Conduits can be constructed of cast in place or manufactured pipe. Conduit outlets are 

usually constructed through dams and can increase the risk of failure of embankment dams 

because of the potential for embankment piping near the conduit. Tunnel outlet works are 

separated from the dam embankment and are generally considered to be a safer than conduits.2

 

3.2 Types of Conduits 

The two general types of outlet conduits through dams are cast in place concrete conduits and 

conduits constructed using manufactured pipe. Pipe materials used to construct outlet conduits 

include of pre-cast concrete, plastic, steel and ductile iron. 

3.3 Tunnels 

Tunnels for dams are usually constructed in hard rock conditions however soft ground tunnels 

have also been used for dam outlet works. Tunnels for dams are usually lined with cast in place 

concrete or welded steel. In some cases (usually in the past) tunnels have been constructed for 

dams without tunnel linings. This practice is not currently accepted for outlet tunnels located 

near embankment dams because of the potential for pressurized water to travel through joints in 

the rock and cause internal erosion of the embankment. 

 

2

 Refer to “Conduits through Embankment Dams” a guideline developed by FEMA containing the design 

requirements for conduits through embankment dams. Final Draft dated February 2005. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 5 of 12

3.4 Embankment Dam Failure Caused by Outlet Conduits 

Outlet conduits have been found to be responsible for approximately 25 percent of embankment 

dam failures. Four possible failure modes associated with outlet conduits that can result in dam 

failure are shown in Figure 1. Two of the failure modes are caused by a structural failure of the 

outlet conduit, the other two failure modes is caused by preferential seepage paths through the 

embankment near the conduit. These failure modes are discussed in detail in the upcoming 

FEMA guideline for the design of outlet conduits.3 

Figure 1 – Failure Modes for Conduits in Embankment Dams. 

3.5 Failure Modes of Tunnel Outlet Works 

Tunnel outlets works adequately separated from the embankment cannot cause the internal 

erosion of the embankment. However, tunnel outlet works constructed too close to a dam can 

result in internal erosion of a dam embankment. Internal erosion of an embankment caused by an 

outlet tunnel is rare but can occur. The outlet tunnel at Willow Creek Dam was constructed under 

the abutment of the dam. Erosion of embankment material through rock joints above the tunnel 

 

3

 “Conduits through Embankment Dams,” FEMA. Final Draft dated February 2005. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 6 of 12

caused a sinkhole at the embankment and required significant repairs to the embankment and the 

tunnel.4

 

3.6 Methods of Preventing Failure of Conduits and Tunnels 

Methods that can be used to reduce the potential for failure of outlet conduits through 

embankment include the following actions: 

• Filter the dam embankment near the outlet conduit to prevent the internal erosion of the 

embankment along or near the conduit.5

• Construct outlet conduits on rock foundations whenever possible. Founding the outlet 

conduit upon a rock will provide several benefits and reduce the potential for settlement 

and cracking of the conduit and therefore reduced potential leakage of from the conduit to 

the embankment or from the embankment into the conduit. Conduits constructed directly 

upon rock foundations can also decrease the contact area between the embankment and 

the conduit and reduce the paths for seepage along the conduit. Constructing the conduit 

into the dam abutment can also reduce the potential for stress concentrations and for the 

embankment near the conduit as seen in Figure 2.6

Figure 2 – Conduit Cast Against Excavated Rock 

• Conduits can be designed with redundant methods to prevent seepage of water from the 

conduit to the embankment and embankment material from piping into the conduit. 

Redundant methods of seepage prevention can include the use of a cast in place concrete 

conduit with water-stopped joints and a welded structural steel liner. This redundant 

 

4

 “Conduits through Embankment Dams,” FEMA. Final Draft dated February 2005. 

5

 Refer to the upcoming FEMA guideline on conduits through embankment dams for guidance on the design of 

filters for outlet conduits. 

6

 Source: “Conduits through Embankment Dams – FEMA Final Draft dated February 2005. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 7 of 12

design method has been used by the Bureau of Reclamation for pressurized conduits for 

higher head dams or outlets founded upon weak compressible foundation. HDPE pipe 

with welded joints and reinforced concrete encasement with water-stopped joints can also 

be used for a redundant outlet conduit design for lower head dams. 

4.0 CONTROL STRUCTURES 

4.1 The Function of Control Structures 

For the purpose of this paper, the term ‘control structures’ is used to refer to the location where 

the discharge from the outlet works is controlled. The flow regulating gates for the dam are 

located at the control structure. The regulating can be located: at a valve house at the 

downstream toe of the dam, at an intake structure or at a gate chamber within the dam. Figure 3 

shows these typical locations for control valves.7

Figure 3 – Various Locations for Control Structures. 

 

7

 Source: “Conduits through Embankment Dams – FEMA. Final Draft dated February 2005. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 8 of 12

4.2 Control Valves for Outlet Works 

The selection of the valve used to control flow is strongly influenced by the location of the 

control structure. 

• Control Valves Located at Intake Structures- When control valves are located at the 

intake structure, the conduit through the dam is usually not pressurized; this type of 

design is commonly used by the Corps of Engineers for their flood control dams. 

Location for the control valve at the intake structure is also common for small dams 

where slide gates are located at the upstream face of the dam. Sluice gates or bonneted 

slide gates are commonly used to control flow from intake towers. The approach 

conditions for control gates located at intake towers are usually less than ideal; these poor 

approach conditions can affect the performance of the gate and can cause cavitation and 

vibration. When intake gates are used to control flows at heads above approximately 40 

feet, care is required in the design of the intake and selection of the control gate.

• Control Valves Located in Gate Chambers- When control valves are located in gate 

chambers the flow upstream of the gate chamber is pressurized and the flow downstream 

of the gate chamber is not pressurized (open channel flow conditions.) This type is 

commonly used by the USBR for their outlet works. The approach flow to valves located 

in gate chamber is usually a pressurized conduit with a transition designed to streamline 

flow to the control valve. Conditions upstream of valves located in gate chambers are 

usually trouble free. However, because of the high velocities that can occur downstream 

of a control valve, cavitation can occur downstream of valves at higher head facilities. 

The bonneted slide gate is commonly selected for this application because they perform 

well under a wide range of operating heads and they produce a compact jet which is 

required to allow open channel conditions in the downstream conduit. 

• Control Valves Located a Valve House at the Downstream Toe of the Dam. The entire 

length of the outlet conduit is pressurized when the control valves are located at the 

downstream toe of the dam. This type of design is often used when the outlet works is 

used to supply a pressurized pipeline or a hydroelectric power plant. This type of design 

is almost exclusively used when the head at the valve exceeds 200 feet. The types of 

valves commonly used for this application include the fixed cone valve, the jet flow gate, 

the bonneted slide gate and the sleeve valve.

4.3 Failure Modes of Control Structures 

Failure modes of control structures are usually associated with the gates, valves, or gate 

operators and control systems. These types of failures do not usually jeopardize the safety of the 

dam, but can limit the ability of the dam to discharge water for normal or emergency releases. 

Photo 2 shows a valve stuck in the closed position because of lack of maintenance. Photo 3 

shows poorly maintained and broken gate operators at an intake tower. Photo 4 shows a failed 

needle valve, this failure resulted in the death of the dam tender. Many of these failures are 

caused poor maintenance and the use of outdated (antique) equipment. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 9 of 12

Photo 2 - Valve stuck in the closed position due to lack of maintenance. 

Photo 3 – Broken Gate Operators at an Intake Tower. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 10 of 12

Photo 4 - Failed needle valve. 

4.4 Methods to Prevent Failure of Control Structures 

Methods that can be used to prevent failures of control structures include the following actions: 

• Proper training of all operations and maintenance personnel 

• Regular periodic exercise of all gates and valves to prevent buildup on seats and other 

moving parts that would inhibit operation (minimum of once per year, preferably 3 or 4 

times per year). 

• Note any unusual noises or vibration during operation of equipment, and investigate any 

new observations. 

• Periodically measure electrical current or hydraulic pressure required for gate and valve 

operation. Compare readings with previous measurements to note any deviations 

• When designing new outlet works or outlet works rehabilitations, use a mechanical 

engineer experienced with the design of gates, valves and mechanical systems for dam 

outlet works. 

• Replace outdated and unreliable gates, valves and operators 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 11 of 12

5.0 ENERGY DISSIPATORS 

5.1 Energy Dissipators and Their Function 

Energy dissipators are used to reduce channel erosion downstream of outlet works. A more 

thorough discussion of the types of energy dissipators used for outlet work and their application 

can be found in the White Paper prepared for this workshop by Dr. Henry T. Falvey titled 

“White Paper on Energy Dissipators.” 

5.2 Types of Energy Dissipators 

The more commonly used energy dissipators used for outlet works are: 

• No energy dissipator is used and water discharges directly to the downstream channel. 

As seen in Photo 5 taken at Hubbart Dam a concentrated jet discharging directly to the 

channel downstream of the gate. Initial operation of the gate caused localized channel 

scour which eventually stabilized. 

Photo 5 – Hubbart Dam gatehouse and stilling basin. 

• Use a valve that produces a dispersed jet to reduce the potential for downstream channel 

erosion. Fixed cone valves are often used because the produce a dispersed jet that 

reduces the potential for downstream channel erosion. The spray produced by the fixed 

cone valve can result in maintenance problems in cold climates because of the icing that 

often occurs downstream of these valves. 
Attachment 4_White Paper 1 - Outlet Works Failure Modes.doc 12 of 12

• Energy dissipating valves - Energy dissipating valves such as the sleeve valve or 

Monovar valves are expensive but can eliminate the need for downstream energy 

dissipating structures. 

• Conventional Concrete Stilling Basin Design- Some commonly reinforced concrete 

stilling basin designs include: the hydraulic jump basin, the Impact basin and the fixed 

cone valve dissipation chamber. 

5.3 Failure Modes of Energy Dissipators 

The principal failure mode for energy dissipators is uncontrolled erosion of the downstream 

channel. 

5.4 Method to Prevent Failure of Energy Dissipators 

The design of energy dissipators for outlet works can usually be accomplished using designs 

standardized by the USBR, USACE, and other large state and federal agencies. These designs 

should be used by the designer whenever possible. 

6.0 CONCLUSIONS 

In order to minimize the occurrence of outlet works failures the following guidelines should be 

followed: 

• Well-trained, experienced engineers should design outlet works; 

• Outlet structures should be located on rock foundations whenever possible; 

• Standardized, reliable designs should be used; 

• Unusual failure modes like those associated with vibration, ice loading, and air/water 

interactions, should be considered in outlet works design; and 

• Redundancy should be built into the design of all critical elements of outlet works 

systems. 

Failures of mechanical equipment can usually be traced to the following causes: 

• Lack of exercise 

• Lack of maintenance 

• Deterioration due to erosion, corrosion, and/or cavitation 

• Neglecting operating observed difficulties 

• Operator error or lack of training 
Attachment 5 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 1 of 9

DESIGN AND CONSTRUCTION OF OUTLET WORKS CONDUITS 

FOR EMBANKMENT DAMS 

FEMA Outlet Works Workshop 

May 25 – 27, 2004 – Denver, Colorado 

By Sal Todaro, P.E. 

1.0 Introduction 

This paper addresses outlet works design for smaller dams owned and operated primarily by 

municipalities and private owners. These smaller dams account for most of the nation’s 

jurisdictional dams. This group of outlet works provides greater design versatility than designs 

produced by federal design agencies and large experienced engineering firms. 

There is no single nationally recognized standard for designing outlet works for dams, and 

inexperienced firms have a greater need for established design guidelines for outlet works. State 

dam safety officials are tasked with reviewing designs that can vary significantly in their design 

approach. On the other hand, outlet works designed by federal agencies and large engineering 

firms use similar designs and are often more consistent in their approach to design. For example, 

outlet works designed by the U.S. Bureau of Reclamation (USBR) are consistent within 

groupings of pressurized and non-pressurized outlet works. 

Not all outlet works should be similarly designed; however, there should be consistent guidelines 

and criteria for preparing these designs. Design guidelines could include recommendations for 

pipe encasement, the need for redundancy, seepage control, methods for addressing compressible 

foundations, pipe selection, joint details, and criteria for reinforced concrete conduit design. 

Consistent guidelines can direct design engineers to safe designs, thus providing state dam safety 

officials assistance in reviewing these designs. Designs developed using thorough, consistent 

standards would result in more consistency between projects and would result in safer, more 

reliable facilities. 

Guidelines currently being prepared by the National Dam Safety Review Board (NSRB) can 

form a basis for design of smaller facilities. The NSRB is preparing a manual for the design, 

construction, maintenance, and repair of conduits for embankment dams. The NSRB manual is 

intended to be used for designing and maintaining outlet works for significant high-hazard dams 

and should be required reading for outlet works designers of both large and small dams. 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 2 of 9

2.0 Reducing Embankment Dam Failures Related to Outlet Works Conduits 

Approximately 25 percent of embankment dam failures are related to embankment piping at or 

near the outlet works (Foster et al.)1

. By reducing this failure mode, significant dam safety 

improvements would result. Primary embankment dam failures at or near outlet works can be 

attributed to structural defects of the conduit, resulting in piping embankment into or along the 

outside of the conduit (see Figure 1). Because of the extensive erosion that can occur from a 

failure, the exact cause is often difficult to determine. The above-referenced research also shows 

that dam failures associated with outlet works occur rapidly and usually do not provide adequate 

time to respond so that dam failure can be prevented. 

Important improvements in outlet works design methods and construction procedures can be 

achieved by determining the best practice for designing and constructing outlet works, preparing 

nationally accepted standards, and by state and federal dam safety officials enforcing these 

standards. 

3.0 Standards Presently Used for Design of Outlet Works 

3.1 General 

Outlet works are often designed using standards not specifically intended for use in designing 

pressurized outlet system for dams. For example, American Water Works Association (AWWA) 

pressure pipe standards were developed for water supply pipelines; these requirements may not 

be appropriate (without modifications) for the design and construction of outlet pipe under 

embankment dams. 

Outlet pipes have different requirements than water supply pipelines, including long-term 

performance requirements and requirements for construction procedures compatible with the 

design and performance of the embankment dam. Some of the variations between water 

pipelines and outlet works include: 

• Outlet pipe buried under an embankment dam are there for the life of the dam (which 

can be greater than 100 years), thus not allowing the accessibility for repairs as that of 

a water pipeline. The dam may need to be breached to replace an outlet conduit. 

• Outlet pipes under dams are subjected to different foundation movements than 

waterlines; this is especially true during an earthquake. 

• Because public safety issues are associated with dams, outlet works have redundancy 

requirements not required for water pipelines. 

• The consequences from an outlet pipe leak through an embankment dam can be much 

more critical than a water supply pipeline leak. 

 

1

 Foster, M., R. Fell, and M. Spannagle. 2000. The statistics of embankment dam failures and accidents. Canadian 

Geotechnical Journal. Volume 37, Number 5, pp. 1000-1024. October. 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 3 of 9

Engineers not experienced in designing dams and outlet works can inadvertently use 

inappropriate design standards. Examples of misapplication of outlet works design standards 

include: 

• State highway department standard plans for culverts and culvert structures. These 

standard drawings are simply referred to in design documents to save the engineer 

from designing the outlet works. Culvert designs for highways were not intended for 

use in dams. 

