Download FINAL APPENDIX A WITH PICTURES C REVISED

modifications developed to account for the distribution of flow under the bridge, the
bridge geometry and the computation of velocity at the bridge abutments. The
ABSCOUR program computes both clear water and live bed scour and selects the
appropriate scour type based on the input information. Careful application of the
ABSCOUR Program will provide the user with insight into the factors affecting
contraction and abutment scour at the bridge site under evaluation. Judgment is needed
to modify input information and the ABSCOUR cross-sections so as to best represent
actual site conditions during a flood event. Computed scour depths provided by the
ABSCOUR Program require evaluation to determine if the results are reasonable.
Abutment scour can be viewed as a combination of contraction scour and local scour.
The ABSCOUR Program computes the total scour at the abutment; therefore, the user
should not add contraction scour to this value as is done in the HEC-18 scour evaluation
procedure.
The following information is needed to provide the input information for the ABSCOUR
program:
Hydrologic estimates of Q100, Q500, Qovertopping and Qdesign
2. topographic map of the stream and its flood plain, the location of the bridge
crossing and stream channel cross-sections,
3. information from the geomorphology report regarding estimated channel
degradation, the channel lateral movement zone, D50 soil particle sizes in the
channel/flood plain and whether the type of scour to be expected is clearwater or live-bed.
4. Surface and subsurface information on channel bed load, flood plain soils,
borings, etc.
5. geometric information about the bridge and approach roads
6. HEC-RAS runs for the given hydraulic conditions including:
7. stream channel cross-sections,
8. hydraulic data tables,
9. reliable bridge tailwater elevations,
10. selection of appropriate approach section and flow distribution, and
11. appropriate flow distribution at bridge with regard to channel, flood plain and
overtopping flows.
1.
II. DEVELOPMENT OF THE INPUT DATA FOR THE ABSCOUR (ABUTMENT
SCOUR) MODEL
SHA has been conducting and reviewing ABSCOUR analyses for a number of years. It
is our experience that one of the biggest sources of error in scour computations is an
incorrect hydraulic model. It is not an easy task to model a 3-D flow pattern with a 1-D
model such as HEC-RAS. In particular, special care needs to be given to the following
three primary sources of error in developing the input data:
Water surface elevation under bridge. The hydraulic model should include a
APRIL 2011
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