**OFFICE OF BRIDGE DEVELOPMENT**

**MANUAL ON HYDROLOGIC AND HYDRAULIC DESIGN**

**CHAPTER 11**

**EVALUATING SCOUR AT BRIDGES**

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**MAY 2003**

**Table of Contents**

11.1 Introduction 4

11.1
General\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
\...\...\...\...\.....4

2.  Qualifications of
    Personnel\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....
    \...\...\...\...\... ..4

3.  Definitions
    \...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....5

11.2 Overview of Scour Concepts and Processes
\...\...\...\...\...\...\...\...\...\...\....
\...\...\...\...\...\...\... .8

11.3 Design
Philosophy\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
\...\...\...\...\..... 11

11.4 Policy
\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....13

11.5 General Location and Design
Features\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....15

11.5.1 Location
Considerations\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...15

11.5.2 Evaluation of Alternative Location
Sites\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...16

11.6 Bridge Scour Evaluation Studies and
Reports\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....17

1.  General\...\...\...\...\...\...\.... \...\....
    \...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....17

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2.  Introduction and
    Background\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....18

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3.  Scope of
    Study\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....18

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4.  Summary and Recommendations\..... \...\...\.....
    \...\...\...\...\...\...\...\...\...\...\...\...\...\...19

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5.  Hydrology
    Study\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....
    .19

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6.  Site
    Investigation\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....22

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7.  Stream Classification, Morphology and Stability
    Study\...\...\...\...\...\...\...\...\....23

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8.  Subsurface Study of Underlying Soils and Rock\...\...\...\...
    \...\...\...\...\...\...\..... .24

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9.  Type, Size and Location of the Bridge
    (TS&L)\...\...\...\...\...\...\...\...\...\...\...\...\.....25

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10. Approach
    Roadways\...\...\...\...\...\...\...\...\...\...\...\...\...
    \...\...\...\...\...\...\...\...\...\...\...\....27

**Table of Contents**

11.6 Bridge Scour Evaluation Studies and Reports (Continued)

11. Hydraulic
    Study\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....28

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12. Scour Evaluation; Development of the Bridge Scour Cross-section.
    \....30

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13. Significance of the Scour
    Evaluation\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...33

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14. Structural and Geotechnical Design Considerations\...\...\...\....
    \...\...\...\...\....34

15. Scour
    Countermeasures\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....34

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16. Appendices\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....37

11.6.17
Documentation\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...37

11.7 Scour Assessment
Studies\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...38

11.8 Changes in Foundation Conditions Due to
Scour\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....38

9.  Scour in "Bottomless"
    Culverts\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...38

11.10 Temporary
Structures\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...38

11.11
References\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....39

Appendix A - Guidelines for Estimating Abutment Scour

Appendix B - Superceded

Appendix C - Estimating Scour in "Bottomless Culverts"

Appendix D - Scour Countermeasures For Piers And Abutments

Appendix E -Guideline for Obtaining Soil Samples In Streams and on Flood
Plains

for Evaluating Scour At Bridges

**11.1 Introduction**

**11.1.1 General**

The intent of this Chapter is to provide policies, guidelines and
methodologies for the evaluation of scour at bridges. Any evaluation of
bridge scour will involve engineering judgment and may include factors
that cannot be readily defined and standardized. This guide identifies
and comments on a number of these factors for (1) the application of
methods used to predict the magnitude of bridge scour, and (2) the
evaluation, location and design of bridge foundations. The guidance
recognizes and stresses that the primary responsibility of the Engineer
is to provide for the safety of the public.

This Chapter is based on and incorporates the recommendations and policy
guidance of various FHWA, AASHTO and ASCE manuals and guidelines. The
FHWA Manuals (References 1, 2, 7 and 15 ) shall serve as basic
guidelines for evaluation of stream stability and scour, except where
different policies and procedures of the SHA are set forth in the
Interim Manual for Hydrologic and Hydraulic Design. For Chapter 11,
these differences apply primarily to estimating scour at abutments.
However, it is the responsibility of the Engineer to assure that the
hydraulic, stream stability and scour evaluation procedures utilize
current technology and are appropriate for the site conditions under
consideration (Sections 11.1, 11.2 and 11.3).

Chapters 8 (Hydrology), 9 (Channels) ,10 (Hydraulic Design of Bridges)
and 17 (Bank Protection and Scour Countermeasures) are closely related
to this chapter and need to be considered as a unit for purposes of
defining the process of locating and designing bridges in flood plains.

**11.1.2 Qualifications of Personnel**

Personnel involved in the evaluation of scour at bridges need to possess
the technical qualifications, including practical experience, education,
and professional judgment, to perform the individual technical tasks
assigned. Interpretation of results and conclusions of scour analyses
shall be accomplished by registered engineers qualified in the
appropriate disciplines. Because of the complexity of bridge scour, it
is recommended that evaluations be performed by an interdisciplinary
team of engineers with the requisite knowledge in structural, hydraulic,
river mechanics and geotechnical engineering.

**11.1 Introduction**

**11.1.3 Definitions**

The following definitions are provided to assure uniform understanding
of some selected terms as they are used in this chapter:

**100-Year Flood** - The flood due to storm, tide or mixed population
flood event having a one-percent chance of occurring in any one year. It
serves for assessing flood hazards and meeting flood plain management
requirements. This will also be the design flood for bridge scour unless
the incipient overtopping flood is of a lesser flow magnitude.

**500-Year Flood** - The flood due to storm, tide or mixed population
flood event having a 0.2 percent chance of occurring in any one year.
This flood may also be selected for the check flood for scour unless the
incipient overtopping flood is of a lesser flow magnitude.

**Aggradation** - A long term general and progressive build-up or
raising of the longitudinal profile of the channel bed due to sediment
deposition.

**Bankfull Flow** - a discharge used to classify and evaluate stream
channel morphology and stability. The bankfull flow normally occurs in
natural, stable channels for a recurrence interval in the range of the
1.5-year flood. This recurrence interval can be considerably lower for
unstable channels. The magnitude of the bankfull flow is usually
obtained from field measurements.

**Bendway Scour** - Scour which occurs on the river bed near the outside
of the bend due to the variable velocity distribution and the resulting
secondary currents which develop across the bendway cross-section.

**Check Flood for Bridge Scour** - This flood serves for investigating
the adequacy of the bridge to remain stable and not fail during a
catastrophic flood event. This flood event represents the *ultimate
loading condition*, since a bridge failure may occur for flood flows of
a greater magnitude:

> -If the incipient overtopping flood is less than the 500-year flood,
> the overtopping flood may represent the worst case condition and serve
> as the check flood for scour,
>
> -If the incipient overtopping flood is greater than the 500-year
> flood, the 500-year flood will normally serve as the check flood for
> scour,

-If the site conditions are such that a more severe flood event is
considered to be warranted, the Engineer may use such a flood event for
the check flood for

**11.1 Introduction (11.1.3 Definitions)**

> scour. This may be the case for a crossing site where the overtopping
> flood has a low recurrence interval and creates less of a hazard than
> is caused by floods with higher recurrence intervals. The Engineer may
> also wish to investigate special conditions caused by ice or debris
> dams, flows on controlled waterways, or locations near confluences
> involving varying tailwaters.

**Contraction Scour** - Scour in a channel or on a flood plain that is
caused by a

constriction or contraction of the flow due to a naturally occurring
confinement of the river or the construction of a bridge or other
feature in the channel or flood plain. To

satisfy the law of continuity of flow, velocities in the contracted
section are higher than in the normal section; consequently, the channel
bottom is subject to higher shear stress forces and tends to scour out.
In a channel, contraction scour usually affects all or most of the
channel width.

**Degradation** - A general and progressive lowering of the longitudinal
profile of the channel bed due to long-term scour and erosion.

**Design Flood** - A flood of a specified recurrence interval used for
sizing the waterway opening for the various functional classes of
highways to satisfy the design policies and criteria of the SHA (See
Chapter 10).

**Design Flood for Bridge Scour** - serves for estimating the total
scour at the bridge and the design of the bridge foundations to resist
damage from the scour. Generally, the magnitude of the design flood for
bridge scour is the lesser of the 100-year flood or the overtopping
flood. If the site conditions are such that a more severe flood event is
considered to be warranted, the Engineer may use such a flood event for
the design flood for bridge scour. This may be the case for a crossing
site where the overtopping flood has a low recurrence interval and
creates less of a hazard than the 100-year flood. The Engineer may also
wish to investigate special conditions caused by ice or debris dams,
flows on controlled waterways, or locations near confluences involving
varying tailwaters.

**Estuary -** A tidal reach at the mouth of a stream.

**Flood Plain** - A nearly flat, alluvial lowland bordering a stream
that is subject to inundation by floods.

**Historic flood** - A recorded past flood event that is useful for
calibrating water surface profiles and evaluating the performance of
existing structures.

