**OFFICE OF BRIDGE DEVELOPMENT**

**BRIDGE SCOUR PROGRAM**

**CHAPTER 11 APPENDIX A**

**PART 2**

**GUIDELINES FOR APPLYING THE ABSCOUR PROGRAM**

**ABSCOUR**

**USERS MANUAL**

![](media/image1.jpeg){width="6.35in" height="3.25in"}

**OCTOBER 2005**

##### **PART 2: GUIDELINES FOR APPLYING THE ABSCOUR PROGRAM**

# [I. Introduction]{.underline}

Available technology has not developed sufficiently to provide reliable
scour estimates at abutments for all possible site conditions. The
policies and guidance in the abutment scour program ABSCOUR and in
Chapter 11 of the SHA Manual of Hydraulic and Hydrologic Design have
been developed with this consideration in mind.

The ABSCOUR program provides for considerable flexibility in the input
format and the computations to permit the user to model field
conditions. However, the user should make a critical review of all scour
computations, using ABSCOUR for sensitivity analyses of input factors,
to evaluate whether the answers obtained are reasonable. Part 2 guide
has been written to assist the user in this evaluation.

> **The user assumes all responsibility for any decisions or actions
> taken as a result of the use of this program.**

Please note that definitions used and references cited refer back to the
text in Part 1.

The discussion on abutment scour in the FHWA HEC-18 Manual (Reference 1)
explains why the early abutment scour equations developed from
laboratory flume studies are generally not reliable for predicting scour
at abutments. The essence of this discussion is that a rectangular flume
with a constant depth and velocity of flow across the width of the flume
does not accurately model the field conditions of a channel and its
flood plain; consequently, the equations developed from these lab
studies generally predict conservative estimates of scour.

In the last several years, various researchers have begun to model
"compound channels" to reproduce more accurately the field conditions of
a channel and its flood plain. Information from these studies has been
used to develop the ABSCOUR software program. The background on the
development of the logic and the equations used in the ABSCOUR analysis
is presented in Part 1 of this Appendix. The Engineer is encouraged to
read and understand this information as well as the information in Part
2, Users Guide, before using the ABSCOUR computer program.

The ABSCOUR program is an expanded application of Dr. Emmett Laursen's
live bed contraction scour equation as presented in the FHWA HEC-18
Manual, with certain modifications developed to account for the
distribution of flow under the bridge, the bridge geometry and the
computation of velocity at the bridge abutments. The ABSCOUR program
computes both clear water and live bed scour and selects the appropriate
scour type based on the input information. Careful application of the
ABSCOUR Program will provide the user with insight into the factors
affecting contraction and abutment scour at the bridge site under
evaluation. Judgment is needed to modify input information and the
ABSCOUR cross-sections so as to best represent actual site conditions
during a flood event. *Computed scour depths provided by the ABSCOUR
Program require evaluation to determine if the results are reasonable.*

*Abutment scour can be viewed as a combination of contraction scour and
local scour. The ABSCOUR Program computes the [total]{.underline} scour
at the abutment; therefore, the user should not add contraction scour to
this value as is done in the HEC-18 scour evaluation*

*procedure*.

The following information is needed to provide the input information for
the ABSCOUR program:

1.  HEC-RAS run for the given hydraulic conditions including:

```{=html}
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```
1.  topographic map of the stream and its flood plain, the location of
    the bridge crossing and stream channel cross-sections,

2.  stream channel cross-sections,

3.  geometric and hydraulic data tables,

4.  reliable bridge tailwater elevations,

5.  selection of appropriate approach section and flow distribution, and

6.  appropriate flow distribution at bridge with regard to channel,
    flood plain and overtopping flows.

