Presentation 18:
Inspection, Maintenance, and Monitoring of Service and Emergency Spillways

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

MWH
Inspection, Maintenance and Monitoring of Service and Emergency Spillways
Daniel L. Johnson
MWH Americas

Current Condition
* Change in Mentality
* Attention paid to safety of dams
* Understanding of design events
* Owners awareness

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Levels of Experience
* Rarity of large flood events
* Denver snowstorm of 1913
* Big Thompson Flood of 1976
* South Platte Flood of 1965
* Events do occur and spillways are leading cause of failures

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Personnel Issues
* Inspection Knowledge Needed
* Failure modes
* Service spillways see more use than emergency spillways
* Emergency spillway may have never been used

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Inspection Issues
* Capability to meet design criteria
* Conditions and components for successful operation
- Located on abutment
- Located on dam
* Condition assessment
* Changes with time

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Inspection Issues
* Observation of operation
* Annual flood
* 5, 10 and 25 year flood
* Normal flows give indication of ability for successful operations
* Normal flows may be most critical for maintenance

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Maintenance
* Maintenance is typically not frequent
* Emergency spillway may be forgotten
* Repairs are necessary to maintain in as-designed condition
* Concrete
	- Movement, foundation erosion, toe and head erosion
* Earth
	- Slope protection, erosion of channel, abutments, toe, head
* Deleterious materials

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Special Issue
* Over-the-dam spillways need additional attention
* Frequent use as they are cost effective using RCC
* Induces new failure mode
* Increases frequency of emergency spillway usage

Monitoring
* Monitoring is needed
	- to estimate performance
	- to set a maintenance/rehabilitation plan
* Measurements of
	- movement, cracking, deterioration, aging issues
* Documentation of
	- surveys, photos, checklists and notes

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Data Usage
* Review of monitoring data
	- by personnel experienced and qualified
* When first gathered to understand current condition
	- as comparison to historic records for evaluating changes
* Reporting of results to owner and safety agencies

Conclusion
* Spillways constructed of engineered materials age
* Criteria may not be up to date
* Modern designs may have less robust components
* Inspection, maintenance and monitoring may be last hope

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Presentation 19:
Unlined Spillway Erosion Risk Assessment

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment 
Johannes Wibowo
Evelyn Villanueva
Don Yule
Darrel Temple

Photo of Tuttle Creek, KS

Unlined Spillway Erosion Risk Assessment
Problem Statements:
* Spillway erosion analysis encounters variable nature of geometry, geologic material and unpredictable flood events.
* Dam Safety Portfolio Analysis needs a tool to determine the probability of spillway damage.
Photo of Painted Rock, AZ

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Research Objectives:
* Develop a tool to assess the probability of damage on unlined spillway erosion * Develop a tool to prioritize unlined spillway/channel remediation projects
Photo of Saylorville, IA

Unlined Spillway Erosion Risk Assessment
Photo of DMAD spillway shortly after failure (1982)

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Canyon Dam Spillway, Texas
Date: July 6, 2002
Flow: 66,000 cfs, 250 yrs flood
Duration: 12 days
Spillway Width: 1260 ft
Material: Limestone

Unlined Spillway Erosion Risk Assessment
Photo of Canyon Dam

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Risk Assessment
Process of Answering Three Questions
1. What can go wrong?
2. What is the likelihood it will go wrong?
3. What are the consequences if it does go wrong?

Unlined Spillway Erosion Risk Assessment
1. What can go wrong?
	* Local Scouring
	* Spillway Breach
	* Headcut Erosion
	* Dam Breach

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
2. What is the likelihood it will go wrong?
	* Uncertainty of Flood Event
	* Uncertainty of Material Parameters
	* Uncertainty of Performance of the Unlined Spillway

Unlined Spillway Erosion Risk Assessment
3. What are the consequences if it does go wrong?
	* Spillway Partial Damage
		- Lightly Damaged
		- Severely Damaged
	* Spillway Breach
		- Population at Risk
		- Loss of Economic Value

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Spillway Erosion Models
* USDA (Temple et al., 1994
* Modified USDA (KCD, 1995; ERDC, 2002)
* Annandale (1995)
* REMR (WES, 1998)

Unlined Spillway Erosion Risk Assessment
Phase of Erosions
Rock
Top Soil
Original Surface
Vegetal Detachment
Head-cut Development
Head-cut Advancement

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Event Tree

Sedimentary
Erosion Occurred
Head-cut Developed
Head-cut Advanced
Spillway Breach
Dam Breach

Volcanic
Spillway Flow
Intact
Local Scour
Big Pot Hole
Spillway Breach
Local Damages
Partial Damages
Dam Breach
Partial Damages

Unlined Spillway Erosion Risk Assessment
Development of Head-cut
Load: Hydrograph
Governing Equations:
Te = y(d = Ad)S
Ds/dt = kd[te tc]

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Parameters
Te = effective stress
Y = unit weight of water
D = normal depth of flow
S = surface slope
De/dt = erosion rate
Kd = detachment rate
Tc = threshold stress

Unlined Spillway Erosion Risk Assessment
Head-cut Advance
Load: Hydrograph
Governing Equations:
Dx/dt = {C(A A0) (A A0) > 0
		0 (A A0) < 0
Dx/dt = Rate of headcut advance
C = empirical parameter
A = Hydraulic attack
A0 = Threshold level

