Project Name

The name of the project


Project Number

The project number used in your organization


Description of the project

A short description of the project and the scheme of the data. Provide the name of the highway and stream being crossed. Include the magnitude and frequency of the flood flow beging evaluated (for example Q100=20,000 cfs)


Bridge Upstream Section Utility

This utility is mainly used to import bridge upstream cross-section data from existing HEC-RAS project file. The data format is the same as the corresponding data at the downstream section of the bridge. This utility may be used to obtain the low chord elevations at the upstream side of the bridge that are part of the ABSCOUR input data. It can also be used to obtain the streambed elevation and low chord elevation at upstream side of the pier that are required for local pier scour analysis.


Critical & Boundary Shear Stress Override Option

Program can calculate the critical shear stress based on median particle size of the bed material and also can calculate the boundary shear stress at the stream bed based on the flow depth and flow velocity.

Critical shear stress (psf)

τ c = 4D50

Boundary shear stress (psf)

τ 1 =γ y1 S

Where: γ  is the water unit weight, y1 is the average flow depth or hydraulic depth and S is the energy slope of the flow.

User can override the above calculated value by choosing this option


Live Bed and Clear Water Scour Override Option

Program use the critical shear stress and the boundary shear stress to determine the scour type, whether it is live bed scour or clear bed scour. When the boundary shear stress at the approach section exceed the critical shear stress at the same section, then the scour is live bed scour. Otherwise it is clear water scour. User can override this scour type by choosing this option


Bridge (Culvert) Section Unit Width Discharge Override Option

Program calculates the unit width discharge through the overbank and channel sections by a weighted average depend on the abutment setback distance and the flow depths in the channel and overbank sections. User can override the unit discharge computed by the ABSCOUR program by choosing this option


Bridge/Culvert Section Critical Velocity Override Option

The bridge/culvert section critical velocity is calculated in the program by using the median particle size under the bridge/culvert. It is only applicable to the clear water scour, most likely in the overbank areas. The user can override the critical velocity by checking this option.

Suggested Values  for Critical Velocity in Cohesive Soils

Neill – Guide to Bridge Hydraulics, June 2001


Sediment Transport Parameter (k2) Override Option

Program calculate the sediment transport parameter as follows

K2 = 0.11(τ c/τ 1 + 0.4)^2.2+ 0.623

User can override this parameter by choosing this option


Clear-water Scour Method Option

The program default to use Neill’s method for clear water scour. User can choose the Laursen’s clear-water scour by choosing this option. See HEC-18 for Laursen’s clear water scour information. Please refer to the Users Manual for additional information.


English or Metric SI Units Option

Choose the input and output data units used


Channel & Overbank Transition Option

Since the program divide the flow into channel and two overbanks. If the abutment is right at edge of the bank, the scour at the abutment will be that for channel. If we setback the abutment a small distance away from the edge of the channel, the scour condition should not be very far away from that for channel. There is a transition zone between the channel flow and overbank flow. The program uses 5 times channel flow depth as the limit of this transition zone. Program calculates four scour cases for each overbank. They are:

1.	Clear-water scour of channel flow

1.	Clear-water scour of overbank flow

1.	Live-bed scour of channel flow

1.	Live-bed scour of overbank flow

The program then interpolates the transition zone after scour flow depth depending on the setback distance and the scour type in channel and in overbank of interest. For example, if the channel if live-bed scour and the overbank is clear-water scour and the setback is in transition zone, then the program will interpolate the after scour flow depth between case 2 and case 3 listed above.

However, when the overbank if armored, then this interpolation should not be applied. This option allows user to specify whether to use above interpolation or not to use the interpolation.


Approach Section Discharge – Left Overbank

The discharge in cfs (cms) of the left overbank in the approach section. See the user’s guide for appropriate selection of the approach section.


Approach Section Discharge – Channel

The discharge in cfs (cms) of the stream channel in the approach section. See the user’s guide for appropriate selection of the approach section.


Approach Section Discharge – Right Overbank

The discharge in cfs (cms) of the right overbank in the approach section. See the user’s guide for appropriate selection of the approach section.


Approach Section Flow Top Width – Left Overbank

The flow width, W1, of the left overbank of the approach section in feet (meters).  Use the active portion of the overbank only

Click here to see the sketch of section


Approach Section Flow Top Width – Channel

The flow width, W1, of the stream channel of the approach section in feet (meters).  Use the active portion of the channel only.

Click here to see the sketch of section


Approach Section Flow Top Width – Right Overbank

The flow width, W1, of the right overbank of the approach section in feet (meters).  Use the active portion of the overbank only.

Click here to see the sketch of section


Approach Section Flow Depth (Hydraulic Depth) – Left Overbank

The flow depth, y1 (hydraulic depth) of the left overbank of the approach section in feet (meters).  The flow depth is typically the flow area, A1, divided by the flow top width, W1.

Click here to see the sketch of section


Approach Section Flow Depth (Hydraulic Depth) – Channel

The flow depth, y1, (hydraulic depth) of the stream channel of the approach section in feet (meters).  The flow depth is typically the flow area, A1, divided by the flow top width, W1.

Click here to see the sketch of section


Approach Section Flow Depth (Hydraulic Depth) – Right Overbank

The flow depth, y1 (hydraulic depth) of the right overbank of the approach section in feet (meters).  The flow depth is typically the flow area, A1, divided by the flow top width, W1.

Click here to see the sketch of section


Approach Section Median Particle Size (D50) – Left Overbank

The median soil particle size of overbank in feet (meters), i.e., D50, for the approach section left overbank.  Refer to Chapter 11, Appendix E, Guideline for Determining Soil Samples in Streams and on Floodplains for more details. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.


Approach Section Median Particle Size (D50) – Channel

The median stream bed soil particle size in feet (meters), i.e., D50, for the approach section channel. Refer to Chapter 11, Appendix E, Guideline for Determining Soil Samples in Streams and on Floodplains for more details. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.


Approach Section Median Particle Size (D50) – Right Overbank

The median soil particle size of overbank in feet (meters), i.e., D50, for the approach section right overbank.  Refer to Chapter 11, Appendix E, Guideline for Determining Soil Samples in Streams and on Floodplains for more details. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.


Approach Section Critical Bed Shear Stress (psf/Pascal) – Left Overbank

This cell is to be used when the user desires to override the critical shear stress computed by the program for the Approach Section.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate (cohesive soils, etc. )


Approach Section Critical Bed Shear Stress (psf/Pascal) – Channel

This cell is to be used when the user desires to override the critical shear stress computed by the program for the Approach Section.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate (cohesive soils, etc. )


Approach Section Critical Bed Shear Stress (psf/Pascal) – Right Overbank

This cell is to be used when the user desires to override the critical shear stress computed by the program for the Approach Section.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate (cohesive soils, etc. )


Approach Section Stream Bed Boundary Shear Stress  (psf/Pascal) – Left Overbank

This cell is to be used when the user desires to override the boundary shear stress computed by the program.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate.


Approach Section Stream Bed Boundary Shear Stress (psf/Pascal) – Channel

This cell is to be used when the user desires to override the boundary shear stress computed by the program.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate.


Approach Section Stream Bed Boundary Shear Stress (psf/Pascal)– Right Overbank

This cell is to be used when the user desires to override the boundary shear stress computed by the program.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate.


Live Bed or Clear Water Scour – Left Overbank

Choose the scour type for the left overbank area


Live Bed or Clear Water Scour – Channel

Choose the scour type for the channel area


Live Bed or Clear Water Scour – Right Overbank

Choose the scour type for the right overbank area


Sediment Transport Parameter – Left Overbank

This cell is to be used when the user desires to override the sediment transport parameter computed by the program.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate.


Sediment Transport Parameter – Channel

This cell is to be used when the user desires to override the sediment transport parameter computed by the program.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate.


Sediment Transport Parameter – Right Overbank

This cell is to be used when the user desires to override the sediment transport parameter computed by the program.  The user is cautioned to use the program override only for those situations where the program formulas are not appropriate.


HEC-RAS discharge under bridge - Left overbank

Enter the portion of the total left overbank discharge that flows under the structure from HEC-RAS output in cfs (cms).

For some cases such as (1) bends; (2) large overtopping flows over approach roadways; or combinations of (1) and (2), the flow distribution under the bridge becomes complex and HEC-RAS may not be able to properly model these conditions.  Under these conditions, user can override the discharge under the bridge to account for such complex flow patterns. Several examples of such problems and suggested solutions are presented in the ABSCOUR Users Manual, Chapter 11, Appendix A.


HEC-RAS discharge under bridge - Channel

Enter the portion of the total channel discharge that flows under the structure from HEC-RAS output in cfs (cms).

For some cases such as (1) bends; (2) large overtopping flows over approach roadways; or combinations of (1) and (2), the flow distribution under the bridge becomes complex and HEC-RAS may not be able to properly model these conditions.  Under these conditions, user can override the discharge under the bridge to account for such complex flow patterns. Several examples of such problems and suggested solutions are presented in the ABSCOUR Users Manual, Chapter 11, Appendix A.


 HEC-RAS discharge under bridge - Right overbank

Enter the portion of the total right overbank discharge that flows under the structure from HEC-RAS output in cfs (cms).

For some cases such as (1) bends; (2) large overtopping flows over approach roadways; or combinations of (1) and (2), the flow distribution under the bridge becomes complex and HEC-RAS may not be able to properly model these conditions.  Under these conditions, user can override the discharge under the bridge to account for such complex flow patterns. Several examples of such problems and suggested solutions are presented in the ABSCOUR Users Manual, Chapter 11, Appendix A.


Override discharge under bridge

For some cases such as (1) bends; (2) large overtopping flows over approach roadways; or combinations of (1) and (2), the flow distribution under the bridge becomes complex and HEC-RAS may not be able to properly model these conditions.  Under these conditions, user can override the discharge under the bridge to account for such complex flow patterns. Several examples of such problems and suggested solutions are presented in the ABSCOUR Users Manual, Chapter 11, Appendix A.


Downstream water surface elevation under bridge

The water surface elevation under the bridge on the downstream side of the bridge..  This information is determined from a water surface profile program such as HEC-RAS.  If the water surface elevation is higher than the low chord of the bridge, use the water surface elevation immediately downstream of the bridge.