• Natural Resources Conservation Service (NRCS) standards for the structural design 

of flood control outlets have been used for designing high-hazard pressurized outlet 

facilities; however, these standards were not intended for such application. 

• NRCS has also developed design standards for articulated outlet pipe design. These 

standards can also be used beyond NRCS’ intended use. These design standards have 

been used for designing a pressurized outlet at a high-hazard municipal dam on a soft 

foundation. These particular standards should not be used for designing high-hazard 

pressurized outlet works. 

• USBR Design of Small Dams. This may be the best general reference for designing 

reinforced concrete outlet conduits. The NRCS and Design of Small Dams show a 

precast concrete outlet pipe detail with partial encasement, which should not be used 

for pressurized outlet works for high-hazard dams (Figure 2). This detail allows 

embankments to be placed directly against the pipe surface, an area where leakage 

from the pipe joint has direct access to the embankment. 

• Precast pipe under medium height to high embankments. For example, reinforced 

concrete pressure pipe is usually not a good selection for a pressurized outlet pipe 

under a medium height dam. An outlet pipe under a 100-foot-high dam can have 

more than 80 gasketed joints, each of which is a potential leakage source into the 

embankment. 

3.2 Need for Design Guideline for the Design and Construction of Outlet Works Conduits 

Nationally recognized design guidelines are needed for designing outlet works at small- and 

medium-sized dams. Dam safety officials could then require and enforce minimum standards for 

all new non-federal designs, and design engineer’s tasks would be simplified by having a 

consistent standard for design, one they know review agencies will accept. Such guidelines 

would reduce the use of inappropriate design standards. The USBR and U.S. Army Corps of 

Engineers (USACE) design standards for outlet works combined with the forthcoming NSRB 

manual can form a basis for preparing a single design and construction standard for smaller dam 

outlet facilities. 

4.0 Foundations for Outlet Works 

Good foundations are important for all civil engineering structures, especially for dams and 

outlet works structures. Design engineers must use appropriate measures in determining the 

foundation and potential movements to help eliminate possible outlet works failures. 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 4 of 9

4.1 Outlets Constructed on Rigid Foundations 

Outlet works structures should be founded on firm non-erodable foundations whenever possible. 

The alignment of an outlet conduit should be based upon the location of competent foundation 

for the conduit. Outlet works designs and construction specifications should allow for 

adjustments in the alignment during construction after the foundation in the areas of the outlet 

has been exposed. Final alignment adjustments to fit field conditions can improve foundations 

and help ensure that the outlet works structures are founded on firm, uniform foundations. 

Refining the outlet works alignment can also reduce the amount of required excavation or the 

amount of backfill concrete required. Figures 3, 4, and 5 show the construction of an outlet pipe 

excavated into the abutment of a dam. 

Locating the outlet works on a firm rock foundation will decrease settlement and potential for 

structural distress, improve performance during seismic loads, and reduce seepage potential 

adjacent to the conduit. 

4.2 Outlets Constructed on Compressible Foundations 

Outlet works structures founded upon compressible foundation require more attention during 

design and construction, and provide more challenge to the designer and more risks to the dam 

owner than those on firm rock foundations. Outlet conduits on soft and erodable foundations are 

subject to foundation movement, thus incurring greater risks of embankment piping and 

structural distress of the conduit than outlet conduits founded on firm rock foundations. 

Foundation movements are especially dangerous for rigid outlet pipes with open gasketed joints; 

Figure 6 shows a photograph of a failed pipe. 

Outlet conduit should be located on uniform foundations whenever possible to prevent abrupt 

changes in conduit settlement. Because geological condition variables can exist along a conduit 

alignment, uniform foundation is often not possible. Figures 7 and 8 show profiles of foundation 

settlements for cast-in-place outlet conduits at several USBR dams on compressible foundations. 

Soft foundations under embankment dams can settle vertically and spread horizontally. Figure 9 

shows settlement profiles of a failed cast-in-place articulated pipe. Note that the settlement of 

the pipe joints is more abrupt than the cast-in-place conduit design. (The longitudinal 

reinforcement provided by cast-in-place conduits appears to have an effect of smoothing out the 

conduit settlements under the embankment.) 

These figures are important information for engineers conducting outlet conduit settlement 

analyses. It is important to note that foundation settlement conditions can vary significantly 

along an outlet conduit, do not conform to more uniform settlements, and concentrated localized 

settlements can be more severe than predicted by analysis. 

Engineers should question the precision or theoretical settlement analysis conducted to estimate 

the foundation movement of an outlet works. This is especially important if the outlet works has 

gasketed joints that can fail or open when subjected to movements larger than their design 

capacity. Figure 9 shows the estimated deflection and actual deflection of a reinforced concrete 

cylinder pipe that failed under a 100-foot-high embankment. This outlet pipe was designed 

using articulated joint design methods. 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 5 of 9

Work by Rutledge and Gould2 has documented longitudinal movement of pipe joints on soft 

foundations. This work shows that settlement and the horizontal joint openings can be erratic 

and non-uniform. Large horizontal movements take place at random selected joints, rather than 

uniformly along the conduit. This type of concentrated movement can open joints of gasketed 

pipe that are not designed for large concentrated horizontal movements. The results of these 

field measurements show that dangerous movements can occur for gasketed pipe outlet works, 

even when the engineer may believe that the pipe has been designed to resist movement 

predicted by analysis. 

Because of the unpredictable random movements that can occur under dam foundations, 

engineers should question the precision of theoretical settlement analysis estimates of outlet 

works conduits. This is especially important if the outlet works has gasketed joints that can fail 

or open when subjected to movements large than their design capacity. 

Techniques historically used by USBR for designing outlet conduits on compressible 

foundations include: 

• Over-excavating foundation and refill with compacted impervious earthfill, 

• Positioning control gates upstream of impervious embankment, 

• Avoiding pressure conduits through impervious embankments unless it is housed in a 

freestanding pipe, 

• Providing steel lining for pressurized conduits, 

• Avoiding pre-cast concrete pipe except for embankments of small heights (10 to 

12 feet), and 

• Protecting the foundation during construction. 

Approaches to Design Outlet Conduits on Soft Foundations

1. Provide a Freestanding Pipe inside of a Reinforced Concrete Conduit. This is a classical 

outlet works design, which locates a freestanding pressurized outlet pipe inside of a castin-place reinforced concrete conduit. The conduit upstream of the embankment case is 

usually pressurized. The design prevents pressurized water from a leaking pipe from 

contacting the embankment. 

2. Provide a Non-Pressurized Outlet Works System. 

3. Welded Steel Outlet Pipe Encased in Reinforced Concrete. This design includes a 

welded steel pipe and reinforced concrete encasement. The theory of this design is that 

the welded steel pipe is ductile, will deform and maintain a watertight conduit, and will 

resist rupture when subjected to foundation movements. The reinforced concrete 

encasement provides a rigid beam to bridge over weak foundation to minimize 

concentrated deflections. The reinforced concrete also provides a redundant water 

passage, capable of resisting internal and external water loads. This design has been 

 

2

 Rutledge, P.C., and Gould, J.P. 1973. Embankment-Dam Engineering. Casagrande Volume. John Wiley & Sons. 

New York City, New York. 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 6 of 9

successfully used by USBR and URS Corporation for new dams as well as retrofits for 

relining outlet works; however, there are no published design criteria for this design 

method. When possible, field joints for welded steel pipe on compressible foundations 

should be welded after embankment construction once the initial foundation settlement 

has occurred. 

4. Articulated Joint Conduit Design. Another outlet design system on soft foundations is 

the use of articulated joints for the outlet conduit. Joint design for outlet work conduits is 

an important area where guidelines are needed, especially for pressurized outlet works on 

compressible foundations. The theory of this design is that the pipe joints are designed to 

deflect and accommodate for longitudinal and rotational deflections of the pipe joints. 

The pipe encasement concrete is provided with open joints to allow the movement of the 

pipe joints. 

There are two important dam safety concerns when an articulated pipe design is used for 

pressurized outlets at high-hazard dams. The first is that theoretically predicting 

deflections and longitudinal movement of joints may not be accurate, and the joints can 

be subjected to movements beyond their safe capacity. The second concern is that the 

encasement concrete at each pipe joint that can provide a leakage path directly to the 

embankment. A gasket or pipe bell failure from excessive settlement will allow 

pressurized flow from the conduit to the embankment. 

5. Treatment of Foundation During Construction. Special construction procedures should 

be provided to protect the foundation integrity, which can degrade when exposed to air, 

moisture, and construction activity. Construction methods used to prepare conduit 

foundations include covering the exposed foundation with a concrete mud mat as soon as 

the excavation to final grade is exposed. 

5.0 Pipe Material Used for Outlet Works Conduits 

The following is a brief description of various pipe material that have been used at embankment 

dams for outlet conduits. The information is intended to form a basis of discussion by group 

members during the conference. 

1. Cast-in-Place Reinforced Concrete and Outlet Works Construction. This design is typical 

construction for larger dam projects, especially projects designed by major federal 

agencies such as USBR and USACE. The design requirements and methods for cast-inplace conduits are well documented in USBR and USACE technical publications. These 

conduits are very versatile in their application and can be designed to fit specific project 

requirements and site conditions. Design alternatives for cast-in-place conduit include 

various standard circular and horseshoe shapes. Cast-in-place conduits can house 

freestanding outlet pipes and can also be steel lined when required for water tightness 

(see No. 2 below). This cast-in-place reinforced concrete conduit is often the best 

technical solution for an outlet works conduit, but is an expensive design alternative. 

2. Welded Steel Pipe Encased in Reinforced Concrete. This alternative is similar to cast-inplace reinforced concrete construction. The primary difference between the two is that 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 7 of 9

steel-lined conduits are designed to be watertight. The encasement concrete is designed 

to resist embankment and foundation loadings, as well as internal and external water 

pressure. The steel liner is usually designed to resist both the internal and external water 

pressure loadings. This design can be used to provide a ductile and watertight conduit for 

compressible foundation conditions. This design has also been used with relatively thin 

non-structural steel linings whose function is to only provide water tightness. 

3. High-Density Polyethylene Pipe. High-density polyethylene (HDPE) pipe has been 

successfully used for low-head water supply outlet works; however, its use for higher 

head and high-hazard dams needs further investigation. This design requires the HDPE 

pipe to be fully encased in concrete. The encasement allows compaction of embankment 

next to the conduit, fills the pipe haunch areas to eliminate this area of difficult 

compaction and potential seepage, prevents pipe deformation from embankment 

loadings, and can provide a redundant conduit system. 

HDPE pipe is a relatively new material to be used for outlet works conduits. Advantages 

of this material include: (1) it is relatively inert and not subject to erosion; (2) it has 

relatively good and because of the smooth wall, lack bond to the concrete and potential 

for radial thermal movement it has potential for; (3) it has fusion-welded joints and is 

seamless; (4) it is ductile and will not crack and act brittle when subjected to differential 

settlements; and (5) it is relatively inexpensive. 

Possible disadvantages include: (1) there is not a long record of its use; (2) it has a large 

expansion coefficient, and the design must account for radial and longitudinal expansion 

and contraction; (3) it has a smooth wall and will not bond to encasement concrete; (4) 

and because of the smooth wall, lack of bond to the concrete and potential for radial 

thermal movement, it has potential for seepage along the outside of the pipe lining, which 

is a design concern especially if large thermal changes can be expected; and (5) buckling 

and collapsing are important design concerns. HDPE pipe has a low modulus of 

elasticity and is relatively weak in buckling when subjected to external water pressure or 

vacuum loads. 

4. Ductile Iron Pipe. Ductile iron pipe has been used for outlet conduits through 

embankment dams. Modern designers encase the pipe in reinforced concrete. Design 

concerns with the use of ductile iron pipe includes corrosion of the pipe and opening of 

the gasketed joints caused by foundation settlement. 

5. Reinforced Concrete Pressure Pipe. RCP pipe is the most commonly used pipe for outlet 

works at small- and medium-head dams. RCP pipe is used for both pressure and nonpressure applications. This design when used for embankment dams requires either 

partial or full encasement in concrete. Design references for outlet works using RCP pipe 

include NRCS and USBR design references. 

A hard rock foundation is the best application for this pipe because RCP is relatively 

rigid. Large deflections can crack pipe joints or open gasketed joints. Abrupt settlement 

changes can occur under embankment dams, and this pipe can act brittle when subjected 

to concentrated vertical movements (refer to Figure 6). 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 8 of 9

The primary advantages of using RCP pipe for outlet conduits are that it is low cost, is 

commonly available, and requires low maintenance. 

Disadvantages of using RCP pipe include: 

• The pipe gasket is the only line of defense against direct seepage into the 

embankment unless the pipe is fully encased with waterstopped encasement joints. 

• RCP pipe was developed for water supply pipelines; however, improved joints can be 

specified for dam use. Each joint is a potential source of leakage. 

• RCP pipe sections are typically only 8 feet long, thus resulting in a large number of 

gasketed joints under the dam. 

• RCP pipe sections provide little or no resistance to longitudinal pipeline movements 

that can result in openings of joints from foundation settlement and spreading joints. 

• RCP outlet pipe under embankment dams can elongate as the dam settles. 

6.0 Tunnel Outlet Works 

Tunnel outlet works are often not considered for dams less than 150 feet high. In the past, tunnel 

outlet works were more common for embankment dams. For example, USBR used a tunnel 

outlet works at an 80-foot-high dam with soft ground tunneling conditions (see Figure 10 [Rye 

Patch Dam Outlet Tunnel]). Tunnels are not usually considered as a design alternative for 

conditions where soft ground conditions are encountered. However, tunnel outlet works have 

several technical advantages when compared to cut-and-cover outlet conduit designs, especially 

for pressurized outlets. These advantages include: 

• A tunnel outlet works is not physically associated with the embankment. Using a 

tunnel outlet will eliminate embankment failure modes associated with conduit outlet 

works. This failure mode accounts for 25 percent of embankment dam failures. 

• Tunnel outlet works often facilitates stream diversion around the dam site during 

construction. 

• Tunnel outlet works construction can take place independently of embankment 

construction. 

• The tunnel outlet works can allow embankment placement, unobstructed by the outlet 

conduit. 

• Tunnel outlets eliminate special compaction requirements required for the conduit. 

• Tunnel outlets eliminate the need for special filter placement and drainage 

requirement, which can also slow embankment construction. 

• Tunnel outlets eliminate the conduit, a critical path item in constructing the 

embankment dam. 
Attachment 5_White Paper 2 - Design and Construction of OW Conduits for Embankment Dams.doc 9 of 9

Disadvantages include: 

• Tunnel outlet works are often more expensive than conduit outlet works, especially 

for smaller diameter outlet works. 

