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