Water Surface Elevation at Datum Point Inside The Culvert

The datum point is a point inside the culvert to be selected by the user.  The datum point is selected to represent an average flow depth inside the culvert.

The following guidance pertains to selection of the datum point.

•	For a short culvert, similar to a bridge, select the datum point at the culvert outlet.

•	For a long culvert, select the datum point downstream of the entrance at least 2 culvert widths in order to be at a point where the flow is contracted.

•	For a long culvert with subcritical flow and a tailwater above critical depth controlled by the downstream channel, select the normal depth in the culvert.  However, for a very high tailwater that submerges the culvert outlet, the flow depth is likely to be closer to the culvert height H.  (The flow depth selected should not exceed the value of H)

•	For a long culvert with subcritical flow in the barrel but critical flow at the outlet, further study needs to be made before calculating scour with this program (See limitations)

•	For a long culvert with supercritical flow in the barrel further study needs to be made before calculating scour with this program (See limitations)


Abutment type

Select the abutment type Vertical for vertical wall abutments without wingwalls or with wingwalls at an angle of either zero or 90 degrees.

Select the abutment type Wingwall for vertical wall abutments with wing wingwalls flared between 0 and 90 degrees.

Select the abutment type Spill-through for abutments constructed with a spill-through slope.


Abutment shape factor, Left and Right Over-banks

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 spill-through 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 spill through slope and (2) the total horizontal abutment and approach road length, X2, that is within the effective flow width of the approach flow.  Please refer to the Figure below for an illustration of the X1 and X2 values.  As indicated in the Figure, measure or estimate  X1 and X2 for spill-through slopes on the ABSCOUR cross-section; not on the actual cross-section:

•	The X1 value for a flared wing-wall is the horizontal distance perpendicular to the flow from the abutment face to the end of the wing-wall

•	The X1 value for a spill-through slope is the horizontal distance perpendicular to the flow between the abutment toe (on the ABSCOUR cross-section) and the location of the water surface line on the spill-through slope.  (In some cases, the water surface may extend back to the abutment.)

•	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 a function of the ratio of X1/X2. Equations 1-29 and 1-30 in the Users Manual compute the value of Kt.  Kt is used in Equation 1-28 to compute the final abutment scour, taking into account the abutment shape.

Figure 2-12 Abutment Shape Factor

(Measure or Estimate the X1 and X2 Values for Spill-Through Slopes Using the ABSCOUR Cross-Sections)


Abutment shape factor (X2)  Left Overbank


Abutment shape factor (X1) – Right Overbank


Abutment shape factor (X2) Right Overbank


Abutment setback (ft/m)

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.

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

•	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.

•	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.

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

Click here to see a definition sketch of setback for an abutment

Click here to see a definition of setback for a bottomless culvert


Right abutment setback (ft/m)

The horizontal distance (ft/m) measured from the right downstream edge of the stream channel to the right abutment face (or culvert wall for bottomless culverts) for vertical or wingwall abutments.

For a spill-through slope, the setback is measured from the right edge of the channel to the point where the spill-through slope intersects the ABSCOUR cross-section.  The setback is negative if the abutment projects into the channel.

Click here to see a definition sketch of setback for an abutment with a spill-thru slope

Click here to see a definition of setback for a bottomless culvert


Embankment skew angle - Left overbank

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


Embankment skew angle – Right Overbank

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


Waterway Area Measured Normal to The Flow At Downstream End of The Structure – Left Overbank

The left overbank flow area under the bridge/culvert at the downstream end of the structure measured normal to the flow.  This area must be measured by the program user.  The area does not include bridge piers or flow above the low chord elevation.

Click here to see the sketch of bridge section.

Click here to see the sketch of culvert section


Safety Factor

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, kf

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 Factor	Incremental 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 conditions (Sharp upstream bend in channel, confluence, unstable reach, lateral migration, etc.)		Add 0.1
Non-tidal rivers 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		Add 0.1
Tidal rivers 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. Guidance on what constitutes a wide flood plain might include the existence of a Rosgen C, E or F type stream with a meander belt; and a  total flood plain width (excluding the channel) equal or greater than 20 times the channel flow depth for the design flood.		Add 0.1

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 = 1.3

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 = 1.4


Waterway Area Measured Normal to The Flow At Downstream End of The Structure – Channel

The channel flow area under the bridge/Culvert at the downstream end of the structure measured normal to the flow.  This area must be measured by the program user. The area does not include bridge piers or flow above the low chord elevation.

Click here to see the sketch of section.

Click here to see the sketch of culvert section


Waterway Area Measured Normal to The Flow At Downstream End of The Structure – Right Overbank

The right overbank flow area under the bridge/culvert at the downstream end of the structure measured normal to the flow.  This area must be measured by the program user. The area does not include bridge piers or flow above the low chord elevation.

Click here to see the sketch of section.

Click here to see the sketch of culvert section


Estimated long-term aggradation (+) or degradation (-)  (ft/m)  - Left Overbank Section

The estimated long-term change in the elevation of the left overbank section under the bridge/culvert over the service life of the structure.  Enter aggradation values as (+) and degradation values as (-).  Sources of information regarding long-term change in the stream channel and flood plain include the scour report, the stream morphology report, bridge inspection records, and studies by the Army Corps of Engineers, etc.

The user needs to decide how to take degradation into account.  The preferred method for evaluating the effect of degradation is to make appropriate changes to the channel and flood plain cross-sections, rerun HEC-RAS and use the revised hydraulic parameters obtained from this revision as input into the ABSCOUR Program to recompute abutment, contraction and pier scour.  Please refer to Users Manual for additional information.


Estimated long-term aggradation (+) or degradation (-)  (ft/m) –  Stream Channel.

The estimated long-term change in the elevation of the stream channel under the bridge/culvert over the service life of the structure.  Enter aggradation values as (+) and degradation values as (-).  Sources of information regarding long-term change in the stream channel and flood plain include the original bridge plans, the scour report, the stream morphology report, bridge inspection records, and studies by the Army Corps of Engineers, etc.

The user needs to decide how to take degradation into account.  The preferred method for evaluating the effect of degradation is to make appropriate changes to the channel and flood plain cross-sections, rerun HEC-RAS and use the revised hydraulic parameters obtained from this revision as input into the ABSCOUR Program to recompute abutment, contraction and pier scour.  Please refer to Users Manual for additional information.


Estimated long-term aggradation (+) or degradation (-)  (ft/m) – Right Overbank

The estimated long-term change in the elevation of the right overbank section under the bridge/culvert over the service life of the structure.  Enter aggradation values as (+) and degradation values as (-).  Sources of information regarding long-term change in the stream channel and flood plain include the scour report, the stream morphology report, bridge inspection records, and studies by the Army Corps of Engineers, etc.

The user needs to decide how to take degradation into account.  The preferred method for evaluating the effect of degradation is to make appropriate changes to the channel and flood plain cross-sections, rerun HEC-RAS and use the revised hydraulic parameters obtained from this revision as input into the ABSCOUR Program to recompute abutment, contraction and pier scour.  Please refer to Users Manual for additional information.


Average Flow Depth at Datum Point Inside Culvert

The datum point is a point inside the culvert to be selected by the user.  The datum point is selected to represent an average flow depth inside the culvert.

The following guidance pertains to selection of the datum point.

•	For a short culvert, similar to a bridge, select the datum point at the culvert outlet.

•	For a long culvert, select the datum point downstream of the entrance at least 2 culvert widths in order to be at a point where the flow is contracted.

•	For a long culvert with subcritical flow and a tailwater above critical depth controlled by the downstream channel, select the normal depth in the culvert.  However, for a very high tailwater that submerges the culvert outlet, the flow depth is likely to be closer to the culvert height H.  (The flow depth selected should not exceed the value of H)

•	For a long culvert with subcritical flow in the barrel but critical flow at the outlet, further study needs to be made before calculating scour with this program (See Limitations)

•	For a long culvert with supercritical flow in the barrel further study needs to be made before calculating scour with this program (See Limitations)


Top width (T) measured normal to the flow (ft/m) – Left overbank

The flow width of the left overbank section under the bridge at the downstream side measured normal to the flow in feet  (meters).

Click here to see the sketch of section.


Top width (T) measured normal to the flow (ft/m) – Channel

The flow width of the channel section between the right and left channel banks under the bridge at the downstream side measured normal to the flow in feet  (meters).

Click here to see the sketch of section.


Top width (T) measured normal to the flow (ft/m) – Right overbank

The flow width of the right overbank section under the bridge at the downstream side measured normal to the flow in feet  (meters).

Click here to see the sketch of section.


Culvert Flow Width (W)

For a rectangular culvert, this is the top flow width. For an arch culvert, this will usually be the width at the spring line of the arch. In any case, use the maximum flow width for the section.  Please note that this width needs to be subdivided into left overbank, channel and right overbank and measured normal to the flow to develop the ABSCOUR cross-section.

Click here to see the sketch of structure cross-section


Low chord elevation at downstream side of bridge (ft/m)– Left Overbank

The average elevation of the low chord (or lowest part of the superstructure) on the downstream side of the bridge over the left overbank section.  This elevation (instead of the downstream water surface elevation) will be used to determine the elevation of contraction and abutment scour when the downstream water surface rises above the elevation of the low chord.

Click here to see definition sketch of the low chord elevation


Low chord elevation at downstream side of bridge (ft/m)– Channel

The average elevation of the low chord (or lowest part of the superstructure) on the downstream side of the bridge over the channel.  This elevation (instead of the downstream water surface elevation) will be used to determine the elevation of contraction and abutment scour when the downstream water surface rises above the elevation of the low chord.

Click here to see definition sketch of the low chord elevation


Low chord elevation at downstream side of bridge (ft/m)– Right Overbank

The average elevation of the low chord (or lowest part of the superstructure) on the downstream side of the bridge over the right overbank section.  This elevation (instead of the downstream water surface elevation) will be used to determine the elevation of contraction and abutment scour when the downstream water surface rises above the elevation of the low chord.

Click here to see definition sketch of the low chord elevation


Median particle size under bridge/culvert, D50 (ft/m) – Left Overbank

The median (D50) particle size in feet (meters) for the material in the left overbank section under the bridge/culvert. For clear water scour conditions the particle size should be representative of the bed material in the bottom of the scour hole. Refer to Chapter 11, Appendix E “Guideline for Determining Soil Samples in Streams and on Flood Plains” for information on collecting soil samples. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.


Median particle size under bridge/culvert, D50 (ft/m) - Channel

The median (D50) particle size in feet (meters) for the material in the channel under the bridge/culvert. For clear water scour conditions the particle size should be representative of the bed material in the bottom of the scour hole. Refer to Chapter 11, Appendix E “Guideline for Determining Soil Samples in Streams and on Flood Plains” for information on collecting soil samples. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.


Median particle size under bridge/culvert, D50 (ft/m) – Right Overbank

The median (D50) particle size in feet (meters) for the material in the right overbank section under the bridge/culvert. For clear water scour conditions the particle size should be representative of the bed material in the bottom of the scour hole. Refer to Chapter 11, Appendix E “Guideline for Determining Soil Samples in Streams and on Flood Plains” for information on collecting soil samples. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.


Bridge/Culvert Section Unit Width Discharge – Left Overbank

Program calculates the unit width discharge through the structure overbanks and channel by a weighted average that depends on the setback distance and flow depth at the bridge/culvert section. The user can override the unit discharge by checking this option


Bridge/Culvert Section Unit Width Discharge – Channel

Program calculates the unit width discharge through the structure overbanks and channel by a weighted average that depends on the setback distance and flow depth at the bridge/culvert section. The user can override the unit discharge by checking this option


Bridge/Culvert Section Unit Width Discharge – Right Overbank

Program calculates the unit width discharge through the structure overbanks and channel by a weighted average that depends on the setback distance and flow depth at the bridge/culvert section. The user can override the unit discharge by checking this option


Critical Flow Velocity Under Bridge – Left Overbank

This cell is activated if the user desires to override the value of the critical velocity computed by the program.  It will be advantageous to use the override for certain conditions, such as cohesive soils, where the critical velocity is based on factors other than particle size.

The override function should be used with caution and with a full understanding of how the ABSCOUR Program works.  Based on the user’s input for the Approach Section, the program determines whether the scour condition under the bridge will be clear water scour or live bed scour.  If clear water scour, the scour depth is determined on the basis that the average velocity of flow is equal to the critical velocity of the D50 particle size in the bottom of the scour hole. The critical velocity is that velocity, which if exceeded, will move the D50 particle. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.

For critical velocities in cohesive soils see this link


Critical Flow Velocity Under Bridge – Channel

This cell is activated if the user desires to override the value of the critical velocity computed by the program.  It will be advantageous to use the override for certain conditions, such as cohesive soils, where the critical velocity is based on factors other than particle size.

The override function should be used with caution and with a full understanding of how the ABSCOUR Program works.  Based on the user’s input for the Approach Section, the program determines whether the scour condition under the bridge will be clear water scour or live bed scour.  If clear water scour, the scour depth is determined on the basis that the average velocity of flow is equal to the critical velocity of the D50 particle size in the bottom of the scour hole. The critical velocity is that velocity, which if exceeded, will move the D50 particle. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.

For critical velocities in cohesive soils see this link


Critical Flow Velocity Under Bridge – Right Overbank

This cell is activated if the user desires to override the value of the critical velocity computed by the program.  It will be advantageous to use the override for certain conditions, such as cohesive soils, where the critical velocity is based on factors other than particle size.

The override function should be used with caution and with a full understanding of how the ABSCOUR Program works.  Based on the user’s input for the Approach Section, the program determines whether the scour condition under the bridge will be clear water scour or live bed scour.  If clear water scour, the scour depth is determined on the basis that the average velocity of flow is equal to the critical velocity of the D50 particle size in the bottom of the scour hole. The critical velocity is that velocity, which if exceeded, will move the D50 particle. Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.

For critical velocities in cohesive soils see this link


Flow Depth Upstream Face of Pier

The flow depth immediately upstream of the bridge at the nose of the pier in feet (meters).


Flow velocity immediately upstream of the pier

The flow velocity immediately upstream of the nose of the pier in fps (mps)


 Width of pier stem

The width of the pier perpendicular to the centerline of the pier in feet (meters).  For a pier with a varying width, use the average pier width between the channel bed (or footing) and the water surface.


Pier Scour Condition

Available pier scour conditions are

•	Pier footing not exposed  (See MDSHA procedures)

•	Pier with exposed footing or pile cap (See MDSHA procedures)

•	Pier with exposed pile cap and pile group (See MDSHA procedures)

•	Multiple columns skewed to the flow

•	Wide pier in shallow water


Units Option

Choose between English Units and Metric SI Units for input and output


Length of Pier Stem

This is the length of the pier measured along the centerline of the pier in feet (meters)


Angle of attack of the flow

The angle between the flow direction and the centerline of the pier in degrees


Override for Angle of Attack Correction Factor (K2)

Normally, the user should provide the angle of attack and let the program calculate the value of K2. This override option for the value of K2 is provided for situations where the computed value of K2 is considered to be excessive.  Such cases are discussed in the FHWA Manual HEC-18 in the discussion about selection of K2.


Pier Stem Nose Shape Correction Factor K1

Square nose: K1=1.1

Round nose: K1=1.0

Circular nose: K1=1.0

Sharp nose: K1=0.9

Group of cylinder: K1=1.0


Stream bed Condition Correction Factor K3

Refer to the FHWA Manual HEC-18 for a discussion of the stream bed conditions associated with typical flood flows frequencies used for estimating pier scour.

Clear-water scour: K3=1.1

Plane bed and Antidune Flow: K3=1.1

Small dune (dune height 10’>H>2’) (3m>H>0.6m): K3=1.1

Medium dune: (dune height 30’>H>10’) (9m>H>3m): K3=1.1 to 1.2 (program use 1.2)

Large dune: (dune height H>30’) (H>9m): K3=1.3


Override Option for the Armoring Correction Factor (K4)

When the user inputs the D50 and D95 of the bed material near the pier, the program will calculate the armoring correction factor, K4. Where warranted for special conditions, the user can override the program and provide a K4 factor in this cell. Normally, this cell should be left blank to allow the program to compute the K4 correction factor.


Median Soil Grain Size D50 for Armoring Factor K4

Median diameter of the bed material, the diameter of which 50% of the sizes are smaller, in feet or meters.  Refer to Chapter 11, Appendix E for guidance on selecting samples of bed material for analysis.


95% Finer Grain Size (D95)

This is the diameter of the bed material of which 95% of the sizes are smaller. Input in feet or meters


Distance from Stream Bed to top of Footing (Exposed Footing Height)

The distance from the stream bed to the top of footing or top of pile cap in feet (meters)


Grain Roughness of Streambed D84

For the case of pier with exposed footing, the grain roughness of the streambed at pier is normally taken as D84 in feet (meters).


Number of Pile Columns

For an exposed pile group this is the number of pile columns perpendicular to centerline of pier


Number of Pile Rows/Number of piles

For an exposed pile group this is the number of pile rows parallel to the centerline of pier. For a pile bent, this is the number of piles spaced along the centerline of the pier.


Pile Center to Center Spacing in The Pier Width Direction

The center to center spacing between piles perpendicular to centerline of the pier in feet (meters).

For battered piles select the average pile spacing as measured at the midpoint of the depth between the channel bottom and the bottom of the pile cap of the pier. If the water surface is below the bottom of the pile cap of the pier, select the average spacing at the average depth.


Pile Center to Center Spacing in The Pier Length Direction

The center to center spacing between piles along the centerline of the pier in feet (meters).

For battered piles select the average pile spacing as measured at the midpoint of the depth between the channel bottom and the bottom of the pile cap of the pier. If the water surface is below the bottom of the pile cap of the pier, select the average spacing at the average depth.


Pile Size in the Pier Width Direction

Pile size (width/depth or diameter) in the direction perpendicular to centerline of pier in feet (meters)


 Pile Size in the Pier length Direction

Pile size (width/depth or diameter) in the direction parallel to the centerline of pier in feet (meters)


Footing/Pile Cap Width

Width of the footing or pile cap size, measured perpendicular to the centerline of the pier in feet (meters)


Footing or Pile Cap Thickness

Thickness of the footing or pile cap in feet (meters)


Pile Group Additional Attack Angle Correction Factor Ka

This correction factor for angle of attack on the pile group is in addition to the K2 attack angle correction factor in HEC-18. This is used in Salim-Jones method. See figure 3 of their paper. Default is 1.0


Option of Debris Between Piles

User can choose to consider debris blocks the spacing between piles or no debris


Contraction and Abutment Scour Introduction

The method presented in this guideline for estimating abutment scour (ABSCOUR) is based on Laursen’s contraction scour equation as presented in the FHWA Publication HEC No. 18, Evaluating Scour at Bridges, Fourth Edition, May 2001. (1). This equation was originally derived by Straub (2) considering that the shear stresses (and thus the rates of sediment transport) in an uncontracted section and a contracted section are the same. It assumes a long contracted channel where the flow is considered to be uniform and the scour depth is constant across the channel section.

The contracting flow at the entrance corner of a channel constriction differs significantly from the conditions described above. The flow velocity across the channel is not uniform. The velocity near the edge of the constriction is faster than that in the midstream. Because of this higher velocity and its associated turbulence, the scour depth near the edge or corner of the constriction is usually deeper than in the center of the channel. The flow pattern at the upstream corner of an abutment will be similar to the flow at the entrance corner of a contracted channel, when the bridge approach roads obstruct overbank flow or the abutment constricts the channel. Local abutment scour can be expected to be deeper than the contraction scour in the center of the channel.

Chang has applied Laursen's long contraction theory to both clear-water and live-bed scour.  He developed a "velocity adjustment factor" kv to account for the non-uniform velocity distribution in the contracted section, and a "spiral-flow adjustment factor" kf at the abutment toe that depends on the approach Froude number.  The value of kv was based on 2-D potential flow theory, and kf was determined by Chang from the analysis of a collection of abutment scour experiments in laboratory flumes.