• Tunnel construction typically involves more cost risk for overruns than a conduit 

outlet works. 

• Few engineering firms maintain qualified staff for planning, design, and construction 

services for tunnel engineering. 

• Design engineers may not have readily available costing information to compare 

costs of conduit and tunnel outlets. 

6.1 Comparing Conduit and Tunnel Outlet Works 

Tunnel outlet works are generally more expensive than conduit outlet works. However, outlet 

works for medium-size dams incorporating modern design methods and redundancy features can 

also be surprisingly expensive. This is especially true if the outlet works conduit is a pressurized 

system and the design incorporates a gate chamber under the dam with a freestanding discharge 

pipe. An important benefit of the tunnel outlet is that it eliminates the failure mode caused by 

the conduit passing through the embankment. 

Cost curves for tunnel outlets based on outlet diameter would be helpful, allowing engineers the 

ability to compare costs for preliminary evaluations of tunnel and conduit outlets. Also, cost 

curves for outlet tunnels combined with design guidelines for tunnel outlet works design can 

provide a method for engineers to properly evaluate tunnels when planning the design of an 

outlet works. 

7.0 Conclusions 

• Outlet works are a significant cause of embankment dam failures. Approximately 25 

percent of embankment dam failures are related to piping failures at or near the outlet 

works (Foster et al.). By reducing this failure mode, significant dam safety 

improvements would result. 

• There is no single nationally recognized standard for designing outlet works for dams. 

Outlet works are often designed using standards not specifically intended for outlet 

works. 

• Nationally recognized design guidelines are needed for outlet works at small- and 

medium-sized dams. Consistent guidelines can direct design engineers to safe 

designs and provide state dam safety officials guidelines for review. Designs 

developed using thorough, consistent standards would result in more consistency 

between projects and result in safer, more reliable facilities. 
Attachment 6 
Attachment 6_White Paper 3 - Gates and Valves.doc 1 of 18 

WHITE PAPER ON GATES AND VALVES 

FEMA Outlet Works Workshop, May 25 – 27, 2004 – Denver, Colorado 

By Lee C. Gerbig, P.E. 

This white paper is an attempt to present the current state of the art with regards to gates 

and valves for dam outlet works systems. 

Introduction 

The purpose of an outlet works system in a storage reservoir is to control the flow of 

water from the dam into a river, canal, or some other type of channel. The outlet works 

releases can be controlled to maintain river flows, provide irrigation water, provide water 

to a water treatment facility, provide water to an industrial user, lower the reservoir in an 

emergency, or any other of a multitude of options. The outlet works system must be 

capable of regulating the flow of water for the intended purpose, and must be very 

reliable to provide the necessary control whenever needed. In many facilities, the outlet 

works equipment is very reliable, requires very little maintenance, and is often taken for 

granted until something goes wrong. 

This paper will discuss several types of gates and valves that can be used in outlet works, 

from the very basic to the more complex systems. The discussion will also include 

different methods of gate and valve operation and control. 

The terms gate and valve are used interchangeably in many designs and existing 

structures. For the purpose here, a gate is a mechanical device with a sliding flat member 

within a square or rectangular framework or structure that controls the flow of water. A 

valve is usually a circular structure with a sliding member, flat or circular, that controls 

the flow of water. 

This paper will be organized into several sections, including gate descriptions, types of 

operators, typical outlet designs, and current developments. 

Gate and Valve Descriptions 

The following are descriptions of the basic types of gates and valves found in current use 

on dams, and some descriptions of older equipment that may be found. First the 

discussion will cover the current gates and valves used. 

1. Current Gates and Valves 

A. Slide Gates - The most common type of gate used for dam outlet works is the 

basic slide gate, also referred to as a sluice gate. A slide gate has a movable leaf 

or disk, which slides against bearing surfaces in the frame. The frame is attached 
Attachment 6_White Paper 3 - Gates and Valves.doc 2 of 18 

to a conduit or wall, which provides support against the water load of the gate. 

The gate leaf is moved by a gate stem, which is operated by a manual handwheel 

or crank, electric motor-operator, or hydraulic cylinder. 

Slide gates have been used for throttling service on dams with up to 500 feet of 

head when the proper design is provided, and slide gates are often used as guard 

gates for outlet works with discharge valves at the downstream end of the conduit. 

Slide gates can be of various types and designs as follows: 

1) Cast Iron Slide Gate – The cast iron slide gate (also known as a cast iron 

sluice gate) has been used for over a century for controlling the flow of 

water. The square or rectangular gate leaf or disk and frame are 

constructed of cast iron (or in some instances, ductile iron or cast stainless 

steel). Reinforcing ribs are included on the gate leaf for required strength 

and a pocket for attaching the gate stem is provided. Bronze seats are 

attached to the downstream side of the leaf and the upstream side of the 

gate frame to provide a low-friction sliding surface, and to provide a 

metal-to-metal seating surface to reduce leakage. The gate stem is usually 

made of stainless steel, but older designs may use bronze or carbon steel. 

For longer length stems, guides with bronze bushings are provided to 

reduce the stresses due to column loading. Wedges are installed around 

the perimeter of the gate leaf to assist sealing when closed, by forcing the 

leaf against the seats. Gate leakage is usually very low (0.1 gal/min. per 

foot of leaf perimeter). These gates are usually very robust and can be 

used for throttling flow in excess of 90 feet of head, and used for guard 

service for heads in excess of 150 feet. 
Attachment 6_White Paper 3 - Gates and Valves.doc 3 of 18 

Photo 1 

Typical Cast Iron Slide Gate 

2) Fabricated Slide Gate – The fabricated slide gate can be made from a 

variety of materials including, steel, stainless steel, and aluminum. The 

fabricated gate uses welded construction for the gate leaf and frame, with 

reinforcing ribs where needed. The gate leaf uses the metal plate as a 

sliding surface against low friction material (UHMW or similar) attached 

to the frame. Neoprene J-type bulb seals are often attached to the 

perimeter of the leaf to provide additional sealing, eliminating the need for 

wedges. Gate stem design and construction is the same as for cast iron 

gates. Gate leakage is approximately 1⁄2 the leakage form cast iron slide 

gates. 

For higher head applications (above 100 feet of head), the slide gates can 

be provided with metal-to-metal seats, eliminating the bulb-type seals. 

a. Bonneted Slide Gate - For high head applications, usually greater 

than 100 feet, bonneted slide gates can be used. Bonneted slide 

gates are similar in design to fabricated slide gates, except that the 

body is fabricated to totally enclose the gate leaf. The gate leaf 

may be fabricated as a flat stainless steel plate without gate seats 

attached or a carbon steel plate with bronze or stainless steel seats 

to slide against bronze or stainless steel seats in the gate body. The 

flat stainless steel plate design will seal at the top at all gate 
Attachment 6_White Paper 3 - Gates and Valves.doc 4 of 18 

openings; whereas the leaf design with seats attached will allow 

water to flow over the top of the leaf at partial gate openings. At 

higher operating heads, stainless steel/bronze seat combinations 

provide low friction, long wear, and cavitation resistant 

characteristics. The bonneted gates use a flush bottom design, with 

no offsets in the flow along the floor of the conduit, providing 

increased discharge capacity and reduced cavitation potential. The 

upstream side of the leaf is designed for reduced downpull and 

effective flow control at all gate openings. 

The bonneted slide gate is usually embedded in reinforced concrete 

to provide additional structural reinforcement for the pressurized 

gate body. 

b. Wheel Gate – Wheel gates are similar in design to slide gates, in 

that there is a square or rectangular leaf, moving vertically over an 

opening in the gate frame. Instead of the leaf sliding on machined 

bearing surfaces, the leaf is fitted with multiple wheels that ride on 

rails attached to the face of the structure. The rails support the 

water load of the gate leaf and frame, and must be installed in good 

alignment to provide a smooth operating surface. The operating 

loads are much lower for a wheel gate, since there is less friction in 

the wheels and axles than there is in the sliding contact on a slide 

gate. Wheel gates can be designed to close by gravity in an 

emergency if necessary. There is usually a bulb-type seal attached 

to the perimeter of the leaf to seal against the gate frame. 

Installation of wheel gates is often difficult because the tracks and frame 

require critical alignment to achieve smooth operation and effective 

sealing. 

Wheel gates can be operated by stem systems similar to the stem design 

for slide gates, and are usually operated by hydraulic cylinders, although 

there are many installations using a wire-rope hoist systems with an 

electric motor operator. Some multiple intake installations use a gantry 

crane that can install one gate in any one of several gate slots when 

needed. 
Attachment 6_White Paper 3 - Gates and Valves.doc 5 of 18 

Photo 2 

Example of a Wheel Gate 

c. Roller Gate – Roller gates are similar to wheel gates, but a roller 

train is used instead of wheels, when moving against the rails. 

Roller gates also have less friction than slide gates, and are capable 

of closure by gravity. Roller gates have fallen out of favor 

because the roller trains are susceptible to corrosion and buildup of 

deposits that increase friction and interfere with operation of the 

rollers and links. 

The operating systems for roller gates are similar to the systems 

used for wheel gates. 

B. Butterfly Valves – Butterfly valves are commonly used at power plants and 

water facilities. A butterfly valve has a circular body with a circular disk in the 

fluidway that rotates around a shaft in the centerline of the valve, perpendicular 

to the flow. The valves may be installed with the shaft vertical, but the valves are 

usually installed with the shaft horizontal. Butterfly valves are relatively 

inexpensive to purchase and install, and as result, they are often used in 

installations that are questionable. The flow velocities through butterfly valves 

are usually limited to 25 to 30 feet per second, although high-performance 
Attachment 6_White Paper 3 - Gates and Valves.doc 6 of 18 

butterfly valves can operate up top 50 feet per second. The flow around the disk 

in the fully open position creates eddies and flow disturbances that can be carried 

to downstream valves unless proper spacing is provided between valves. 

Butterfly valves can be provided with resilient seats for extremely tight shutoff 

capability. 

Butterfly valves are sometimes used for controlling flow, but that has to be done 

with caution. The rotation of the disk does not provide good flow control 

throughout the operating range, resulting in most of the flow control in the midposition of operation. It is also difficult to introduce air into the water flow 

immediately downstream of the valve to prevent cavitation. 

Butterfly valve operating forces are very low, with very little friction and 

hydraulic effects. They may be operated manually, by electric motor-operators, 

or by hydraulic operation. Some butterfly valves are counterweighted to close 

upon loss of power in an emergency situation. 

C. Gate Valve – A gate valve is a variation of a slide gate, with a circular body and 

disk. The disk is often wedged into tapered seats to provide tight shutoff. Gate 

valves are often used for guard gates upstream of regulating valves, or used for 

throttling flow in low-head applications. Flow through the valve is fairly 

unrestricted except for the slot around the disk seats. They are relatively 

inexpensive and easy to install, but the larger valves can become quite heavy.

Knife gates are often manually operated, but the larger sizes are often operated by 

electric motor-operators or hydraulic operation. 

D. Knife Gate – A knife gate is another variation of a slide gate, but is not 

commonly used in the hydropower or dam industry. A knife gates has a circular 

opening in the gate body with a very thin disk that moves across the flow, 

resulting in a very thin valve, face to face. The wetted parts are often made of 

stainless steel to prevent corrosion, and resilient seats are often used for tight 

shutoff. Flow through the valve is fairly unrestricted, with just slight offsets in 

the flow at the seat area. 

Another variation of the knife gate is a “Thru Port Knife Gate, which has a disk 

with a circular opening which matches the pipe diameter to provide an 

unobstructed flow through the valve, resulting in no flow losses or cavitation 

potential. 

Knife gates may be operated manually, by electric motor-operators, or hydraulic 

operation. 

E. Fixed-Cone Valve – A fixed-cone valve is commonly used for regulating the 

flow of water from an outlet works in a dam. A fixed-cone valve is a cylindrical 

valve with a sliding sleeve that covers the opening at the downstream end of the 
Attachment 6_White Paper 3 - Gates and Valves.doc 7 of 18 

valve. A cone (usually 90 degrees) is installed at the end of the circular structure 

that disperses the flow radially through the opening between the valve body and 

the sleeve. Flow can be controlled very well at all but the smallest valve 

openings. Seals are provided between the valve body and sleeve to reduce 

leakage. Operating forces are low, since there is only the friction between the 

moving parts and the seals, with little hydraulic forces involved. The valves have 

a high coefficient of discharge, and do not dissipate much energy. They tend to 

produce considerable spray, as a result of the cone at the end. 

Fixed-cone valves can be furnished with hoods attached to reduce the spray, while 

some other installations use an energy dissipation chamber to contain the spray 

and reduce the energy at the downstream end of the structure. Some attempts 

have been made to operate fixed-cone valves submerged, but that requires careful 

study and evaluation. 

Fixed-cone valves can be operated manually or with an electric motor-operator 

through a miter-gear box and shafts or by hydraulic operation. 

Photo 3 

Fixed-Cone Valve with a Hood 

F. Jet-Flow Gate – A jet-flow gate is a variation of a slide gate used as a free 

discharge gate. The gate has a circular opening with a sliding disk that moves 

over a floating, sloping orifice shaped opening. The sloping orifice concentrates 

the flow to help introduce air to prevent cavitation. There is very little energy 

dissipation, resulting in fairly high flow coefficient. Operating forces are high, 
Attachment 6_White Paper 3 - Gates and Valves.doc 8 of 18 

due to the friction between the disk and the seats, and between the disk and the 

orifice. 

Jet-flow gates have also been used in submerged applications, by expanding the 

downstream discharge pipe to three times the orifice diameter and keeping the 

discharge pipe very short to allow recirculation and prevent cavitation. 

Jet-flow gates can be operated manually, by electric motor-operators or by 

hydraulic operation. 

Photo 4 

Motor-Operated Jet-Flow Gate 

G. Sleeve Valve – Sleeve valves are used as energy dissipaters, both in-line and at 

the end of a conduit. Sleeve valves consist of a section of perforated pipe with 

small tapered holes and a sliding sleeve to uncover the ports required to provide 

the desired discharge. The sleeves are provided with seals to minimize leakage. 

The in-line type valves usually operate with the flow on the exterior of the 

perforated section, flowing inward to dissipate the energy and then the flow 

moves downstream within the conduit. The valves at the end of a conduit are 

usually installed in a vertical position, with the flow moving outward from inside 

the conduit into a stilling well to dissipate energy. Sleeve valves are very 
Attachment 6_White Paper 3 - Gates and Valves.doc 9 of 18 

expensive to purchase, because the manufacturing process is difficult and the 

materials are usually stainless steel and other corrosion/cavitation resistant 

materials. Installation costs are nominal, since the valves are usually just 

connected to the conduit with appropriate flanges. 

Sleeve valves are usually operated to provide precise flow control; they are 

usually operated by either electric motor-operators or hydraulic operation, with 

remote position sensing provided. 

