FLOOD03064 - Laserfiche WebLink

Home Browse Search

y, = depth of floe,d plain flo." at the abutment Ys = scour depth Description K, VERTICAL-WALL ABU"MENT VERTICAL-WALL ABU~:MENT WITH WING WALLS SPILL-THROUGH ABUTMENT 1.0 0.82 0,55 TABLE 4.1 ABUTMEIlT SLOPE COEFFICIENTS Froehlich (28) suggested that scour depth be increased by y,/6 if there are dunes in the main channel upstream of the abutment. CLEAR-WATER SCOUR AT A1' ABUTMENT Use Equation 7 for live-bed scour since Froehlich's clear-water scour equation presented in Appendix B potentially decreases scour at abutments due to the presence of coarser material, This is unsubstantiated by field data, however, and Froehlich's clear- water scour equation is not recommended. F. STEP 6. COMPUTE LOCAL SCOUR AT PIERS 1. General, Local scour at piers is a function of bed material size, flow characteristics, fluid properties and the geometry of the pier. The subject has been studied extensively in the laboratory, but there is limited field data. As a result of the many studies, there are many equatio~s. In geheral, the equations are for live-bed scour in cohesionless sand bed streams, which give similar results. The FHWA (29) compared many of the more common equaticns in 1984. Comparison of these eqLations is given in Figures 4.4 and 4.5. Some of the equations tave velocity as a variable (normally in the form of a Froude m:.mber), However some equations, such as Laursen's do not incluc.e velocity. A Froude number of 0.3 was used in Figure 4,4 for purposes of comparing commonly used scour equations. In Figure 4,5 the equations are compared with some field data measurements, As can be seen from Figure 4.5, the Colorado state University (CSU) equation encloses all the points, but gives lower values of scour than Jain's, Laursen's and Niell's equations. The CSU equation includes the velccity of the 48