Laursen’s Live Bed Contraction Scour Equation

Laursen’s equation for estimating live bed scour in a contracted section in a rectangular channel can be expressed in the following form:

y2/y1 = (W1/W2) ^k2

where

y1, y2 = flow depths in the approach section and the contracted section.

W1, W2 = channel widths of the approach section and the contracted section

k2= experimental constant related to sediment transport

(Please note that this equation is a simplified form of Equation (5-2) in HEC-18 for a contraction of a rectangular channel with a uniform bed-material. )

The ratio of q2/ q1 may be substituted for W1/ W2 , and above equation may be written as :

y2/y1 = (q2/q1)^k2

where q1,q2 = unit discharges in the approach section and the contracted section

This is a comparative equation, equating the rates of sediment transport at the uncontracted and contracted sections. The equation applies to both clear-water and live-bed cases to the extent that the shear stresses in the two sections are considered equal. These cases are solved in ABSCOUR program by assuming that the critical velocity has been reached at the contracted section.

The contracted section, Section 2, is best represented for most cases as the downstream end of the bridge where the flow is still contracted. The upstream uncontracted section, Section 1, is typically considered to be about one bridge length upstream where the flow is uniform and not influenced by the bridge contraction.


Upstream Approach Section

Convert the actual cross-sections from the water surface profile model computer program to ABSCOUR model cross-sections for the subareas of the left overbank, main channel and right overbank facing downstream. Represent each subarea as a rectangle having a width and average depth. Obtain the width (W) and flow area (A) of each subarea from the output tables of the water surface profile model. Compute the average depth of flow for each subarea as (y)ave = y = A/W. In most cases this average depth will be close to the hydraulic flow depth which is computed as A/T where T is the top width of the flow in the subsection. The estimate of this value of hydraulic depth from the HEC-RAS model is acceptable for use in the model.

The ABSCOUR estimating procedure is based on the consideration that the cross-section at the approach section remains constant in the reach between the approach section and the bridge. Select the upstream model cross-section with this consideration in mind. However, the ABSCOUR program does take into account whether there is water behind the abutments in determining the extent of abutment scour. For bridges located on bends, the distribution of contraction scour needs to be assessed with regard to the effect of bendway scour (7).

Verify that values used for y (depth), V (velocity), W (width), q (discharge per foot of width) and Q (discharge) are consistent to assure that Q = VA (where A = W*y ) and q = V*y for each cross-section subarea.

Click here to see the sketch for sections

Click here to the the profile through the bridge


Bridge (Contracted) Section

A basic limitation of the current programs used to model flow through bridges such as HEC-RAS and WSPRO is that they provide for the distribution of flow under the bridge based on conveyance calculations. This approach does not appear to adequately reflect the three dimensional flow patterns observed in the field at bridge contractions. For scour calculations, it is important 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 channel. Converging flows under bridges with abutments near the channel banks tend to distribute uniformly, with higher local velocities observed at piers and abutments. 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. On the other hand, if an abutment is set well back from the channel bank, the overbank flow and the main channel flow tend to remain separated from each other and do not mix as the flow passes under the bridge. This concept is applied in the ABSCOUR model in the following manner.

Click here to see the sketch for sections

Click here to the the profile through the bridge


Short Abutments Setback Case (Case A)

When setback distance of abutments from channel no greater than 5 times the flow depth in the channel at Section 2, it is considered short setback case. When the setback is more than 75% the upstream flow width of the overbank, it is considered long setback case. The medium setback case is located between the short and long setback cases.

The assumption used for developing the velocity and flow distribution under bridge for the short setback is that the channel and overbank flow is entirely mixed together. The average velocity of flow (Vave ) under the bridge can be calculated as:

Va = Q / A

where

Q = total flow under the bridge, and

A = total channel and flood plain flow area under the bridge.

The discharge per foot (q) is computed as

q = Va * yo

where

yo = average depth of flow on the flood plain or in the channel

Note that the value of yo can be expected to be different for the left overbank, channel and right overbank areas.


Long Abutment Setback Case (Case C)

When setback distance of abutments from channel is greater than 75% of the flow width of overbank section at Section 2, it is considered the long setback case.

For this case, the assumption is made that the flow on the flood plain at the approach section remains on the flood plain as it flows under the bridge. Similarly, the flow in the main channel at the approach section remains in the channel under the bridge. Accordingly, the following relationship will hold true for flows on either the right or left flood plain subsections for the approach section (1) and the bridge section (2):

Q1 = Q2

or

q1 *W1 = q 2 *W2	(4b)

The discharge, Q, in any cross-section subarea (channel, overbank area) is computed as:

Q = q* W	 (5)

where

W = width of the cross-section subarea.

The average velocity is

  (Va)i=Qi/Ai

 Where (Va)i is the velocity at sub-area i, Qi is the discharge under bridge of the sub-area, Ai is the area of the sub-area.


Medium Abutment Setback Case (Case B)

When the abutment setback is between 5 times the channel flow depth at the bridge section and 75% of the upstream overbank flow width, it is considered to be the medium setback case.

The average velocity at the overbank area is adjusted as following equation

Vmix=Qt/At

(Va)c=Vmix

Vo=Qo/Ao

(Va)o=Vmix-(Vmix-Vo)/(0.75*Wo-5*yc)*(Setback-5*yc)

Where:

Vmix is the velocity of the totally mixed flow condition., i.e., average total flow under bridge.

(Va)c is the average channel flow velocity and is equals to Vmix in this case

Vo is the overbank flow assume separate flow condition (long setback condition)

(Va)o is the average overbank velocity for this medium setback case

This case provides for a smooth transition between the short and long setback cases.

There is a special case where the above interpolation method for a medium setback may over-predict the flood plain velocity.  The ABSCOUR program prints a warning for this case when the overbank flow velocity is greater than the channel flow velocity

The special case applies to crossings of wide swamps/flood plains where most of the flow is carried on the flood plain and where the channel flows in a meandering pattern with shallow depths, low velocities and small discharges

The recommended procedure in this case is to use the override procedure to input a unit discharge for the medium setback abutment under the bridge.  Calculate the unit discharge under the bridge as indicated below for an example where the medium setback is on the left overbank:

q (left overbank under bridge) = Q (left overbank at approach section)/ medium setback distance between channel bank and the left abutment.

The user needs to evaluate the answer obtained by the override procedure to see if it is reasonable. For example, compare the override unit discharges with the unit discharges obtained from the HEC-RAS program.


Contraction Scour Adjustment for Pressure Flow

(y2)adj=y2*Kp

Where:

y2 is the after scour flow depth for open channel flow calculated by Laursen’s Eqaution

Kp is the pressure flow adjustment factor.

kp = 1.1     when (y2)us>1.2*Hc

kp is linearly varied between 1.0 to 1.1 when (y2)us is between Hc and 1.2*Hc

Parameters already defined for open channel flow click here.

The net contraction scour for pressure flow is

(ys)adj=(y2)adj-(y0)adj

where:

(ys)adj = net depth of contraction scour adjusted for pressure flow

(y0)adj = adjusted depth of flow at downstream face of the bridge sections.


Determination of Sediment Transport Parameter, k2

The value of k2 varies from 0.637 to 0.857 depending on the critical shear stress of bed material to the boundary shear stress in the normal channel section. For clear-water flow it is 0.857 and for live-bed flows it is less depending on the ratio of shear stress to the critical shear stress of the bed material. Laursen (2) established the variation of k2 value as a function of τc/τ1 as shown in Figure 2.24 in ASCE Manual on Sedimentation (2). This curve may be approximated by the following equation for the English System:

k2= 0.11(τc/τ1 + 0.4)^2.2 + 0.623

where τc is the critical shear stress and τ1 is the boundary shear stress in the upstream or normal channel section. If τc is equal to or greater than τ1, then clear water scour can be expected to take place at the bridge, and the value of k2 should be selected as 0.857.

Critical shear stress, τc, may be calculated by several methods. For non-cohesive materials and for fully developed clear-water scour, Laursen (1) used the following simple equation for the English System:

τc = 4D50

where D50 is the median particle size in the section (channel bed or overbank area)  under consideration. On overbank areas, estimating the critical shear stress may also involve consideration of the flood plain vegetation.

The boundary shear stress, τ1, in the approach channel or overbank subarea may be calculated as:

τ 1 =γ y1 S

Where: γ  is the water unit weight, y1 is the average flow depth or hydraulic depth and S is the energy slope of the flow.


Contracted Section Overbank Flow Depth Adjustment

For a short abutment setback, the flow depth at overbank area at downstream end of the bridge shall be adjusted to account for a gradual change from channel flow depth to overbank flow depth. A constant bank slope is used to calculate this transition.

For the purpose of the adjustment, the setback limit is the horizontal bank slope length. When the abutment setback exceeds this limit, no adjustment is required. When the abutment setback is between 0 and this limit, the program adjusts the overbank flow depth as follows,

(y0)adj=yc-setback/Z

Where Z= bank slope horizontal projection when vertical projection is 1.

yc= before scour downstream section average channel flow depth

setback= the distance from the edge of channel to the face of the abutment for vertical and wingwall type  or  toe of the slope for the spillthrough type

 (y0)adj= adjusted downstream section overbank average flow depth.


Adjustment for Short Setback Abutment (Case A)

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)

Please note that any correction for pressure flow is already taken into account in the contraction scour computations.


Adjustment Factor For Velocity

The average flow velocity used in contraction scour analysis applies only to a long contraction where the flow velocity is considered uniform. For flow constricted by an abutment, the velocity across the section is not uniform because the velocity at the face of the abutment is higher. The average velocity used for contraction scour, accordingly, needs to be modified to account for the higher velocity and resulting deeper scour at the abutment.

The two dimensional potential flow pattern around a rectangular abutment was used for evaluating the velocity distribution across the contracted section. A study of the velocity distribution in this constricted section (3, 4) applying the principles of potential flow revealed that the ratio of the velocity at the toe of the abutment to the mean velocity of the flow in the contracted section can be approximated by the following equation:

kv = 0.8(q1 /q2)^1.5 + 1

where kv = ratio of velocity at the abutment toe to the mean velocity in the contracted section.

           q1 = unit discharge at the left or right of the approach channel

           q2 = unit discharge at the left or right of the channel at contracted section

This equation was developed considering flow through a simple contraction, where the unit discharge of the approach section is less than that in the contraction, q1  < q2 .

The values of kv should be limited to the range of values between 1.0 and 1.8. If the computed value is less than 1.0, use a value of 1.0; if the computed value is greater than 1.8, use a value of 1.8.


Adjustment Factor For Spiral Flow At Abutment Toe

The above discussion with respect to the velocity coefficient reflects the limited analysis available using two-dimensional flow concepts. The flow at an abutment toe is in spiral motion, which is three dimensional. Accordingly, a factor for adjusting two dimensional flow to three dimensional flow needs to be added to Laursen’s Equation. Available scour data for vertical-wall abutments were analyzed (5). The analyses resulted in the following two equations for determining the value of the spiral flow adjustment factor, kf .

For clear-water scour:

kf = 0.13 + 5.85F

For live-bed scour:

kf = 0.46 + 4.16F

where

kf = experimental coefficient for spiral flow at the abutment toe.

(The values of kf should range from 1.0 to 4.0. If the computed value is less than 1.0, use a value of 1.0; if the computed value is greater than 4.0, use a value of 4.0)

F = Froude number of the flow in the approach overbank subarea (see note below).

F = V1/ (gy1)^0.5

where 	V1 is the average velocity, y1 is the average depth in the approach subarea and g is the gravitational constant.

Note: above equation for kf is derived from the data by correlating to Froude number at the approach OVERBANK area. When the abutment encroaches more than 10% of the channel width, this equation may not applicable.


Adjustment Factor For Embankment Angle

When the approaching embankment is not perpendicular to the flow direction, the scour depth need to be adjusted by embankment angle factor ke,

Ke=(a/90)^0.13

Where:	a = embankment angle in degrees. See following figure for definition

ke = embankment angle factor


Abutment Scour Equation - Vertical Wall Abutments

The adjustment factors presented above are combined with Laursen’s contraction scour equation to develop the equation for abutment scour:

y2a/y1 = kp*kf (kv*q2/q1)^k2

where:

y2a = 	total depth of flow at the bridge abutment after scour has occurred

y1 = 	initial depth of flow at the approach section prior to the occurrence of scour.

q2 = 	unit width discharge at the bridge section

q1 =	unit width discharge at the approach section

kp =	pressure flow adjustment factor

kf =	spiral flow adjustment factor

kv =	velocity adjustment factor

k2=	sediment transport experimental constant


Computation Of Abutment Scour Depth For Open Channel Flow

For conditions of open channel flow at a bridge, the net scour depth for open channel flow is:

ysa = y2a – (yo)adj

where

ysa = 	 net depth of abutment scour before adjustment

y2a = 	depth of open channel flow at the bridge after abutment scour has occurred (see details here)  and

(y0)adj = adjusted initial depth of flow at the immediate downstream end of the bridge sections prior to the occurrence of scour. Click here for the depth adjustment.

The above scour depth shall be adjusted for the abutment shape, the embankment angle and factor of safety,

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

Where:

FS= factor of safety

Kt= abutment shape factor

Ke=embankment angle factor


Computation Of Abutment Scour Depth For Pressure Flow

For conditions of pressure flow, abutment scour equation needs to be adjusted to account for the effect of the modified flow conditions by multiplying by the pressure coefficient, kp:

(y2a)adj= y2a*Kp

where

y2a = open channel after scour flow depth for abutment scour (see details here)

(y2a)adj= pressure flow after scour flow depth for abutment scour

kp = 1.1     when (y2)us>1.2*Hc

kp is linearly varied between 1.0 to 1.1 when (y2)us is between Hc and 1.2*Hc

(y2)us = The flow depth immediate upstream of the bridge

Hc= low chord height of the bridge from the stream bed or overbank

 kp ,has been determined experimentally (6), and is between range of 1.0 to 1.2. An average value of 1.1 is used in this program.

For conditions of pressure flow at a bridge, using the depths determined from above equation, the scour depth can be expressed as:

ysa = (y2a)adj – (y0)adj

where

ysa = net depth of abutment scour

(y0)adj = adjusted depth of flow at downstream face of the bridge sections. Click here for the depth adjustment

The above scour depth shall be adjusted for the abutment shape, the embankment angle and factor of safety,

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

Where:

FS= factor of safety

Kt= abutment shape factor

Ke=embankment angle factor


Modification For Wing-wall And Spill-through Slopes Kt

The ABSCOUR program calculates a Kt value based on the ratio=X1/X2 designated as SF in the equation below,

For an abutment with a spill-through slope:

  Kt  = 0.55 +0.05 (1/(SF) - 1)

For abutments with wing-walls:

  Kt  = 0.82 + 0.02((1/(SF) - 1)

For vertical wall abutments,

  Kt  = 1.0

If SF<0.1, then Kt=1.0


Contraction Scour – Approach Section Covered with Vegetation or Armored

Laursen's contraction scour equation, as mentioned above, assumes the bed materials and the shear stresses in the approach and the contracted sections are the same. Where the bed material of the approach section is not the same as the contracted section, the following equation is used. Where the upstream overbank section is covered with vegetation and no sediment is transported (clear water scour), either Neill's or Laursen’s clear water scour equations may be used in determining contraction scour. The bed material in the contracted section will be eroded until (1) the bed shear is reduced to its critical value, or (2) the flow depth increases until it reaches the depth where the shear stress is reduced to the value of the critical shear stress. The clear water contraction scour equation, then may be reduced to the following form:

y2 = yc

or

y2/yc = 1

where yc = flow depth in contracted overbank or channel section when bed shear is at the critical value.


Abutment Scour – Approach Section Covered with Vegetation or Armored Bed

The same analogy may be used in estimating abutment scour for a channel where the upstream bed or flood plain is covered with vegetation and no sediment is transported regardless the magnitude of the shear stress on the upstream bed. In such a case, the following equation should be used to compute the abutment scour depth.

y2/yc = kf (kv)^0.857

The value of the contraction scour, yc, can be determined by using either Neill's or Laursen's clear water scour equations and can be calculated as

yc=q2/Vc

where:

            q2 is the unit width discharge at bridge, Section 2.

           Vc is the critical velocity, and

            y2 is the flow depth at the abutment toe after scour has occurred.

The value of kf is dependent on the intensity of the spiral flow in the approach flow, and can be calculated by using the Froude number of the approach flow (See details).

The value of kv is related to the contraction ratio of the approach flow (See details).


Abutment Scour Computation: Step 1. Water Surface Profile

Prepare a water surface profile using HEC-RAS or WSPRO to model flow conditions  upstream of, through and downstream of the bridge.  The discharge selected for analysis should represent the worst case scour condition for the bridge.  This may require more than one run to develop the anticipated worst case conditions at the bridge. Typically, scour studies should evaluate the 100-year, the 500-year and the overtopping floods.

Carefully select Manning’s “n” values for the flood plain areas.  There appears to be a tendency of some engineers to underestimate the flood plain roughness, thereby assigning too much flow to the flood plain areas.  This results in higher estimates of overbank contraction scour and abutment scour.

The approach section must be selected so that it is upstream of the influence of the bridge contraction.


Abutment Scour Computation: Step 2. Development of Model Cross-sections

1.	Select the two cross-sections required to run the ABSCOUR program:

•	one section at the upstream approach section (Section 1) as defined by the water surface profile model, normally WSPRO or HEC-RAS,

•	one section under the bridge at the downstream end of the bridge, Section 2.

2.	Convert the actual cross-sections to  ABSCOUR model cross-sections  for the subareas of the left overbank, main channel and right overbank.  Represent each subarea as a rectangle having a width and average depth.  Obtain the top width (T) and flow area (A) of each subarea from the output tables of the water surface profile model.  Compute the average depth of flow or hydraulic depth for each subarea as yave =   y1 = A/T.

3.	Modify section 1 and 2 as necessary to reflect field conditions. 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.  The user has the option of selecting a unit discharge different from that computed by the program if the site under study varies somewhat from the assumptions used in the model. The flow width under the bridge must extend to the abutment. If it does not reach the abutment, the program will not compute abutment scour.

Click here to see the sketch for sections

Click here to the the profile through the bridge


Abutment Scour Computation: Step 3. Hydraulic Data for the Approach Cross-section

1.	The water surface profile models compute flow velocities, depths and discharges for the approach section on the basis of conveyance calculations.  Modify these values, if necessary to fit the ABSCOUR cross-sections as defined in Step 2.

2.	Verify that values used for y (depth), V (velocity), W (top width), q (discharge per foot of width) and Q (discharge) are consistent. 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.  Please note that flow width (W) and top width (T) are used interchangeably in the ABSCOUR Program

3.	Determine the D50 median grain size for material on the overbank areas and in the channel from field samples taken at the approach section (See also Step 5).  Please refer to chapter 11 Appendix E of the H&H Manual which contains a guideline for obtaining soil samples for scour analysis.


Abutment Scour Computation: Step 4. Hydraulic Data for the Bridge Section

Using the output data from the water surface profile model, enter the depth of flow, yo in the channel and on the right and left flood plains (1) immediately upstream of the bridge near its upstream face and (2) at a point immediately downstream near the downstream face.   The upstream depth will be used to evaluate pressure scour, and the downstream depth will be used as the basis for evaluating contraction scour in the channel and at the abutments.  The ABSCOUR program uses an adjusted yo value based on a linear interpolation between the channel depth and the flood plain depth to minimize the abrupt change in depth at the channel bank.

The flow width under the bridge is also required as input data. The flow width shall be measured normal to the flow for skewed bridges. This width times the yo at the downstream bridge section should equals the flow area under the bridge.


 Abutment Scour Computation: Step 5 Soils Data at the Bridge Section

Determine the D50 median grain size for material on the overbank areas and in the channel from field samples taken at the bridge site.  Locate the sample sites to be representative of the cross-section at the downstream end of the bridge.

Please note that this information is of particular importance in the event that clear water scour will occur for the worst case scour event.  Clear water scour estimates are very sensitive to particle size and vegetative cover.  For this case, determine the particle size of the bed material at the estimated depth of the contraction scour.  If the subsurface material varies with depth, this estimate may require a trial and error approach, utilizing the information from the soil borings.  Refer to Appendix C of Chapter 11 for additional discussion on the estimation of the critical velocity for particle movement.  We recommend that you independently check the field conditions and make your own determination of critical velocity; then use the over-ride feature to enter your value if it is different from the value computed by the program.