H. Clamshell Gate – The clamshell gate is a special design used by the U.S. Bureau 

of Reclamation. The clamshell gate consist of two leafs operating in an arc over 

the end of a conduit; machined to match the arc of the gate leafs. The leafs move 

in a symmetrical manner to provide uniform flow between the two leafs. There is 

very little energy dissipation, since the flow is much like the flow from an open 

pipe, so an energy dissipation basin or structure is required. Clamshell gates 

work very well in a submerged installation, providing excellent recirculation to 

prevent cavitation. 

Clamshell gates are usually operated by electric motor-operators or hydraulically 

operated. 

I. Radial Gate – Radial gates have been used for many installations at spillways, 

canal check structures, and canal turnouts. They have also been used 

occasionally for outlet works with a top-seal arrangement in a pressurized 

structure. A radial gate consists of a curved plate with reinforcing ribs, and arm 

structure to provide lateral support, and trunnion pins to allow rotation of the gate 

structure. Resilient seals are provided at the bottom and sides to reduce leakage, 

and smooth wall plates are often used to provide a good sealing and sliding 

surface. 

Operation of radial gates is provided by wire-rope hoists using either manual 

operators or electric motor-operators, or by hydraulic cylinders attached to the 

gate structure. 

Variations of radial gates with a top seal have been used for medium head (100 to 

150 feet) outlet works regulation. The use of the top seal provides a sealing 

surface around the entire perimeter of the gate leaf. The top seals have been 

difficult to maintain, however, and leakage can be a problem. 
Attachment 6_White Paper 3 - Gates and Valves.doc 10 of 18 

Photo 5 

Example of Radial Gate 

 

J. Hinged Crest Gate – Hinged crest gates are used at the crest of a spillway, to 

allow flow overtopping the gate to regulate the reservoir or pool elevation. The 

gate consists of a flat or slightly curved plate structure, with a hinge system at the 

bottom to allow the gate to rotate from near vertical to horizontal. The crest gates 

are usually wide and short to provide regulation of long spillway structures. 

The operation of crest gates varies considerably. Some gate use torque tube 

arrangement with a hydraulic operator or electric motor-operator at one end. 

Some other installations use hydraulic cylinders, mounted either underneath the 

panel or mounted above the gate on a deck. Still others use a pneumatic bladder 

arrangement under the panel to regulate the gate. 

2. Older Equipment 

A. Needle Valves – Needle valves have been used primarily as free discharge valves 

for outlet works, but on some occasions they have been used for guard valves for 

turbines in power plants. Needle valves are cylindrical valves with an inner 

plunger or needle, with a tapered nose, that moves back and forth to regulate flow 

through the valve. Most needle valves were operated by pressurizing the opening 

or closing chambers within the valve, with reservoir water from within the valve. 

A type of feedback control system was built-in to try to maintain the position of 

the needle. The needle valves were very troublesome, due to build-up of 

corrosion on closely fitting plungers and bodies, operating clearances that 

increased with usage and produced large internal leaks, and control systems with 
Attachment 6_White Paper 3 - Gates and Valves.doc 11 of 18 

close tolerances that tended to plug up or corrode. Some needle valves have 

experienced catastrophic failure due to air entrapment that created large 

unbalanced forces in the opening and closing chambers. As a result, the valves 

slammed shut and ruptured the valve body or upstream piping due to water 

hammer. 

Some needle valves are mechanically operated, and do not experienced the 

operating difficulty experienced with water-operated valves. Many needle valves 

have been replaced with more modern, trouble-free valves, with better flow 

characteristics. The needle valves also experienced problems with cavitation on 

the needles, due to sub-atmospheric pressure, and leakage past the seats due to 

erosion form high-velocity flow. 

B. Hollow-Jet Valves – Hollow-jet valves have been used for many years by the 

U.S. Bureau of Reclamation and some other entities. The valves are designed as a 

type of reverse needle valve, with the seal at the upstream end of the needle 

instead of downstream, but they are operated either hydraulically or by electric 

motor-operator. 

Hollow –jet valves provided higher discharge coefficients than needle valves, but 

they are very costly to fabricate because of the complex shapes used in the needle 

portion and upstream body. The hollow-jet valves experience seat erosion at 

small openings, and can have cavitation damage at any slight offsets in the 

downstream bodies. 

Gate Operating Systems 

There are several types of operating systems available to operate gates and valves. 

Principally, there are four basic types; manual operators, electric motor-operators, 

hydraulic operators, and pneumatic operators. 

1. Manual Operation 

Manual operation is usually accomplished with a crank or handwheel connected 

to the gate or valve operating mechanism with threaded stems or a gearbox 

arrangement. (Manual operation is also possible with hydraulic operators, and 

will be discussed under hydraulic operation). 

a. Slide gates, gate valves, knife gates, jet-flow gates, and some sleeve 

valves and butterfly valves use a rising stem design, where an operating 

stem rises and lowers to open or close the gate or valve. The stem has 

threads machined onto the upper end, and a crank, handwheel, or gearbox 

is connected to the stem with an operating nut. 
Attachment 6_White Paper 3 - Gates and Valves.doc 12 of 18 

b. Fixed-cone valves, clamshell gates, and some sleeve valves and butterfly 

valves use rotary motion from the crank or handwheel and rotating shaft to 

convert to linear motion through a gearbox arrangement. 

c. Radial gates, wheel gates, roller gates, and crest gates often use a wire –

rope hoist design that can be manually operated with a crank or handwheel 

that turns the wire-rope hoist drums. Usually wheel gates and roller gates 

are too large to operate manually, in a practical sense. 

The crank or handwheel should be sized to operate the equipment easily and 

provide sufficient force to operate the equipment. The handwheels should be at 

least 12 to 15 inches in diameter for ease of use. Equipment with cranks or 

handwheels is usually designed to operate with no more than 40 pounds of pull on 

the handle or handwheel rim, but the equipment must be designed for much more 

effort (80 to 120 pounds), since some operators can exert much more than 40 

pounds effort if the equipment is stuck or difficult to operate. 

The size of the equipment using manual operation should be carefully considered. 

Although some gates and valves can be operated manually, operation may be so 

slow that it is not practical, and if operation is too difficult, the equipment will not 

be exercised as it should. 

Photo 6 

Manual Operator 

2. Electric Motor-Operators 
Attachment 6_White Paper 3 - Gates and Valves.doc 13 of 18 

Electric motor-operators are used in similar manners as the manual operators, that 

is, either linear motion with threaded stems or rotary motion converted to linear 

motion. 

An electric motor-operator consists of a high-torque electric motor, gearbox for 

suitable gear reduction and operating speed, pushbuttons, limit switches to control 

stopping positions, torque switches to protect equipment from overloads, 

indicating lights, local position indicators, an in some instances, remote position 

indicators. The high torque electric motors are used to initially start equipment 

open or closed, and operate best on three-phase power due to the high current 

draw during initial operation. The motor-operators will function on single-phase 

power, but size and output is limited. 

The electric motor-operators are usually designed for outdoor applications, with 

weatherproof enclosures. Submerged operation is usually not an option, except 

for extremely short duration. 

Handwheels are usually provided on the units for manual operation if power is 

interrupted. 

Photo 7 

Electric motor-Operator 

 

3. Hydraulic Operators 

Hydraulic operation is most often used on large gates and valves operating at high 

head, but is becoming more common for small equipment at low heads for 

particular installations. 
Attachment 6_White Paper 3 - Gates and Valves.doc 14 of 18 

Hydraulic operation typically uses one or more hydraulic cylinders attached to the 

gate or valve, to provide linear motion. The hydraulic cylinder(s) are connected 

to a central hydraulic power unit, with rigid or flexible hydraulic lines. The 

hydraulic line connections should be welded where possible, to reduce the 

potential for leaks at the joints. The hydraulic cylinders can be made fairly 

compact, since high operating pressures are available (2,000 to 3,000 psi, with 

some systems approaching 5,000 psi). The hydraulic cylinders can be designed 

for submerged operation, and can be furnished with electronic sensors for remote 

gate or valve position sensing. 

The hydraulic power units (HPUs) consist of an oil reservoir, one or more electric 

motor-driven hydraulic pumps, control valves, filters, pressure switches to control 

pump and valve operation, relief valves to limit operating pressure, and 

sometimes accumulators to store hydraulic pressure. HPUs can be designed to 

operate more than one gate and/or valve at a facility, by simply adding the 

required control valves, and can be located at fairly long distances from the 

equipment being operated. 

Manual operation can be provided by using a hand pump system instead of an 

HPU. Also, a hand pump may be provided with an HPU for operation during 

electric power interruption. Hand pumps can provide slow operating speeds, but 

some pumps utilize a two-stage pump, with a larger output available at low 

pressure. 

Accumulators provide operating flexibility when need by providing for limited 

operation during electric power interruption, or for modulating service without 

running the hydraulic pumps and motors frequently. 

Improved hydraulic oils can provide better outdoor performance, and 

biodegradable oils are available where environmental concerns arise. 

Some older gates and valves operated on hydraulic systems using water instead of 

hydraulic oil. The water is stored in tanks above the dam, or the pressure is built 

up by pumps. The principles are the same, but the water systems are usually 

manually operated by opening and closing control valves. The water systems 

have operated for many years, and some are still in service, but corrosion, worn 

and corroded seals, and leaky piping have made many of the systems lacking in 

reliability. Many of the water components, such as cylinders, are very large, since 

high pressure, produced in oil hydraulic systems, is not available. Most water 

systems operated in the range of several hundred psi, instead of several thousand 

for an oil hydraulic system. 
Attachment 6_White Paper 3 - Gates and Valves.doc 15 of 18 

Photo 8 

Hydraulic Power Unit with Accumulators 

4. Pneumatic Operators 

Pneumatic operators are rarely used for operating gates and valves; although one 

successful gate manufacturer currently designs and fabricates hinged crest gates 

using pneumatic bladders at low pressure (Approx. 20 psi). 

The problem with pneumatic operation in typical gates and valves is that, with the 

air, being compressible, it is difficult to maintain a set position. If operating 

forces are high, pneumatic cylinders and control systems become quite large, 

since air pressure is limited to approximately 125 psi. Compressed air is also 

dangerous to work with, because the expansion of the air when released can cause 

great damage. 

Moisture in the compressed air can also create problems at cold temperatures, and 

can cause corrosion to occur in pneumatic equipment. 

5. Portable Operators 

There are several types of portable operators available to work in conjunction 

with manual operators. The manual crank or handwheel operators can usually be 

provided with a square nut or fitting for adapting to a portable operator. 

A. Portable Drill – A portable drill unit is available, which can be mounted on 

a tripod and attached to the crank or handwheel shaft on a manual operator. 
Attachment 6_White Paper 3 - Gates and Valves.doc 16 of 18 

Electric power must be available, and output is usually limited to 

approximately 80 lb. ft. of torque. The drill unit is usually comparable to a 

1⁄2-inch heavy-duty drill. 

B. Portable Hydraulic Operator – Portable hydraulic operators are available, 

which also connect to the shaft of the manual operator. The portable 

hydraulic operator usually uses a small (5 hp) gasoline engine, hydraulic 

pump, control valve, and a hydraulic motor with flexible hoses to clamp to 

the manual operator. These units are usually mounted on a cart that can be 

easily transported to where needed. 

Photo 9 

Portable Hydraulic Operator 

Another type of portable hydraulic operator connects to a standard hydraulic 

system with quick-connect fittings or other connections, to power a 

hydraulic system or to take the place of the main HPU during an electric 

power interruption. 

C. Chain Saw Operator – Chain saws have been modified to replace the 

cutting chain assembly with a chain-driven operating nut to attach to the 

shaft of a manual operator. These units are lightweight, but very noisy. 

Trends 

In the design of outlet works gates and valves, and outlet works arrangements, there are 

several trends that have been noted. 
Attachment 6_White Paper 3 - Gates and Valves.doc 17 of 18 

1. Remote Operation – More and more designs are being prepared with remote gate 

position sensing and remote or automated operation. The modern gate operating 

systems have reasonably inexpensive means of providing remote gate position 

sensing, and consequently remote control is more attractive. More precise control 

of water flow is provided by remote or automated control, since there is faster 

response to changing conditions. The remote operating systems have also become 

more reliable. Remote sensing is achieved by several means: 

A. Electric Motor-Operators – Electric motor-operators can be furnished 

with position transducers producing a 4 to 20 Ma signal that provides a 

continuous output signal of the gate or valve position. This signal can be 

transmitted by hard wire, optic cable, or radio signal to a control panel or 

computer many miles away. The transducers are built into the operators 

for integral operation, and can be easily modified to produce stepping 

functions or signals at certain openings. 

B. Hydraulic Cylinders – Hydraulic cylinders can be provided with linear 

position transducers also producing a 4 to 20 Ma signal, that function just 

as the transducers on the motor-operators. These transducers san be used 

on hydraulic cylinders submerged in the reservoir for underwater 

operation. 

C. Inclinometers – Some gates that operate with limited rotation, such as 

radial gates and crest gates, can use a transducer known as an inclinometer, 

which measures the angle of a gate member and converts that to gate 

position. These transducers also produce a 4 to 20 Ma signal that functions 

just as the other transducers. 

2. Hydraulic Operation – There appears to be more acceptance on smaller projects 

for hydraulic operation of gates and valves. This practice is favorable for several 

reasons: 

A. Submerged Operation – Hydraulic operators can be designed for 

submerged operation, which may be preferable in some instances. 

Submerged hydraulic cylinders can eliminate long operating stems, and the 

resultant alignment problems. Some stem installations are difficult to 

design, considering access and maintenance issues. 

B. Improved Hydraulic Fluids – Developments in recent years have provide 

hydraulic fluids that are suitable for outdoor use, at most operating 

temperatures. Biodegradable hydraulic fluids are now available that reduce 

the problems with oil leakage in storage reservoirs. 
Attachment 6_White Paper 3 - Gates and Valves.doc 18 of 18 

C. Better Seals – Seals and pipe joint connections have improved to reduce 

leakage and oil spills. Welded pipe connections have virtually eliminated 

oil leaks, and tubing and fitting connections are improved. 

D. Automated Operation – Hydraulic operation lends itself very well to 

automated operation. Where gates and valves are frequently operated for 

optimum flow control, there are no stem threads to wear out, and 

accumulators in the hydraulic systems can reduce pump starts and stops. 

The design of hydraulic cylinders allows for frequent operation without 

undue wear. 

3. Foreign Design and Manufacturing – In recent years there has been a shift in 

gate and valve design and fabrication to foreign countries. Two large gate 

manufacturers, Hydro Gate and Waterman Industries, have gone through 

bankruptcy. Hydro Gate reorganized and was purchased by the Henry Pratt 

Company, and Waterman is trying to reorganize and scale back, eliminating much 

of their engineering staff. 

Asian countries, such as Korea and Japan, have been designing and/or fabricating 

gates and valves for many years. India is now trying to expand their expertise in 

gate and valve design and manufacturing. Some work is still produced in Europe, 

notably England and Sweden. 

There is a lack of gate designers in the U.S. now, with many steel fabricators 

trying to bid on work, but there are no designers to be found for jobs involving 

design-and-build. Many of the old gate and valve manufacturers have 

disappeared, such as Bingham-Willamette, Allis-Chalmers, Pelton, and GoslinBirmingham, with no new companies arising to take their place. 