```{=html}
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```
2.  Bridge Plans

```{=html}
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```
2.  Surface and subsurface information on channel bed load, flood plain
    soils, borings, etc.

# [II. DEVELOPMENT of the Input Data for the ABSCOUR (Abutment Scour) Model]{.underline}

SHA has been conducting and reviewing ABSCOUR analyses for a number of
years. It is our experience that one of the biggest sources of error in
scour computations is an incorrect hydraulic model. It is not an easy
task to model a 3-D flow pattern with a 1-D model such as HEC-RAS. In
particular, special care needs to be given to the following three
primary sources of error in developing the input data:

-   [Water surface elevation under bridge]{.underline}. The hydraulic
    model should include a sufficient reach downstream of the bridge to
    establish a reliable tailwater elevation at the downstream side of
    the bridge. Guidance on the required length of the downstream reach
    is provided in Chapter 4 of the H&H Manual.

-   [Flow distribution for overtopping flow]{.underline}. The Engineer
    needs to develop a rational flow distribution to account for the
    flow through the bridge and the flow over the bridge and approach
    roads. A trial and error approach to the HEC-RAS runs is often used
    to obtain a balanced flow condition.

-   [Approach section]{.underline}. Selection of a cross-section and of
    hydraulic flow parameters that are representative of the flow
    distribution in the approach section is essential to the scour
    evaluation. (See Step 3 below).

The guidance below, provided in a step-by-step format, is offered to
assist the user in applying the ABSCOUR Program to a specific bridge
site. The user is referred to Part 1 for a discussion of definitions and
the derivation of equations used for scour calculations.

*Help Options*

There are two sources of help. Short help is available for most input
cells by placing the cursor on the cell and pressing the F-1 key. More
detailed help is available from the HELP tab on the Menu Bar at the top
of the ABSCOUR screen. It is a good idea to use the short help (F-1 key)
to check the text and sketches for clarification of the information to
be provided in the cell.

## [A. STEP ONE - HYDRAULIC MODEL]{.underline} 

Prior to entering data for the ABSCOUR Model, the user will need to
obtain hydraulic data as discussed below:

### *A.1 Water Surface Profile*

*Prepare a water surface profile using HEC-RAS or other program to model
flow conditions upstream of, through and downstream of the bridge.
Discharges selected for evaluation of scour should include the
overtopping flow, Q-100 and Q-500 in order to develop the anticipated
worst case scour conditions at the bridge.*

Field check Manning's "n" values to obtain the proper flow distribution
between the channel and flood plains. Use sufficient downstream
cross-sections to establish reliable bridge tailwater elevations.

### *A.2 Development of ABSCOUR Model Cross-sections*

Two cross-sections are required to run the ABSCOUR program:

-   **Section 1: Upstream approach section**. This section should be
    upstream of the area of influence of the bridge contraction and
    should be representative of the approach flow conditions. In some
    cases, the user may need to modify the actual approach section so
    that it is representative of actual approach flow conditions. (See
    Step 3).

1.  **Section 2: Downstream** **Bridge Section.** This section is
    located under the bridge *at the downstream end*.

## [B. STEP TWO -PROJECT INFORMATION MENU]{.underline}

Figure 2-1 shows the ABSCOUR Project Information Menu screen. The
following section explains each of the input parameters.

![](media/image2.png){width="6.278472222222222in"
height="3.908333333333333in"}

**Figure 2-1: ABSCOUR Project Information Screen**

### *B.1 Project Name and Description* 

Use this input to provide information on the project, bridge number,
magnitude and frequency of the flood being evaluated, special conditions
used in the analysis, etc. Since the user may make several ABSCOUR runs,
this section can be used to detail the flood-frequency and magnitude,
and any special conditions or modifications used in the analysis. This
approach will help to clearly delineate and identify the various runs.

### *B.2 Over-Rides* 

The ABSCOUR Program contains various over-ride features to allow the
user flexibility in making the scour evaluation. The user is cautioned
to use the over-ride features only after giving full consideration to
the consequences of this approach. *Problems with the program output or
with unrealistic scour estimates can often be traced to improper use of
the over-ride functions*. We recommend that none of these features be
used on the initial run. They are provided primarily to assist in the
evaluation of a bridge with special problems or flow conditions. We
suggest that users contact the Office of Bridge Development for guidance
on using the over-ride functions. The common overrides include:

2.  **Critical and Boundary Shear Stress:** For use where these values
    have been measured and determined to be reasonable.

-   **Live Bed/Clear Water:** Use to change the determination made by
    ABSCOUR regarding the scour condition - live bed scour or clear
    water scour. A common use for the override is made for the condition
    on flood plains where there are low flow velocities and depths
    coupled with heavy vegetative cover, and a clear water scour
    condition is considered reasonable.

-   **Bridge Section Unit Flow Values:** This over-ride can be useful in
    conducting sensitivity analyses of complex flow patterns. For
    example, consideration of higher unit flow values on the outside of
    a bend.

-   **Bridge Section Critical Velocity:** This over-ride should be
    helpful in evaluating the characteristics of the critical velocity
    for cohesive soils.

-   **Sediment transport parameter:** Not recommended for use unless the
    engineer has specialized knowledge of the sediment transport
    characteristics of the stream.

-   **Two-**Dimensional Flow. For studies utilizing 2-D flow models, the
    user can input directly, the velocity of flow measured at the
    abutment face.

*B-3 Other Options*

-   **Clear water scour method:** The SHA has experienced reasonable
    results in the use of Neill's equation for evaluating clear water
    scour. In general, Laursen's equations result in much deeper scour
    estimates for very fine grained, non-cohesive bed material in
    channels. The user is encouraged to compare both methods.

-   **Unit Option:** The user can choose between Metric and English
    units.

## [C. STEP THREE - APPROACH SECTION]{.underline}

Figure 2-2 shows the input screen for the Approach Section. In order to
enter the data for this sheet, the actual cross-section must be
converted to the ABSCOUR model cross-section for the sub-areas of the
left overbank, main channel and right overbank. Refer to Figure 2-3 for
a definition sketch of the conversion from the actual cross section to
the ABSCOUR cross section. The User has the option of superimposing the
actual cross-section on the ABSCOUR cross-section for comparison
purposes by using the importing function of ABSCOUR. Represent each
sub-area as a rectangle having a width and average depth. Obtain the top
width (T) and flow area (A) of each sub area from the HEC-RAS Program.
Be careful not to include ineffective flow areas. Compute the average
depth of flow or hydraulic depth for each sub-area as y~ave~ = A/T.

The model assumes an ideal one-dimensional flow pattern with a straight
channel. The occurrence of a bend would affect the flow distribution in
the reach of the stream under study. Refer to the discussion included
under [Upstream Bridge Data]{.underline} for ideas on how to modify flow
distributions to account for 2-D flow patterns in the reach of the
stream upstream of the bridge.

The ABSCOUR program uses Laursen's live bed contraction scour equation
to determine scour. This equation serves to compare the unit discharges
and scour in the approach section and in the contracted (bridge)
section, assuming similar bed materials and hydraulic conditions. The
best results will be obtained by selecting an approach section where the
flow pattern and bed condition is similar to the bridge section, keeping
the following considerations in mind:

1.  The approach section should be in a relatively straight reach and be
    representative of the upstream channel and flood plain. (If the
    bridge is in a bend, the approach section may be selected in an
    upstream bend with a similar configuration).

2.  The cross-section should be perpendicular to the stream tube lines.

3.  The approach section should be near the bridge, but far enough
    upstream to be out of the influence of the bridge contraction.

4.  If upstream conditions are complex, select the approach section one
    bridge length upstream and reevaluate the ineffective flow areas in
    the analysis. Refer also to the discussion under Upstream Bridge
    Data for ideas on complex flow patterns.

![](media/image3.png){width="4.377083333333333in"
height="2.5722222222222224in"}

**Figure 2-2: ABSCOUR Approach Section input sheet**

![](media/image4.wmf)

**Figure 2-3: Definition Sketch for ABSCOUR Approach Section (Looking
Downstream)**

*(Please note that W and T may be used interchangeably in figures and
equations to designate a channel or floodplain width)*

### *C.1 Enter Approach Section Data*

The water surface profile models compute flow velocities, depths and
discharges for the approach section on the basis of conveyance
calculations. Modify these values as necessary to fit the ABSCOUR
cross-sections as discussed above.

Verify that values used for y (depth), V (velocity), T (top width), q
(discharge per foot of width) and Q (discharge) are consistent (q =
V\*y; Q= q\*T).

As a general rule, information on each channel and overbank subsection
is readily available from the output tables of the water surface profile
model. For example, HEC-RAS computes the area of each subsection as the
top width times the hydraulic depth. With a known area, hydraulic depth,
and discharge provided for each subsection of the approach
cross-section, the user can readily obtain the velocity and unit
discharge values needed for the program.

3.  **Approach Section Water Surface Elevation:** This elevation is used
    as a datum for importing the HEC-RAS cross-section for the approach
    section. It is a good idea to compare the ABSCOUR and HEC-RAS
    cross-sections.

-   **Discharge, Q:** Enter the approach section discharge for the left
    overbank, channel and right overbank in cfs or cms.

-   **Flow Top Width, W:** From HEC-RAS, obtain the flow top width for
    the left overbank, channel and right overbank. Be careful not to
    include ineffective areas in the top width computations.

-   **Average Flow Depth (Hydraulic Depth):** From HEC-RAS, obtain the
    hydraulic depth for the left overbank, channel and right overbank.
    Be careful to adjust the hydraulic depth to account for any
    ineffective flow areas.

-   **Median Bed Grain Size, D50:** Determine the D50 median grain size
    for material on the overbank areas and in the channel from field
    samples taken at the approach section. (Guidance on collecting
    samples and measuring D50 is provided in Appendix E of Chapter 11).

**Average Bank Slope, Z:** Enter the average bank slope of the stream in
the vicinity of the bridge. . The program uses this information in
evaluating scour when the abutment is close to the channel bank. The
average bank slope (Z) of the left side of the channel is the horizontal
projection of the slope when vertical is 1. The slope is used to adjust
the ground line between the channel and the flood plain. The adjustment
modifies the idealized ABSCOUR rectangular sections in order to model a
more reasonable geometry for the bank condition. This adjustment
provides for a better prediction of the abutment scour depth for
abutments with short setbacks. as explained in Attachment 1

The bank slope also determines the relative effect of the channel scour
on scour at the abutment for abutments with short setbacks. Steeper
slopes such as 1:1 will reduce the effect of channel scour whereas
flatter slopes such as 4:1 will increase the effect of channel scour.
The bank slope can be used as a variable in sensitivity analyses of
factors affecting abutment scour. See Contraction Scour, Adjustment for
Short Setback Abutment (Case A).

-   **Average Energy Slope:** This value is used in computing the
    boundary shear stress. Enter the average energy slope of the flow in
    the stream reach between the approach section (1) and the downstream
    bridge section (2). Refer to Figure 2-4 for details. The average
    energy slope is computed as:

> S~ave~ = (Energy Line Elevation Section 1 - Energy Line Elevation
> Section 2)/ L
>
> where L = distance between Sections 1 and 2.
>
> Please note that alternative methods may be more appropriate for some
> flow conditions, especially for backwater conditions. The computed
> value should be compared with information obtained from the HEC-RAS
> runs.

![](media/image5.wmf)

> **Figure 2-4: Average Energy Slope**

-   **Scour Parameter Button:** Click on the scour parameter button to
    view ABSCOUR scour parameters computed from the approach flow
    conditions. Refer to Figure 2-5. As noted earlier, over-riding any
    of these values should be undertaken with caution and an
    understanding of the flow and sediment transport conditions. For
    example, if the computations indicate live bed scour on the flood
    plain and the flood plain is covered with heavy vegetation with
    attendant low velocities, it is likely that clear water scour will
    actually occur on the flood plain. The scour parameter can be
    over-ridden to indicate clear water scour for the flood plain
    approach flow.

![](media/image6.png){width="6.379861111111111in"
height="3.9965277777777777in"}

**Figure 2-5: Scour Parameter Table**

Please be aware that the sediment transport parameter, k~2~, represents