**11.1 Introduction (11.1.3 Definitions)**

**Limit State** - A condition in which the forces stabilizing a
structure are equally balanced by the forces tending to destabilize the
structure. This represents the ultimate loading condition for the
structure, since any increase in the destabilizing forces will cause the
structure to fail (No reserve capacity).

**Local Scour** - Scour in a channel or on a flood plain that is caused
by an obstruction to the flow such as a pier or abutment, and is
therefore localized in the immediate vicinity of the obstruction.

**Mixed Population Flood Event** - A flood event which involves the
mixing of two type of flows such as tidal flow and riverine flow, or
snow melt and rainfall runoff.

**Overtopping Flood** (Incipient overtopping flood) This flood is
determined by finding the maximum flow that will be accommodated by the
bridge without overtopping of the bridge, its approach roads or other
drainage divide. It serves for assessing risks to highway users and
damage to the bridge and its roadway approaches. The magnitude and
frequency of the overtopping flood is a function of the highway/bridge
design and must be determined from hydraulic analysis. The incipient
overtopping flood may be designated as both the design flood for bridge
scour and the check flood for bridge scour if overtopping occurs for a
flood with a recurrence interval less than the 100-year event.

**Preliminary Bridge Plans -** For the purpose of this guide,
preliminary plans are developed to the extent of depicting the type,
size and location (TS&L) of a proposed structure.

**Scour -** Erosion due to flowing water, usually considered as being
localized as opposed to general bed degradation,

**Scour Prism (total scour line) -** A 3-dimensional shape or solid
comprised of the scourable material located above the elevation of the
total scour line.

**Superflood** - A term used to denote a flood with a recurrence
interval greater than a 100-year flood that is used as the check flood
for scour.

**Thalweg -** The line or thread extending down a channel that follows
the lowest elevation of the bed.

**11.2 Overview of Scour Concepts and Processes**

Scour is the result of the erosive action of flowing water, excavating
and carrying away material from bed and banks of streams and other
waterways. Different materials scour at different rates. Loose granular
soils are rapidly eroded by flowing water, while cohesive or cemented
soils are more scour resistant. However, ultimate scour in cohesive or
cemented soils can be as deep as scour in sand bed streams. Scour will
reach its maximum depth in sand and gravel bed materials in hours;
cohesive bed materials in days; glacial tills, sandstones and shales in
months, limestones in years and dense granites in centuries.

Total scour at a highway crossing is comprised of three elements
(Reference 1):

. Aggradation and degradation or long term stream bed elevation changes:
These are long term stream bed elevation changes due to natural causes
or to development of the river, its flood plain or watershed.
Aggradation involves the deposition of material eroded from other
sections of a stream reach, whereas degradation involves the scouring or
lowering of the stream bed.

. Contraction Scour: Contraction scour in a natural channel involves the
removal of material from the bed and banks across all or most of the
channel width. This component of scour results from a contraction of the
flow resulting from a natural constriction or a constriction caused by
the bridge and its roadway approaches. Contraction scour is caused by
increased velocities and the resulting increased shear stresses on the
bed and banks.

. Local scour: Local scour involves removal of material from around
piers, abutments, spurs and embankments. It is caused by an acceleration
of flow and resulting vortices induced by the flow obstructions.

In addition to the types of scour mentioned above, naturally occurring
lateral migration of a stream may erode the approach roadway or change
the total scour by changing the flow angle of attack. Factors that
affect lateral movement also affect the stability of a bridge. These
factors include the geomorphology of a stream, location of the crossing
with respect to the stream plan form, flood characteristics,
characteristics of the bed and bank materials and land use in the
watershed basin (See Chapter 9).

Case studies of various bridges in the United States which have
collapsed as a result of scour graphically illustrate how one or more of
the elements of scour discussed above contributed to the bridge failure.
For example, the collapse of the New York State Thruway bridge over
Schoharie Creek on April 5, 1987 has been attributed directly to local
scour at the bridge piers. The failure of the U. S. Highway 51 Hatchie
River bridge on April 1, 1989, has been attributed to a combination of
(a) lateral shifting of the stream channel induced by contraction scour
and (b) local scour of bridge piers that had originally been built on
the flood plain but which became river piers due to the shift

in the stream channel.

**11.2 Overview of Scour Concepts and Processes**

Long term stream bed elevation changes may be the natural trend of the
stream or may be the result of some modification to the stream or its
watershed. The problem for the engineer is to estimate the long term bed
elevation changes that will occur over the life of the structure. This
involves assessing the present state of the stream and its watershed,
and then evaluating the probable effect of anticipated future changes in
the river system (Reference 2).

Contraction scour occurs when the shear stress created by flowing water
acting on the bed and bank material exceeds the ability of the material
to resist this force; consequently, the material moves downstream. There
are two types of contraction scour to be considered. Live bed scour
occurs when there is stream bed sediment being transported into the
contracted section from upstream. Most streams and rivers carry a
sediment load during floods, and bridges spanning such streams are
likely to be subject to live bed scour. Clear water scour occurs when
there is an insignificant amount of stream bed sediment being
transported into the contracted section. Clear water scour might be
expected to occur at relief bridges on flood plains or at constrictions
in some tidal waterways.

Bendway scour occurs primarily on the outside of bends due to the
formation of secondary currents created by the bend. For bridges located
on bends subject to contraction scour, the engineer will need to use
judgment in the distribution of contraction scour at the bridge section
to be consistent with the anticipated bendway scour (Reference 18).

Pressure flow occurs when the water surface elevation of the upstream
face of the bridge is higher than the low chord of the bridge
superstructure. This condition results in a vertical contraction of the
flow, causing the flow velocity to increase and to enter the bridge at a
downward angle to the horizontal. Both of these factors contribute to an
increase in the depth of contraction scour under the bridge.

The basic mechanism causing local scour at a pier is the formation of
vorticies at their base. The formation of these vortices results from
the pileup of water on the upstream surface and subsequent acceleration
of the flow around the nose of the pier. The action of the vortex
removes bed material from around the base of the pier. When the
transport rate of the sediment away from the pier caused by the vortex
is greater than the transport rate of sediment into the region around
the pier, a scour hole develops. As the depth of the scour hole
increases and widens, the strength of the vortices is reduced, thereby
reducing the transport rate of sediment out of the scour hole. At the
same time, the widened scour hole is able to capture a greater amount of
the bed load moving past the pier. Eventually, an equilibrium condition
is established and scouring ceases.

**11.2 Overview of Scour Concepts and Processes**

Abutment scour is perhaps the most complex and difficult aspect of scour
at a bridge to predict. It can be viewed as a combination of contraction
scour and local scour.

Of the three elements listed above which contribute to the total scour
at a bridge pier or abutment, local scour depths are usually much larger
than the other two elements.

The configuration of a scour hole at a bridge or abutment can be
expected to change with time. The scour hole initially forms and
increases in size and depth as flood flow increases at the bridge. The
scour hole generally reaches its maximum depth near the peak of the
flood hydrograph, and then partially refills during the recession of
flow from the peak. For this reason, post flood measurements of the bed
surface of the scour hole may neither reveal the maximum depth of scour
experienced at the bridge nor be indicative of the maximum threat to the
stability of the bridge. Maximum scour depths during floods are best
obtained by on-site personnel making continuous measurements or by the
installation of scour monitoring equipment that are programmed to make
continuous readings during floods.

**11.3 Design Philosophy**

There are a number of considerations involved in the design of a bridge
waterway and its roadway approaches. Each highway agency has developed
its own standards and criteria for sizing of the waterway opening and
for determining an acceptable recurrence interval of the overtopping
flood. Higher standards are set for major interstate highways and
expressways while lower standards are usually set for minor local roads
with low traffic counts. Various other factors may be considered in the
selection of the design flood for the bridge waterway area such as the
uses of the road (school bus route and access for fire and emergency
vehicles ), the character and size of the river, the proximity of other
river crossings and the detour routes that will be available for use in
the event that the roadway and bridge are inundated and closed to
traffic. Bridge owners often use some type of a formal or informal risk
assessment to organize and document the decision-making process for
selection of the design flood and overtopping flood.

A different approach is used in the design of the bridge substructure to
resist scour. All bridges should be designed to remain stable for the
worst case scour condition which can reasonably be expected to occur at
the crossing. The added cost of making a bridge less vulnerable to scour
damage is usually small in comparison to the total costs of a bridge
failure including:

-   The personal costs of the injuries or deaths caused by a bridge
    collapse,

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-   Reconstruction of the bridge,

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-   Added time and travel costs incurred by road users on detour routes
    while the bridge is being repaired or reconstructed, and

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-   Indirect economic losses to a local community or an entire region
    due to lost business opportunities.

There are many sources of uncertainty in predicting the worst case scour
condition and the depth of the resulting scour. These include model,
parameter, hydrologic and hydraulic uncertainties. The extent of these
uncertainties can vary significantly from bridge to bridge depending
upon the site conditions and the difficulties in estimating the
parameters used in the scour estimate.