4. Reduced Government Involvement – Most of the government agencies, such as 

the Bureau of Reclamation, Corps of Engineers, and Tennessee Valley Authority, 

have cut back on their dam design and construction phases of work, resulting in 

less research on new equipment and operation. The government agencies were in 

the forefront of new developments for many years, but that development has really 

decreased. 
Attachment 7 
Attachment 7_White Paper 4 - Energy Dissipators.doc 1 of 11 

White Paper on Energy Dissipators

FEMA Outlet Works Workshop, May 25 – 27, 2004 – Denver, Colorado 

By 

Henry T. Falvey1

INTRODUCTION 

Water passes over or through a dam in a controlled manner through three possible paths. 

These paths are over the spillway, through the penstock, or through an outlet works. The 

purpose of the spillway is to pass flood events past the dam without overtopping the 

structure. Water passing through penstocks goes to hydroelectric turbines to generate 

electricity. Water passing through outlet works is used to control the water level in the 

reservoir and to provide water for irrigation, municipal, or minimum downstream flow 

requirements. 

The outlet works conduit normally passes through the body of the dam. Upstream of the 

centerline of the dam, the pressure in the conduit is almost balanced by the external 

pressure from the water in the dam. However, in the section downstream of the dam 

centerline, the internal pressure can be much higher than the external pressure. A guard 

gate is often placed near the centerline of the dam to avoid the risk of cracks forming in 

the downstream conduit and washing out an earth dam. The conduit downstream of the 

guard gate is either a steel pipe located in a tunnel or it is a chute that forms a part of the 

tunnel. If the downstream conduit is a steel pipe in a tunnel, then the control gate 

structure is located near the downstream toe of the dam. However, if the downstream 

conduit is a chute, then the control gate is located immediately downstream of the 

emergency gate. With a concrete dam, the location of the guard and control gates is not 

critical. They can be found at the axis of the dam or at the downstream face. 

Almost every outlet works requires an energy dissipator so that the high-energy flow 

from the reservoir does not damage the downstream channel or structures. Technically, 

energy is not dissipated in the process. The hydraulic energy is converted into heat 

energy. However, the heat rise in the conversion process is normally so low that it can be 

neglected. Therefore, hydraulic engineers normally refer to the energy conversion process 

as dissipation of the hydraulic energy. The conversion is accomplished by 

• Creating fine grain turbulence, or 

• Creating a loss through a change in momentum of the flow. 

The different types of energy dissipators that have been used at outlet works include, 

• Stilling basins, 

 

1

 Henry T. Falvey & Associates, Inc, Conifer, CO 80433. Email: Falvey@members.asce.org. Tel & Fax: 

303 808-4920 
Attachment 7_White Paper 4 - Energy Dissipators.doc 2 of 11 

• Baffled Apron Drops, 

• Stepped spillways, 

• Impact basins, 

• Stilling wells, 

• Various types of valves, 

• Sudden enlargements 

• In-line orifices 

• Flip buckets, and 

• Plunge pools. 

The selection of the type of energy dissipator that is used at an outlet works depends upon 

head, discharge, and economical considerations. Often the energy dissipator that is used 

for a specific installation consists of a combination of the types listed above. 

The design of the stilling basins, impact basins, flip buckets, and baffled apron drops has 

been thoroughly discussed in USBR Engineering Monograph No. 25.2

 General 

guidelines for the design of outlet works can be found in the US Army Corps of 

Engineers Engineering Manual EM 1110-2-16023 and in the USBR publication “Design 

of Small Dams.”4

 Many of the specialized types of dissipators are described in sitespecific studies. 

The purpose of this paper is to present a “state of the art” review of current energy 

dissipation devices with respect to their design and selection. 

DESIGN FEATURES 

Stilling basins. The USBR defines three types of stilling basins. The Type I Basin is 

simply a hydraulic jump on a horizontal apron, Figure 1. The energy dissipation is 

accomplished by turbulence within the jump. The Type II Basin contains blocks (chute 

blocks) at the end of the basin and a dentated end sill, Figure 2. The chute blocks create 

shear zones that generate fine grain turbulence. The purpose of the end sill is to prevent 

downstream erosion and does not contribute significantly to energy dissipation. The 

Type III Basin contains an additional set of blocks (baffle blocks) within the horizontal 

apron to create additional turbulence, Figure 3. Cavitation and transverse loads on the 

baffle blocks have required the development of special shapes for high head installations. 

The addition of the chute blocks and the baffle blocks permits the length of the stilling 

basin to be reduced. The energy dissipation for these structures is based on the change in 

momentum of the water through the structure. 

 

2

 Peterka, A.J., 1978, “Hydraulic Design of Stilling Basins and Energy Dissipators.” This monograph is 

available over the Internet at http://www.usbr.gov/pmts/hydraulics.lab/publications.html. 3

 This can be obtained on the Internet at http:/www.usace.army.mil/usace-docs/eng-manuals/em1110-2-

1602. 

4

 Hoffman, C.J., 1987, “Outlet Works,” Chapter X of Design of Small Dams.” Third Edition. This book is 

available over the Internet at http://www.usbr.gov/pmts/hydraulics.lab/publications.html. 
Attachment 7_White Paper 4 - Energy Dissipators.doc 3 of 11 

 Figure 1. Type I Basin Figure 2. Type II Basin 

Figure 3. Type III Basin 

Baffled Apron Drops. The baffled apron drop that is shown in Figure 4 is extremely 

useful in dissipating energy by exerting a force on the water that passes through the 

baffles. The drops are also useful in controlling the air content of the water. With 

approximately seven rows of baffles, the air content of the water becomes saturated. That 

is, if the water is supersaturated or if the water needs more air the baffled apron drop will 

either release air or entrain air to improve the water quality. 

Figure 4. Baffled Apron Drop 
Attachment 7_White Paper 4 - Energy Dissipators.doc 4 of 11 

Stepped Spillways. Methods to design a stepped spillway as shown in Figure 5 can be 

found in an excellent book edited by Minor and Hager.5

 Although the title indicates that 

the designs are for spillways, the methods outlined in the book can be applied equally 

well to the design of dissipators of outlet works. If the flow depth relative to the step 

height is less than about 0.8, then the energy dissipation is primarily due to changes in 

momentum. This regime is known as nappe flow. For greater depths, energy dissipation 

is primarily due to boundary shear. This regime is known as skimming flow. The exact 

value of the coefficient 0.8 depends upon the slope of the chute. 

Figure 5. Stepped Spillway 

Impact Basins. With an impact basin, the energy is dissipated by turbulence that is 

generated when a jet of water impacts against a vertical wall. After hitting the wall, the 

jet is deflected upward and downwards. A horizontal projection is provided on the top of 

the wall to increase the turbulence in the upstream pool. Although the USBR limits the 

dimensions and discharges to relatively low energy levels, the basins have been used 

successfully at much higher energy levels by scaling the recommended dimensions to the 

larger sizes. The USBR defines impact basins as Type VI Basins, Figure 6. The force that 

the baffle wall exerts on the incoming water jet changes its momentum and thus generates 

the loss. 

Figure 6. Type VI Basin 

 

5

 Minor, H.E, and Hager, W.H., 2000, Hydraulics of Stepped Spillways, A.A. Balkema, Brookfield. 
Attachment 7_White Paper 4 - Energy Dissipators.doc 5 of 11 

Stilling Wells. For low discharges for flows into canals, a stilling well as shown in Figure 

7 is frequently used. The design of stilling wells has been outlined by Falvey6

 and Burgi7. 

The energy dissipation is achieved by the change in momentum between the water 

entering the well through the pipe with the diameter and the flow up the square well with 

a side length of width b. The corner fillets and the pedestal in the invert tend to 

maximize the circulation and energy dissipation in the lower portion of the well. The 

flow is controlled by changing the opening, a, using a sleeve valve on the bottom of the 

down comer pipe. Problems have been experienced with the valve operating mechanism 

due to the turbulence in the stilling well. 

Figure 7. Stilling Well 

 

Valves. With high-head and high flow applications, fixed-cone valves are normally used 

for energy dissipation. These valves use the principle of impinging jets and baffles to 

provide the energy dissipation as shown in Figure 8.8 The ring deflector was damaged 

during operation of the energy dissipator because of a weld failure on the steel cladding.9 

Subsequent inspection found that a large void had been left in the concrete behind the 

steel liner when it was placed. Various modifications of this design concept have been 

developed including hoods on fixed-cone valves that discharge into the atmosphere as 

shown in Figure 9.10 The distance between the end of the fixed cone valve and the hood 

is critical. Severe blowback has been experienced in prototype installations where this 

distance was off by only an inch or two. 

 

6

 Falvey, H. T. 1962, “Hydraulic Model Studies of the Wanship Dam Vertical Stilling Wells, USBR 

Hydraulic Laboratory Report, HYD 481. 

7

 Burgi, P.H., 1973, “Hydraulic Model Studies of Vertical Stilling Wells, USBR Hydraulic Laboratory 

Report, REC-ERC-73-3. 

8

 Colgate, D., 1963, “Hydraulic Model Studies of the River Outlet Works at Oroville Dam,” USBR 

Hydraulic Laboratory Report, HYD 508. 

9

 DeVries, J.J., Hameed. T., and Cheema, M.N., 1994, “Hydraulic Model Study of the Oroville Dam River 

Outlet Works,” Center for Water and Wildland Resouces, University of California, Davis. 10 Beichley, G.L., 1970, “Hydraulic Model Studies of an Energy Dissipator for a Fixed Cone Valve at the 

Ute Dam Outlet Works,”“ USBR Hydraulic Laboratory Report, REC-ERC-70-11. 
Attachment 7_White Paper 4 - Energy Dissipators.doc 6 of 11 

Figure 8. Fixed-cone valve installation 

Figure 9. Hooded Installation of a Fixed-cone Valve 

For high-head and low-flow requirements, the valves are designed on the principle of 

providing small orifices through which the water flows.11, 12 The sudden expansion at the 

downstream side of the orifice is the mechanism that generates the energy dissipation. 

The valves are normally placed in-line as shown in Figures 10 and 11. However, a 40-

inch MonovarTM valve was used at Terminus Dam in California. In this case, the valve 

discharged freely into the downstream pool. 

Figure 10. Orifice valve 

 

11 Burgi, P.H., 1977, “Hydraulic Tests and Development of Multijet Sleeve Valves,” USBR Hydraulic 

Laboratory Report, REC-ERC-77-14. 

12 Frizell, K.W., “Laboratory Testing of an 8-inch MonovarTM Valve.” 
Attachment 7_White Paper 4 - Energy Dissipators.doc 7 of 11 

Figure 11. MonovarTM Valve 

Sudden Enlargements. A sudden enlargement is a very effective method of dissipating 

energy. The sudden enlargement can occur within or at the end of a pipeline. Two 

examples of sudden enlargements are shown in Figures 12 and 13. The proportions 

shown in Figure 12 are those that minimize the formation of cavitation in the sudden 

enlargement.13 Nevertheless, some ranges of operating conditions will still generate 

cavitation within the structure. The sudden enlargement at the end of the pipeline places 

all the cavitation formation in the downstream tailrace.14 At the Crystal Dam outlet, long 

snake-like vapor filled cavities can be seen during operation. However, these cavitation 

vortices apparently do not generate any adverse effects. 

Figure 12. A sudden Enlargement in a Pipeline with Jet Flow Gates 

 

13 Isbester, T.J., 1974, “Hydraulic Model Studies of the Teton Canal Outlet Works Energy Dissipator,”“ 

USBR Hydraulic Laboratory Report, REC-ERC-74-16. 

14 Burgi, P.H., and Fujimoto, S., 1973, “Hydraulic Model Studies of Crystal Dam Spillway and Outlet 

Works, Colorado River Storage Project,” USBR Hydraulic Laboratory Report, REC-ERC-73-22. 
Attachment 7_White Paper 4 - Energy Dissipators.doc 8 of 11 

Figure 13. A Sudden Enlargement at the End of a Pipeline 

In-line Orifices. Another example of the use of sudden enlargements to dissipate energy 

is the placement of a series of in-line orifices in a pipeline as shown in Figure 14.15

Figure 14. In-line orifices 

This solution was used at Seven Oaks Dam outlet to provide low flows under a high 

head. Instead of a valve, the terminating structure was an impact basin. Detailed 

computations were necessary to determine the proper number and orifice diameters to 

provide the required energy dissipation under a wide range of reservoir heads without 

generating cavitation downstream of the orifices. 

Elbow Dissipator (ECI Concept)

The energy loss for a 90-degree miter bend is approximately 

g

V H

2

1.1

2

∆ ≈

 

15 Ball, J.W., 1963, “Progress Report on Hydraulic Characteristics of Pipeline Orifices and Sudden 

Enlargements used for Energy Dissipation,”“ USBR Hydraulic Laboratory Report, HYD 519. 
Attachment 7_White Paper 4 - Energy Dissipators.doc 9 of 11 

Placing several 90-degree miter bends in a concrete block, as shown in Figure 15, can 

dissipate relatively high heads. The concrete block must be designed to withstand the 

internal pressures and be massive enough to resist overturning from the uneven thrust 

produced as the pressure reduces at each elbow. To date, this concept has not been model 

tested or installed at any prototype location. 

Figure 15. Elbow Energy Dissipator 

Flip Buckets. A flip bucket is often used at the end of an outlet chute to throw the flow 

away from the toe of the dam. These are used in conjunction with plunge pools described 

below. Actually, the flip bucket is not an energy dissipation device although it is 

designated as one in the USBR Engineering Monograph No. 25, as shown in Figure 16. 

Dentates are frequently used to spread the compact jet as it enters the plunge pool as 

shown in Figure 17.16 The literature frequently refers to the energy dissipation effects of 

the jet within the air due to the spreading jet. Actually, very little energy dissipation 

occurs because of the air-water interaction as the jet flies through the air. The principle 

effect of the dentates is to increase the area over which the jet enters the plunge pool. 

This decreases the energy per unit area over which the energy must be dissipated. 

Figure 16. USBR Type X Basin 

 

16 Cooper, D.R., 1992, “Outlet Works for Seven Oaks Dam, Santa Ana River, San Bernardino, County, 

CA,” US Army Corps of Engineers, Technical Report HL-92-14. 
Attachment 7_White Paper 4 - Energy Dissipators.doc 10 of 11 

Figure 17. Dentates at the End of Chute 

Plunge Pools. A typical plunge pool installation in connection with a flip bucket is shown 

in Figure 18. During the development of a plunge pool, the material that is excavated by 

the plunging jet can form a dam downstream of the plunge pool that will raise the tail 

water sufficiently to drown out the flip bucket. Therefore, many plunge pools are preexcavated to alleviate this possibility. The depth and side slopes of this type of plunge 

pool are determined by the soil characteristics. 

