Local scour around bridge piers in pressure flow

The failure of many bridges can be attributed directly to local scour around piers and abutments during floods. In an attempt to predict, control and manage scour around piers, many investigations have been performed. However, none of these investigations were of scour with pressure flow. Pressure flow exists when the bridge deck is submerged. Herein is presented the first investigation of local scour around bridge piers in pressure flow.;Local scour depth and width, bed disturbance and velocity values were much larger in pressure flow than those in free flow. In free flow, no contraction scour occurred and only local pier scour occurred. In pressure flow, local scour around the pier occurred which was composed of contraction and local pier scour. Scour also occurred at the wall of the flume. Maximum scour depth was increased by a factor 2 to 4 depending on Froude number and bridge opening. Maximum scour hole width was increased by a factor 2 to 3 depending on scour depth. For the same velocity, local contraction and wall scour increased as the bridge opening decreased.;Curves of pressure flow coefficient are provided to predict local scour depth in pressure flow, and also to correct free flow CSU and Froehlich scour equations for scour with pressure flow. Empirical relationships were developed in pressure and free flow, expressing the depth and width of scour as a function of flow depth, Froude number, pier width, and bridge opening.;Twenty-five clear water scour experiments were performed. Fifteen with pressure flow and ten with free flow. In the experiments, flow depth ranged from 0.8 to 2.0 feet, velocity from 1.0 to 3.0 feet/sec, and Froude number from 0.128 to 0.59. The bed material used was pea size gravel of median diameter 3.2 mm. The pier (0.5 foot width, 6.0 feet long) was aligned with the flow and without footing.