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 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 Highways in the River Environment 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.

The ABSCOUR clear water scour equations compute scour by comparing the average flow velocity with the critical velocity (competent velocity) of the D50 particle in the channel bed or overbank area at the estimated depth of scour.


Abutment Scour Computation: Step 6. Enter the Bridge Data

Specify the setback distance of the abutments on the right and left overbank areas at the downstream end of the bridge.  If the abutment projects into the channel, the distance of this projection should be entered as a minus value. For vertical wall abutments the setback is the distance from the edge of the channel to the abutment face.  For abutments on spill through slopes, measure the setback distance as the horizontal distance between the point where the spillthrough slope intersects the ABSCOUR cross-section to the point where the water surface intersects the spillthrough slope or the abutment face. Click here to see the definition sketch.

Specify the Abutment Type as Indicated:

·	vertical wall abutment

·	vertical wall abutment with wingwalls

·	abutment on a spill through slope

·Specify the upstream and downstream average low chord elevations for the left overbank, channel and right overbank subareas. Select these values carefully, especially for bridges on a grade.  The program will compute pressure scour if the value of yo, depth of flow at the upstream face of the bridge exceeds the elevation of the low chord.  Under certain conditions, the program will use the elevation of the downstream low chord to adjust the elevation of contraction and abutment scour.  This adjustment is typically required for bridges on a grade where only a portion of the low chord is submerged.


Sections


Profile


Plot Scour Cross-Section

The following guidance is provided with regard to drawing 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.

•	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.

•	 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 bottom of scour hole.

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

2.	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.


References

1.   FHWA, “Evaluating Scour at Bridges,” HEC No. 18, Fourth Edition, May 2001

2.  Vanoni, Vito A., Manual on Sedimentation, Sedimentation Engineering, ASCE Hydraulic Division, 1975.

3.  Kirchhoff, Robert H., Potential Flows, Computer Graphic Solutions, Marcel Dekker, Inc. New York, 1985.

4.  Milne-Thomson, L. M., Theoretical Aerodynamics, Fourth Edition, Macmillan, London, 1968.

5.  Palaviccini, M., “Scour Predictor Model at Bridge Abutments,” Doctor of Engineering Dissertation, The Catholic University of America, Washington, D.C., 1993.

6.  Chang, Fred, “ Analysis of Pressure Scour,” Unpublished Research Report,  1995.

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

8.  Maryland State Highway Administration, Office of Bridge Development, Manual for Hydrologic and Hydraulic Design, 2003.

9. Peggy A. Johnson, Pier Scour At Wide Piers, ASCE North American Water and Environment Congress, June 1996.

10. FHWA, “Bridge Scour and Stream Instability Countermeasures,” HEC No. 23, Fourth Edition,  May 2001

11. “Guide to Bridge Hydraulics”, Second Edition, Transportation Association of Canada, 2001.


Pier Local Scour Introduction

This program implements five pier scour conditions set forth in HEC-18, March 2001 Edition.

All methods are based on the basic FHWA HEC-18 scour equations with additional design procedures to account for complex pier geometry and bed load conditions. User should read and understand the HEC-18 pier scour equations. This program implements the HEC-18 pier scour equations based on the input provided by the user.


Local Pier Scour

•	Introduction

•	Pier foundation not exposed to the flow

•	Pier with footing or pile cap exposed to the flow

•	Pier with pile cap and pile group exposed to the flow

•	Multiple columns skewed to the flow

•	Wide Pier in Shallow water

•	SHA procedures for computing scour for complex piers


Pier with no exposed footing

ys/y1=2.0*K1*K2*K3*K4*[(a/y1)^0.65]*(Fr1^0.43)

Where ys= Local pier scour depth

y1= flow depth just upstream of the pier

K1= Correction for pier nose shape

K2= Correction for attack angle

K3= Correction for stream bed condition

K4= Correction for armoring

a= pier width

V1= Flow velocity just upstream of pier

Fr1= Froude number = V1/(g*y1)^0.5

Units to be consistent in ft & ft/sec or m & m/sec. Refer to HEC-18 for guidance in determining the factors in this equation.


Pier with exposed footing or pile cap

HEC-18 uses superposition method to determine the total scour for this case. The total scour is the sum of the scour components for the pier stem in the flow and the component for the footing or pile cap in the flow.

ys= (ys) pier + (ys) pc

Where:	ys is the total scour depth

(ys) pier is the scour component for the pier stem in the flow

(ys) pc is the scour component for the footing or pile cap in the flow

Please read HEC-18 for details on how to calculate (ys)pier and (ys)pc .


Multiple Columns Skewed To The Flow (Pile Bents)

HEC-18 uses equivalent pier width with the projection area the same as the total projection area of the piles, and the basic pier scour equation applies. When there is no debris, if the multiple columns are spaced 5 diameters or greater apart, the maximum scour depth can be limited to 1.2 times that for local scour of a single column (pile). Note, when the attack angle exceeds 5 degrees, the nose shape of the column will no longer have any impact and K1 will be adjusted to 1.0.   Since the attack angle is already accounted in the equivalent pier width, K2 will also be assigned a value of 1.


Pier With Exposed Pile Cap and Pile Group

HEC-18 uses a superposition method to determine the total scour for this case. The total scour is the sum of the scour components for the pier stem in the flow, the component for the pile cap in the flow and the scour component for the pile group in the flow.

ys= (ys) pier + (ys) pc + (ys) pg

Where:	ys is the total scour depth

(ys) pier is the scour component for the pier stem in the flow

(ys) pc is the scour component for the pile cap in the flow

(ys) pg is the scout component for the pile group in the flow

Please read HEC-18 for details on how to calculate (ys)pier, (ys)pc and (ys)pg


Wide Pier in Shallow Water

This method introduces a correction factor for a wide pier in shallow water. The experimental data used in the analysis have flow depth to pier width ratios in the range of 0.2 and 0.8. Therefore use of this data outside of these limits is not recommended.

The correction factor is as follows

Kw= 2.58*(y1/a)^0.34*Fr1^0.65      for V1/Vc<1

Kw= (y1/a)^0.13*Fr1^0.25               for V1/Vc>=1

Where y1= flow depth upstream of pier

 a= pier width

 Fr1= Frounde Number

V1= Flow velocity upstream of pier

Vc= Critical velocity

The Neill’s critical velocity is used as the default in the program. The above correction factor Kw is applied to basic pier scour equation to get the scour depth for wide pier in shallow water.

Limitations of Kw correction factor:

•	Only sand bed streams were available for calibration with (a/D50)>50. Use on gravel bed streams should be avoided or used cautiously.

•	y1/a values less than 0.8

•	Froude Number less than 1

Use of the correction factor outside these limits is not recommended. The program will assign Kw to 1 if the about limitation is exceeded.


About ABSCOUR

The abutment and culvert scour equations in this program are based on research studies conducted by Fred Chang.  The Maryland Bridge Scour Program represents a cooperative effort by Fred Chang and Stanley Davis of the Maryland SHA Office of Bridge Development, Chao Chen of the consultant firm of The Wilson T. Ballard Company and Kelly Brennan of the consultant firm of Parsons Brinkerhoff Quade & Douglas, Inc.


Channel & Overbank Transition Option

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 rectangular shapes, the calculated overbank scour may be based on clear water scour (as determined from the Approach Section calculations) when it is actually subject to live bed scour conditions from the main channel. There is obviously a transition zone between the no setback case and the long setback case. To make a smooth transition, the following scheme is used to calculate the scour in this transition zone. This scheme applies to both contraction scour and abutment scour, although the details are somewhat different. The program provides this option to disable this interpolation. In case of armored overbank and vegetation covered overbank, user should choose to disable this interpolation.


How To Use Help

This help includes documentation of the program and the context sensitive help on each required input data.

For the documentation, user can browse through the topics starting from introduction and using browsing buttons on the menu bar (<< for backward, >> for forward) to read topics in sequential order. User can also chose a specific topic to read from the table of contents. The topics are grouped into two major groups; one for abutment scour and one for pier scour. Some general topics are not grouped. User can read the ungrouped topics from the table of contents.

Click here to start browsing abutment scour documentation

Click here to start browsing pier scour documentation

Click here to start browsing bottomless culvert scour documentation

For the help on the input data, user can use <Tab> key to focus the input data or using mouse to point to the input data. The program will display a short hint at the status bar at bottom of the input form. For more extended help, while the input data is in focus, press <F1> key, program will display the extended help on the help window.


Bottomless Culvert Scour

The flow distribution in the channel and on the flood plain approaching the inlet of a bottomless culvert is similar to that in a channel contracted by a pair of vertical wall abutments. The upstream cross section of the channel and flood plain is generally wider than the culvert width and the flow velocity is lower than the velocity in the culvert. This creates a backwater condition upstream of the culvert and some of the sediment carried in the flow tends to deposit on the channel bed and the flood plain in the approach section. Consequently, the flow entering the culvert carries less sediment and approaches clear water scour condition.

Further, small culvert-sized streams in Maryland generally have well vegetated overbank areas. For worst case scour conditions, much of the flood flow will be contained in these overbank areas in the upstream approach section. As a result, the bed load delivered to the culvert from overbank flow is likely to be small in comparison with the total flow.

Based on the foregoing discussion, it is considered appropriate to utilize abutment scour equations in estimate scour in bottomless culverts and to assume a clear-water scour condition for the analysis. Estimated scour depths should be on the conservative and reasonable side.

If the designer is confident that live bed scour conditions will prevail for worst case scour conditions, the bridge abutment scour procedure can be used.

The cross-section of a typical bottomless culvert installation can be accessed by clicking on the link at the end of this discussion.  The actual channel cross-section is converted to the ABSCOUR cross-section in the same manner as is done by the ABSCOUR Program for a bridge opening.  Scour at the culvert walls corresponds to scour at the bridge abutments, and is computed in a similar manner.  Therefore, the terms ‘abutment’ and ‘culvert wall’ are used interchangeably in the help screens.

