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~o= Median dianeter of bed material at relief bridge, Use a weighted average of the material in the scour zone. W2 = Bottom width of the relief bridge less pier widths, Dimensions are in feet. Note. The depth y, is the depth upstream of the relief bridge that has active flow, d. CASE 4. CONTF~CTION SCOUR, RELIEF BRIDGE WITH BED MATERIAL TRANSPORT. (LIVE-BED SCOUR) Case 4 is similar to case 3 but there is sediment transport into the relief opening (live-bed scour). This case can occur when a relief bridge is over a secondary channel on the flood plain, Figure 4.1. Hydraulically this is no different from case 1 but analysis is required to determine the flood plain width associated with the relief opening and the flow distribution going to and through the relief bridge, This information could be obtained from WSPRO (23), Use the equation given for Case 1 with appropriate adjustments of the variables. 3. Other Contraction Scour Conditions. Contraction scour resulting from variable water surfaces downstream of the bridge is analyzed by determining the lowest potential water-surface elevation downstream of the bridge in so far as scour processes are concerned. Use the FHWA WSPRO (21) computer program to determine the flow variables, such as velocity and depths, through the bridge, With these variables, determine contraction and local scour depths. Contraction scour in a bendway resulting from the flow through the bridge being conCentrated in one area is analyzed by determining the superelevation of the water surface on the outside of the bend and estimating the resulting velocities and depths through the bridge. The maximum velocity in the outer part of the bend can be 1.5 to 2 times the mean velocity. A physical model study can also be used to determine the velocity and scour depth distribution through the bridge for this case. Estimating contraction scour for unusual situations involves particular skills in the application of principles of river mechanics to the specific site conditions and such studies should be undertaken by engineers experienced in the fields of hydraulics and river mechanics. Highways in the River Environment (7) will be of great assistance. 43