a complex function. The Level 2 analyses provided by HEC-RAS and ABSCOUR
offer a reasonable approach for estimating this function. However, the
water surface profile and hydraulic variables are assumed to be fixed
for the HEC-RAS/ABSCOUR analysis, remaining constant for changes in the
particle size of the bed load. This limitation can be minimized by
making small changes to the HEC-RAS runs to account for varying 'n'
values, but such refinement is normally unnecessary. However, we have
observed an unusual and special condition for live bed scour while
running sensitivity checks. For certain combinations of hydraulic flow
conditions, a slight increase in the D50 particle size will result in an
[increase]{.underline} in the scour depth. This result, of course is the
opposite of what we would expect. The anomaly is typically small and can
be modified by the user to obtain a reasonable answer. The user has the
option of overriding the calculated values and substituting other values
for the critical shear stress and boundary shear stress. A first step in
the evaluation of these parameters would be to refine the boundary shear
stress as calculated by ABSCOUR (τ~1~ = γ RS~ave~) at the approach
section by obtaining more detailed information about the flow in the
channel reach between Section 1 and Section 2.

## [D. STEP FOUR - DOWNSTREAM BRIDGE DATA]{.underline}

### *D.1 Enter the Downstream Bridge Data*

Figure 2-6 shows the ABSCOUR input screen for the downstream bridge
data. Figure 2-7 shows the definition sketch for the downstream bridge
data. Please note that the program is set up to input the flow estimates
under the bridge as computed by the HEC-RAS model. However, the user has
the option of using the over-ride cells to select a different flow
distribution where there is a question regarding the HEC-RAS
distribution which is based on conveyance calculations. Examples include
a bridge on a bend where the user may expect a larger portion of the
flow to move to the outside of the bend, a complex overtopping situation
or an upstream confluence. See also the discussion on balancing the
flows regarding the Upstream Bridge Data Card.

![](media/image7.png){width="5.616666666666666in"
height="3.8041666666666667in"}

**Figure 2-6: Downstream Bridge Input Screen.**

![](media/image8.wmf)

**Figure 2-7: Definition sketch for Bridge Section**

*(Please note that W and T are used interchangeably in figures and
equations to designate a channel or floodplain width)*

-   **Downstream water surface elevation under bridge:** Enter the
    information from the hydraulic model. Check that there are enough
    downstream cross-sections to provide for a reliable estimate of the
    tailwater elevation. Please note that the measurement is to be made
    at the downstream side of the bridge and on the inside of the
    bridge. For pressure flow conditions, enter the water surface
    elevation immediately downstream from the bridge.

> The downstream water surface elevation serves as the datum for all
> ABSCOUR computations.

-   **Scour Definition Sketch Button:** This button provides a quick
    reference to scour terms when using the program.

-   **Waterway Area** *(**Measured normal to the flow):***
    [Measure]{.underline} the waterway area bounded by the water surface
    elevation and the channel cross-section for the right overbank
    section, channel section and left overbank section. (Typically, this
    information cannot be directly obtained from the HEC-RAS Tables. The
    bridge plans or the HEC-RAS cross-sections provide good information
    for use in measuring the waterway area). Please note that for
    pressure flow conditions where the water elevation is above the low
    chord, the top of the waterway area will be defined by the low
    chord.

-   **Top Width, W or T,** *(**Measured normal to the flow):*** Measure
    the top width for the channel and the right and left overbank areas
    under the bridge. Judgment needs to be applied in obtaining this
    information. In some cases, the left and right overbank top widths
    may be very small, and it may be more reasonable to model the
    channel so as to incorporate these small overbank areas as a part of
    the main channel. If there is a pier within the limits of the
    ABSCOUR cross-section, the top width should be adjusted to subtract
    the pier width.

> The program will compute the hydraulic depth for each downstream
> sub-area (left overbank, channel and right overbank) as y =A/T.

-   **Low Chord Elevation:** Enter the average low chord (lowest
    superstructure element) elevation at the downstream side of the
    bridge for the left overbank section, right overbank section and
    channel section. Refer to Figure 2-8.

![](media/image9.wmf)

**Figure 2-8: Average Low Chord Elevation**

-   **Abutment Type:** Select the abutment type (Vertical Wall,
    Wing-wall or Spill-through Slope)

-   **Setback:** Setback is the horizontal distance measured from the
    channel bank or edge of channel to the abutment:

-   For a vertical wall or a wing wall abutment, measure the setback
    from the channel bank to the face of the abutment.

1.  For a bottomless arch culvert, measure the setback from the channel
    bank to the culvert wall

2.  For an abutment on a spill through slope, measure the setback from
    the channel bank to the point where the ground line intersects the
    spill-through slope. If the ABSCOUR cross-section is above the
    existing ground, use the ABSCOUR cross-section to define the ground
    line. If the ABSCOUR cross-section is below the existing ground, use
    the existing ground to define the ground line.

3.  If there is a pier on the over-bank section, the pier width should
    not be included in the top width value T. This may result in a
    condition that the top width as measured from the channel edge will
    not extend to the abutment, and abutment scour will be computed as
    zero. For this case, the setback distance needs to be adjusted to
    equal the top width, T.

4.  If the abutment projects into the channel beyond the channel bank,
    enter the setback as a negative number.

![](media/image10.png){width="6.09375in" height="2.1354166666666665in"}

**Figure 2-9: Illustration of Setback**

-   **Median particle size:** This value is important for clear water
    scour and should represent the particle size at the
    [bottom]{.underline} of the scour hole. This is typically a trial
    and error process to balance particle size with depth of scour. This
    selection of particle size is often a judgment call due to the lack
    of good soils data at a distance of 5, 10 or 15 feet below the
    channel bed. A conservative approach is recommended where there is
    limited data on which to select a particle size.

> A complicating factor in selecting a representative particle size for
> clear water scour is the potential for armoring of the channel bed. A
> discussion of this consideration is presented in Part 1 of Appendix A.
> However, a comprehensive treatment of the armoring of channel beds is
> beyond the scope of this guide, and the user is referenced to the FHWA
> publication HDS 6, River Engineering for Highway Encroachments or
> similar texts on river mechanics to evaluate this condition. *In
> general, great reliance should not be placed on the expectation that
> armoring of the bed will limit the extent of contraction scour.*
>
> For particle sizes of about 0.1 mm or less, soil behaves more like a
> cohesive material and the assumption of a cohesionless bed material
> used in the ABSCOUR computations becomes less valid. For silt and clay
> soils, the User is referred to the discussion in Attachment 4. When a
> critical velocity of such soils can be determined, the override
> function of ABSCOUR can be used to calculate scour based on the
> critical velocity of the soil.

-   **Estimated long term degradation/aggradation:** The Engineer will
    need to make an evaluation of the stream morphology and utilize
    available information to determine a best estimate of future
    conditions. If this value is significant, it may be appropriate to
    modify the channel profile and rerun the water surface profiles.

-   **Safety Factor:** Please refer to the table in Attachment 3 and the
    accompanying examples for guidance in selecting a safety factor for
    the abutment scour estimations

-   **Over-rides:** Please note that the user has the option on the
    Project Information Card of selecting a unit discharge under the
    bridge that is different from that computed by the program if the
    site under study varies from the assumptions used in the model. An
    example would be a bridge crossing located in a bend with higher
    unit discharges on the outside of the bend. If the override is
    selected, then the input cells are displayed on the Downstream
    Bridge Data Card. *(Use all over-ride features with caution).*

## [E. STEP FIVE - UPSTREAM BRIDGE DATA]{.underline}

### *E.1 Enter the Upstream Bridge Data*

Figure 2-10 shows the input screen for the upstream bridge data.

![](media/image11.png){width="5.116666666666666in"
height="3.3520833333333333in"}

####### **Figure 2-10: Upstream Bridge Data Input Screen**

-   **Water surface elevation upstream of the structure:** The water
    surface elevation just upstream of the structure is determined from
    the water surface profile (HEC-RAS) model. The ABSCOUR program
    compares this elevation with the upstream bridge low chord or
    culvert crown elevations to determine whether pressure flow occurs.
    If so, a pressure scour coefficient factor is computed.

-   **Low chord elevation at upstream side of bridge:** The average
    elevation of the low chord (or lowest part of the superstructure) on
    the upstream side of the bridge over the channel and left and right
    overbank sections. The elevation of the low chord is used by the
    program to determine whether the bridge will be subject to pressure
    flow. If pressure flow exists, the program adjusts the predicted
    scour value to account for pressure flow. Refer to Figure 1-21.

![](media/image12.wmf)

**Figure 2-11: Input Values for Low Chord Elevations**

**Abutment shape factor Left and Right Overbanks:** Abutment scour is
reduced by a streamlined shape that facilitates a smooth transition of
the flow and a corresponding reduction in turbulence. Two common
examples of streamlined abutment shapes are vertical wall abutments with
flared wing walls and abutments placed on spillthrough slopes. The
effectiveness of the abutment shape in reducing scour depends on two
factors: (1) the horizontal length, X1, of the streamlined portion of
the abutment or spillthrough slope and (2) the total horizontal abutment
and approach road length, X2, that is within the [effective
flow]{.underline} width of the approach flow. Please refer to Figure
2-12 for an illustration of the X1 and X2 values. As indicated in the
Figure, measure X1 and X2 on the [ABSCOUR cross-section]{.underline};
not on the actual cross-section:

1.  The X1 value for a flared wing wall is the [horizontal distance
    perpendicular to the flow]{.underline} from the abutment face to the
    end of the wing wall

2.  The X1 value for a spillthrough slope is [the horizontal distance
    perpendicular to the flow]{.underline} between the abutment toe (on
    the ABSCOUR cross-section) and the location of the water surface
    line on the spillthrough slope. (In some cases, the water surface
    may extend back to the abutment.)

3.  A vertical wall abutment without wing walls or with a 90 degree wing
    wall is not a streamlined shape and has an X1 value of zero.

The shape factor, Kt, is defined as the ratio of X1/X2. Equations 1-29
and 1-30 compute the value of Kt. Kt is used in Equation 1-28 to compute
the reduction in scour due to abutment shape.

![](media/image13.png){width="5.669444444444444in"
height="7.6506944444444445in"}

Figure 2-12 Abutment Shape Factor

Selection of X~1~ and X~2~ Measurements

-   **Embankment skew angle:** The angle measured from the flow
    direction to the centerline of the left or right approach roadway
    embankment, in degrees. Refer to sketch Figure 2-13. The embankment
    angle is used to account for the effect of the orientation of the
    embankment on the contracting approach flow. For an embankment
    angled downstream, the scour depth is decreased; for an embankment
    angled upstream, the scour depth is increased. Please note that the
    embankment skew angle may be different from the abutment skew angle.

![](media/image14.png)

**Figure 2-13: Embankment Skew Angle**

-   **Future lateral migration of the channel:** Lateral migration needs
    to be considered for bridges and culverts. The structure is fixed,
    but the channel is free to modify its bed and banks over time.
    Design considerations for piers and abutments relative to channel
    migration are presented in the SHA Chapter 11 Scour Manual and in
    the FHWA publications HEC-18 and HEC-20. The stream morphology
    study, including the evaluation of the stream location over time,
    may provide an insight as to future trends of the stream channel and
    guidance on providing for an adequate abutment setback and scour
    protection.

### F. STEP SIX - PIER DATA

Figure 2-14 depicts the Pier Data Card. It is used to input information
on the bridge piers into the ABSCOUR Program so that a complete scour
cross-section under the bridge can be generated for the scour report.
The User needs to calculate the elevation of total pier scour
(contraction scour elevation - local pier scour) before entering
information on the Pier Data Card. Use the Pier Local Scour module to
calculate local scour. Obtain the contraction scour at the pier from the
ABSCOUR output. Once this is done, the following information needs to be
supplied:

Column 1 - A listing of pier numbers beginning with the pier closest to
the left abutment looking downstream (already listed).

Column 2 - The Pier ID number depicted on the plans

Column 3 - The elevation of the bottom of the scour hole at the pier.
This needs to be the elevation of the total scour depth - the sum of
local scour plus contraction scour.

Column 4- Distance from the left abutment face to the centerline of the
pier.

1.  For the special case of a spill-through slope at which the water is
    bounded by the slope instead of the abutment face, one more piece of
    information needs to be input into the cell at the top of the card:
    Distance from left edge of water to left abutment face
    (spill-through slopes only). This step locates the left abutment
    with regard to the edge of water.

![](media/image15.jpeg){width="5.997222222222222in"
height="3.308333333333333in"}

**Figure 2-14 Pier Data Card**

G. STEP 7 ACTUAL SECTIONS

The Actual Sections menu allows the user to import HEC-RAS
cross-sections into the ABSCOUR program and to superimpose the HEC-RAS
(Actual) Sections on the ABSCOUR (Computed) Sections. This option can be
exercised for both the APPROACH SECTION 1 and the BRIDGE SECTION 2. The
user can view and compare the fit between the Actual and ABSCOUR
sections by accessing the DRAW option on the top MENU bar for the
Approach Section, Bridge Section and Scour Section.

![](media/image16.jpeg){width="5.750694444444444in"
height="3.9972222222222222in"}

**Figure 2-15 Actual Sections**

The user can use this information to advantage in making an evaluation
of the ABSCOUR scour computations:

1.  Identify errors in the input data for the ABSCOUR cross-sections

2.  Compare how well the ABSCOUR Section fits the Actual Section.

3.  Determine if "fine tuning" adjustments in scour elevations should be
    made in order to match the actual cross-section more closely.

> The ABSCOUR program develops a rectangular area of contraction scour
> as top width x average depth. For a bridge location on a bend, a more
> accurate representation of contraction scour might be developed by
> pro-rating more of the scour on the outside of the bend, keeping the
> scour area constant. The actual cross-section can serve in developing
> a more accurate representation of the distribution of contraction
> scour under the bridge for such non-symmetrical channels. The actual
> cross-section may also serve to provide insight regarding adjustments
> to the elevation of the abutment scour hole in instances where there
> is a significant difference between the actual and ABSCOUR sections at
> the abutment.

[Application]{.underline}

Clicking on the Actual Sections Menu brings up two tables: the "Approach
Section Looking Downstream" and the "Bridge Section Looking Downstream"
The top of each table provides cells to input the beginning
cross-section station (Left Bank Station) and the ending cross-section
station (Right Bank Station). Additional cells are provided to input
Manning "n" values for the channel and left and right flood plains.

The body of each table consists of 3 columns: the designated point
number, its station and elevation. The information in this table can be
filled in manually or imported directly from the appropriate HEC-RAS
model. It is useful to run the example problem included with the ABSCOUR
program to view the format for the data in a typical table.

[Manual Input:]{.underline} Input the data in the same manner as is
depicted by the table for the example problem:

[Import Cross-section Data]{.underline} Use of the import function is
recommended, since it is much easier to do. This function imports the
actual cross section of the stream at the approach and at the bridge. At
the bridge, the program will also import the bridge deck data from
HECRAS. Note, only the geometry file of the last selected plan in HECRAS
project will be used.

To import the approach section, select the HECRAS project file in the
open file dialog. The program will read the current active plan of
HECRAS project and generate a list of available cross sections. The User
can then choose the cross section of the desired approach section on the
list. The imported data includes the station and elevation of the ground
point in the cross section and the left bank and right bank point
station.

For the bridge section, the program will search through the geometry
file of the current active plan of HECRAS project and find the available
bridges. If more than one bridge exists, a list of bridges will be
generated and the user can select the appropriate bridge. If there is
only one bridge, the program will import the bridge data without asking.
The bridge data includes the downstream section (or upstream section for
the upstream tool) and the bridge deck high chord and low chord
elevations. The left bank and right bank point stations are also
obtained. If the left bank and right bank stations do not match the
ABSCOUR stations used in the scour analysis, the user can make the
following adjustment: Change the HEC-RAS stations to match the ABSCOUR
section.

# 

# 

# 

# [III. COMPUTATIONS AND PROGRAM OUTPUT INFORMATION]{.underline}

Please note that the ABSCOUR program presents computations with up to
three decimal points. However, final scour values used for design should
be rounded off to the nearest foot, since the assumption of accuracy of
scour estimates to a tenth of a foot is not valid.

## [A. ABSCOUR Output]{.underline}

After entering the data on the input menus as described in Steps 1
through 5, click on the RUN button to compute the scour. If the program
inputs are correctly entered, the output file appears. If there are any
of the input items are not filled in, an error message will appear
prompting the user to correct the input files. All input data and output
computations are summarized in the output report.

Figure 2-14 shows the screen that appears after running the ABSCOUR
program. The user can scroll down through the output to look at input
data, output data and program notes. The output can be sent directly to
a printer or it can be saved as a text file so that it can be inserted
into an electronic report.

**Figure 2-14: Sample ABSCOUR Output Screen**

![](media/image17.jpeg){width="5.816666666666666in" height="4.25in"}

The ABSCOUR output file contains the scour calculations necessary for
inclusion in the scour report. Each line of the output file has an
accompanying line number for easy identification. Many of the formulas
and the adjustment parameters are shown in the output file reference.

ABSCOUR can also generate plots of the approach section, bridge section
and predicted scour. Figures 2-15, 2-16, and 2-17 show the plots created
for the approach section, bridge section and bridge scour section
respectively. The plots may be printed directly from the program to a
specified scale or the user may export \*.dxf files for inclusion in
AutoCAD or Microstation. The cursor can be used to determine various
elevations and distances depicted on the plots.

If the HEC-RAS Approach Section and Bridge Section have been imported
into ABSCOUR, they will be included in the above noted Figures.
Comparison of these cross-sections will be helpful in evaluating the
answers obtained from the program.

![](media/image18.png){width="6.358333333333333in"
height="2.515277777777778in"}

**Figure 2-15: Sample Approach Section Plot**

![](media/image19.png){width="5.78125in" height="4.128472222222222in"}

**Figure 2-16: Sample Bridge Section Plot**

![](media/image20.png){width="6.36875in" height="4.625in"}

**Figure 2-17: Sample Scour Prediction Plot**

The output sheets are labeled in the same manner as the input menu
cards. The following is a summary of the sample output sheets included
below. Please note that the line numbers and descriptions may vary
slightly from run to run, depending on the input data.

INPUT DATA

1.  Project information - Lines 1-17

2.  Project Options - Lines 18-28

3.  Approach Section Data - Lines 29-39

4.  ABSCOUR Over-rides Lines 40-51

5.  Downstream Bridge Data Lines 52- 70

6.  Upstream Bridge Data Lines 71 - 86

OUTPUT COMPUTATIONS AND RESULTS

1.  Approach Section Lines 94 - 102

2.  Downstream Bridge Computations, Lines 103- 113

3.  Downstream Contraction Scour Computations, Lines 114 - 126

4.  Total Bridge Scour at Abutments, Lines 127 - 140.

A sample of an ABSCOUR output report is presented on the next two pages.

1:
\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*

2: \* Maryland State Highway Administration \*

3: \* Abutment Scour Program \*

4: \* Version 7 Build 1.01, August 25, 2005 \*

5:
\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*

6:

7: Time stamp: 10/04/2005 10:41:08 AM

8:

9: Input Data:

10:

11: Project information:

12:
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

13: Project name: Example Bridge

14: Project number:

15: Description: Example showing the actual cross section and bridge
data

16: 100 year storm

17:

18: Project options:

19: Program calculates critical and boundary shear stresses at approach
section

20: Program decides the scour type as either live bed or clear water
scour

21: Program calculates the unit width discharge at the bridge section

22: Program calculates critical velocity at bridge section

23: Program calculates sediment transport parameter k2

24: Program calculate the flow velocity at abutment face

25: Clear-water scour uses Neill\'s method

26: English Units

27: Section orientation is looking downstream

28:

29: Approach Section Data:

30:
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

31: Left Channel Right

32: \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

33: Approach section discharge (cfs): 6897 3233 2145

34: Approach section top width (ft): 463.5 46.68 183.72

35: Approach flow depth (hydraulic depth) (y1) (ft): 5.14 10.81 4.46

36: Approach median particle size, D50(ft): 0.0515 0.0515 0.0515

37: Bank slope (Z) in the vicinity of the bridge (Z=H/V): 3 3

38: Energy slope (S) at approach section: 0.05

39:

40: ABSCOUR Overrides

41:

42: Reserved for override approach critical shear stress

43: Reserved for override approach boundary shear stress

44: Reserved for override scour type

45: Reserved for override sediment transport parameter

46: Reserved for override location header

47: Reserved for override unit width discharge

48: Reserved for override critical velocity

49: Reserved for override 2-D velocity at abutment

50: Reserved for override average velocity in portion of bridge

51:

52: Downstream Bridge Data:

53:
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

54: Downstream water surface elevation under bridge: 288.64 ft

55:

56: Left Channel Right

57: \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

58: HEC-RAS discharge under Bridge (cfs): 5897 4233 2145

59: Waterway area (A) measured normal to flow (sf): 1023.46 546.42 76.85

60: Top width (T) measured normal to flow (ft): 172.10 65.00 13.10

61: Hydraulic depth (A/T) (ft): 5.95 8.41 5.87

62: ABSCOUR X-Section elevation (#54-#61) (ft): 282.69 280.23 282.77

63: Abutment type: Vertical Vertical

64: Setback (- for an abutment in channel) (ft): 168.7 12.3

65: Low chord elevation downstream side of bridge (ft): 289.75 289.16
288.96

66: Correction factor for low chord submergence (#54-#65\>0)(ft): 0.00
0.00 0.00

67: Median particle size under bridge, D50(ft): 0.0515 0.0515 0.0515

68: Estimated long-term aggradation(+) or degradation(-) (ft): 0 0 0

69: Safety factor (typical ranges 1.2 to 1.4): 1.2

70:

71: Upstream Bridge Data

72:
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

73: Water surface elevation upstream side of bridge: 289.53 ft

74:

75: Left Channel Right

76: \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

77: Water depth at upstream side of bridge (#73-#62) (ft): 6.84 9.30
6.76

78: Low chord elevation upstream side of bridge (ft): 289.41 289.41
289.41

79: Low chord height (#78-#62) (ft): 6.72 9.18 6.64

80: Pressure flow, Yes or NO: (Yes if #77\>#79) Yes Yes Yes

81: X1: (ft): 1 1

82: X2: (ft): 1 1

83: Ratio (X1/X2): 1.00 1.00

84: Embankment skew angle (degrees): 90 90

85: Is future lateral migration of channel likely to occur?: No

86:

87: Output Computation And Results

88:

89: Approach Section:

90:

91: Total approach discharge (cfs): 12275

92: Left Channel Right

93: \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

94: Approach average flow velocity (fps): 2.895 6.407 2.618

95: Approach unit width discharge (cfs/ft): 14.88 69.259 11.675

96: Approach section depth (ft): 5.14 10.81 4.46

97: Approach section Froude Number: 0.225 0.3434 0.2184

98: Approach section critical shear stress(psf): 0.206 0.206 0.206

99: Approach boundary shear stress(psf): 16.0368 33.7272 13.9152

100: Approach sediment transport parameter (k2): 0.639 0.638 0.639

101: Scour type: Live Bed Live Bed Live Bed

102:

103: Downstream Bridge Computations:

104:

105: Total discharge under Bridge (cfs): 12275

106: Left Channel Right

107: \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

108: Method of computing flow velocity adjustment: Medium Setback Short
Setback

109: Flow velocity (fps): 9.413 10.233 10.233

110: Adjustment to hydraulic depth (y0)adj (ft): 5.947 8.406 5.866

111: Unit width discharge (#110\*#109)(cfs/ft): 55.977 86.024 60.032

112: Critical velocity (fps): N/A N/A N/A

113:

114: Downstream Contraction Scour Computations:

115:

116: Left Channel Right

117: \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

118: Clear water scour flow depth (y2)(ft): 8.53 12.122 9.032

119: Live bed scour flow depth (y2)(ft): 11.98 12.414 12.695

**120: Interpolated scour flow depth (y2)(ft): 11.98 12.414 12.528**

121: Pressure flow coefficient (Kp): 1.009 1.007 1.009

122: Adjusted scour flow depth (y2)adj (#121\*#120(ft): 12.087 12.495
12.641

**123: Contraction scour depth (ys) (#122-#110\>0)(ft): 6.141 4.089
6.775**

124: Final contraction scour depth (#123\*#69)(ft): 7.369 4.906 8.13

125: Contraction scour elevation(#54-#110-#124-#66)(ft): 275.324 275.327
274.644

126:

127: Total Bridge Scour At Abutment:

128:

129: Left Channel Right

130: \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

131: Abutment local velocity factor (Kv): 1.11 1.069

132: Abutment spiral flow factor (Kf): 1.396 1.369

133: Pressure flow coefficient (Kp): 1.009 1.009

**134: Abut. scour flow depth
(y2a)adj(#120\*#132\*#131\^#100\*#133)(ft): 18.035 18.052**

135: Initial abutment scour depth (ysa)(#134-#110\>0)(ft): 12.088 12.185

136: Coefficient for abutment shape factor (Kt): 1 1

137: Coefficient for embankment angle (Ke): 1 1

138:

**139: Final abutment scour depth (ysa)adj(#135\*#136\*#137\*#69)(ft):
14.506 14.622**

140: Abutment scour elevation(#54-#110-#139-#66)(ft): 268.187 268.151

The following comments highlight certain ABSCOUR output computations and
results:

-   **Project Information:** Use this section to outline the primary
    factors of interest in the scour evaluation: flood flow, project
    description and any special conditions to be evaluated (discharge,
    trial selections for soils, types of scour).

-   **Project Options:** This section prints the options used by the
    program.

###### **Input Data**

-   **Approach Section Data:** These numbers reflect the information
    provided by the User for the Approach Section. An important item to
    check here (Line 101) is whether the flow is live bed or clear
    water.

-   **ABSCOUR Overrides:** This summary should always be reviewed to
    make sure that the User is aware of any overrides input into the
    program.

-   **Downstream Bridge Data:** This summarizes the information used to
    construct the ABSCOUR cross-section under the bridge. It computes a
    correction factor for the case where the downstream water surface is
    higher than the elevation of the low bridge chord.

-   **Upstream Bridge Data:** This is a summary of the information
    needed to compute the shape factor for the bridge and to determine
    if pressure flow will occur.

###### **Output Computations and Results**

-   **Approach Section:** This is a summary of the data used to compute
    the unit discharges at Section 1 and to develop the computations to
    determine if the flow condition is for live bed or clear water
    scour.

-   **Downstream Bridge Computations:** Based on the abutment setback
    and channel flow depth, the program computes the flow distribution
    and velocities as described in Part 1 for short setback,
    intermediate setback or long setback. There are 16 possible
    combinations of flood plain geometry and abutment setback distances
    that are utilized in the ABSCOUR Program to compute the appropriate
    velocity used in the scour equations. These combinations are
    presented in Attachment 1.

> For clear water scour, the critical velocity is computed from
> Laursen's equations or Neill's curves. The ABSCOUR program computes
> contraction scour depth by setting the average flow velocity equal to
> the critical velocity (Neill's competent velocity) of the D50 stone
> size.
>
> An adjustment is made for the hydraulic depth at the abutment if the
> abutment is within the limits of the bank slopes line 106.

-   **Downstream Contraction Scour Equations:** Line 118 and 119 reflect
    computed contraction scour for clear water and live bed,
    respectively, and Line 120 provides for an interpolated scour depth
    depending on the scour conditions. In the Case C example presented
    above, there is live bed scour on the overbank and in the channel.
    The live bed scour flow depth in the channel (line 119) is 12.4
    feet; and on the left overbank at a distance of 5yo (34 feet) it is
    5.9 ft. The abutment setback for the left overbank is 169 feet. The
    program makes a parabolic interpolates between the two scour values
    to compute the contraction scour flow depth in the left overbank as
    12.0 ft. In some cases, the flow width under the bridge for one or
    more abutments may be less than the abutment setback. When this
    occurs, the program assumes that there is no water behind the
    abutment and the abutment scour is calculated as zero. Consequently,
    the extent of scour at the abutment is limited to the value of the
    contraction scour. In general, this case is more likely to be based
    on user error than on an actual field condition.

-   **Total Bridge Scour at Abutment:** The abutment scour flow depth
    (y2a) at the abutment (line 134) is computed by multiplying the
    adjusted contraction scour flow depth determined in line 122 by the
    Kv and Kf factors using the procedure explained in line 134 (see
    also Equation 1-23 or 1-24).

> The computations for final abutment scour depth (Line 139) is
> explained in Equation 1-28 and also by the accompanying notes on Line
> 139. Please note that SHA uses a minimum (default) scour depth of 5
> feet

-   **Program Sketches:** After running the program, the user can click
    on the "DRAW" button on the "Menu Bar at the top of the screen.
    Three options are presented: Approach Section, Bridge Section and
    Scour Results. We recommend careful inspection of each of these
    sketches to check for a reasonable representation of the actual
    HEC-RAS sections and to view a depiction of the scour cross-section.
    This exercise is well worthwhile to assure that there are no obvious
    errors in the input data.

> Please note that the user can input the results of the pier scour
> modules into the ABSCOUR bridge cross-section (Scour Results) to
> prepare a complete scour cross-section at the bridge. However, the
> pier scour elevations apply to the upstream side of the bridge whereas
> the abutment scour elevations are computed at the downstream side of
> the bridge. Combining these results provides a simplified and
> conservative means of evaluating the scour. The user is encouraged to
> redraw the scour cross-section to develop a more readable sketch and
> to account for such features as rock.

The following guidance is provided with regard to refining the scour
cross-section.

1\. For vertical wall abutments, the preferred method is to plot values
of y2 under the bridge, measuring down from the water surface at the
downstream side of the bridge.

2 Where coefficients have been used to modify the depth of scour, ys ,
plot the scour depth using the same datum as was used to define yo.

3 Where the abutment scour is deeper than the channel scour, use an
angle of 30 degrees to define the sides of the scour hole. Use a nominal
value of 5 feet to determine the width of the bottom of scour hole.

4 Where the abutment scour depths are at a higher elevation than the
channel contraction scour, use a smooth curve to define the transition
area.

5 For an abutment on a spill through slope, draw an arc with center at
the toe of the abutment slope and with a radius equals the abutment
scour depth. The arc represents the possible sliding surface of the soil
on the slope and also represents the possible scour hole.

## [B. ABSCOUR Program Logic]{.underline}

The following discussion is provided for insight into the logic used by
the program in computing flow distribution and velocity distribution at
the bridge.

A current limitation of the HEC-RAS program used to model flow through a
bridge is that it provides for the distribution of flow under the bridge
based on conveyance calculations. This approach does not reflect the
three dimensional flow patterns actually observed in the field at bridge
contractions. To obtain reasonable estimates of scour depth, it is
necessary to account for the high local flow velocities and turbulence
near the abutments caused by the contracting flow in the overbank areas
upstream of the bridge.

Findings from recent laboratory studies of compound channels indicate
that the velocity of flow under a bridge tends to be highest at the
abutments (due to rapid acceleration and turbulence of the overbank flow
entering the bridge contraction) and in the thalweg section of the
channel. This phenomenon has been observed in field surveys conducted by
the U. S. Geological Survey and is consistent with the theory of
potential flow at a contraction. The procedure used by the ABSCOUR
Program to determine the flow distribution under the bridge is explained
in Part 1 of this guideline.

## [C. EVALUATION OF THE PROGRAM OUTPUT]{.underline}

### *C.1 Overrides*

A special message indicating that "OVERRIDE IS ACTIVE" is printed when
the user over-rides the computer values. *Any over-ride function should
be used with caution, and the logic of the over-ride carefully checked
in this evaluation phase.* Please be aware that the sediment transport
functions and the hydraulic flow conditions must be compatible. If the
user imposes unrealistic conditions on the program, the resulting scour
estimates will be in error.

### *C.2 Bridge Section Data*

Based on the user's input data, the program determines the discharge,
unit discharge and velocity of flow for each cross-section sub-element
under the bridge. As noted earlier, the widths input by the user and the
abutment setbacks should be measured normal to the direction of the
approach flow.

The method of analysis (Method A, Short Setback, B Long Setback, or
Method C, Intermediate or Transition Setback) is determined on the basis
of a comparison of the abutment setback with the depth of flow in the
main channel at the bridge (Section 2) as previously described. The unit
discharges, q, and velocity, V ~2~ are computed from the equations set
forth in Part 1. Attachment 1 provides detailed examples of how the
computations are made for various combinations of channel and overbank
geometry, and abutment setback.

The critical velocity required for the incipient motion of the D~50~
particle size for flow under the bridge for clear water scour is
computed from the particle size of the channel bed or flood plain
material and the flow depth using Neill's competent velocity curves. An
over-ride table is provided to allow the user to change this value to
account for cohesive soils or other factors. This over-ride process is
the same as that for the scour parameter table. The user is also given
the option of using Laursen's relationship for clear water scour.

### *C.3 Contraction Scour Table*

The value of y~2~ in this table is the vertical distance between the
water surface and the stream bed after contraction scour has occurred.
The program calculates this value using the Equations in Appendix A. The
scour depth y~s~ is the depth of contraction scour:

**y~s~ = y~2~ - y~o~ (2-1)**

> where:

y~s~ = depth of contraction scour

y~2~ = vertical distance from the water surface to the stream bed after
contraction scour has occurred, and

y~o~ = depth of flow under bridge before scour occurs (Bridge Section
Data)

Please note that the output table will indicate whether or not pressure
scour is computed in accordance with the procedure in Part 1.

### *C.4 Abutment Scour Table*

*The abutment scour depth, (ysa )~adj~ represents the total scour,
including contraction scour and local scour which is predicted to occur
at the abutment. It does not include long term degradation, which the
user must account for in the final scour evaluation. The scour depth
elevation is the elevation the Engineer should use to evaluate scour.*
It reflects all of the adjustments made by the program to account for
the various factors affecting abutment scour. These adjustments include
the following:

-   For a skewed embankment crossing, the ABSCOUR program will adjust
    the computed scour by a skew coefficient in accordance with the
    procedure set forth in FHWA Hydraulic Engineering Circular 18, 2001
    Edition. The user must enter the theta angle of the orientation of
    roadway with respect to the direction of the flow.

-   The program increases scour depths where necessary to account for
    the effects of pressure flow,

-   An abutment shape factor is used to evaluate the effect of the
    abutment shape on the predicted scour.

-   A safety factor, input by the user, is applied to increase the
    calculated scour depth. This safety factor permits the user to apply
    judgment to the design considerations based on the site conditions,
    reliability of available data and the risks to the bridge, the
    transportation system and the traveling public.

### *C.5 Scour Depth Elevation*

The scour depth elevation is used for plotting the scour cross-section
and for evaluating the scour.

### *C.6 Scour Cross-Section*

A scour cross-section is plotted as a part of the program output data.
Development of the scour cross-section is discussed in Part 1 and in
Reference 1. The user will need to evaluate and possibly change this
scour cross-section to reflect field conditions.

As noted earlier, the scour cross-section assumes that the computed
scour occurs at the downstream cross-section. Actually, the computed
deepest pier scour usually occurs near the upstream side of the bridge.
The deepest scour for vertical wall abutments can be expected to occur
near the upstream side of the bridge whereas the deepest scour for
spillthrough slopes can be expected to occur on the downstream side of
the bridge.

Where rock of varying elevations and resistance to scour is encountered,
the user needs to develop a 3-D representation of the "Scour
Cross-section", and to use a different method for computing scour in
rock.

### *C.7 Evaluation of the Computed Scour Values*

Use the computed values of scour from the ABSCOUR program *as a guide*
in the design of the bridge abutment, keeping the following
considerations in mind:

-   the SHA policies and procedures set forth in Chapter 11, Bridge
    Scour,

-   the guidance in the FHWA HEC-18 Manual regarding abutment scour
    (Reference 1).

-   the need to provide some form of scour countermeasure to protect the
    bridge abutment and inhibit the formation of a scour hole. Base the
    design of the riprap on the anticipated contraction scour depths
    near the abutment. Use the utility section of the program to compute
    the minimum D50 size of the riprap for each abutment. These
    calculations are based on the procedures set forth in the 2001
    edition of HEC-23. Use this information to select the appropriate
    riprap size, typically Class 2 or 3.

There are factors which can affect the extent of contraction scour and
abutment scour at a bridge that are not directly computed by the ABSCOUR
model. However, various procedures have been suggested in this manual to
permit the user to take some of the factors into consideration in the
scour evaluation:

> \- the possible effect of nearby adjacent piers in modifying flow
> patterns and resultant abutment scour (engineering judgment; model
> studies)
>
> \- effect of bends and upstream tributaries in the distribution of
> contraction scour (bendway scour) and the effect of a severe angle of
> attack causing flow to impinge directly on the abutment. These
> conditions may increase scour at abutments located on the outside of
> bends. (See Attachment 2 and Reference Numbers 1, 2, and 8).

\- effect of ice or debris in clogging a waterway opening, deflecting
channel currents and increasing flow velocities and resulting scour (See
HEC-18).

> \- effect of two dimensional flow patterns, especially for wide flood
> plains, in modifying the flow conditions at a bridge (See Attachment
> 2; use a 2-D model).

\- effect of confluences or other geomorphological features affecting
the lateral migration of stream channels (See Attachment 2).

> \- the method does not directly address critical shear stress or
> critical velocity for cohesive soils or rock. The user is provided a
> means of partial evaluation of this condition by use of the over-ride
> functions.

The engineer also needs to keep in mind the limitations of the ABSCOUR
model used to estimate the depth of clear water scour. The concept is
that the area under the bridge will scour and thereby increase the flow
area while decreasing the flow velocity. This process will continue
until the flow velocity is below the critical velocity needed to move
the selected D50 particle size under the bridge. The model application
is likely to result in high clear water scour depths for high flow
velocities in fine-grained non-cohesive soils. The following factors
need to be evaluated in this regard:

\- The particle size should be representative of the soil at the
elevation of [the bottom of the scour hole]{.underline}. Armoring of the
stream bed may inhibit the depth of the scour.

> \- SHA's experience on Maryland streams is that critical velocities
> for fine particle sizes are best modeled by Neill's curves in
> Reference 8 (See Appendix C and the Maryland SHA Scour notebook,
> Reference 9). The user has the option to use Neill's method or
> Laursen's method for clear water scour.

\- The hydrograph for the worst case scour conditions should be
considered. For flashy streams on small watersheds, the time period
during which scouring velocities actually occur may be relatively short,
especially for overbank areas.

> \- The conditions for clear water or live bed scour are not always
> clear cut, and it is possible that both types of scour may occur
> during different stages of a flood hydrograph. The user is encouraged
> to evaluate both

cases.

As indicated above, a limited flexibility has been built into the
ABSCOUR program to allow the engineer to account for some of the above
factors. The engineer is encouraged to consider all information obtained
from field and office studies, the limitations of the scour model, and
apply judgment in the selection of an appropriate abutment design *The
user needs to select and input a safety factor on the Bridge Data card
consistent with the uncertainties of the scour parameters at the site
and the importance of the bridge under design.*

The ABSCOUR program requires accurate hydrologic, hydraulic and soils
data in order to compute accurate contraction and abutment scour depths.
The extent to which the Engineer can obtain accurate data will vary from
site to site. In some cases, for example subsurface soils data, it may
not be practical to obtain a complete and accurate description of all
the input parameters. However, the use of incomplete or inaccurate input
data may significantly affect the accuracy of the ABSCOUR output results
of predicted scour depths. The Engineer needs to exercise judgment to
arrive at a practical solution to this problem.

A big advantage of the ABSCOUR program is the ease of checking the
sensitivity of the scour estimate to the different input parameters.
Where there is a question about the value of the input parameter, the
recommended procedure is to input the best estimate of the value and
then check the sensitivity of the scour depths for reasonable maximum
and minimum values of the parameter

# [IV QUESTIONS TO ASK AND FACTORS TO CONSIDER IN REVIEWING THE ABSCOUR OUTPUT]{.underline}

1.  Is the ABSCOUR model being used the most up-to-date version?

> ***Check for updates on the web at
> http://www.wtbco.com/scour/scour.html***

2.  Are the contraction scour and abutment scour values reasonable? If
    not, what are the likely sources of error in the input data that are
    creating what appears to be high or low scour values?

3.  Have you checked the performance history of the original structure
    being replaced or of other nearby bridges? What historical
    information is available on scour or on bridge failures during
    previous floods?

4.  Does the hydrology study provide for reasonable estimates of flood
    magnitudes? Follow the latest Maryland Hydrology Panel
    Recommendations. (Use of TR-20 by itself may overestimate the
    magnitude of flood discharges and corresponding scour depths).

5.  Does the HEC-RAS analysis provide reasonable values for flow
    distribution and energy slopes? Is the approach section and bridge
    section reasonable representations of actual effective flow
    conditions during a major flood? Do you need to modify the Approach
    Section or select a different section? How reliable is your estimate
    of the tailwater elevation at the bridge? Do you have a reasonable
    flow distribution model for overtopping flow at the bridge?

6.  How accurate and complete are the soils data? This is particularly
    important for clear water scour conditions. Are there other sources
    of information regarding subsurface conditions that may corroborate
    the values used in the scour evaluation?

7.  Do borings and subsurface investigations indicate the presence of
    rock? Have you consulted a geologist if RQD values are less than
    75%? Is the rock erodible or scour resistant? How does the rock
    affect the scour cross-section under the bridge? If the rock is
    erodible, have you used Annandale's Erodibility index method or
    other methods to assess the extent to which it will scour?

8.  Have you made sensitivity analyses to evaluate the field conditions
    you are modeling? For example, (a) live bed vs. clear water
    scour; (b) Neill's curves VS Laursen's curves for clear water scour,
    etc.

9.  The ABSCOUR program develops a rectangular area of contraction scour
    as top width x average depth. For a bridge location on a bend, a
    more accurate representation can be developed by pro-rating more of
    the scour on the outside of the bend, keeping the scour area
    constant. You may wish to use the actual cross-section instead of
    the ABSCOUR cross-section to draw the contraction scour under the
    bridge. Importing the HEC-RAS bridge section will be helpful in this
    respect.

# [V. COMPUTATION OF PIER SCOUR]{.underline}

## [A. Pier Local Scour Introduction]{.underline}

The computations in the Pier Local Scour Module are based on the
research reported by the FHWA in HEC-18, Evaluating Scour at Bridges,
May 2001 Edition.

The FHWA method and scour equations account for complex pier geometry as
well as bed load conditions. The User is referred to HEC-18 for a
discussion on the research used to develop the pier scour equations. The
Maryland program simplifies the work required to obtain pier scour
depths.

The procedure for computing pier scour is outlined below:

![](media/image21.jpeg){width="4.836805555555555in"
height="2.8090277777777777in"}

Option 1

Option 1 computes local scour for the pier stem only. Fill in the
required information and hit the run button. If the scour computed by
Option 1 is deeper than the top of the footing/pile cap, continue on to
Option 2 below.

![](media/image22.jpeg){width="5.999305555555556in" height="3.76875in"}

![](media/image23.jpeg){width="5.995138888888889in"
height="3.790277777777778in"}

Option 2

1\. Rerun the program using Option 2 with the following modifications.

2\. Set a new bed elevation one foot below the top of the pier
footing/pile cap

3\. Set the approach flow depth, y1, equal to the distance between the
water surface and the new bed elevation.

4\. Let the unit flow discharge, q, (cfs/ft, cms/m) just upstream of the
pier be the same as that used for Option 1.

5\. Compute a new approach flow velocity as V1=q/y1

3.  If the modified scour depth is above the bottom of the footing/pile
    cap, use this value: If the modified scour depth is below the bottom
    of the footing, continue with step 5.

Option 3

5.  Rerun the program using Option 3 with the following modifications.

6.  Set a new bed elevation one foot below the bottom of the footing.

7 Set the approach flow depth, y1, equal to the distance between the
water surface and the new bed elevation

8.  Compute a new approach flow velocity as V1=q/y1

9.  Compute the final local scour depth and elevation for Option 3 using
    the scour parameters determined above

10. As a check on the computations, subtract the scour depth determined
    from options 1, 2 or 3 from the contraction scour depth and compare
    the scour elevation of the two alternatives. Use the worst case
    (deepest) scour elevation.

![](media/image24.jpeg){width="5.992361111111111in"
height="3.7743055555555554in"}

> .

# 

# 

# 

# [VI. UTILITY MODULE]{.underline}

## [A. RIPRAP]{.underline}

The Utility module provides a means of selecting the D50 size of riprap
for abutments culverts and piers. The computations for the riprap D50
size for piers and abutments use the procedures set forth in the 2001
edition of HEC-23. Use this information to select the appropriate riprap
size, typically Class 2 or 3. The computations for the D50 size for
bottomless culverts are based on a cooperative FHWA-Maryland SHA
research study conducted in the FHWA Hydraulic Laboratory.

The process for using this module is the same as for the other modules
previously discussed. The various input cells are to be filled in; then
the "COMPUTE" button is clicked to make the calculation

![](media/image25.jpeg){width="5.998611111111111in"
height="4.420833333333333in"}

## [B. CRITICAL VELOCITY]{.underline}

This is a handy tool for calculating the critical velocity for a channel
bed, given the D50 particle size and the flow depth. Calculations are
based on Neill's competent velocity curves (Reference 11). Short Help
(F-1 key) and Regular Help are available for this module.

## 

## ![](media/image26.jpeg){width="4.041666666666667in" height="2.5625in"}

## 

## 

## [C. SCOUR IN ROCK]{.underline}

The Utility Module provides a methodology for the computation of scour
in rock entitled ROCK SCOUR. Many of the bridge designs developed by the
Office of Bridge Development involve foundations in rock, and most of
these bridges will require a scour evaluation. The SHA has adopted the
Erodibility Index Method as the best guideline currently available for
evaluating scour in rock. Engineers making scour evaluations in rock are
expected to use the Erodibility Index Method. The Rock Scour Program in
the Utility Section of the ABSCOUR program provides for a convenient
method of making the necessary calculations..

The Erodibility Index Method was developed by Dr. George Annandale,
currently the President of Engineering and Hydrosystems, Inc. of
Littleton Colorado. The SHA applies this method in accordance with the
manual prepared by Dr. Annandale for the SHA dated April, 1999 and
entitled "Calculation of Pier Scour Using the Erodibility Index Method"
Engineers making scour evaluations in rock for the SHA are expected to
be familiar with the information in this manual.

### *C.1 Application of the Erodibility Index Method*

The Erodibility Index Method involves the following steps:

1.  Calculation of the Erodibility Index of the rock, based on its
    physical characteristics and orientation with respect to the flow
    direction of the water.

2.  Calculation of the stream power of the flow in the stream or river
    for the hydraulic conditions under investigation.

3.  Calculation of the modified stream power at a pier or abutment due
    to the effect of the obstruction on the flow. These modified values
    are calculated by a series of equations developed in the FHWA
    Hydraulic Laboratory for different types of piers under different
    flow conditions.

> The piers scour equations are recommended for design when used with
> caution and the application of engineering judgment.
>
> [The abutment scour equations should not be used for
> design]{.underline}. The SHA has derived the abutment scour equations
> from the rectangular pier equations developed by the FHWA lab studies,
> and there are no data at this time to assure that this approach is
> valid. However, these equations can be useful of in comparing the
> estimated scour in rock with the equivalent scour in sand. This
> information can serve as one factor in making an engineering judgment
> regarding scour at abutments founded in rock.
>
> Using the empirical relationships presented in the Erodibility Index
> Method described above, a comparison can be made between stream power
> and the ability of the rock to resist the hydraulic forces. If the
> rock at the surface of the stream cannot withstand the hydraulic
> forces of the water, it will scour and a scour hole will form at the
> base of the pier or abutment. As the scour hole deepens, the stream
> power at the bottom of the scour hole diminishes in accordance with
> the relationships determined by the FHWA studies. At some point, the
> hydraulic power of the water and the resistance of the rock will
> achieve a balance, and the scour will end.
>
> A safety factor should be applied to the above scour evaluation, to
> take into account the limited understanding of and experience with
> evaluating the resistance of rock to scour. This safety factor should
> be determined on a case by case basis; however, the current SHA
> thinking is to use a safety factor in the range of 2 to 5, with a
> range of 2 to 3 being used for most bridges.

## [D. STREAM POWER CALCULATIONS]{.underline}

The hydraulic calculations are relatively straight-forward and consist
of the following:

1.  Inputting the velocity, hydraulic radius and energy slope of the
    flow so that the program can calculate the stream power (Pa): Pa =
    γVRS.

-   For piers, select a section just upstream of the bridge to compute
    the stream power.

```{=html}
<!-- -->
```
-   For abutments, select the downstream section under the bridge
    (Section 2 as defined in the ABSCOUR Program) to compute the stream
    power.

2.  Selecting the pier type along with the angle of attack of the flow.

3.  Calculating the maximum scour in sand for the selected foundation
    geometry and flow conditions.

-   For piers, select a section just upstream of the bridge to obtain
    the hydraulic values in the pier scour equation. Use the Pier Scour
    Module in the ABSCOUR Program to calculate the scour depth in sand.

```{=html}
<!-- -->
```
-   For abutments, select the downstream section under the bridge
    (Section 2 as defined in the ABSCOUR Program) to compute the maximum
    scour in sand. Use the ABSCOUR Program to make this computation.

## [E. ERODIBILITY INDEX CALCULATIONS]{.underline}

**Computations of the Erodibility Index of the rock should be made only
by engineers or geologists with knowledge and experience in evaluating
the properties of rock.** It is the practice of the Office of Bridge
Development to meet with the SHA geologists for the purpose of:

1.  inspection of the rock cores, and

2.  selection of appropriate rock characteristics for purposes of
    computing the erodibility index of the rock.

> The steps for computing the Erodibility index are outlined below:

### *E.1 COMPUTING THE ERODIBILITY INDEX FOR ROCK*

Please note that the erodibility index can be expected to vary with the
depth of the rock below the channel. Typically it will increase, but
this is not necessarily true in all cases.

In conducting studies of scour in rock, it is necessary to compute the
erodibility index for the same elevation at which the rock scour will
occur. Normally this will involve a trial and error approach using the
computer program.

#### The references below pertain to appropriate tables and pages in Dr. Annandale's manual "Calculation of Pier Scour Using the Erodibility Index Method" The Erodibility Index is computed from the following equation:

#### 

####  **K = Ms Kb Kd Js (2-2)**

where:

K = erodibility index

Ms = mass strength number

Kb = block size factor

Kd = inter-particle bond shear strength number

Js = relative ground structure number

### *E.2 DESIGN PROCEDURE*

STEP 1 DETERMINE (Ms) THE MASS STRENGTH NUMBER

> This value is selected from Table 5, Intact Material Strength Number
> Ms for Rock, Page 18.

STEP 2 COMPUTE Kb, THE BLOCK SIZE FACTOR:. Kb = RQD/Jn

-   RQD = Rock quality designation where RQD \> 5. This is obtained by
    [qualified engineers and geologists]{.underline} through an
    inspection of rock cores taken at the bridge site

-   Obtain Jn , the joint set number, from Table 7, page 21

STEP 3 COMPUTE KD, THE INTER-PARTICLE BOND SHEAR STRENGTH NUMBER, Kd =
Jr/Ja

-   Obtain the joint roughness number, Jr, from Table 8, page 26

-   Obtain the joint alteration number, Ja, from Table 9, page 27

STEP 4 COMPUTE Js, THE RELATIVE GROUND STRUCTURE NUMBER,

> The information required to obtain Js is obtained from Table 10, the
> Relative Ground Structure Number Table, page 29.
>
> The value of Js depends upon the appropriate selection of the
> following rock properties:

-   Dip direction in direction of stream flow or dip direction against
    direction of stream flow (degrees)

-   Dip angle of closer spaced joint set (degrees)

-   Ratio of joint spacing , r

The computer program provides the user with a convenient method to
compute and compare the erodibility index and the stream power, and to
determine the extent to which the rock will scour for the given
conditions. The method allows the user to select an appropriate safety
factor to be considered in applying the results of the evaluation. Use
the following input menu cards:

PROJECT DATA CARD

1.  Project description

2.  Pier or Abutment Data

HYDRAULIC DATA

1.  Input the data described above in Stream Power Calculations

2.  Input the desired safety factor

ROCK DATA

#### Input the data as described in the above section on computing the erodibility index for rock.

After inputting the above noted data, click the run tab, and then the
output tab to obtain the scour report. The program will compute the
depth of scour in rock along with the computed safety factor.

##### **ATTACHMENT 1: COMPUTATION OF THE VELOCITY OF** 

##### **FLOW USED IN THE ABUTMENT SCOUR COMPUTATIONS.**

# [I. COMPUTATION OF VELOCITY AND SCOUR]{.underline} 

Field observations of flows at bridge crossings in wide streams revealed
that the flow in the overbank sections is contracted by the abutment and
moves toward the main channel where it mixes with the main channel flow.
When the abutment setback from the main channel was less than five times
the flow depth in the channel, the flows were well mixed and the flow
velocity in the channel and overbank became uniform. If the abutment
setback was large, being located near the edge of the flood plain, the
flows in the main channel and in the overbank section remained separated
as they passed under the bridge. These findings are utilized in
computing flow velocity in ABSCOUR program.

Abutment setbacks are classified into three categories: short,
intermediate, and long setbacks. The term short setback is used to
define the condition where the setback is equal to or less than five
times the channel flow depth (5y~0~). The term long setback is used to
define the condition where the setback being is equal or greater than
75% of the overbank width (0.75W). A setback between these two limits is
defined as an intermediate setback.

For short setbacks, the velocity (V) is computed as a uniform velocity
(V=Q/A ) in the waterway area under the bridge(A) where Q is the
discharge through the bridge.

For long setbacks, the velocity in the overbank is computed
independently from the channel flow. It is based only on the discharge
and flow area of the overbank section.

For intermediate setbacks, the velocity is computed by interpolating the
velocity of the mixed flow (at a setback distance of 5y~o~ from the
channel bank) with the velocity of separate flow (at a setback distance
of 0.75W).

In each case above, the unit flow discharge under the bridge is computed
by multiplying the velocity and flow depth (q = V\* yo). For short
setbacks very close to the channel banks and within the limits of the
bank slope, the flow depth is adjusted to reflect the actual location
within the bank area. Finally, the scoured flow depth, y~2~, used to
define contraction scour is computed by using the appropriate scour
equation:

-   Laursen's equations for live-bed contraction scour, or

-   The user's choice of Laursen's equation or Neill's competent
    velocity equation to compute clear-water contraction scour.

When the abutment has no setback (is at the channel bank), the scour at
the overbank will be equal to that for channel. When the setback is
small, the scour at the overbank will be very close to the scour in the
channel. However, due to the idealization of channel and overbank flow
into the rectangular shapes for the ABSCOUR cross-section, the
calculated overbank scour may be based on clear water scour (as
determined from the Approach Section calculations) whereas it may be
subject to live bed scour from the main channel. Some transition is
needed between the no setback case and the case where the abutment is
set well back on the flood plain.

The limit of the transition zone is defined as five times the flow depth
in the downstream channel. When there is no setback, the channel scour
flow depth (y2) is used for the contraction scour. When the abutment
setback on the flood plain exceeds the limit of the transition zone,
separate flow is assumed between the channel and the flood plain and no
interpolation is required. When the setback is within this transition
zone of from zero to 5yo, the following scheme is used to compute
contraction scour:

ABSCOUR separately calculates both clear water scour flow depth and live
bed scour flow depth for (1) the channel section and (2) the overbank
section

The channel contraction scour flow depth (y2) is the scour when the
setback is equal to or less than zero - that is no setback case.

The overbank contraction scour flow depth (y2) is the overbank scour
when the setback is located on the flood plain beyond the channel banks
a distance equal to 5 times the flow depth in the downstream channel (SB
= 5yo)

There are four combination of overbank scour in the transition zone:

1 clear water scour with no setback

2\. clear water scour with setback = 5yo

3\. live bed scour with no setback

4\. live bed scour with setback = 5yo

The computed overbank contraction scour will be interpolated between
these four cases, depending on the setback distance and the scour type
(live-bed or clear water at overbank and channel). For example:

When the channel is live bed and the overbank is clear water, then the
overbank contraction scour for the actual setback (between 0 and 5 times
channel flow depth) will be interpolated between case 3 ( live bed scour
with no setback) and case 2 (clear water scour with setback = 5yo).

The interpolation depends on the distance that the abutment is set back
from the channel bank and the scour type at the overbank and channel
sections.

A parabolic interpolation is used for the contraction scour flow depth
calculation (y2) since this method provides for a smooth transition that
approximates the scour depths computed through the application of
Laursen's contraction scour equations. The following parabolic equation
is used for interpolation.

y2=(y2)bank + ((y2)channel - ((y2)bank)\*(1-(setback)/(5\*yo))\^p

Where: p=4.5-Z and p is limited to the values of 1\<=p\<=4

Z is the approach section bank slope H/V

(y2)bank is the scour flow depth at setback=5y0

(y2)channel is the scour flow depth with no setback

Please note that the bank slope determines the shape of the parabola and
therefore the relative effect of the channel scour on scour at the
abutment. Steeper bank slopes such as 1:1 will reduce the effect of
channel scour whereas flatter slopes such as 4:1 will increase the
effect of channel scour. The bank slope can be used as a variable in
sensitivity analyses of factors affecting abutment scour.

The contraction scour flow depth is modified as necessary to take into
account the effect of any pressure scour and to apply a safety factor to
the design.

Next, the abutment scour flow depth (y2a) is computed directly from the
interpolated contraction scour value:

y2a =( kf \* (kv)\^k2 ) \* (contraction scour)

Abutment scour (ysa) = y2a - (yo)adj , where (yo)adj = flow depth before
scour occurs.

The final or adjusted abutment scour value (ysa)adj is determined as

(ysa)adj = Kt \* Ke \*FS \* ysa

Where

Kt = modification for abutment shape

Ke = modification for embankment skew

FS = factor of safety.

ysa = initial abutment scour estimate noted above (ysa = y2 - (yo) adj)

The logic presented above is based on the assumption that the overbank
area is wide and that 0.75W \> 5y~0~. A special case may exist for a
narrow flood plain where 0.75W \< 5y~0~. In this instance, no
intermediate zone exists and the interpolation scheme for the
intermediate setback cannot be applied. If the setback is equal or
larger than 5y~0~, the velocity and resulting contraction scour depth is
computed assuming that the setback is equal to 5y~0~. If the setback is
smaller than 5y~0~, the velocity and scour depth are computed the same
as it would be for the short setback case.

Here are some example problems to illustrate the computation of flow
velocity and contraction scour for various setback distances from the
channel bank.

# [II. EXAMPLE PROBLEM 1]{.underline}

GIVEN:

  ------------------- ----------------- ----------------- -----------------
                      LEFT OVERBANK     CHANNEL           RIGHT OVERBANK

  APPROACH SECTION                                        

  \- DISCHARGE cfs    600               1600              1200

  \- TOP FLOW WIDTH   80                20                100
  ft                                                      

  \- HYDRAULIC DEPTH  4.8               9.8               3.8
  ft                                                      

  UNIT DISCHARGE (q1) 7.5               80                12
  cfs/ft                                                  

                                                          

  BRIDGE SECTION                                          

  \- DISCHARGE cfs    600               1600              1200

  \- TOP FLOW WIDTH   80                20                100
  ft                                                      

  \- HYDRAULIC DEPTH  5                 10                4
  ft                                                      
  ------------------- ----------------- ----------------- -----------------

# [III. COMPUTATION OF CONTRACTION SCOUR:]{.underline}

Computations for the contraction scour flow depths, y~2~, for the right
overbank section are presented for different abutment setbacks. The left
abutment is kept at a fixed location with its setback at a distance of
20 ft from the channel edge. The methods of computation are demonstrated
only for the right overbank. Contraction scour of the left overbank for
different setbacks can be computed in the same way by keeping the right
abutment at the actual fixed location.

## [A. Short Setback - CASE A in Figure A1-1]{.underline}

Since the channel depth is 10 feet, any setback less than (5 X 10 = 50)
feet is a short setback.

Let the setback of the right abutment be 30 ft. Since the left abutment
setback is also short, being 20 feet, the velocity is computed as if all
flows are mixed. The contraction scour depth then shall be computed by
interpolating the contraction scours at the setbacks set at the channel
edge and at five times the channel flow depth, 5y~0.~

Step 1. Compute flow velocity.

> As the setback of the left abutment is short as well as the right
> abutment, total flow will be mixed.

[For right setback of 30 ft:]{.underline}

V2= Q/A = (3400)/(20\*5+20\*10+30\*4)=8.1 ft/s

Step 2. Compute Unit discharges, q2 = V\*yo

[For setback of 0 ft:]{.underline}

q2=8.1\*10=81 cfs/ft

[For setback of 50 ft:]{.underline}

q2=8.1\*4=32.4 cfs/ft

Step 3. Compute contraction scour depth

> The ABSCOUR program will compute two scour depths for each setback for