Click here for a definition sketch of a bottomless culvert cross-section

The following guidance pertains to selection of the datum point.

·	For a long culvert, select the datum point downstream of the entrance at least 2 culvert widths in order to be at a point where the flow is contracted.

·	For a long culvert with subcritical flow and a tailwater above critical depth controlled by the downstream channel, select the normal depth in the culvert.  However, for a very high tailwater that submerges the culvert outlet, the flow depth is likely to be closer to the culvert height H.  (The flow depth selected should not exceed the value of H)

·	For a culvert with subcritical flow in the barrel but critical flow at the outlet, further study needs to be made before calculating scour with this program (See limitations below)

.	For a culvert with supercritical flow in the barrel further study needs to be made before calculating scour with this program (See limitations below)

Contraction Scour

The contraction scour for a bottomless culvert is computed in the same manner as the vertical wall abutment scour case for a bridge opening. Please refer to the abutment scour section for further details. The analysis assumes clear-water scour and a vertical wall abutment.

Wall (Abutment) Scour

The wall area at the culvert inlet is a region of higher velocity flow due to the rapidly contracting flow and the resulting vortex action.  This is similar to the flow at a vertical wall abutment, resulting in localized scour that is deeper than the contraction scour in the channel.  The SHA abutment scour equations can be used to estimate the scour depth at the culvert wall near the culvert entrance. Please note that the computed abutment scour represents the total scour at the abutment.  It should not be added to the contraction scour. This is accomplished as follows: the contraction scour depth y2 computed above is multiplied by the correction factors, Kv and Kf to account for the higher velocity and vortex flow near the culvert wall. These correction factors are computed by the following equations (See also Appendices A, B and C of Chapter 11):

   Kv = 0.8(q1/q2)^1..5 + 1

    Kf = 0.13 + 5.85F1

Where

q1 = unit discharge of the overbank approach flow at the approach section 1 (cfs/ cfm per foot)

q2 = unit discharge next to the wall at the datum point within the culvert width (cfs/ cfm per foot).

F1 = Froude Number of approach flow: F1= V1/(gy1)^0.5

V1 = velocity of flow, ft/s

y1 = flow depth, ft

The term Kv is related to the effect of the higher flow velocity which occurs near the culvert wall.

The term Kf  is related to the effect of vortex flow on scour at the corner of the culvert.  The limits of the Kf value range from 1.0 to 4.0. If the value computed by the above equation is less than 1.0, use a value of 1.0. If the value computed is greater than 4.0, use a value of 4.0

The scour depth at the culvert walls, y2a can be written as:

  Scour depth, y2a = Kp Kf Kv^0.857y2

Where

y2a = total water depth at the culvert wall measured from water surface to the channel bed after scour has taken place.

y2 = total water depth at the datum point in the culvert measured from water surface to the channel bed after contraction scour has taken place.

Kp, the pressure flow coefficient, is defined below

Pressure flow coefficient Kp:

Pressure flow for the bottomless culvert is also different from that for the bridge. Following are the assumptions used in calculating the pressure flow coefficient Kp,

·	Pressure flow is assumed to occur only when the upstream flow elevation is higher than the crown of the culvert at the entrance.

·	The pressure coefficient Kp will be the same for overbank and channel sections inside the culvert

·	Kp varies from 1.0 (when the water elevation just reaches the crown) and increases to 1.1 when the submergence reach 1.2Hc where Hc is defined as the vertical distance from the channel bed to the culvert crown.  This is similar to the height of the low chord for a bridge.

Limitations to the bottomless culvert scour analysis.

Scour depth in the bottomless culvert program is computed using the clear water scour equations (Neill or Laursen).  The concept is that as scour increases, the flow area inside the culvert increases and the flow velocity decreases (V = Q/A).  When the flow velocity is reduced to the competent (or critical) velocity of the bed material, the scour will stop.

In order for this concept to work, there must be a downstream control that will maintain  flow depths inside the culvert so that velocity decreases as scour increases.  If there is no control downstream, then it is likely that channel degradation will occur downstream, through and upstream of the culvert.   If the user encounters this flow condition in the culvert, the bottomless culvert program should not be used to design the culvert since it does not account for the anticipated degradation.  (It may still be of use in estimating the magnitude of contraction and abutment scour.)

Hydraulic flow conditions should be check for several discharges to verify that the downstream controls are effective for low and intermediate flows as well as the design flow.

For conditions of supercritical flow, the user is encouraged to consider one of the following options:

·	Select an alternative culvert design

·	Modify the bottomless culvert design.  Construct downstream controls to minimize degradation and scour of the downstream channel.  Provide riprap to control scour and stabilize the culvert outlet.  Provide riprap in the culvert barrel of adequate D50 size to resist movement for design conditions.

Evaluation of the Computed Contraction and Abutment Scour

The contraction and abutment (or wall) scour is based on the use of the ABSCOUR cross-section as described in the section on bridge scour.  The user will need to compare the ABSCOUR cross-section with the actual cross-section and make a judgment as to the need to modify the scour values computed by the program to account for the actual field conditions.


Abutment Riprap Design

Abutment Riprap Design

This utility is based on The FHWA Publication HEC-23, March 2001 edition, Design Guideline 8 Section 8.7, Sizing Rock Riprap at Abutment.  These equations, along with additional guidance are also contained in the SHA Hydrologic and Hydraulic Manual Chapter 11, Appendix D

To size the riprap at an abutment, the user needs to know the characteristic average velocity in the contracted section.   This velocity depends on the setback distance of the abutment, which will be used to determine whether the flow at the abutment is a mixture of channel flow and overbank flow, or whether it is comprised only of overbank flow.

As explained in the F-1 help information, the method of computing the depth and velocity of the flow depends on the setback of the abutment from the channel.

Case a: Short setback on both abutments

If the setback is less than five times the hydraulic depth of flow in the channel, the flow is defined as mixed flow.  This means that the channel flow is mixed with the overbank flow.  For this case use the hydraulic flow depth in the channel at the downstream side of the bridge.  The velocity of flow (V) is defined as

V= Q/A where Q = the total flow through the bridge and A = the total waterway area of the flow.

Case b: Medium or long setback

If the setback is greater than five times the hydraulic depth of flow in the main channel, the characteristic flow depth is defined as the average depth of flow (hydraulic depth) in the overbank area between the abutment and the edge of the channel.  Similarly, the flow velocity is defined as the average flow velocity in the overbank area and is computed as follows:

 V = Q/A = Q overbank/ (overbank hydraulic depth x overbank top width between the abutment and the channel).

Case c: Short setback on this abutment but medium or long setback on other abutment

The average flow velocity is defined as the average mixed flow of this overbank area and the channel flow.

V=(Q overbank + Q channel)/(Overbank flow area + Channel flow area).

Normally, the data can be imported from the abutment scour analysis simply by pressing the [Import] button. If an abutment scour data file is not available, then the characteristic flow depth and flow velocity for the abutment can be input into the cells provided to calculate the D50 riprap size.  Select these values using the guidance in the F-1 tabs.

Selection of size of riprap.

The Utility program provides the minimum diameter riprap size to resist the calculated velocities.  If the calculated value of D50 is 16 inches or less, use Class 2 riprap.  If the calculated value of D50 is significantly over 16 inches, use Class 3 riprap.


Distance from Front End of Pier Stem to Front End of Footing

This is the horizontal distance from the upstream end of the pier stem to the upstream end of the footing or pile cap in feet or meters.


SHA Procedures for Computing Scour for Complex Piers

The following procedures are recommended by MDSHA and may not necessarily produce the exact same results as HEC-18. Figure 1 illustrates the various options discussed below.

Option 1 computes local scour for the pier stem only.  However, if the total scour (local scour plus contraction scour) computed by Option 1 is deeper than the top of the footing/pile cap, follow steps outlined below to obtain the best estimate of the scour depth

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

6.	If the modified total scour depth (local scour plus contraction scour) 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 7.

Option 3

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

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

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

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

11.	Compute the final local scour depth for Option 3 using the scour parameters determined in Steps 7 through 10.  Add this to the contraction scour to determine the total pier scour.


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 elevation to determine whether pressure flow occurs.  If so, a pressure scour coefficient factor is computed.


Is future lateral migration of the channel likely to occur?  (Channel)

Future lateral migration of the channel 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.

The ABSCOUR output sheet reflects the user’s yes or no answer to this question.  Support for this conclusion and further discussion of lateral movement should be included in the scour report. Please refer to Users Manual for additional information.


Distance From Left Edge of Water to Left Abutment Face (Spill-through Slopes Only)

Leave this cell blank for vertical wall abutments.  The table below can be used directly for vertical wall abutments because the setback is measured to the face of the vertical wall abutment.

For a spill-through abutment slope, enter the horizontal distance between the abutment face and the intersection of the water surface with the spill-through slope.  If the water surface elevation is above the spill-through slope so that it reaches the abutment, leave this cell blank.


Average Bank Slope (in the vicinity of the bridge) – Left Overbank

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.

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).

Click here to see a sketch of the average bank slope


Average Bank Slope (in the vicinity of the bridge) – Right Overbank

The average bank slope (Z) of the right 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.

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).

Click here to see a sketch of the average bank slope


Low chord elevation at upstream side of bridge - Channel

The average elevation of the low chord (or lowest part of the superstructure) on the upstream side of the bridge over the channel section.  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.

Click here to see definition sketch of the low chord elevation


Low chord elevation at upstream side of bridge - Left overbank

The average elevation of the low chord (or lowest part of the superstructure) on the upstream side of the bridge over the left overbank section.  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.

Click here to see definition sketch of the low chord elevation


Low chord elevation at upstream side of bridge - Right overbank

The average elevation of the low chord (or lowest part of the superstructure) on the upstream side of the bridge over the right overbank section.  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.

Click here to see definition sketch of the low chord elevation


Average Bank Slope Sketch


Approach Section Sketch


Sketch of Waterway Area and Top Width Measured Normal to The Flow At Downstream End of The Bridge


Abutment Setback


Right Abutment Setback and Shape Factor for a Spill-Thru Slope


Scour Definition Sketches

Click the item below to view the definition sketch

Definition of ABSCOUR Cross-Section

Definition of Contraction Scour Terms

Definition of Abutment Scour Terms


Definition of ABSCOUR Cross-Section

The ABSCOUR program utilizes the same equations to compute scour on bridge abutments and culvert walls.  The sketches below are designed to assist the ABSCOUR user in determining the proper input values to use in the model.