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Erosion Model  Threshold Line
Maximum qH
Erodibility Index Kh
Eroded
Threshold Line
Not Eroded

Unlined Spillway Erosion Risk Assessment
Erodibility Index Kh
Kh = Ms x Kb x Kd x J6
Ms = Material Strength Number
Kb = Block Size Number
Kd = Joint Shear Strength Number
Js = Joint Orientation Number

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Maximum Hydraulic Attack 
E = w x q x h
E= Maximum Hydraulic Attack
W = Unit weight of water
Q = Unit discharge
H = Energy line drop

Unlined Spillway Erosion Risk Assessment
Logistic Regression
* Regression for Binary Outcomes
	- Occurrence (Erosion)
	- Non-Occurrence (No Erosion)
* User of Logistic Regression Method
	- Medical
	- Business
* Probabilistic Liquefaction Analysis

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Logistic Regression
* Odds ration p/1-p
* Logit transformation
Ln[p/1-p] = b0 + blx
p = 1/1 + cxp [-(b0 + blx)]
p = probability of occurrence
b0-b1 = regression parameters
x = independent variable

Unlined Spillway Erosion Risk Assessment
Multiple Logistic Regression
P=1/1 = exp [-(b0 +b1x1 + b2x2 + .. + bnxn)]
p = probability of occurrence
b0 bv .., ba = regression parameters
xl, x2, .., xq = independent variables

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Multiple Logistic Regression for Spillway Erosion
P = 1/1 + exp [-(b0 + b1Ka + b2qH)]
Kh =  Erosion Index, Material Resistance
qH = Maximum aH, Hydraulic Attack

Unlined Spillway Erosion Risk Assessment
Result of Multiple Logistic Regression
p = 1/1 + exp [-(1.71  3.9Kh + 3.364 qH)])]
p = probability of erosion
Kh = Erosion Index, Material Resistance
aH = Maximum aH, Hydraulic Attack

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Logistic Regression for ERDC Threshold

Unlined Spillway Erosion Risk Assessment
Logistic Regression for Annandale Threshold

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways


Unlined Spillway Erosion Risk Assessment
Independent Variables
* Hydrograph
	- Peak unit discharges (cfs/ft)
	- Flood durations (hrs)
* Spillway Geometry
	- Lengths (ft)
	- Slopes (degrees)
* Material Index
	- Erosion Indexes

Unlined Spillway Erosion Risk Assessment
Ordinal Logistic Regression
Sj = F (Material, Peak Discharge, Duration, Average_Slope, and Length)
Data: Case Histories (USDA and COE)
Damage Levels:
No Damage 0  0.05%
Lightly Damage 0.06 - 15%
Moderately Damage 16  40%
Severely Damage 41  75%
Breach 76  100%

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Ordinal Logistic Regression
Sj = 1.515 Log_Kh + 8.635 Log_q  1.581 Log_Dura + 0.807 Slope_av + 3.975 Log_Length
Probability Formulation:
No Damage = 1/ (1 + exp (Sj-k1))
Lightly Damage = 1/ (1 + exp (Sj-k2)) - 1/ (1 + exp (Sj-k1))
Moderately Damage = 1/ (1 + exp (Sj-k3)) - 1/ (1 + exp (Sj-k2))
Severely Damage = 1/ (1 + exp (Sj-k4)) - 1/ (1 + exp (Sj-k3))
Breach = 1 - 1/ (1 + exp (Sj-k4))
K1, k2, k3, and k4 = boundary parameters from regression

Unlined Spillway Erosion Risk Assessment
Input
Unit Disch. (cfs/ft) 112.1 41.8 104.4 163.5
Duration (hours) 120 570 210 3
Erosion Index1 Kh 1/ 5340 28 103 0.01
Ave. Slope (deg) 1.4 1.32 14.04 1 7.2
Length (ft) 2200 520 230 1340 155
Probability Output
No Damage 0.001 0.990 0.000 0.029 0.000
Lightly 0.019 0.009 0.002 0.275 0.000
Moderate 0.305 0.001 0.047 0.609 0.000
Severe 0.629 0.000 0.639 0.085 0.003
Breach 00.046 0.000 0.312 0.002 0.997

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Photo of the Bluestone, WV dam
Photo of the Dalles, OR dam
The Dalles, OR Q = 2,290,000 cfs Bluestone WV Q = 430,000 cfs
Erosion Index (Kh) 1960 2734
Stream Power (Kw/mw) 125.4 22.3
Probability of Erosion 0.012 0.000

Unlined Spillway Erosion Risk Assessment
Simulation Using USDA Model
* Monte Carlo
* Latin Hyper-Cube
No Damage 0  0.05%
Lightly Damage 0.06  15%
Moderate Damage 16  40%
Severely Damage 41  75%
Breach 76% - 100%

Unlined Spillway Erosion Risk Assessment
Prioritizing Process
Ranking the outcome:
Risk = P occurrence * P failure * Consequences

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

Unlined Spillway Erosion Risk Assessment
Future Research
* Erosion Index needs to be refined
* Geophysical Exploration will be useful for volcanic areas
* Effect of spillway channel geometry (curving, narrowing)
* Three dimensional erosion (side erosion)

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways