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 14 Appendix B of the H&H Manual which contains a guideline for obtaining soil samples for scour analysis.