Bridge Opening:

The hydraulic flow depth (yo) at the downstream side of the bridge equals A/T.  The solid black line represents the bottom of the ABSCOUR cross section and the dashed brown line represents the existing bed elevation.

Bottomless Culvert Opening:

The hydraulic flow depth (yo) at the downstream side of the bottomless culvert equals A/W.  Notice that on the left overbank, the cross hatched area is equal to the light blue shaded area on the ABSCOUR cross section.


Definition of Contraction Scour

Scour parameters are shown for the left overbank and main channel only.  The same variables also apply to the right overbank section.


Definition of Abutment Scour

Scour parameters for abutment scour are shown for the left overbank section only.  The same variables also apply to the right overbank section.


Low Chord Elevation Sketch


Depth of Flow on the Downstream Side of the Bridge/Culvert

For abutments with a short setback (setback less than five times the hydraulic flow depth in the channel) or a negative setback use the hydraulic depth in the channel

For abutments with an medium or long setback, use the average depth of flow (hydraulic depth) in the overbank section between the channel and the abutment.

Please note that this information can be obtained from the ABSCOUR calculations.


Characteristic Average Flow Velocity on Downstream Side of Bridge

For an abutment with a short setback (setback less than five times the hydraulic flow depth in the channel) on both abutments, use the average flow velocity through the bridge (V = Q/A, where Q = discharge through the bridge and A = flow area under the bridge).  This is computed by the ABSCOUR program.

For an abutment with an medium or long setback, use the characteristic average flow velocity on the flood plain between the channel and the bridge abutment.

This flow velocity is computed as follows:

 V = Q/A = Q overbank/ (overbank hydraulic depth x overbank top width between the abutment and the channel.)

For an abutment with short setback, but other abutment with medium or long setback,  the flow velocity is calculated as follows:

V=(Q overbank + Q Channel)/(Flow Area of Overbank + Flow area of the channel)

Please note that this information can be obtained from the ABSCOUR calculations


Flow Depth

Typically, This is the average depth of flow in the channel or in the overbank area at the location where the determination of critical velocity is desired.


Median particle size (D50) (ft/m)

Guidance on selection of  the median particle size (D50) of the material in a channel or overbank area is provided in Chapter 11, Appendix E.  Please note if the particle size is smaller than the limit for fine sand (0.0006 ft.), then evaluate the cohesive properties of the soil when estimating its critical velocity.


Flow Velocity at Pier

The desired velocity is the velocity immediately upstream of the pier.

For a relatively straight channel, this velocity may be selected from the velocity distribution plots in HEC-RAS.  It may be determined more accurately from a 2-D hydraulic analysis.

The FHWA Publication HEC-23 suggests using the average channel velocity (Q/A) multiplied by a coefficient depending on the pier location.  This coefficient ranges from 0.9 for a pier near the bank to 1.7 for a pier in the main current of flow around a sharp bend.


Footing/Pile Cap Length

This is the length of the footing or pile cap measured along the center line of the pier in feet (meters)


Elevation of Culvert Crown at Datum Point

The elevation of the culvert crown at the datum point is not used directly in the scour computations.  However, the elevation of the water surface at the datum point cannot exceed the elevation of the culvert crown.  When the water surface is at the crown, use the perimeter of the culvert to define the waterway areas for the channel and overbank sections.

Click here to see definition sketch of the culvert crown elevation


Elevation of Culvert Crown at the Entrance

The elevation of the crown (or highest point inside the culvert) at the culvert entrance (Hc). This elevation will be used to determine the effect of pressure scour when the upstream water surface rises above the elevation of the crown.  Use this same elevation for both channel and overbank areas.

Click here to see definition sketch of the culvert crown elevation


Specific Gravity of the Riprap

The specific gravity of the riprap rock depends on the minerals it contains. Most common one is the quartz and may be estimated to be about 2.65.


Average Energy Slope between Approach Section and Bridge Section

Input the average energy slope between the approach section 1 and the bridge section 2. The bridge section is located inside of the downstream end of the bridge. The sketch below illustrates the computation of the average energy slope.

    Average Energy Slope Computation


Import Cross Section From HECRAS Project File

This program can import the HEC-RAS cross section of the stream at the approach and at the bridge. At the bridge, the program will also import the bridge deck data.

The User needs to be careful to select the appropriate HEC-RAS plan when using the import function.  Before importing the HEC-RAS cross-sections into ABSCOUR Program, we recommend that the user follow the procedure outlined below to make sure that the desired HEC-RAS plan is imported into ABSCOUR:

•	run and save the HEC-RAS Program with the geometry file and the flow file that is to be used in the scour analysis.  This file should be considered the current active plan.

•	Close the HEC-RAS program. This step establishes the desired plan (geometry and flow files) as the current active plan.

•	(Note: An alternative approach to avoid the potential for importing incorrect geometry is to save the desired HEC-RAS model as a Scour Model using File/Save Project, and deleting all plans and geometry files except the plan that scour is to be computed for.)

•	Now use the import procedure in ABSCOUR to select the desired project file and the appropriate approach section and bridge section.

-	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 points in the cross section and the left bank and right bank point stations.

-	For the bridge section, the program will search through the geometry file of the current active plan of HEC-RAS project and find the available bridges. If more than one bridge exists, a list of the bridges will be generated and the user can pick the desired bridge. If only one bridge exists, 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. Note, if the left bank and right bank do not match what is used in the scour analysis, the user needs to adjust these two stations as explained below

 -	The Program uses the left bank and right bank stations of the actual cross-section to calculate the station at center of the channel. The ABSCOUR cross section is then drawn based on this station. If no actual cross section is available, this station will be set to 0. If the channel top width as defined in the HECRAS cross section is the same as in the ABSCOUR cross section, then the ABSCOUR cross-section and the actual HECRAS cross-section should match at the channel banks. If these two channel widths are different, the user can adjust the left and right bank stations to match the ABSCOUR cross-section.

-	The program assumes that the orientation of the cross-section is the same in the HEC-RAS Program and the ABSCOUR Program, whether looking downstream or looking upstream. In case they are different, change the ABSCOUR input data to match the HEC-RAS data. This should not normally be a problem since the input data in ABSCOUR is usually obtained from the HEC-RAS output.

•	If the user inadvertently imports the wrong approach or bridge section station, the problem can be corrected by repeating the import procedure and selecting the correct station.

•	To graphically check the sections, click on Tools/Draw Section/OK.


Draw Cross-Section and Scour Profile

User can draw the approach cross-section, bridge cross-section and the bridge cross-section with the scour profile.

The ABSCOUT cross-section will be superimposed with the actual cross-section if available. The actual cross-section can be imported from the HECRAS project file or it can be input by user. Program use the left bank and right bank stations of the actual cross-section to calculate the station at center of the channel. The ABSCOUR cross section is then drawn based on this station. If no actual cross section is available, this station will be set to 0. If the channel top width as defined in HECRAS cross section is the same as in the ABSCOUR cross section, then the ABSCOUR cross-section and the actual HECRAS cross-section should match at the channel banks. If these two channel widths were different, user shall adjust the left and right bank stations to match the ABSCOUR cross-section. The actual cross-section will be drawn as long dash line. For cross-section at bridge, the top (high chord) and bottom (low chord) of the bridge deck will be drawn in dash-dot-dot pattern line.


 Approach Section Water Surface Elevation

This elevation is not used in the scour analysis; however, it is used as the baseline to tie in the ABSCOUR cross-section with the actual (HEC-RAS) cross-section.


Actual Approach Section Left Bank Station

The channel left bank station for the actual approach section. Note if the top width of the imported HEC-RAS channel is different from the ABSCOUR section, change the HEC-RAS stations to match the ABSCOUR section.


Actual Approach Section Right Bank Station

The channel right bank station for the actual approach section. Note if the top width of the imported HEC-RAS channel is different from the ABSCOUR section, change the HEC-RAS stations to match the ABSCOUR section.


Actual Bridge Section Left Bank Station

The channel left bank station for the actual downstream bridge section. Note if the top width of the imported HEC-RAS channel is different from the ABSCOUR section, change the HEC-RAS stations to match the ABSCOUR section.


Actual Bridge Section Right Bank Station

The channel right bank station for the actual downstream bridge section. Note if the top width of the imported HEC-RAS channel is different from the ABSCOUR section, change the HEC-RAS stations to match the ABSCOUR section.


Actual Approach Section Manning Roughness

This is the Manning roughness n value of the left overbank, channel or the right overbank of the actual approach section. These values can be imported from the HECRAS project and were used in the discharge distribution calculation and the section drawing in the tools menu of the actual approach section. These values are not used in scour analysis.


Actual Bridge Section Manning Roughness

 This is the Manning roughness n value of the left overbank, channel or the right overbank of the actual bridge section. These values can be imported from the HECRAS project and were used in the section drawing in the tools menu of the actual bridge section. These values are not used in scour analysis.


Computation of Kv for 2-D Flow Models

If the ABSCOUR user selects a 2-D model instead of a 1-D model such as HEC-RAS for the hydraulic analysis, kv should be computed by a different procedure.  The 2-D model can be used to measure directly the velocity of flow at the face or toe of the abutment (Vface) and the average flow velocity in the adjacent contracted section. Referring back to equation 1-16a, kv is defined as the ratio of velocity at the abutment face or toe to the mean velocity (Vave) in the adjacent contracted section. Both of these parameters can be measured using the 2-D model. The procedure to calculate kv is described below:

1. Select the override option for 2-D flow on the Project Information Card

2. Step 1 above will open two cells on the Downstream Bridge Data Card:

•	Enter the calculated/measured flow velocity at the abutment face/toe in the cell designated Vface

•	Enter the calculated/measured average flow velocity in the adjacent contracted section in the cell designated Vave

3. The ABSCOUR program will then calculate kv using Equation 1-16b:

kv =  Vface/Vave                                                                                            1-16b


Critical Velocities in Cohesive Soils

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:

•	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.

•	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.