Model uncertainty results from using a model or equation form that may
not be representative of the physical processes which occur in the
river. In the case of bridge scour, most of the current models have been
developed from small scale laboratory experiments which are then
extrapolated to the prototype scale. Complexities in the field cannot be
modeled entirely in the laboratory and the degree of accuracy of the
model in predicting phenomena at prototype structures is often unknown.

**11.3 Design Philosophy**

If the field conditions differ significantly from the laboratory
conditions used to calibrate and develop the scour estimating equations
or procedures, the model uncertainty is increased to the extent that it
may overshadow all other types of uncertainty.

When using scour equations developed from laboratory studies, the
engineer should consider the following questions:

-   What are the laboratory conditions used to develop the equations?

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-   Can the field conditions be represented in a way that is compatible
    with the laboratory conditions?

If the answer to the second question is no, then additional study and
evaluation of the problem is most likely warranted.

Parameter uncertainty results from an inability to accurately assess
parameters and model coefficients required in the model. For example,
for many bridges, the pier width is not a simple measurement but rather
an estimation of the size of the obstruction that the piles and pile
caps or footings create in the flow field.

Hydrologic uncertainty results from the difficulty in accurately
estimating the 100-year and 500-year flood peaks due to relatively short
gaging station records or to a lack of runoff data for the watershed.

Hydraulic uncertainty is the result of attempting to estimate the flow
depth and velocity for a specific discharge at a specific site. This
uncertainty increases when there are limited data available for use in
calibrating flood stage with discharge.

Economic uncertainty results from the large number of alternative
choices and decisions involved in the location and design of a bridge
and its roadway approaches. The Engineer is expected to achieve a design
that is safe, compatible with the river environment and cost-effective.

It is the responsibility of the engineer to use judgment in the
development and evaluation of scour estimates. This judgment should
extend to the evaluation of the interactions of the uncertainties
discussed above, along with the sensitivity of the scour estimate to
changes in these uncertainties. The basis for decisions and assumptions
relating to the predicted scour depths and the resulting foundation
design should be clearly documented in the project records.

**11.4 Policy**

The following policies apply to the preparation of scour evaluation
reports and to the design of bridge foundations to resist damage from
scour.

1.  The primary responsibility of the Engineer is to provide for the
    safety of the public (Section 11.1).

2.  Structures shall be located and designed, to the extent practicable,
    to minimize obstructions to flood flows so as to minimize the scour
    potential (Section 11.6).

3.  A scour evaluation report or a scour assessment report is to be
    prepared for every project involving a structure crossing of a
    stream. The report shall address the considerations set forth in
    Sections 11.6 or 11.7.

4.  Scour evaluations and assessments shall be performed by an
    interdisciplinary team of engineers with the requisite knowledge in
    structural, hydraulic, river mechanics and geotechnical engineering
    (Section 11.1.2).

5.  The structural, hydraulic and geotechnical aspects of foundation
    design shall be coordinated and differences resolved prior to the
    approval of the TS&L (Section 11.6.9).

6.  The FHWA Manuals (References 1, 2, 7 and 15 ) shall serve as basic
    guidelines for evaluation of stream stability and scour, except for
    the specific policies and procedures of the SHA set forth in this
    chapter and its appendices, primarily with regard to abutments.
    However, it is the responsibility of the Engineer to assure that the
    hydraulic, stream stability and scour evaluation procedures utilize
    current technology and are appropriate for the site conditions under
    consideration (Sections 11.1, 11.2 and 11.3).

7.  Structures shall be designed to be stable for worst case conditions
    for the design flood for scour and the check flood for scour
    (Section 11.6 and 11.8).

8.  Spread footings for piers on soil or erodible rock shall be located
    so that the bottom of the footing is below scour depths determined
    for the check flood for scour (Section 11.6.12).

9.  Spread footings for piers or abutments on scour resistant rock shall
    be designed and constructed to preserve the integrity of the
    supporting rock (Section 11.6.12).

10. Abutments shall be protected with riprap or other appropriate scour
    countermeasure unless an alternative design is approved by the SHA
    (Section 11.6.12).

**11.4 Policy**

11. Where practical, spread footings for abutments on soil or erodible
    rock shall be located so that the bottom of the footing is below
    scour depths determined for the check flood for scour. Engineering
    judgment must be used to determine the practicality of such designs.
    Where this objective is not achieved, the Engineer should document
    in the scour report the reasons for not placing the footing below
    the total scour line. As a minimum, for abutments, spread footings
    founded on soil or erodible rock shall be placed below the elevation
    of contraction scour and long term degradation as determined for the
    check flood for scour, and appropriate countermeasures shall be
    provided to minimize the scour potential (Section 11.6.12).

12. Deep foundations shall be designed to place the footing below the
    estimated scour depth for contraction scour and long term
    degradation. This is to minimize the potential for undermining of
    the footing and, for abutments, resultant loss of the approach
    roadway. Deep foundations for abutments on spillthrough slopes shall
    be designed to remain stable if the spillthrough slope blows out and
    leaves the piles exposed (Section 11.6.12).

13. The scour evaluation report shall include a study to evaluate the
    stability of the waterway and to assess the effect of the proposed
    highway/structure construction on the waterway (Section 11.6.7).

14. Scour Evaluation Reports shall be submitted to the SHA for review
    and approval. The final report shall be submitted both as a hard
    copy report and on a computer disk. In addition, the Hydraulics and
    Hydrology (H&H) plan sheet and summary report presented in Chapter 4
    shall be filled out and included as a part of the final submission
    (Section 11.6.17). The scour cross-section is an integral part of
    every scour evaluation report

**11.5 General Location and Design Features**

**11.5.1 Location Considerations**

Rivers are dynamic, continually changing their dimensions, patterns and
profile in response to changes in the discharge and sediment load
carried by their channels. Bridges are static and remain in a fixed
location. The task of the bridge designer is to locate and size the
structure to remain stable throughout its design service life, and to
accommodate the anticipated flood discharges, sediment loads and
accompanying changes in the river which will occur during this time.

*It is generally safer and more cost effective to avoid hydraulic
problems through the selection of favorable crossing locations than to
attempt to minimize the problems at a later time in the project
development process through design measures.*

*To the extent practicable, bridges over waterways and their approach
roadways on flood plains shall be located to provide for the desired
level of traffic service and safety and to*:

-   be consistent with flood plain management objectives and strategies,
    discouraging uneconomic, hazardous or incompatible use and
    development of flood plains,

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-   avoid changes to the bankfull flow conditions,

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-   minimize changes to existing flood flow distribution and elevations
    in the channel and on the flood plain, upstream, through and
    downstream of the bridge, particularly where such changes will
    adversely affect improved properties in the flood plain,

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-   minimize adverse changes to wetlands and other sensitive
    environmental features within the river 's flood plain,

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-   select favorable crossing sites where the hydraulic flow conditions
    and resulting scour can be analyzed and accounted for within a
    reasonable degree of confidence,

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-   avoid locations of complex or hazardous hydraulic flow conditions
    (stream confluences, alluvial fans, sharp bend sections, alignments
    with high skew angles, sections with special problems with ice
    build-up or debris accumulation, etc.) conducive to the development
    of hazardous scouring at

    bridge abutments and piers.

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-   achieve the above objectives in a cost-effective manner, giving
    appropriate attention to their importance as well as the costs of
    construction, operation, maintenance and inspection associated with
    the structure.

5.  **General Location and Design Features**

**11.5.2 Evaluation of Alternative Location Sites**

The choice of location of bridges shall be supported by analyses of
alternatives. The scope of such studies should be commensurate with the
risks involved. Studies of alternative crossing locations should include
assessments of:

-   the hydrologic, hydraulic and morphological characteristics of the
    waterway and its flood plain, including channel stability, flood
    history and in estuarine crossings, tidal ranges and cycles,

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-   the effect of the proposed bridge on flood flow patterns and the
    resulting scour potential at bridge foundations,

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-   the potential for creating new or augmenting existing flood hazards,

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-   environmental impacts on the waterway and its flood plain, and

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-   life cycle costs of each alternative.

    Experience at existing bridges in the vicinity of the project should
    be used to evaluate the reasonableness of all computations and
    conclusions regarding the proposed structure and can be helpful in
    selecting the type, size and location of the structure.

    Further guidance on procedures for evaluating the location of
    bridges and their approaches on flood plains is contained in Chapter
    9, Channels, and Chapter 10, Hydraulic Design of Bridges. Other
    references include those of the Federal Highway Administration and
    the American Association of State Highway and Transportation
    Officials (AASHTO). See References 1, 2, 3, 4, 14 and 15.