Figure 18. Plunge Pool in Connection with a Flip-Bucket 
Attachment 7_White Paper 4 - Energy Dissipators.doc 11 of 11 

Another type of plunge pool is shown in Figure 19.17 High-pressure fluctuations develop 

on the invert with this design. Sufficient anchorage and thickness of the floor slabs must 

be provided to prevent their loss during high flows. 

Figure 19. Plunge Pool for an Outlet in a Concrete Dam 

CONCLUSIONS 

Wide varieties of structures are available to designers for energy dissipation of high head 

flows from outlet works. Many of these are described in books and monographs. 

However, many designs have been developed for site-specific installations that have not 

appeared in the literature. Some of these designs have not been verified by either model 

or prototype tests. 

 

17 Bollaert, E., 2004, “Folsom Dam, Numerical Modeling of Hydrodynamic Forces on the Stilling Basin” , 

Aquavision Engineering, Lausanne, Switzerland
Attachment 8 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 1 of 8 

White Paper on Rehabilitation of Pipe Conduit 

Spillways Through Dams 

FEMA Outlet Works Workshop, May 25-27, 2004 

Denver, Colorado 

 By James R. Crowder, P.E. 

Schnabel Engineering 

Included in the aging infrastructure of the Southeast United States are hundreds of dams in 

excess of thirty (30) years old. Many of these aging dams are earthfill embankments constructed 

with corrugated metal pipe (CMP) spillway conduits passing through the base/foundation of the 

embankment. Deterioration of the conduit may become a problem when a spillway conduit has 

reached or exceeded its design service life, 

which is generally considered to be 25 to 30 

years for CMP. The deterioration generally 

consists of leaking joints, corrosion and holes 

in the pipe. Corrugated Metal Pipes (CMP) are 

the most susceptible to deterioration. 

Corrugated Metal Pipes were used extensively 

in small earthen embankment dams in the 

southeast, ranging in height from 10 to 50 feet, 

in the past 30 to 50 years. The main risk 

associated with the deterioration of these 

conduits is piping of the surrounding 

embankment soils into the conduit, which can 

lead to eventual failure of the earthen 

embankment. 

This paper discusses the repair options that are 

available to address these deteriorated 

conduits. The options discussed below are 

based upon the experiences of the author in the 

Southeast United States and are mainly limited 

to CMP’s in earthen dams having a maximum 

height of less than 50 feet. Included in the 

discussion of each of the options are 

advantages, disadvantages, and design 

considerations. 

• Option 1 – Replacement (Cut and Cover) 

• Option 2 - Slipline the Existing Conduit 

• Option 3 – Grouting Along the Exterior of the Conduit 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 2 of 8 

• Option 4 - Abandon the Existing Conduit 

• Construct Siphon Spillway 

• Construct “Short” Riser and New Outlet Conduit. 

Option 1 - Replacement 

Replacement, as the name suggests, consists of excavation of the earthen embankment, removal 

of the deteriorated conduit, installation or construction of a new conduit, and backfill of the 

excavation. The side slopes of the excavation through embankment dams are typically limited to 

two horizontal to one vertical (2:1) or flatter, in order to facilitate compaction of the replacement 

backfill against the existing embankment. On relatively high embankments in narrow valleys, 

the excavation required to remove 

the existing low-level conduit may 

necessitate the removal of nearly 

the entire dam. 

The replacement option typically 

offers the greatest operational 

flexibility among the spillway 

rehabilitation options. 

Appurtenances with the ability to 

completely drain the lake can be 

provided, if desired. Spillway 

capacity can be increased, or in 

some cases, controlled to allow for 

flood control improvements or 

storm water management. 

Removing and replacing the existing conduit typically offers the most thorough rehabilitation 

option for impaired conduits through dams, although this option can be cost-prohibitive. 

The advantages of the replacement option include: 

• Ability to inspect or observe the foundation of the dam 

• Ability to repair areas that may have been damaged by the deteriorating spillway 

• Ability to dramatically change the capacity of the principal spillway 

• Ability to add or improve seepage control measures along the conduit. 

• Capacity to fully drain lake can be provided 

The disadvantages of the replacement option include: 

• Potential high cost of excavating the existing embankment fill to remove the existing 

conduit 

• The creation of potential seepage paths along the surface of the excavation if proper 

bonding of the replacement backfill is not achieved. 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 3 of 8 

• The lake must be drained, and remain in a drained condition throughout construction. 

The draining of the lake may be detrimental to both surrounding property values, as well 

to the aquatic life that relies on the lake. 

• Difficulty in diverting normal flows and flood flows during the construction operations 

• Loss of the use of the crest of the dam for pedestrian or vehicular traffic 

• Excavation through the dam may cause conflicts with existing utilities or seepage control 

measures that may be present in the dam 

Option 2 - Sliplining 

Sliplining a pipe consists of placing a new pipe with a smaller diameter inside the existing 

deteriorated conduit and grouting the annular space between the two pipes. The sliplining 

technique can be used in lieu of the replacement or cut and cover method for the rehabilitation of 

the existing outlet pipe. 

Depending on the pipe used, 

sliplining may increase the 

hydraulic capacity of the outlet 

pipe, even when reducing the 

inside diameter of the original 

pipe. The reason the hydraulic 

capacity may be increased, even 

when the diameter of the pipe has 

been decreased, is due to the 

decrease in roughness of the pipe. 

Corrugated metal pipes have a 

relatively high roughness value as 

a result of the corrugations. 

The liner will typically have a 

roughness value of approximately 1⁄2 of 

the corrugated metal pipe. When 

designed and installed properly, the 

slipline technique will add additional 

years of service life to the existing pipe 

conduit spillway. A sand diaphragm 

drain is typically installed around the 

downstream portion of the existing 

conduit to limit the potential for soil 

piping as the CMP eventually corrodes 

and deteriorates. 

The advantages of sliplining are many when compared to Option 1. The advantages include: 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 4 of 8 

• No excavation required through the dam. In cases when either pedestrian or vehicular 

traffic is common or required along the crest of a dam this advantage may be critical. 

• In some cases, a lake can be 

maintained at or near full when 

sliplining. When rehabilitating 

a lake in a residential area or 

adjacent to a golf course green, 

aesthetics are commonly the 

controlling factor. Keeping a 

lake full of water during 

summer months, as opposed to 

having a mud flat, is attractive 

to many clients. In addition, the 

presence of a full pool allows 

for the maintenance of the 

existing aquatic habitat within the lake. 

The disadvantages of sliplining can be project threatening. The common disadvantages include: 

• Specialized contractors are required for the installation of the liner and grouting of the 

annular space. Specialized contractors may increase the construction cost. 

• Complete annular space grouting can be a challenge in certain instances. 

• Use of excessive pressure during the grouting of the annular space may cause failure 

or collapse of the liner. Failure of the liner could be critical and could require 

excavation and removal of the conduit system. 

• Partial collapse, irregularities, offset joints or bends in the existing pipe conduit may 

prevent the installation of an adequately sized sliplined pipe. 

• Inability to observe and address internal issues associated with the embankment dam. 

Option 3 – Grouting along the Exterior of the Conduit 

Conduits exhibiting seepage through joints 

or deteriorated portions of the pipe walls can 

be remediated by injecting materials through 

the walls of the pipe in an attempt to fill 

voids that may have been created and to plug 

openings in the pipe walls or joints. 

Cementitious or chemical grouts are 

typically utilized for the purpose. Injection 

of the materials can be performed either 

from the surface of the embankment or from 

the interior of the conduit. Selection of the 

approach to injecting the grout depends upon 

the availability of working room for grouting 

equipment and personnel (i.e. conduit size), 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 5 of 8 

the depth of the conduit below the embankment surface, and if the remediation is performed with 

the lake full. 

Injection of the grout from the interior of the pipe typically offers a better opportunity to deliver 

the grout to specified areas. Drilling through the embankment to grout along the conduit is 

usually performed for small diameter conduits or when the pipe is very shallow within the 

embankment. The greater the depth of fill over the conduit, the less certainty the grouting 

procedures are impacting the targeted areas. Drilling through the embankment is not practicable 

for situations in which the lake cannot be drained and portions of the conduit within the upstream 

slope of the dam are to be grouted. 

Although costs for this type of rehabilitation are typically lower than the cost of other options, 

this approach is generally considered somewhat of a temporary repair. This option does not 

extend the design life of the material of the conduit. If the damage to the conduit has created 

voids in the surrounding backfill, it is difficult to be certain that the grouting has filled all of the 

voids. 

This option retains the stage/discharge relationship of the existing spillway, which maintains any 

flood control benefits downstream of the dam. However, if the existing spillway capacity is 

insufficient to meet current project requirements, there is no opportunity to alter the capacity of 

the existing conduit. 

To summarize, the advantages to this remediation option include; 

• Relatively low initial cost. 

• Ability to make multiple repairs, if initial attempts are not completely successful 

• Work can be performed without draining the lake, although flow through the conduit 

needs to be controlled when working on the interior of the conduit. 

• Access along the top of the dam may not be impaired. 

The disadvantages to grouting along the conduit exterior include: 

• Inability to be certain that potential voids have been filled. 

• Does not typically provide a permanent repair for an aged conduit. 

• Inability to address spillway capacity issues. 

Option 4 – Abandonment of Existing Conduit and Construction of a Siphon 

Abandonment consists of grouting full the existing corrugated metal pipe with either a 

sand/cement grout or a gravel/sand/cement grout. In either scenario, an expansive chemical 

admixture is typically included in the grout mix to control or reduce shrinkage during curing of 

the grout. As with the sliplining option, a sand diaphragm drain is typically installed around the 

downstream portion of the existing conduit to limit the potential for soil piping as the CMP 

eventually corrodes and deteriorates. 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 6 of 8 

Siphons, as the principal spillway for the rehabilitation of dams, have been used successfully in 

the southeast, but are generally limited to small drainage basins with relatively small peak 

inflows. Because the soil loading 

is relatively small, siphons can be 

constructed of flexible conduit 

such as PVC, HDPE, or Ductile 

Iron. Typically, siphon spillways 

in these applications do not 

exceed 12-inches in diameter. 

However, in some instances, 

siphons as large as 15 to 18 inches 

in diameter have been 

successfully utilized in small 

dams. Dam owners and 

surrounding property owners 

should be aware that the use of 

siphons, as opposed to the more 

traditional pipe-and-riser spillway systems, generally results in a lake that will fluctuate more as 

a result of the inherit inefficiency of the siphon until priming occurs. Siphons prime with a head 

or water surface elevation between 1 and 1-1/4 times the diameter of the siphon above the siphon 

invert. Once a siphon primes, there is very little increase in outflow as the lake surface rises. 

The advantages of grouting full the existing corrugated metal pipe and installation of a siphon 

include: 

• Lake does not have to be completely drained. Maintaining a partial pool allows for 

the maintenance of some of the aquatic habitat. 

• Installation of siphons can be performed in a relatively short amount of time and are 

typically cost-effective. 

• No visible outlet structure in the lake. 

• Allows for the removal of relatively cool water from deeper areas within the lake to 

promote cold water fish habitat downstream. In areas where trout populations are 

threatened by increasing water temperature, the use of siphons can be used to combat 

the rise in stream temperature. 

• Specialty contractors are not required if quality engineering oversight is available 

during construction. 

The disadvantages of this option include: 

• Some excavation of the dam is required. If the dam is utilized as either a pedestrian 

path or vehicular path, some interruption of service should be anticipated. 

• Inefficient removal of water at heads below 1 to 1-1/4 times the diameter of the pipe, 

which causes excessive fluctuations in the water surface when compared to pipe and 

riser spillways. 

• Not cost effective for large watersheds. 

• Can be susceptible to vandalism, unless protective measures are taken. 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 7 of 8 

• Limited ability to drain lake deeper than 20 to 25 feet. 

• Inability to keep lakes drained 

• Susceptible to blockage with ice unless special provisions are implemented during 

design and construction 

• Some underwater work may be required 

Option 4 – Abandonment of Existing Conduit and Construction of a Short 

Riser and Pipe Spillway 

This option can be considered a modified version of the traditional pipe-and-riser spillway 

system or an economical hood inlet spillway system. The grouting procedure is the same as for 

option 3. The riser and pipe inverts for this option are set higher in the dam instead of being set 

along the fill/foundation interface. Usually, the riser invert is set a vertical distance below 

normal pool equal to 1/5 to 1⁄4 of maximum water depth at the upstream toe of the dam. For 

example, if the lake depth were approximately 20 feet, the riser invert would be set 4 to 5 feet 

below normal pool. The limiting height of the riser is dictated by both pipe diameter and 

location of the upstream pipe 

invert (entrance to the pipe) 

relative to the hydraulic grade 

line. Subatmospheric 

pressure conditions, that can 

accompany entrance 

conditions for steep pipes 

should be avoided. Also, the 

conduit invert should be a 

minimum depth of 1.5 

diameters below the pool 

elevation to avoid the capture 

of air due to surface water 

drawdown, and associated 

“slugging” under full flow 

conditions. 

Installation requires excavation of the embankment below normal pool. This excavation depth is 

greater than that required for the installation of a siphon but significantly less than what would be 

required for the removal of the deteriorated conduit. Utilization of this system may require the 

use of bends or elbows along the conduit to allow for the discharge of water at or near the 

downstream toe. A gate is usually installed on the upstream edge of the riser to allow for partial 

draining of the lake. Complete draining of the lake is not feasible with this system. 

The main advantage of this system is its ability to simulate the hydraulic characteristics and 

efficiencies of the traditional pipe-and-riser systems located through the base of the dam. Unlike 

the siphon, both the traditional pipe-and-riser system and this option can accommodate a wide 

variation in base flows into the lake without the corresponding large fluctuations in lake level. 
Attachment 8_White Paper 5 - Rehabilitation of Pipe Conduit Spillways Through Dams.doc 8 of 8 

Because soil loading is relatively moderate, flexible conduits such as ductile iron, PVC, and 

HDPE may be utilized. 

The advantages of such a system include the following: 

• Limited embankment excavation 

• Limited lake level fluctuation, as compared to a siphon spillway. 

• Can be installed without complete draining of the lake. 

• Cost-effective in that the riser height is limited. 

The system does have disadvantages, such as: 

• Cannot be utilized to drain the lake below the invert elevation of the riser. 

• Conduit bends/elbow can be expensive 

• Riser location within the upstream slope allows for an increase chance for vandalism 

in that the riser is only located a short distance from shore 

• Foundation soils for the riser base may be soft which can cause settlement problems 

or raise costs due to over-excavation 

• The height of fall in the conduit is limited. Pipe or culvert spillways should not be 

used for drops, from riser invert to pipe outlet, greater than about 25 feet due to the 

danger of cavitation. 

