Enhanced Abutment Scour Studies for Compound Channels

Experimental results and analyses are given in this report on bridge abutment scour in compound channels. Experiments were conducted in a laboratory flume with a cross section consisting of a wide floodplain adjacent to a main channel. The embankment length, discharge, sediment size, and abutment shape were varied, and the resulting equilibrium scour depths were measured. Water-surface profiles, velocities, and scour-hole contours were also measured. In the report, a methodology is developed for estimating abutment scour that takes into account the redistribution of discharge in the bridge contraction, abutment shape, sediment size, and tailwater depth. The independent variables in the proposed scour formula are evaluated at the approach-channel cross section and can be obtained from a one-dimensional water-surface profile computer program such as the Water-Surface Profile Program (WSPRO). The proposed scour evaluation procedure is outlined and illustrated, including consideration of the time required to reach equilibrium scour. The proposed methodology is applied to two cases of measured scour in the field. Research objectives include: Investigate the effects of flow distribution, as affected by abutment length, on clear-water abutment scour in a compound channel for abutment lengths that terminate on the floodplain as well as encroach on the bank of the main channel. Quantify the effects of floodplain sediment size on abutment scour. Explore the influence of abutment shapes, including wingwall, vertical-wall, and spill-through shapes, on equilibrium scour-hole depth and scour-hole form. Determine the relative importance of the live-bed scour case compared to the clear-water case when there is sediment transport in the main channel and the abutment encroaches on the main channel. Combine the experimental results into a methodology for assessing field abutment scour and test it on available field data.