    **11.6 Bridge Scour Evaluation Studies and Reports**

    **11.6.1 General**

    Each bridge scour evaluation report should be developed to address
    the stability of the structure for the perceived worst case scour
    conditions at the site, the resulting effect on the safety of the
    traveling public and the measures to be taken to assure the
    appropriate degree of inspection and maintenance of the structure.
    The studies and information that should be addressed in scour
    reports are listed below:

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-   Introduction and background

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-   Scope of study

### Summary and Recommendations

-   Hydrology study,

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-   Site investigation,

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-   Stream classification, geomorphology, and stability study. (The
    scope of such studies are defined by the level of detail required
    and may range from simple field classifications and evaluations
    (Level 1 analyses) to detailed stream sedimentation studies (Level 3
    analyses),

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-   Subsurface investigation of underlying soils and rock, based on
    borings,

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-   Type, size and location of the bridge, including the geometry and
    dimensions of the superstructure and substructure elements and any
    proposed scour countermeasures,

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-   Line, grade and typical sections of the approach roadways,

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-   Hydraulic study, including evaluation of the consistency of the
    proposed design with the objectives of State and Federal policies
    and criteria regarding flood plain management,

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-   Scour evaluation and development of the scour cross-section under
    the bridge,

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-   Significance of scour evaluation,

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-   Structural and geotechnical design considerations,

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-   Scour countermeasures,

**11.6 Bridge Scour Evaluation Studies and Reports (11.6.1 General)**

-   Appendices, and

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-   Documentation

Each of these studies or report items, along with references of manuals
and guidelines which provide further direction and guidance, is
addressed in the following sections.

**11.6.2 Introduction and Background**

This section should provide information on the highway section under
design, the traffic it carries and other information regarding the need
for the bridge project.

**11.6.3 Scope of Study**

The accuracy of the scour evaluation for a proposed bridge site is
highly dependent upon the accuracy of the hydrologic and hydraulic data
used in the equations for estimating the scour depths.

The scope of the bridge scour studies will depend upon the degree of
accuracy to be achieved in the analysis. It is expected that water
surface profiles will be developed for the reach of the river under
study. One-dimensional hydraulic models such as the Corps of Engineers
HEC-RAS model is commonly used for this purpose (See References 10 and
12). However, sites with complex flood flow patterns may warrant the use
of a two dimensional model, such as the FESWMS model, to establish the
hydraulic flow conditions (Reference 11). For tidal flows, different
methodologies involving one and two-dimensional models are needed to
define the hydraulic flow conditions. See Chapter 10 and References 11
and 13. Where conditions warrant, such as a major structure with complex
hydraulic flow conditions, physical models can be used to more
accurately evaluate the flow and resulting scour in the bridge opening.
The selection of the study scope and the methodologies to be used in the
analysis are among the most important decisions to be made in the
preparation of a scour evaluation report.

One-dimensional models are based on the assumption that the slope of the
energy line of the flood flow at any cross-section is the same across
the entire cross-section. For many streams and rivers, this assumption
provides for a reasonable approximation of the relationship between the
flow in the river and the flow on the flood plain. Various techniques
are built into the models to account for the more complex flow patterns
which occur just upstream of a bridge crossing. One-dimensional models
serve best for streams with relatively narrow flood plains where most of
the flood flow is carried by the main channel.

**11.6 Bridge Scour Evaluation Studies and Reports 11.6.3 Scope of
Study**

Two-dimensional models are developed to account for changes in the
energy slope

longitudinally along the river and laterally between the channel and the
flood plain. A

two-dimensional network of triangles or quadrilaterals is established
and the flow is calculated into and out of each of the network elements.
The output is a plan view of the river and flood plain with vectors
depicting the direction and velocity of the flow at each of the network
elements. The water surface elevation and the magnitude of the flow is
also provided at each network element.

While the two-dimensional model provides more information for use in a
scour analysis, the cost and the time required for its application is
greater. The engineer must decide which type of model is most
appropriate for use based on traffic service and safety considerations,
the extent of the flooding hazard and the particular site conditions at
the proposed river crossing.

**11.6.4 Summary and Recommendations**

It is helpful to readers of the report to place the summary and
recommendation section near the beginning of the report to provide an
overview of the results of the scour evaluation. It should include a
drawing of the scour cross-section which depicts the horizontal and
vertical locations of the various foundation elements in relation to the
estimated scour. Proposed scour countermeasures should also be included.

**11.6.5 Hydrology Study** Appendix A presents a convenient method of
summarizing hydrologic, hydraulic and bridge scour data that is
currently being used by a state highway agency.

SHA policies and procedures regarding Hydrology Studies are addressed in
detail in Chapter 3, Policy, Chapter 8, Hydrology and in Chapter 10,
Hydraulic Design of Bridges. The following discussion provides an
overview of hydrologic considerations for bridge scour and stream
stability study reports.

The assessment of hydrologic site conditions necessarily involves many
assumptions. Key among these assumptions involves the prediction of flow
magnitudes for floods with high recurrence intervals, e.g., the 500-year
flood or other superfloods. The runoff from a given storm can be
expected to change with the seasons, immediate past weather conditions
and long-term natural changes or development of the watershed.

The ability to statistically predict such rare flood events is a
function of the adequacy of the data base of past floods, and such
predictions often change as a result of the addition of new data. The
above factors make the check flood investigation of scour an important,
but highly variable, safety criterion.

The assumptions used by the engineer to predict the magnitude and
frequency of flood

6.  **Bridge Scour Evaluation Studies and Reports 11.6.5 Hydrology
    Study**

peaks used for the design and check floods for scour must be reasonable
with respect to the data, conditions and projections available at the
time of the bridge design. These assumptions need to be identified and
documented in the hydrology study and report.

The general trend is that the magnitude of scour increases with the
magnitude of the river discharge, but this may not always be true. It is
possible that the worst-case scour condition may occur for a discharge
other than the design flood for scour or the check flood for scour. Such
conditions may be most likely to occur at confluences or in controlled
rivers with highly variable tailwater. It is the responsibility of the
Engineer to verify that the flows selected for analysis represent
worst-case conditions for scour.

The recurrence intervals, periods and elevations of storm tides used to
predict tidal scour should be correlated with the hurricane or storm
tide elevations as reported in studies by the Federal Emergency
Management Agency (FEMA), the Corps of Engineers (COE), the National
Oceanic and Atmospheric Administration (NOAA) or other agencies.

The SHA and the Maryland Department of the Environment have jointly
developed procedures for estimating peak flood flows in Maryland using
the Maryland GIS HYDRO 2000 Program. Detailed guidance on the
methodology to be used in estimating flood peaks is presented in the
publication "Application of Hydraulic Methods in Maryland dated February
1, 2001. Bankfull stages and discharges are normally obtained from field
surveys as discussed in Chapter 9, Channels

The scope of the hydrology study should be determined on the basis of
the functional highway classification, the applicable federal and state
requirements, and the flood hazards at the site. A range of flood flows
should be determined to evaluate (1) the hydraulic adequacy of the
proposed waterway opening of the bridge, (2) the effect of the bridge on
the river, (3) the effect of the river on the bridge, and (4) the worst
case conditions for designing the bridge foundations to resist scour.
The development of a flood frequency plot is recommended to depict the
magnitude of flood flows for recurrence intervals from the 1.5-year
flood (typically bankfull stage for natural rivers) to the 500-year
flood.

The riverine flood flows, storm tides or mixed population events listed
in Section 11.1.3, Definitions, should be considered for use for the
evaluation of stream stability and the design of the bridge waterway and
foundations (See Reference 4). *Particular attention*

*should be given to selecting design and check flood discharges for
mixed population flood events.* For example, flow in an estuary may
consist of both tidal flow and runoff from the upland watershed. If the
mixed population flows are both dependant on the occurrence of a major
meteorological event, such as a hurricane, the relative timing of the
individual peak flow events needs to be evaluated and considered in
selecting the design discharge. This is likely to be the case for flows
in an estuary. If the events tend

6.  **Bridge Scour Evaluation Studies and Reports 11.6.5 Hydrology
    Study**

to be independent, as might be the case for floods in a mountainous
region caused by rainfall runoff or snow melt, the Engineer should
evaluate both events independently and then consider the probability of
their occurring at the same time.

Table 1 provides an overview of the considerations normally used in the
selection of the design flood for scour and the check flood for scour.

Table 1

Criteria for Selecting the Design and Check Floods for Scour

  ----------------------- ----------------------- -----------------------
  Magnitude of the        Design flood for scour  Check flood for scour
  incipient overtopping                           
  flood (Q~ot~)                                   

  Q~ot~ \< Q~100~ \<      Q~ot~ , Q~100~ or flood Q~100~, Q~500~ or flood
  Q~500~                  selected by the         selected by the
                          Engineer\*              Engineer\*

  Q~100~ \< Q~ot~ \<      Q~100~ or flood         Q~ot~ ,Q~500~ or flood
  Q~500~                  selected by the         selected by the
                          Engineer                Engineer\*

  Q~100~ \< Q~500~ \<     Q~100~ or flood         Q~500~ or flood
  Q~ot~                   selected by the         selected by the
                          Engineer                Engineer
  ----------------------- ----------------------- -----------------------

\*Caution should be exercised when selecting the incipient overtopping
flood as the design flood for scour or the check flood for scour. It is
recommended that the Engineer evaluate the incipient overtopping flood,
the 100-year flood and the 500-year flood in the process of determining
the worst case condition. It is recognized that available technology has
not developed sufficiently to provide fully reliable scour estimates for
the case where the magnitude of the 100-year or 500-year flood exceeds
the magnitude of the incipient overtopping flood.