> two sediment transport modes (live-bed and clear-water). All together
> four values will be included on the output sheet. For this example,
> only the live-bed contraction scour computations for the two setbacks
> will be presented. The sediment transport coefficient, k2, is computed
> as 0.638.
>
> [For setback 0 ft:]{.underline}

Approach section y1=9.8 ft; q1=1600/20=80 cfs/ft

Bridge section y2 =(to be computed) ; q2=8.1\*10=81 cfs/ft

> [Computation by Lausen's Equation, for a setback equal to zero (at the
> channel bank):]{.underline}

y2/y1=(81/80)\^0.638=1.01

y2=1.01\*9.8=9.89 ft

[For setback 50 ft:]{.underline}

Approach section: y1=3.8 ft ; q1=12 cfs/ft

Bridge section: y2= value to be computed; q2= 8.1\*4=32.4 cfs/ft

> y2/y1=(32.4/12)\^0.638=1.88
>
> y2=1.88\*3.8=7.14 ft

Step 4. The contraction scour for the setback of 30 ft requires
interpolation. ABSCOUR will use two appropriate values based on the
modes of sediment transport in the channel and the overbank, one at 0 ft
setback and another at 50 ft setback. In this example, only the live bed
condition is used. The contraction scour for a setback at 30 ft is
calculated as:

y2=7.14+(9.89-7.14)\*((50-30)/(50-0))\^2.5=7.14+0.278=7.42 ft

## [B. Intermediate Setback of 70 Feet -Wide Overbank Section - CASE B in Figure A1-1]{.underline}

The Intermediate Setback zone exist only for an overbank wider than 6.67
y~0~. For this example the channel flow depth is 10 ft and the right
overbank at bridge is 100 ft. The intermediate zone exists. The
computation of contraction scour depth for the right setback of 70 ft is
as follows:

Step 1. Compute flow velocity

For an intermediate setback, the flow is neither mixed nor separate. It
will gradually change from mixed flow to separate flow. ABSCOUR first
computes the mixed flow velocities at 5y~0~=50 ft setback and separate
flow velocity at 0.75W=75 ft setback. Then, the velocity at 70 ft
setback will be computed by linear interpolation.

[For 50 ft setback:]{.underline}

V2= Q/A = (600+1600+1200)/(20\*5+20\*10+50\*4)=6.8 ft/s

[For 75 ft setback:]{.underline}

V2=Q/A = 1200/(75\*4)=4 ft/s

[For 70 ft setback:]{.underline}

by linear interpolation

V2=4+(6.8-4)\*(75-70)/(75-50)=4.56 ft/s

Step 2. Compute unit discharge

q2= V\*y0 = 4.56\*4=18.27 cfs/ft

Step 3. Compute contraction scour depth

y2/y1=(18.27/12)\^0.638=1.31

y2=1.31\*3.8=4.98 ft

## [C. Long Setback CASE C in Figure A1-1]{.underline}

For a long setback, the flow in the overbank is considered independent
and not affected by the channel flow. For the setback of 80 ft, the
contraction scour will be

Step 1. Compute unit discharge

q2=1200/80=15 cfs/ft

Step 2. Compute contraction scour

y2/y1=((15/12)\^0.638=1.15

y2=1.15\*3.8=4.37 ft

## [D. Special Case Intermediate Setback-Narrower Overbank - CASE D in Figure A1-1]{.underline}

When the setback \> 5y~0~ in a narrow overbank section (width \<
6.67y~0),~ there is no intermediate flow; consequently, the normal
interpolation does not apply. For this case

(Figure 1c ), ABSCOUR will compute contraction scour assuming that the
setback is equal to 5y~0~ for a conservative approximation. For example,
the contraction scour for a setback of 60 ft in a 65ft-wide overbank in
Figure 1c will be computed the same as that for a setback of 50 ft.

Step 1. Compute flow velocity assuming the setback is at 5y0=50ft

V2=(600+1600+1200)/(20\*5+20\*10+50\*4)=6.8 ft/s

Step 2. Compute unit discharge

q1=1200/65=18.46 cfs/ft

q2=6.8\*4=27.2 cfs/ft

Step 3. Compute scour depth

y2/3.8=(27.2/18.46)\^0.638=1.28

y2=1.28\*3.8=4.87 ft

Figure A1-1 illustrates the four contraction scour examples presented
above for varying setback distances. Figure A1-2 illustrates the
resulting contraction scour for these cases, although the details of the
abutment scour calculations are not presented.

The general procedure to compute the abutment scour flow depth is:

y~2a~ = kf \*(k~v~)^k2^ \*(contraction scour)

The final abutment scour depth is computed using the equations presented
in Part 1.

![](media/image27.wmf)

![](media/image28.wmf)

##### **ATTACHMENT 2: COMPLEX APPROACH FLOW CONDITIONS**

The ABSCOUR Program computations are based on rectangular sections for
the channel and overbank areas in the approach section and the bridge
section with a straight channel reach between the sections. However, the
user has considerable flexibility in assigning input values on the
ABSCOUR menu cards so that the program can be used to model much more
complex flow patterns. Examples of these flow patterns might include:

1.  a bridge on a bend in the channel,

2.  large overtopping flows on one or both approach roads,

3.  the confluence of a tributary stream just upstream of the bridge,
    and

4.  combinations of the above conditions.

The accuracy of the modeling for such cases will depend on the skill and
experience of the user in evaluating flood flows. It requires the user
to be able to visualize the flow condition so as to select a reasonable
flow distribution at the bridge. In some cases, the momentum equation or
other computational methods can be employed to assist with this
visualization.

The ABSCOUR computations are illustrated in the table below, with all
numbers representing flood flows in cfs:

  -------------------- ----------------- ------------- -------------------
                       LEFT OVERBANK     CHANNEL       RIGHT OVERBANK

  APPROACH SECTION     500               2000          250

  OVERTOPPING          300               0             0

  BRIDGE SECTION       500 - 300 = 200   2000- 0 =     250 - 0 = 250
                                         2000          
  -------------------- ----------------- ------------- -------------------

The user inputs the discharges for the approach section flows and the
bridge flows, based on the results obtained from the HEC-RAS runs. As
discussed earlier, the HEC-RAS program computes flow on the basis of
conveyance. For complex, rapidly changing conditions upstream of the
bridge, conveyance calculations may not represent the worst-case scour
conditions.

Four examples are presented below to discuss the evaluation of the
HEC-RAS flow distribution and to suggest approaches to use in arriving
at the worst-case scour condition as a part of the sensitivity
assessment of the scour calculations.

# [I. Example 1: Typical Flow Distribution]{.underline}

![](media/image29.png){width="6.009722222222222in"
height="4.823611111111111in"}

**Figure A2-1: Flow re-distribution example**

Example 1 presents information obtained from HEC-RAS for a straight
reach, depicting the flow distribution at the approach and bridge
sections. In the HEC-RAS model, overtopping flow is subtracted from the
approach flow to compute the flow through the bridge. This appears to be
a reasonable flow distribution at the bridge to use in the ABSCOUR
computations

# 

# 

# 

# 

# [II. Example 2: Unbalanced Flow Condition]{.underline} 

The sketch on the left depicts discharge values obtained from HEC-RAS
for the approach and bridge sections. Note that there is 300 cfs at the
approach on the left overbank section (looking downstream) and 400 cfs
of overtopping flow at the left bridge section. HEC-RAS distributes the
flow under the bridge according to conveyance, and may underestimate the
flow at the right abutment.

![](media/image30.wmf){width="6.013888888888889in"
height="4.608333333333333in"}

**Figure A2-2: Flow re-distribution examples**

By inspection, some of the overtopping flow on the left is coming from
the main channel and the right overbank section. A rapid shift of the
flow from left to right occurs in order to meet the HEC-RAS distribution
based on conveyance. This redistribution of flow may not actually occur.
Accordingly, the user may wish to consider the consequences of a greater
flow on the right overbank section. A trial flow distribution, as
depicted on the right sketch, can be selected for a worst case type of
analysis. These values may be input instead of the HEC-RAS values to
assess worst case scour at the right abutment. The total flow through
the bridge remains the same in both cases, as does the total overtopping
flow. The difference is that the user can modify the program to provide
a different flow distribution under the bridge.

[III Example 3: Bend in the River]{.underline}

For a bridge located on a bend in the river, particularly a sharp bend,
momentum forces may affect the flow distribution under the bridge. More
flow may move to the outside of the bend than is indicated by the
HEC-RAS conveyance calculations. This condition can be investigated in
the ABSCOUR model by changing the HEC-RAS flow distribution.

[IV Example 4 Confluence Upstream of the Bridge]{.underline}

There can be a great deal of uncertainty about the flow distribution at
a bridge located just below the confluence of two streams. The location
of the confluence is likely to shift over time. Further, the time of
concentration of the two streams is likely to vary, affecting the
quantity and distribution of flood flows. A worst-case type of scour
analysis is recommended for this type of situation. Consider using two
or more flow distributions, assuming (1) a worst case condition for the
left abutment and then (2) a worst case condition for the right
abutment.

# **[ATTACHMENT 3]{.underline}** 

# **[SAFETY FACTORS]{.underline}**

In developing the ABSCOUR equations for estimating abutment scour,
available information from laboratory studies collected by the
consultant firm of GKY and Associates was used as a means of evaluating
the model. These laboratory tests were conducted in simple rectangular
straight channels (laboratory flumes) with uniform flow. A total of 126
data points were used to develop the envelop equation describing the
value of the coefficient for the spiral flow adjustment factor, k~f~

Natural rivers are not accurately represented by the simple flow
conditions modeled in a laboratory flume. For practical design, use of a
safety factor is suggested to take into account the effect of complex
flow patterns which can be expected to occur at bridges abutments. .

The guide in the table below is suggested for assigning a safety factor
for the ABSCOUR program calculations for abutment scour.

SELECTION OF SAFETY FACTORS

  ------------------------------------------ ------------- ---------------
  Channel Description at Bridge Site         Base Safety   Incremental
                                             Factor        Safety Factor

  Straight channel with uniform flow.        1.0           Add 0.0

  Moderately meandering upstream channel                   Add 0.1

  Severely meandering upstream channel                     Add 0.2

  Channel with complex approach flow                       Add 0.1
  conditions (Sharp upstream bend in                       
  channel, confluence, unstable reach,                     
  lateral migration, etc.)                                 

  Non-tidal river with wide flood plains and               Add 0.1
  complex two dimensional river and flood                  
  plain flow patterns that may change with                 
  river stage where a 2-D analysis is                      
  appropriate but not available                            

  Tidal river with wide tidal flats or                     Add 0.1
  wetlands and complex two dimensional river               
  and flood plain flow patterns that may                   
  change with river stage where a 2-D                      
  analysis is appropriate but not available                
  ------------------------------------------ ------------- ---------------

This table is used in the following manner. The user reviews the site
conditions or descriptors which are present at the bridge site under
consideration, and selects all of the factors in the table that apply to
the crossing site under consideration. The user then adds up the
applicable incremental factors and adds the total to the base safety
factor of 1.0 to compute the overall safety factor for the site. The
method is illustrated by the following examples:

1.  A stream with a relatively straight, stable upstream reach: SF = 1.

2.  A stream with a moderately meandering channel: SF= 1 +0.1 = 1.1

3.  A stream with severe upstream meanders SF = 1 + 0.2 = 1.2

4.  A stream with severe upstream meanders plus a complex upstream
    approach flow pattern due to a confluence:

> SF = 1 + 0.2 + 0.1 = 1.4

5.  Non-tidal river with moderately meandering channel with wide flood
    plains and complex two dimensional river and flood plain flow
    patterns that may change with river stage where a 2-D analysis is
    appropriate but not available SF = 1 +0.1 +0.1 =1.2

6.  Severely meandering tidal river with wide tidal flats or wetlands
    and complex two dimensional river and flood plain flow patterns that
    may change with river stage where a 2-D analysis is appropriate but
    not available. The bridge site is located just below the confluence
    of two rivers.

> SF = 1.0 + 0.2 +0.1 + 0.1 = 1.5

**ATTACHMENT 4**

**[CRITICAL VELOCITIES IN COHESIVE SOILS]{.underline}**

There are no definitive data available for determining critical
velocities in cohesive soils. In an unpublished paper (Permissible Shear
Stresses/Critical Velocities, 2005) Sterling Jones, Research Engineer,
FHWA, has collected and commented on various methods available in the
literature regarding this subject. The Office of Bridge Development has
conducted limited tests of critical velocities in cohesive soils using
the EFA Apparatus in the SHA Soils Lab. On the basis of this existing
information, OBD recommends the following:

1.  For preliminary guidance on estimates of critical velocities in
    cohesive soils, use the figure below developed from information in
    Neill's "Guide to Bridge Hydraulics, Second Edition, June 2001"
    (Please note that there are two lines drawn close together for the
    top two curves representing two different soil types. The top line
    is comprised of straight lines drawn through the data points in
    Neill's table. The lower line is a curve mathematically fitted to
    the data points.

2.  For more refined estimates of the critical velocity of cohesive soil
    layers at a bridge site, take Shelby Tube samples of the various
    soil layers and test them in the EFA Apparatus in the SHA Soils Lab.

![](media/image31.wmf)