The owners of most old embankment dams with problem CMP conduits have a number of 

concerns or issues that will usually involve several of the options described above. The engineer 

should attempt to learn of these concerns or issues for each dam owner and present the 

advantages and disadvantages of each of the available repair options. In many instances, 

education of the owner by the engineer may be required to repair the dam in a manner consistent 

with good engineering practices that also meets the requirements or requests of the owner. 
Attachment 9 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 1 of 15 

White Paper on Outlet Works Inspection 

Federal Emergency Management Agency 

 Outlet Works Workshop 

Denver, Colorado 

 

May 25 – 27, 2004 

Prepared by Chuck R. Cooper1

 

Introduction

Outlet works are generally inspected as part of a thorough inspection of the dam and its 

features. Outlet works are typically either cut-and-cover or tunnels. This white paper 

will focus on the inspection of cut-and-cover outlet works. A cut-and-cover outlet works 

is typically comprised of the following components: 

(1) Entrance channel 

(2) Intake structure 

(3) Conduit(s) 

(4) Terminal structure 

(5) Downstream channel 

The purpose of an outlet works inspection program is to ensure that conduits through 

dams are safely and efficiently operated and maintained. Structural defects and 

deterioration develop progressively over time. A trained and experienced inspector can 

identify defects and potential problems before existing conditions in the dam and conduit 

become serious. This white paper will address the following specific topics relating to 

outlet works inspection: 

• Frequency of inspection 

• Systems, methods, and techniques used for inspection 

• Design criteria to accommodate inspection 

This white paper will not address the inspection of the entrance and downstream 

channels, since they rarely develop dam safety concerns. 

Frequency of Inspection 

Periodic inspection may reveal trends that indicate more serious problems are developing. 

Inspection intervals may vary depending on the overall conditions determined from 

previous inspections and the existence of any dam safety concerns. Periodic inspections 

can vary in scope and purpose and by the organization or personnel (damtender, 

agency/district level, etc.) performing the inspection. 

 

1 Civil Engineer, U.S. Bureau of Reclamation, P.O. Box 25007, Denver, Colorado 80225-0007 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 2 of 15 

Types of inspections 

Dam safety organizations and dam owners may employ a variety of inspections during 

the life of an outlet works. These inspections may include the following types 

(Reclamation, 1988a): 

(1) Initial or formal. – Initial or formal inspections include an in depth review of all 

pertinent data available for the outlet works to be inspected. Design and 

construction data are evaluated relative to current state-of-the-art to identify 

potential dam safety problems or areas requiring particular attention. A thorough 

onsite inspection of all features is conducted, and an attempt is made to operate all 

mechanical equipment through their full operating range, if possible. 

(2) Periodic or intermediate. – Periodic or intermediate inspections are conducted 

between formal inspections. An in depth review is made of all pertinent data 

available on the outlet works to be inspected. However, the data review focuses 

on the current status of the outlet works and the data are not evaluated relative to 

current state-of-the-art criteria. A thorough onsite inspection of all features is 

conducted. All mechanical equipment may not be tested during any one 

inspection. Some equipment may be operated at another time or during the next 

inspection. 

(3) Routine. – Routine inspections are typically conducted by field or operating 

personnel. The primary focus is on current condition of the outlet works. 

Available data may not be reviewed and evaluated prior to the inspection, 

depending on the inspector’s familiarity of the outlet works. Inspections may be 

scheduled on a regular basis or performed in conjunction with other routine tasks. 

(4) Special. – A special inspection is conducted when a unique opportunity exists for 

inspection. For example, if low water conditions exist in a reservoir exposing a 

normally inundated structure, a special inspection may be arranged. 

(5) Emergency. – An emergency inspection is performed when an immediate dam 

safety concern is present or in the event of an unusual or potentially adverse 

condition (e.g., immediately following an earthquake). 

The actual terms and meanings used to define the types of inspection may vary between 

dam safety organizations and dam owners. 

Factors influencing scheduling of inspections 

Scheduling of periodic outlet works inspections may be influenced by (Reclamation, 

1988b): 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 3 of 15 

(1) Sufficient notice. – Dam owners and operators may need sufficient time to make 

necessary arrangements, such as pre-inspections associated with lockout/tagout 

and confined space entry, or special equipment or approval for unwatering 

conduits, terminal structures, or pools. This process could require several weeks 

or months depending on the facility. 

(2) Scheduling access. – Access for the inspection should be scheduled when most or 

all of the major components of the outlet works system can be examined. Some 

features such as intake structures and upstream conduits are usually submerged 

and not accessible. Downstream conduits and terminal structures may or may not 

be able to be unwatered and made accessible for inspection. The dam owner or 

operator may be requested to provide notification when reservoir conditions 

permit or when the reservoir can be drawn down to allow the inspection to be 

performed. If the feature to be inspected is normally inundated and inaccessible, 

certain factors (Reclamation, 1985 and 2001) should be considered in determining 

the extent and frequency for inspection such as: 

a. Results of previous “hands on” inspection or evidence from the inspection 

of the normally accessible portions of the feature. Inspection of the 

normally accessible portion of a feature may provide information on the 

probable condition of the inaccessible portion. 

i. Condition of the concrete. – Cracking, joint separation, or 

significant deterioration. 

ii. Condition of the embankment and foundation. – Excessive post 

construction settlement or alignment distortion of the downstream 

conduit. Excessive embankment settlement or the existence of 

sinkholes on the upstream face along the alignment of the outlet 

works. 

iii. Observed seepage. – Seepage or wet areas observed at the 

downstream toe of the embankment. 

iv. Flow conditions. – Changes in the discharge capacity of the outlet 

works. 

 

v. Damage and deterioration. – Damage or deterioration of gates and 

metalwork. 

vi. Water quality. – Water quality known to be detrimental to 

concrete, conduit linings, or waterstops. Excessive amounts of 

sand or other material transported by the discharge. 

 

b. Operational history and performance of the feature, since its previous 

inspection. 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 4 of 15 

c. Relative costs for providing access for inspection of the feature, including 

costs associated with lost water and power revenues. 

d. Age of the feature. 

e. Design and construction considerations. 

i. Changes in standards or guidelines. – Design criteria, construction 

techniques, and/or quality of material at the time of construction 

fail to meet current standards or guidelines. 

ii. Foundation conditions. – The conduit was constructed on 

foundation of varying compressibility where there is a potential for 

differential settlement. This may result in cracking of the conduit 

or excessive opening of joints. Differential settlement is also 

possible between the conduit and gate chamber. 

iii. Foundation faults. – The conduit crosses a foundation fault where 

there is the potential for movement or disruption. 

iv. Unfavorable stresses. – The conduit located where conditions are 

conducive to arching and resulting in unfavorable stresses in the 

embankment and/or conduit. These stresses could be conducive to 

hydraulic fracturing of the embankment or stress concentrations on 

the conduit. 

v. Filters. – Lack of adequate filters and drainage material around the 

conduit downstream from the impervious zone of the embankment 

to safely convey seepage or leakage along the conduit to an exit 

point. 

f. Critical function of the feature. 

g. Any site conditions which exist that may compromise the safety of the 

feature. 

(3) Operation. – Certain problems may not normally appear when the feature is dry 

that appear when the feature is being operated. Also, when a feature is operating 

during a period of higher than normal releases may provide information that may 

not have been available to previous inspections. 

The opportunity to optimize both access and operation during a single inspection 

typically is not possible. Inspection objectives may have to alternate from one inspection 

to the next. This may necessitate the need for scheduling “special” inspections during 

unusual conditions, in addition to regular inspections to provide a comprehensive view of 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 5 of 15 

the outlet works safety. Special inspections may be required after floods, seismic 

activity, or other unusual events. 

Periodic inspections by selected organizations 

The frequency of periodic inspections varies between organization and dam owners. 

Emergency situations may require much more frequent inspections, such as daily or 

hourly. Situations can arise suddenly that cause serious damage in a short period of time. 

Examples of these problems are operations at full discharge capacity, seismic activity, or 

other special conditions. The need for special inspections should be evaluated after 

occurrence of any of these situations. 

A sampling of periodic inspections as required by selected organizations: 

The Bureau of Reclamation employs the following process (Reclamation, 2001) to 

monitor its significant and high hazard dams and attempt to detect any potential dam 

safety deficiencies: 

• Annually. - Annual inspections are performed by inspectors who are generalist 

(as opposed to specialist) engineers very familiar with the dam and its operations, 

and can readily distinguish changes from year to year. All inspectors attend 

regular training in dam safety inspections. 

• Periodic. - On a 6-year cycle (alternating with the comprehensive facility review 

(CFR), each dam is examined by a team originating in a Reclamation Regional 

Office, including the regional examination specialist. This examination is 

referred to as a Periodic Facility Review (PFR) and includes a rather thorough 

review and reporting of all past dam safety and operation and maintenance 

recommendations. 

• Comprehensive. - On a 6-year cycle (alternating with the PFR), each dam is 

examined/evaluated by a team of specialists from Reclamation’s Technical 

Service Center that includes an Instrumentation Engineer, an Examination 

Specialist, Mechanical Engineer, and a Senior Dam Engineer. This examination 

is referred to as a CFR and includes not only the PFR activities, but also includes 

technical evaluation of all design, construction, and analysis of the dam. 

The Natural Resource Conservation Service (NRCS) requires the sponsor/owner to be 

responsible for making inspections after they are turned over to the sponsors/owners 

(NRCS, 2003). Special, annual, and formal (once every 5 years) inspections are 

performed by personnel trained in conducting the inspections. If requested by the 

sponsor/owner, NRCS may participate in inspections; provide training to ensure that the 

sponsor/owner understands inspection techniques and the importance of completing 

corrective action; and provide technical assistance to address specific O&M needs. If an 

inspection reveals an imminent threat to life or property, the sponsor/owner shall 

immediately notify all emergency management authorities. 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 6 of 15 

The U.S. Army Corps of Engineers (USACE) performs periodic, intermediate, and 

informal inspections on the basis of project size, importance, or potential hazard 

(USACE, 2004): 

• Initial periodic inspection. – The first periodic inspection and evaluation of a new 

earth or rockfill dam is carried out immediately after topping out of the 

embankment prior to impoundment of the pool. 

• Second periodic inspection. – The second periodic inspection for new dams is 

performed no later than one year after impoundment is initiated. 

• Subsequent periodic inspection. – Subsequent periodic inspections are performed 

at one-year intervals for the next two years. The next two inspections are 

performed at two-year intervals and then extended to a maximum interval of five 

years. More frequent inspection intervals are scheduled, if conditions warrant. 

• Intermediate inspection. – For projects on a five-year inspection cycle, an 

intermediate inspection of all or some of the features may be scheduled, if 

warranted. Selection is based on consequences of failure, age, degree of routine 

observation, a natural event (e.g. earthquake), performance record and history of 

remedial measures. Intermediate inspections are also made of any portion of a 

project exposed during dewatering that could not be accomplished during 

scheduled periodic inspection. 

• Informal inspection. – Frequent informal inspections are performed by 

appropriate employees at the project. The purpose of informal inspection is to 

identify and report abnormal conditions and evidence of distress. 

Systems, Methods, and Techniques Used for Inspection

The success of an inspection will be dependent upon good preparation and planning. 

Any inspection should consider: 

• Selection of the inspection team. – The members of the inspection team will vary 

depending on the needs and resources of the organization or dam owner, type of 

the inspection, results of the data review, and any special requirements. 

• Review of project data. – The amount of available data may vary greatly from 

outlet works to outlet works. The extent of project data review and evaluation 

depends on the type of inspection to be conducted. 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 7 of 15 

• Preparation of an inspection plan. – A detailed inspection plan should be prepared 

to identify all features to be inspected, problem areas, and areas of potential 

problems. The inspection plan will also identify special logistics, access, or 

equipment requirements. An inspection checklist is typically prepared as part of 

an inspection plan. The checklist is used to identify specific inspection objectives 

and is also useful in developing the final inspection report. 

Methods used for the inspection of the various features of an outlet works are mainly 

dependent upon accessibility. Factors influencing accessibility include: 

• Inundation. – Reservoir operations and water levels may make some features 

unavailable for normal inspection and require specialized inspection services 

(e.g., dive team, remotely operated vehicles). 

• Confined space. – Certain features may require Occupational Safety and Health 

Administration (OSHA) confined space permitting for man-entry, lockout/tagout 

procedures, and preparation of a job hazard analysis (JHA). An alternative to 

man-entry is the use of specialized inspection services (e.g., closed circuit 

television). 

• Size constraints. – Limitations in size may prevent man-entry and require 

specialized inspection services (e.g., closed circuit television). 

Inspection of intake structures

In most cases, due to the intake structure’s location in the reservoir, it is either partially or 

fully inundated. If the intake structure is partially inundated, inspection of the structure 

above the water level will be fairly straight forward. However, inspection of the portion 

of the structure below the water level such as the entrance, trashracks, fish screens, ice 

prevention systems, gates/valves, stoplogs, and bulkheads will require specialized 

inspection services. If the intake structure is a tower, it may have a wet well or some 

other access to the control mechanism. Closure of a guard gate or bulkhead may provide 

the ability for inspection of the interior of the tower. 

Problems common to intake structures include deterioration, damage, and misalignment. 

Description of more specific problems related to trashracks, fish screens, ice prevention 

systems, gates/valves, stoplogs, bulkeads, and bridges are beyond the scope of this white 

paper. 

Inspection of conduits 

Generally, outlet works conduits with diameters 36 inches or larger can be inspected by 

man-entry, if proper OSHA precautions are taken. Conduits with diameters smaller than 

36 inches are inaccessible for man-entry and require specialized inspection services. 
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Problems common to conduits include obstructions, defective joints, cracking, 

deterioration, and mechanical equipment operation. Signs of these types of problems 

include: water ponding on the invert of the conduit which could be an indication of 

settlement-related problems in certain reaches of the conduit, metallic corrosion of pipe 

or exposed reinforcement, discoloration or staining of concrete surfaces, damaged 

protective coatings, deformation of the conduit circumference, chemical deterioration of 

concrete, joint separation, leakage into or out of the conduit, misalignment of conduit 

sections, plugged drain holes, voids behind the concrete near any observed cracks, joint 

separations, or misalignments, spalled concrete from compression, reinforcement 

corrosion, drummy or hollow-sounding concrete, erosion, abrasion, or damage in 

concrete downstream of gates, offsets, and/or changes in slope, cavitation damage, and 

binding of mechanical equipment. 

Inspection of terminal structures

The terminal structure may be dry or partially inundated depending on the time of year 

and the schedule of releases through the outlet works. If the terminal structure is partially 

inundated, inspection of the structure above the water level will be fairly straight forward. 

However, inspection of the portion of the structure below the water level such as the 

basin, chute blocks, baffle blocks, or end sill will require specialized inspection services. 

Problems common to terminal structures include deterioration, damage, obstructions, 

misalignment, backfill and foundation deficiencies. 

Specialized inspection 

Specialized inspection includes the use of a dive team, climbing team, remotely operated 

vehicle (ROV), or closed circuit television (CCTV). 

(1) Dive team 

Certain factors should be evaluated when considering a dive inspection: 

a. Depth. - As the depth below the water surface increases, the difficulty of 

performing the dive increases. Divers have a limited amount of time on a 

given dive, and that time decreases with the increased pressures on deeper 

dives. Also, as the dive becomes deeper, more of the allowable dive time 

is spent descending. Allowable dive times can be increased by means 

such as using mixed gas, or diving in a pressurized “newt suit.” This 

increased dive time at depth comes at an increased cost due to 

requirements for items like larger dive crews, more specialized equipment, 

and a limited number of companies that can actually do the work. 

b. Altitude. - The altitude of the feature to be inspected can greatly affect the 

viability of a dive inspection. This could really be considered a subfactor 

of the depth factor. Due to the lower atmospheric pressure at higher 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 9 of 15 

altitudes, a diver has an even more limited dive time associated with a 

given depth of dive. 

c. Access. - Often the entrances to a feature are equipped with trashracks on 

the inlet side. The ability to remove enough of the trashrack bars to allow 

easy entry and egress is important. Since divers in such an environment 

will be utilizing some type of surface-supplied breathing gas, it is 

important that the access point be such that the hoses will be able to be fed 

into the conduit without hanging up. 

d. Leakage and currents. - Leakage of downstream gates or valves within a 

conduit is a safety factor that can affect whether a dive inspection can be 

safely performed. Any inspection of this type should be performed such 

that the diver enters the conduit against any current and then exits with the 

current. For an outlet works, a submerged conduit will more than likely 

need to be entered from the upstream end. Therefore, the condition of the 

gates and how much amount of leakage is critical with respect to the 

viability of a dive inspection. 

e. Visibility. - The distance a diver can see is important to whether a dive 

inspection is advisable. In the event of zero visibility, little reason would 

exist to pursue a dive inspection, as the shear magnitude of the entire 

surface of a feature would be extremely difficult to inspect by feel. Also, 

in a circular conduit a diver does not have a real edge or other reference 

point to keep track of any findings. If a dive inspection is planned for a 

conduit, consideration should be given to making a large release prior to 

the inspection as a means of flushing sediments from the conduit and then 

allowing some amount of time for the water to settle out prior to diver 

entry. This time will depend on the type of sediments in the water, but 

could vary from a day to a week. 

f. Size. - A conduit should be large enough, so the diver can turn around 

inside and exit head first. The diameter required for inspection would 

depend on the size of the individual diver and also the type of dive 

equipment required. 

g. Length. - As with depth, the conduit length becomes a factor relating to 

the amount of time the diver has available at depth. If the conduit is 

extremely long, it can take much more time to inspect than the diver has 

available. The available dive time for a long conduit can be increased, but 

this can be costly and have extreme safety concerns since the diver does 

not have a direct path to the surface. 

(2) Climbing team 
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Although not often required, a climbing team may be utilized to perform inspection of the 

inaccessible portions of intake towers and the walls of terminal structures. 

(3) Remotely operated vehicle 

Inspection using an ROV is a good alternative to dive inspection when conditions such as 

depth, diameter, or length become prohibitive. The ROV can be used to inspect the 

submerged portions of intake structures, conduits, and terminal structures. Divers can be 

used to assist with upstream access by opening trashracks and/or guiding the ROV 

through the racks. Access is also possible from downstream conduit, if the section of 

conduit between the upstream guard gate and downstream regulating gate can be 

unwatered and the ROV placed inside. Cables can be threaded through a special manhole 

bulkhead and the conduit can be rewatered, so the ROV can travel upstream and perform 

the inspection. 

Some of the limitations of an ROV inspection include visibility and the size of the 

conduit. If the conduit diameter is large, the ROV inspection is much more likely to be 

limited to one small path along the conduit, where as a diver can cover a much larger path 

or wider swath as the diver moves down the conduit. Because the diver can use their 

sense of feel, in a limited visibility situation they are able to possibly locate more items of 

interest and then focus in on them with their other senses. With an ROV in a limited 

visibility situation, the only area inspected is the small area directly in front of the 

camera, basically whatever the ROV is bumping into. Sometimes, it can be difficult with 

the ROV to tell exactly where the view seen on the monitor is within the conduit, and 

since ROV’s often rely upon a compass, the steel in the conduit lining and/or concrete 

reinforcement can affect the navigation. If CCTV equipment is used in lieu of an ROV, 

the length of cable tether can be measured to determine the location within the conduit. 

(4) Closed circuit television 

CCTV can be used to inspect the submerged portions of intake structures, conduits, and 

terminal structures. CCTV inspection can also be used in structures or conduits where 

confined space entry issues may require permitting prior to physical entry. The OSHA 

regulations define a confined space as having limited access and egress, and not being 

designed for continuous human habitation. This would include not only small conduits, 

but also larger diameter conduits where risks, costs, or system complexity may make 

remote inspection more advantageous. 

CCTV inspection equipment consists of a video camera attached to a self-propelled 

transport vehicle (crawler). The transport vehicle and camera are commonly referred to 

as a camera-crawler. An operator remotely controls both the transport vehicle and 

camera. The camera can provide both longitudinal and circumferential views of the 

interior of the feature. Video images are transmitted from the camera to a television 

monitor, from which the operator can view the conditions within the conduit. The video 

images are recorded onto videotape, compact disc, or digital versatile disc (DVD) for 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 11 of 15 

future evaluation and documentation. The operator can add voice narrative and 

alphanumeric captions or notations as the inspection progresses. 

Depending on the model, camera-crawlers used in conduits with smaller diameters 

(generally about 4 to 14 inches) have cameras with some pan, tilt, and zoom capabilities, 

a wide range of tether pulling capacity (200 to 1,000 feet), and some steering capabilities. 

Camera-crawlers used in conduits with diameters of 15 inches or larger are steerable, 

have a greater cable tether pulling capacity (500 to 1,500 feet), and have cameras that can 

provide a wider array of optical capabilities, including pan, tilt, and zoom. Actual tether 

limits obtainable in the field, vary greatly depending upon a number of factors such as 

conduit diameter, bends, invert slopes, and existing invert conditions. 

In large diameter conduits, the video camera can be attached to a scissor mechanism 

mounted to the transport vehicle. The scissor mechanism, controlled by the operator, can 

raise or lower the video camera as needed for inspection. In addition, the video camera 

usually has a high powered zoom, which can be used to provide closeup views of areas 

that might be difficult for the transport vehicle to get near. These features allow 

examination of very large conduits with diameters as large as 40 or 50 feet. 

If required, some models of camera-crawlers allow for the attachment of retrieval tools 

such as alligator clamps, grippers, and magnets. These tools can be used to remove light 

debris or damage. The attachment of any type of retrieval tool will require additional 

clearance within the conduit to operate the retrieval tool. Some models of crawlers have 

robotic cutters attached to them. These cutters can be used to remove debris or 

protrusions in concrete, steel, or reinforcement. 

Sometimes the conduit is so small that a transport vehicle cannot be used, or 

obstructions/invert conditions exist that prevent the transport vehicle from traversing the 

conduit. For these types of situations, small color video cameras (1.5 to 3 inches in 

diameter) can be attached to metal or poly-vinyl chloride (PVC) poles and manually 

pushed up the conduit. Push poles are normally used for straight sections of conduit. 

The use of poles for advancement is generally limited to about 400 feet. If bends exist in 

the conduit, a flexible snake device (spring steel wire, coiled wire, or flexible 

polypropylene-jacketed fiberglass push rod) can be used instead of the push poles. The 

color cameras are connected to a video cassette recorder and to a television monitor. The 

snake devices are generally limited to about 75 to 200 feet. 

Design Criteria to Accommodate Inspection

Intake structures

Locating submerged intake structures can be very difficult and time consuming. On a 

large dam where the structure is a fair distance from the shore, it is advantageous to have 

some reference points to line up on from two directions. Ideally, if these are permanent 

then they can be used to triangulate in on. For intake structures on small dams where the 

structure is closer to shore, a series of permanent markers can be utilized as a center point 
Attachment 9_White Paper 6 - Outlet Works Inspection.doc 12 of 15 

and a measured length of rope can be used to locate the structure in a radial search using 

the marker as the center point. The series of markers would allow ease of use with 

several water surface elevations. With either of these methods, good documentation is 

needed, so the same points can be used each time. Another possible method is the use of 

an embedded air line which can be used to supply bubbles which can be used to locate it. 

This, of course, requires a compressor, but a small compressor can typically be brought 

on site. Another alternative for locating intake structures involves the use of the Global 

Positioning System (GPS). GPS is a worldwide radio-navigation system formed from a 

constellation of 24 satellites and their ground stations. GPS uses these satellites as 

reference points to calculate positions to a high degree of accuracy. 

Access entry locations (e.g., trashracks) should be in an easily locatable position, such as 

the corner next to the tower. Also, the entry location should be oriented, so it can easily 

be found from inside the structure. If a hatch is near a concrete column, then the column 

can be used for reference. If the hatch is located in the center of the structure it is 

difficult for the diver to determine where the hatch located in the trashracks. The size of 

the hatch or opening provided in the trashracks is recommended to be about 3.5- by 3.0 

feet. The larger hatch size also is beneficial if divers are inserting an ROV. 

The trashracks used for entry locations need to be easily opened or removable by the 

diver. 

Conduits 

If a conduit is going to require inspection by a diver the diameter shouldn’t be less than 

48 inches, so the diver can turn around in the conduit. 

The Bureau of Reclamation performed a series of tests in 2002 to evaluate the 

performance capabilities using camera-crawlers in double walled high density 

polyethylene (HDPE) pipe. The results of these performance tests are generally 

applicable for other conduit materials. The following is a summary of those tests: 

(1) Diameter. - The minimum recommended conduit diameter to successfully 

accommodate camera-crawlers is 8 inches. Although camera-crawlers are 

available for conduits smaller than 8 inches, they can be very limited in cable 

tether pulling capacity and generally do not have sufficient traction for use in 

conduit inspection. In addition, the cameras typically only have a fixed lens and 

the transport vehicle is not steerable. Camera-crawlers used in conduits with 

diameters between 8 and 12 inches generally have cameras with some pan, tilt, 

and zoom capabilities, but generally are not steerable. Camera-crawlers used in 

conduits with diameters of 15 inches or larger are steerable, have a greater cable 

tether pulling capacity, and have cameras which can provide a wider array of 

optical capabilities including pan, tilt, and zoom. Where practical, the use of 

conduits with diameters 15 inches or larger is strongly encouraged. This allows 

for the use of more powerful and versatile camera-crawlers. 
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(2) Bends. - The maximum recommended bend angle to successfully accommodate 

CCTV inspection equipment is 22.5 degrees. In conduits with diameters of 8 and 

10 inches, the camera-crawler cannot be navigated around bends greater than 

45 degrees, since the camera cannot clear the conduit crown as it travels through 

the bend. If sharper bends are required in conduits with diameters of 8 and 10 

inches, a series of 22.5-degree bends are recommended. Each bend should be 

connected to a minimum 5-foot length of conduit to allow the camera-crawler to 

navigate around the bend segment and provide adequate crown clearance. 

Conduits with diameters of 12 inches or larger can have bends that exceed 22.5 

degrees, but drag friction then reduces the cable tether pulling capacity by as 

much as 75 percent. 

(3) Invert slope inclination. - The maximum recommended invert slope inclination to 

successfully accommodate camera-crawlers is 5 degrees. The difference in invert 

slope inclination between flat and 10 degrees can reduce cable tether pulling 

capacity by as much as 70 percent depending upon the conduit diameter, degree 

of conduit bend, and the invert condition. Flat to 5-degree invert slopes would 

appear to be the most reasonable inclination. Slopes with inclinations greater than 

10 degrees are not recommended, due to the significant loss of traction that occurs 

when camera-crawlers are pulling long cable tethers. If slopes greater than 

5 degrees are required, upstream access locations should be provided within the 

conduit. 

(4) Distance between access entry locations. - The maximum distance between access 

entry locations can range between 500 and 2,000 feet, but highly depends upon 

the conduit diameter, bends, invert slopes, and invert conditions. The designer 

will need to take these limitations into account when selecting the appropriate 

distance between access entry locations. In conduits with diameters of 8, 10, and 

12 inches, the maximum distance should not exceed about 1,000 feet. This 

assumes that access is available on both ends of the conduit. If access will only 

be available on the downstream end of the conduit, then the maximum distance 

should be limited to about 500 feet. In conduits with diameters of 15 and 18 

inches, the maximum distance should not exceed about 2,000 feet. This assumes 

that access is available on both ends of the conduit. If access will only be 

available on the downstream end of the conduit, then the maximum distance 

should be limited to about 1,000 feet. 

Terminal structures

Often the most difficult aspect for dive inspection of terminal structures is accessing the 

water. Carrying all of the heavy dive equipment down a riprap slope is a difficult and 

often dangerous task. Providing some means for more easily crossing the riprap such as 

stairs or a narrow concrete section would be useful. Fluctuation of the water level in the 

terminal structure may cause this “path” to become green with algae and result in a very 

slick surface. 
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Other innovative technologies

Other innovations in inspection systems are currently under development for sewer 

pipelines and for the oil and gas industry. These systems may eventually prove 

applicable to outlet works inspection. These systems involve state-of-the-art lasers 

scanners (digital imaging) and gyroscope technology. Laser scanner systems allow the 

operator to see the total conduit surface with color-coding of conduit defects on a digital 

computer image. Data processing and report preparation are completed using a 

manufacturer’s proprietary software. Currently, laser scanners are not readily adaptable 

for conduit inspection, since they have some difficulties identifying infiltration, 

corrosion, and conduit ovality. Laser scanners also are limited to conduits in the range of 

8 to 24 inches in diameter. Costs are generally 50 to 75 percent higher than for CCTV. 

However, the major benefit of laser scanners is the ability to produce a digital record, 

which reduces the subjective interpretation of results. Computerized assessment will gain 

wider acceptance as a reliable inspection and evaluation tool as further technological 

advancements are made (Civil Engineering Research Foundation, 2001, . 

There also are a number of geophysical and non destructive testing (NDT) technologies 

that could have application for the inspection of outlet works. These technologies 

include: acoustic and ultrasonic testing, seismic tomography, sonar, electrical resistivity, 

ground penetrating radar, mechanical and sonar calipers, hydro-acoustic surveys, and 

radiography. 
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References

Bureau of Reclamation, Assistant Commissioner – Engineering and Research Center, 

Technical Memorandum No. 6, Guidelines for Determining Whether Normally Inundated 

Outlet-Works Features Should be Examined, 1985. 

Bureau of Reclamation, Training Aids for Dam Safety Module, Inspection of Spillway 

and Outlet Works, 1988a. 

Bureau of Reclamation, Training Aids for Dam Safety Module, Preparing to Conduct a 

Dam Safety Inspection, 1988b. 

Bureau of Reclamation, Directives and Standards FAC 01-07, Review/Examination 

Program for High- and Significant-Hazard Dams, 2001. 

Civil Engineering Research Foundation, CERF Report No. 40551, Evaluation of SSET: 

The Sewer Scanner and Evaluation Technology, 2001. 

Natural Resource Conservation Service, National Operation and Maintenance Manual, 

2003. 

U.S. Army Corps of Engineers, EC 1110-2-1156, Draft Safety of Dams – Policy and 

Procedures, 2004.