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.6 Site Investigation**

Table 1 provides an overview of the considerations normally used in the
selection of the design flood for scour and the check flood for scour.
It is recommended that the Engineer evaluate the overtopping flood, the
100 year flood and the 500 year flood in the process of determining the
worst case condition. It is recognized that available technology has not
developed sufficiently to provide fully reliable scour estimates for the
case where the magnitude of the design or check flood for scour exceeds
magnitude of the overtopping flood.(See 5.6 Site Investigation).

A site-specific data collection plan should be developed for the
proposed bridge crossing. Development of this plan is discussed in
detail in Chapter 6, Data Collection. The following information should
be considered in the preparation of the bridge scour report:

-   collection of aerial and/or ground survey data for appropriate
    distances upstream and downstream from the bridge for the main
    stream channel and its flood plain,

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-   estimation of roughness elements for the stream and the flood plain
    within the reach of the stream under study,

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-   subsurface information including borings, soil and/or rock samples,
    and soil and/or rock testing,

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-   factors affecting water stages, including high water from streams,
    reservoirs, detention basins, tides and flood control structures and
    operating procedures,

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-   stream confluences,

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-   existing studies and reports including those conducted in accordance
    with the provisions of the National Flood Insurance Program or other
    flood control programs,

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-   improved properties on the flood plain,

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-   available historical information on the behavior of the stream and
    the performance of existing structures during past floods, including
    observed scour, bank erosion and structural damage due to scour,
    debris or ice,

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-   information on the stream channel including the bankfull
    cross-section, slope, plan form and materials in the stream bed and
    banks,

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-   Special studies may be warranted in some cases to collect detailed
    information regard the potential effect of ice, debris or bed forms
    to affect stream stability and the extent of scour at the bridge.

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.6 Site Investigation**

Information obtained from field inspections and office reviews needs to
be carefully identified and documented so that the source data used in
the bridge scour evaluation can be identified and located at a later
date.

**11.6.7 Stream Classification, Morphology and Stability Study**

A study is needed to evaluate the stability of the waterway and to
assess the effect of the proposed highway/bridge construction on the
waterway. The results of this study should include an assessment of the
magnitude of the long term aggradation or degradation of the stream bed
at the bridge site. It should also include an assessment and projection
of the long term tendency of the stream to move laterally as a result of
meander patterns, potential cutoffs of goose neck bends, etc. ( See
References 2, 4 and 15). This information may be of prime importance in
the location and sizing of the bridge. In some instances, it may
indicate that a crossing should not be attempted for the reach of the
stream under study.

A stable stream can be defined as one with the ability, over time, to
carry discharges and sediment loads in a manner that maintains its
dimension (cross-section), pattern (plan form) and profile (slope) so
that the channel neither aggrades nor degrades. If the stream to be
crossed is stable, appropriate design measures need to be taken to
assure that the highway/bridge construction maintains the existing flow
conditions. Since minor changes in any of the above factors may cause a
stream to meander and change the lateral location of its bed and banks,
the likely future lateral movement

of the stream needs to be considered in the bridge location and design.

If the stream is unstable, appropriate measures need to be taken to
protect the structure from anticipated aggradation or degradation of the
channel bed, or from lateral movement and sloughing of its banks as a
result of the stream instability.

The Maryland SHA is developing detailed procedures for the evaluation of
stream stability, particularly with regard to the use of the Rosgen
Stream Classification System to identify stream types and their probable
responses to changes induced by highway construction. Additional
guidance on this subject is included in Chapter 9, Channels.

There are three levels of analysis that can be applied to stream
stability studies (Reference 2):

**Level 1 - Application of simple geomorphic concepts and other
qualitative analyses**

The data required for preliminary stability analysis includes maps,
aerial photographs,

6.  **Bridge Scour Evaluation Studies and Reports**

**11.6.7 Stream Classification, Morphology and Stability Study**

notes and photographs of the river from field inspections, limited
measurements of the channel and floodplain, samples of the channel bed
and banks, historic channel profile

data and information on development in the watershed and how it has been
changing over time. Level One information can be used to classify the
stream type and make qualitative judgments regarding its stability (See
also Chapter 9 and Rosgen criteria).

**Level 2 - Application of basic hydrologic, hydraulic and sediment
transport engineering concepts**

Additional information for Level Two analyses includes determination of
bankfull flow, more detailed information of the make-up of the channel
bed and banks, flow duration

curves, flood frequency plots, channel cross-sections, roughness values
for the channel, etc. The acquisition of Level Two information enables
the engineer to make quantitative calculations and estimates regarding
sediment transport rates in the channel, the probable effects of the
stream on the proposed structure, and the probable effects of the
proposed structure on the stream.

**Level 3 - Mathematical and physical model studies**

Application of mathematical and physical model studies requires the same
basic data as a Level Two analysis, but in much greater detail. This
results in a much larger scope of work.

The Engineer must determine the appropriate level of study for the river
crossing. Most bridge scour studies consist of some combination of Level
1 and Level 2 analyses. For unstable streams or flow conditions,
application of some combination of Level Two and Level Three studies may
be needed to assess the probable future changes to the plan form and
profile of the stream, and to determine countermeasures to be
incorporated in the design, or at a future time, for the safety of the
bridge and approach roadways.

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.8 Subsurface Study of Underlying Soil and Rock**

Exploration of underlying soil and rock is essential for the
geotechnical design of the bridge foundations, and subsurface
exploration studies are normally planned and carried out by the
geotechnical and bridge foundation engineers (See Reference 20). This
information is also important in predicting the extent of scour which
will occur in the channel and at the various piers and abutments. The
question of the timing of the soil exploration needs to be considered
early in the project development process, since the use of this
information for the hydraulic and scour studies is often required at an
earlier date than for the foundation design. In some cases, it may be
necessary to obtain preliminary information for the scour analysis,
followed up at a later date by final borings for the individual
foundation elements. This type of approach requires coordination with
the geotechnical engineers.

It is helpful to prepare subsurface profiles of the various soil layers
and the location of the rock for purposes of evaluating the effect of
the soils and rock on the total scour to be expected at the bridge
crossing.

**11.6.9 Type Size and Location of the Bridge(TS&L)**

It is necessary to have information about the bridge dimensions and
geometry for purposes of estimating contraction scour and local scour at
piers and abutments. However, it is desirable to conduct the scour
evaluation at an early stage in project development so that changes to
the bridge design can be more easily made, if necessary, to avoid
serious scour problems at piers or abutments. The TS&L stage of the
project development process is a desirable stage for the conduct of the
scour analysis. When complete information on the factors affecting scour
is not available, it may be appropriate to make preliminary scour
studies for purposes of establishing the significance of the scour
problem. The estimates of scour can then be refined at a later date when
more information becomes available.

*The structural, hydraulic, and geotechnical aspects of foundation
design need to be coordinated and differences resolved prior to the
approval of the TS&L.*

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.9 Type Size and Location of the Bridge(TS&L)**

In developing the bridge TS&L, consideration should be given to the
following general

design concepts to reduce the vulnerability of the bridge to scour
damage:

-   set deck elevations as high as practical for the given site
    conditions to minimize overtopping by floods, and to provide for
    freeboard for passage of ice and debris,

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-   utilize relief bridges, guide banks, dikes and other river training
    devices to reduce the turbulence and hydraulic forces acting at the
    bridge abutments,

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-   utilize continuous span designs, anchor superstructures to their
    substructures where subject to the effects of hydraulic loads,
    buoyancy, ice or debris impacts or accumulations, and provide for
    venting and draining the superstructure,

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-   locate abutments back from the channel banks to minimize problems
    with ice/debris build up, scour or channel stability, or where
    special environmental or regulatory needs must be met, e.g.,
    spanning wetlands, and

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-   where practical, limit the number of piers in the channel, using
    cylindrical piers; where cylindrical are not feasible, streamline
    pier shapes and align piers with the direction of flood flows; avoid
    pier types that collect ice and debris; locate piers beyond the
    immediate vicinity of stream banks,

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-   design piers on flood plains as river piers and locate their
    foundations at the same depth as the river piers if it is likely
    that the stream channel will shift during the life of the structure
    or that channel cutoffs are likely to occur,

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-   Where ice or debris build-up is likely to occur, their effects
    should be minimized by providing for freeboard and streamlining
    bridge elements. Furthermore, anticipated ice and debris build-up
    needs to be accounted for in determining scour depths and hydraulic
    loads.

    The stream hydrology, hydraulics and geomorphology need to be
    considered in the selection of a structure location, size and type
    that is compatible with the existing stream conditions. This
    includes consideration of the items listed below:

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-   whether the stream reach is degrading, aggrading or in equilibrium,

-   for stream crossing near confluences, the effect of the main stream
    and the tributary on the flood stages, velocities, flow
    distribution, vertical and lateral movements of the stream, and the
    effect of the foregoing conditions on the hydraulic design of the
    bridge,

    **11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.9 Type Size and Location of the Bridge(TS&L)**

-   location of a favorable stream crossing site, taking into account
    whether the stream is straight, meandering, braided or transitional;
    or use of control devices to protect the bridge from existing or
    anticipated future adverse stream conditions,

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-   the effect of any proposed channel changes,

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-   the effect of dredging, aggregate mining or other operations in the
    channel,

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-   potential changes in the rates or volumes of runoff due to land use
    changes,

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-   the anticipated effect of the structure on the existing stream plan
    form, profile and cross-section and anticipated changes to the
    structure due to future changes in the stream geomorphology.

    **11.6.10 Approach Roadways**

    It is necessary to have information about the line, grade and
    typical section of the approach roads to the bridge for the entire
    flood plain in order to evaluate the effect of the proposed
    construction on flood plain flows. Highway embankments on flood
    plains serve to redirect overbank flow, cause it to flow generally
    parallel to the embankment and return to the main channel at the
    bridge. For such cases, the highway designs should include
    countermeasures where necessary to limit damage to highway fills and
    bridge abutments. Such countermeasures may include:

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-   relief bridges,

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-   retarding the velocity of the overbank flow by promoting growth of
    trees and shrubs on the flood plain and highway embankment within
    the highway right-of-way or constructing small dikes along the
    highway embankment,

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-   slope protection of riprap or other types of countermeasures

    Where bridges are subject to overtopping, develop the roadway/bridge
    profile so that one or both roadway approaches are at a lower
    elevation than the bridge. This provides for overtopping of the
    roadway approach section(s) before overtopping of the bridge, thus
    providing relief from the hydraulic forces acting on the bridge.
    This is particularly important for streams carrying a heavy ice or
    debris load which may clog its waterway opening.

    Special hydraulic design problems may need to be dealt with on wide
    flood plains including the need for relief structures and the
    possible pocketing of flood plain waters

    at skewed crossings.

    **11.6 Bridge Scour Evaluation Studies and Reports**

    **11.6.11 Hydraulic Study**

    Agreement should be reached at an early stage of project development
    regarding the analytical models and techniques to be used and
    whether they are consistent with the required level of accuracy.
    Where appropriate, consideration should be given to the use of
    laboratory studies to determine information that cannot be obtained
    from analytical models. The hydraulic study needs to be performed to
    a sufficient degree of accuracy so as to (1) size the waterway
    opening and (2) evaluate scour at the bridge. These two aspects of
    the bridge design are interdependent and should not be considered as
    separate, independent studies. Where use is made of existing flood
    studies, their accuracy should be evaluated at an early stage in the
    design process so that any necessary additional studies can be
    carried out in a timely manner.

    Specific guidance and policies for the conduct of hydraulic studies
    are set forth in Chapter 3, Policy, Chapter 9, Channels and Chapter
    10, Hydraulic Design of Bridges.

    Special procedures are set forth in Chapter 10 to define worst case
    scour conditions for tidal waterways. Once the worst case flow
    conditions are established, the scour evaluation will normally
    follow the same procedure used for riverine channels.

    **11.6.11.1 Hydraulic Considerations for Sizing the Bridge
    Waterway**

    The design process for sizing the bridge waterway should include:

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-   the evaluation of flood flow patterns in the main channel and flood
    plain for existing conditions, and

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-   the evaluation of trial combinations of highway profiles, alignments
    and bridge lengths for consistency with the design objectives.

    Trial combinations should take the following into account:

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-   increases in flood water surface elevations caused by the bridge,

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-   changes in flood flow patterns and velocities in the channel and on
    the flood plain,

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-   location of hydraulic controls affecting flow through the structure
    or long-term stream stability,

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-   clearances between the flood water elevations and low sections of
    the superstructure (free board) to allow passage of ice and debris,

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-   need for relief structures on the flood plain,

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-   need for protection of bridge foundations and stream channel bed and
    banks, and

    **11.6 Bridge Scour Evaluation Studies and Reports**

    **11.6.11.1 Hydraulic Considerations for Sizing the Bridge
    Waterway**

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-   evaluation of capital costs and flood hazards associated with the
    candidate bridge alternatives through risk assessment or risk
    analysis procedures.

    **11.6.11.2 Hydraulic Considerations for Conducting Bridge Scour
    Evaluations**

    Once a preliminary bridge alternative and waterway opening has been
    selected, it should be evaluated for its adequacy to resist scour.
    In this evaluation, particular attention needs to be given to
    developing accurate estimates of the following hydraulic parameters
    at (1) the upstream approach section, (2) the bridge crossing
    and (3) the downstream full valley section for bridge scour studies.
    Estimated scour depths will be quite sensitive to these values:

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-   Unit flow discharges (discharge per foot) on the left overbank,
    right overbank and main channel,

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-   Flow depths and velocities for the unit flow discharges noted above,

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-   Vegetation, its critical shear strength and roughness for overbank
    areas,

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-   Particle size distribution and shear strength of soils in the
    channel bed and banks and in the overbank areas,

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-   Flow distribution and velocity distribution in the section
    immediately upstream of the bridge and in the bridge cross-section,

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-   Tailwater elevations in the downstream section,

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-   Superstructure geometry and its effect in the initiation of pressure
    scour.

### Trial combinations should take the following into account: {#trial-combinations-should-take-the-following-into-account .unnumbered}

-   increases in flood water surface elevations caused by the bridge,

-   changes in flood flow patterns and velocities in the channel and on
    the flood plain,

-   location of hydraulic controls affecting flow through the structure
    or long-term stream stability,

-   clearances between the flood water elevations and low sections of
    the superstructure (free board) to allow passage of ice and debris,

-   need for relief structures on the flood plain,

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-   need for protection of bridge foundations and stream channel bed and
    banks, and

-   evaluation of capital costs and flood hazards associated with the
    candidate bridge alternatives through risk assessment or risk
    analysis procedures.(5.11.2 Hydraulic Considerations for Conducting
    Bridge Scour Evaluations)

Once a preliminary bridge alternative and waterway opening has been
selected, it should be evaluated for its adequacy to resist scour. In
this evaluation, particular attention needs to be given to developing
accurate estimates of the following hydraulic parameters at (1) the
upstream approach section, (2) the bridge crossing and (3) the

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.11.2 Hydraulic Considerations for Conducting Bridge Scour
Evaluations**

downstream full valley section for bridge scour studies.

Every effort should be made to minimize changes to existing flood flow
patterns and elevations in the channel and on the flood plain, upstream,
through and downstream of the bridge, particularly where such changes
will adversely affect improved properties in the flood plain.

**11.6.12 Scour Evaluation; Development of the Bridge Scour
Cross-.Section**

Scour at bridge foundations is to be investigated for the two conditions
presented below (See Reference 4). Section 11.1.3 establishes the
hydrologic definitions of these flood flows.

**11.6.12.1 The design flood for scour**

The stream bed material in the scour prism above the total scour line
shall be assumed to have been removed for design conditions. For piles
and other types of deep foundations, the normal foundation design
procedures and criteria are to be followed, except that no soil support
is to be considered for soils in the scour prism above the total scour
line (See Section 11.7.0).

**11.6.12.2 The check flood for scour**

The stream bed material in the scour prism above the total scour line
shall be assumed to have been removed for this condition. For piles and
other types of deep foundations the normal foundation design procedures
and criteria are to be followed, except that:

-   no soil support is to be considered for soils in the scour prism
    above the total scour line. Deep foundations, such as piles, shall
    be designed using normal foundation design procedures, but with no
    soil support available above the total scour line.

-   excess reserve beyond that required for stability under this
    condition is not

necessary (See Section 11.7.0).

**11.6.12.3 Pier Footings**

Spread footings on soil or erodible rock shall be located so that the
bottom of footing is below scour depths determined for the check flood
for scour. Spread footings on scour-resistant rock shall be designed and
constructed to maintain the integrity of the supporting rock.

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.12.3 Pier Footings**

Deep foundations with footings should be designed to place the top of
the footing below the estimated contraction scour depth and anticipated
long term degradation to minimize obstruction to flood flows and
resulting local scour. Even lower elevations should be considered for
pile supported footings where the piles could be damaged by erosion and
corrosion from exposure to stream currents.

Where conditions dictate a need to construct the top of a footing to an
elevation higher than that described in the preceding paragraph,
attention shall be given to the added scour potential created by the
exposed footing. The additional scour depth caused by the footing may
affect the type, number and length of foundation elements, such as
piles. For a spread footing design, the additional scour depth caused by
the exposed footing may require placement of the footing to a greater
depth below the channel bed.

**11.6.12.4 Abutments**

Available technology has not developed sufficiently to provide reliable
abutment scour estimates for all hydraulic flow conditions, such as at
locations in areas of turbulence due to converging or diverging flows.
The preferred design is to place abutment footings on scour resistant
rock or on deep foundations and to protect the abutment side slopes and
end slopes with riprap or other appropriate type of scour
countermeasure. The toe or apron serves as the base for the slope
protection and must be carefully designed to accommodate scour while
maintaining the support for the slope protection (References 1,2, 7, 15,
19 and 20). Where spread footings are placed on erodible soil, it
becomes even more important to protect the abutment side slopes and end
slopes with riprap or other appropriate type of scour countermeasures.

Where practical, spread footings for abutments on soil or erodible rock
shall be located so that the bottom of the footing is below scour depths
determined for the check flood for scour. Engineering judgment must be
used to determine the practicality of such designs. Where this objective
is not achieved, the Engineer should document in the scour report the
reasons for not placing the footing below the total scour line. As a
minimum, the tops of spread footings founded on soil or erodible rock
shall be placed below the elevation of contraction scour and long term
degradation as determined for the check flood for scour, and appropriate
countermeasures shall be provided to minimize the scour potential.

Consideration should be given to designing the piles under abutments on
spillthrough

slopes so that the abutment remains stable if the spillthrough slope
blows out and leaves the piles exposed.

The SHA has developed a Computer Software Program, the latest version of
which is

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.12.4 Abutments**

ABSCOUR 6, for use in determining the extent of contraction scour and
abutment scour at a bridge. Detailed information on this program is
contained in the program help files and in Appendix A. The SHA expects
that this program will be used, along with other information to evaluate
scour at bridges and to design stable foundations.

**11.6.12.5 Other considerations**

When fenders or other pier protection systems are used, their effect on
pier scour and

collection of debris shall be taken into consideration in the design.

**11.6.12.6 Scour Evaluation Procedure**

The design flood for scour shall be determined on the basis of the
Engineer\'s judgment of the hydrologic and hydraulic flow conditions at
the site . The recommended procedure is to evaluate scour due to the
specified flood flows and to design the foundation for the event
expected to cause the deepest total scour. *The FHWA Hydraulic
Engineering Circular Number 18 (See Reference 1) is recommended as a
primary source of information and guidance for conducting a scour
evaluation*. The recommended procedure for determining the total scour
depth at bridge foundations is as follows:

-   evaluate the long-term channel profile aggradation or degradation
    over the service life of the bridge,

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-   evaluate the long-term channel plan form changes over the service
    life of the bridge,

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-   consult with the SHA on the findings of the above evaluations;
    determine jointly the need for any revised cross-sections to reflect
    anticipated long term changes,

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-   determine the combination of existing or likely future conditions
    and flood events that might be expected to result in the deepest
    scour for design conditions,

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-   determine water surface profiles for a stream reach that extends
    both upstream and downstream of the bridge site for the various
    combinations of conditions and events under consideration, and
    select the worst case condition for scour. Where the worst case
    condition is not obvious, select several cases for detailed study,

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-   determine the magnitude of contraction scour and local scour at
    piers and

    **11.6 Bridge Scour Evaluation Studies and Reports**

    **11.6.12.6 Scour Evaluation Procedure**

> abutments. This evaluation is to include consideration of the effects
> of bendway scour and pressure scour as appropriate (See Appendices A
> and B),

-   Determine the total scour which will occur at the bridge and plot
    the total scour line for both the design flood for scour and the
    check flood for scour.

    The Engineer needs to decide the most appropriate method for
    combining the various scour elements. In most cases, a reasonable
    approach will be to add the long term degradation and contraction
    scour, and then superimpose the local scour configuration to develop
    the total scour line. (See Reference 1).

    Foundation designs should be based on the total scour depths
    estimated by the above procedure, taking into account appropriate
    geotechnical safety factors. Where necessary, to minimize or avoid a
    hazard resulting from scour, bridge modifications may include:

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-   relocation of the crossing to avoid an undesirable location.

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-   relocation or redesign of piers or abutments to avoid areas of deep
    scour or overlapping scour holes from adjacent foundation elements,

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-   enlargement of the waterway area or addition of guide banks, dikes
    or other river training works to provide for smoother flow
    transitions or to control lateral movement of the channel, or

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-   installation of scour countermeasures.

    Foundations shall be designed to withstand the conditions of scour
    for the design flood and the check flood. In general, this will
    result in deep foundations. The design of the foundations of
    existing bridges that are being rehabilitated should consider
    underpinning or supplemental bents if the scour evaluation indicates
    the need. Riprap and other scour countermeasures in combination with
    monitoring may be appropriate if underpinning is not cost effective.

    **11.6.13 Significance of the Scour Evaluation**

    The bridge scour cross-section, as described in the previous
    section, should be carefully reviewed to determine if the results
    are reasonable to use for design purposes. This review is
    particularly important where the engineer has utilized computer
    software to (1) determine the hydraulic parameters for the
    worst-case scour conditions, and (2) calculate the expected scour
    depths. The following considerations are recommended for this
    review:

    **11.6 Bridge Scour Evaluation Studies and Reports**

    **11.6.13 Significance of the Scour Evaluation**

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-   Evaluate the results of the scour analysis, taking into account the
    probable accuracy of the variables in the methods used, whether or
    not the scour prediction equations are appropriate for the given
    site conditions, the available information on the behavior of the
    watercourse, and the performance of existing structures during past
    floods. Also, consider present and anticipated future flow patterns
    in the channel and its flood plain,

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-   Develop a mental picture of the existing flow patterns, how they
    will be affected by the bridge and how these changed conditions will
    affect the stability of the bridge,

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-   Modify the scour evaluation as necessary to assure that the
    evaluation is consistent and reasonable.

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-   Modify the bridge design or location as necessary to satisfy
    concerns raised by the stream stability scour analyses.

    **11.6.14 Structural and Geotechnical Design Considerations**

    Since the results of the scour evaluation will have an effect on the
    foundation design of the structure, it is important that persons
    conducting the hydraulics, structures and geotechnical studies work
    together in the conduct of their respective studies. As noted
    earlier in this guide, the structural, hydraulic and geotechnical
    aspects of foundation design need to be coordinated and differences
    resolved prior to the approval of the TS&L.

    **11.6.15 Scour Countermeasures**

    Scour countermeasures are features incorporated into the design of a
    bridge for purposes of preventing, delaying or reducing the severity
    of hydraulic problems and resulting scour. Specific guidance, policy
    and design procedures for scour countermeasures are included in
    Chapter 17, Scour Countermeasures. An overview of scour
    countermeasures is presented below:

    The recommended solutions (See References 1 and 4) for minimizing
    scour damage at new bridges include:

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-   Locating the bridge to avoid adverse flood flow patterns,

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-   Streamlining bridge elements, using features such as round piers, to
    minimize obstructions to the flow,

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-   Designing pier foundations to be stable for the worst case scour
    condition so that

    **11.6 Bridge Scour Evaluation Studies and Reports**

    **11.6.15 Scour Countermeasures**

> reliance on riprap or other similar types of scour countermeasures is
> not necessary (Section 5.12.3 ), and

-   Designing abutment foundations in accordance with Section 11.6.12.4,
    using stone riprap, or other types of materials to protect the
    highway embankment and

    channel banks adjacent to the abutment.

    For existing bridges, the alternatives available for protecting the
    bridge from scour are listed below in a rough order of cost:

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-   Monitoring scour depths and closing bridge if excessive,

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-   Providing riprap or other types of scour countermeasures at piers
    and abutments, when combined with monitoring,

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-   Constructing guide banks (spur dikes) to streamline the flow and
    reduce scour at the abutments,

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-   Constructing channel improvements,

-   Strengthening the bridge foundations,

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-   Constructing sill or drop structures to control degradation and
    scour, and

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-   Constructing relief bridges or lengthening existing bridges

    A number of considerations may affect the selection of an
    appropriate scour countermeasure including:

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-   The type of scour problem to be addressed, and the assessment of the
    risk involved,

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-   The experience in the area or region with the success or failure of
    various types of scour countermeasures,

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-   The design criteria and specifications of the bridge owner and the
    ability of the contractor to construct with available materials the
    installation envisioned by the designer, and

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-   The availability and cost of alternative scour countermeasures.

    These factors warrant careful consideration by the engineer during
    the design and

    **11.6 Bridge Scour Evaluation Studies and Reports**

    **11.6.15 Scour Countermeasures**

    installation of the countermeasure. Periodic inspection and
    evaluation of such

    countermeasures are necessary to assure that the installation is
    intact and is able to

    perform its function of scour protection.

    There are currently a considerable number of types of scour
    protection that can be used in place of riprap including sheet
    piling, grout bags, gabions, articulated revetment

    units, concrete linings, and various other types of commercially
    made materials Guidance on the selection, design and installation of
    various types of scour countermeasures and filter materials is
    included in the Federal Highway Administration Publications (See
    References 1,2, 3, 7, 15, 16, 17, 18, 19 and 21).

    Where available, stone riprap is often used as a scour
    countermeasure at bridges. The equations in Reference 1 are
    presently recommended for the selection of an appropriate riprap
    size for protecting bridge piers and abutments, since they are based
    on the Ishbash criteria for incipient motion of particles. Equations
    which utilize the Shields parameter, for example those contained in
    Reference 19 for design of bank protection, are dependent on the
    designer's selection of the hydraulic roughness factor and the
    Shields parameter which can range from 0.040 to 0.15. Therefore,
    engineers have been more cautious about general application of this
    type of design approach to all conditions for bank stabilization. A
    number of researchers have developed riprap design equations and the
    results obtained from these equations can vary widely with regard to
    the recommended riprap size. There is a need for a research study to
    make an objective evaluation of the existing procedures for sizing
    of riprap and to recommend an approach for standardizing these
    equations to the extent practicable.

    A recurring question with regard to the use of sheet piling is
    whether it should be pulled or cut off below ground elevation and
    left in place. This decision requires the exercise of engineering
    judgment with regard to the particular site conditions and the
    following considerations:

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-   pulling the sheet piling may permit the contractor to get salvage
    value or possible reuse at another site; however, the process of
    pulling the sheet piling tends to disturb the ground around the
    bridge foundation element and could accelerate local scour.

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-   Sheet piling left in place could be exposed by contraction scour and
    general degradation, thereby effectively increasing the width of the
    foundation element. This condition may cause additional local scour.

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-   Sheet piling left in place and backfilled and protected with an
    erosion resistant material may serve as an effective countermeasure,
    particularly if it is not exposed much above the stream bed.

**11.6 Bridge Scour Evaluation Studies and Reports**

**11.6.15 Scour Countermeasures**

There are available a number of types of natural or bio-engineering
materials which, if properly designed and installed, can serve
effectively as bank protection materials. These include such features as
willow planting, root wads and other vegetative type treatments.
Installation of natural materials requires special experience and
training, and it may be prudent to use a design-build type of contract
for these materials.

These natural or environmental treatments are not recommended for use at
locations involving the integrity of the highway/ bridge or the safety
of the persons using the highway. However, they can serve effectively in
other less critical locations where a failure will not endanger lives or
developed properties.

**11.6.16 Appendices**

Appendices should include input to and output from computer models,
plots of the location of subsurface borings along with the results of
tests and measurements made of the soil and rock, scour computations,
etc.

**11.6.17 Documentation**

Project documentation involves a system for keeping track of all
significant field data, assumptions, the computer models used, input to
and output from the computer models and other information on which the
scour estimates are based. It is desirable to keep this information on
file as hard copy as well as on computer storage disks.

The SHA has developed a Hydraulics and Hydrology ( H&H ) Sheet and
Report which is to be completed as a part of the hydrology, hydraulic
and scour evaluation for bridge structures. The report is presented in
Chapter 4, Documentation. The H&H Sheet is to be included as a part of
the permanent bridge plans for each bridge design.

**11.7 Scour Assessment Studies**

In some instances, the interdisciplinary scour team may determine that a
full scour evaluation report is not needed to assure that a structure
can be designed and constructed in a manner that meets the intent of
this guideline. An example of this type of situation might include
replacement of a superstructure on an existing foundation over a stable
stream when the existing piers and abutments are founded on erosion
resistant rock and the footings are located below the channel thalweg.
For such situations, a scour assessment report may be prepared which
generally addresses the topics listed in Section 11.6 and provides the
judgmental factors which serve to support the adequacy of the proposed
design. If the scour assessment report cannot provide a reasonable basis
on which to support the proposed design, then a more detailed scour
evaluation report should be undertaken.

8.  **Changes in Foundation Conditions Due to Scour**

Scour is not a force effect, but by changing the conditions of the
substructure it may have a significant effect in altering the force
effects acting on structures. The AASHTO Standard Specifications set
forth detailed requirements for applying loads and load factors to
bridge foundations (Reference 4). The consequences of changes in
foundation conditions resulting from the design flood for scour shall be
considered. Structures will be designed under this provision using
normal design considerations and factors of safety selected by the
foundation engineer. The assumption is made that all material in the
scour prism has been removed and is unavailable for foundation support.

The effect of the check flood for scour is to be considered with respect
to the stability of structures over water. In the evaluation of this
condition, the assumption is made that all material in the scour prism
has been removed and is unavailable for foundation support. The
structure is to remain stable for this condition, but is not required to
have any reserve capacity to resist loads.

**11.9 Scour In "Bottomless" Culverts**

Refer to Appendix C for policy and guidance on the design of culverts on
footings.

**11.10 Temporary Structures**

Temporary structures for the Contractor's use or for accommodating
traffic during construction shall be designed with regard to the safety
of the traveling public and adjacent property owners as well as
minimization of impacts to the stream channel and its flood plain (See
Reference 4). The SHA may permit revised design requirements consistent
with the intended service period for and flood hazard posed by the
temporary

structure. Contract documents for the temporary structure shall
delineate the respective responsibilities and risks to be assumed by the
SHA and the contractor.

**11.11 References**

1\. Federal Highway Administration, Hydraulic Engineering Circular
Number18, Evaluating Scour at Bridges, Fourth Edition, May 2001.

2\. Federal Highway Administration, Hydraulic Engineering Circular
Number 20, Stream Stability at Highway Structures, Third Edition, March
2001.

3\. AASHTO Model Drainage Manual, 1998.

4\. AASHTO Standard Specifications for Highway Bridges, Sixteenth
Edition, 1996 and including all Interim Revisions through 2002.

5\. U. S. Geological Survey, National Flood Frequency Manual and
Software Program, 1994.

6\. Federal Highway Administration Memorandum from Chief, Bridge
Division, to Regional Federal Highway Administrators dated July 19, 1991
on the subject of Scourability of Rock Formations.

7\. Federal Highway Administration HEC-23, \"Bridge Scour and Stream
Instability Countermeasures\", Second Edition, March 2001.

8\. Federal Highway Administration Recording and Coding Guide for the
Structure Inventory and Appraisal of the Nation\'s Bridges, 1995.

9 U.S. Code of Federal Regulations, 23 CFR 650, Subpart A.

10\. Corps of Engineers HEC-RAS (River Analysis System) Users Manual,
Version 3.0.1, March 2001

11. Froehlich, D. C., Finite Element Surface-Water Modeling System,
    FESWMS-2DH, Version 2 Users Manual, FHWA Research Report, 1996. (See
    also Reference 12 below)

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12. Surface Water Modeling System (SMS (Reference Manual, Brigham Young
    University, Version 3.1.5, 2003

13\. Federal Highway Administration, Users Manual for WSPRO, \"A
Computer Model for Water Surface Profile Computations, October 1997.

14\. Corps of Engineers Hydraulic Engineering Center, UNET\-- One
Dimensional Unsteady Flow Through a Full Network of Open Channels,
User\'s Manual,

> CPD-66, Version 3.1, 1996.

11. **REFERENCES**

16\. FHWA, Highways in the River Environment. February 1990.

17\. Neill, C. R., Guide to Bridge Hydraulics, Roads and Transportation
Association of Canada, 2001 Edition.

18\. AASHTO Highway Drainage Guidelines, 1998.

19\. Maynord, Steven T., Toe Scour Estimation in Stabilized Bendways,
Technical Note, Journal of Hydraulic Engineering, August 1996.

20\. Federal Highway Administration, HEC-11, Design of Riprap Revetment,
March, 1989.

21\. AASHTO Manual on Subsurface Investigations, 1988.

22 FHWA Publication No. HI-95-038, Geosynthetic Design and Construction
Guidelines, May, 1995.

23\. Tidal Hydraulic Modeling for Bridges, Users Manual, Zevenbergen et.
al. (Ayres Associates), December, 1997.

24. Development of Hydraulic Computer Models to Analyze Tidal and
    Coastal Stream Hydraulic Conditions at Highway Structures, Report
    prepared for the South Carolina DOT, Zevenbergen et. al., December
    1997.

25. Applied River Morphology, David Rosgen, Wildland Hydrology, Pagosa
    Springs, Colorado, 1996.

26\. Stream Channel Reference Sites, An Illustrated Guide to Field
Techniques, United States Department of Agriculture, Forest Service,
General Technical Report RM-245, April, 1994.