Clear-Water Contraction Scour under Bridges in Pressure Flow

Bridges that become partially or totally submerged during large floods are subject to pressure-flow scour, a condition where the flow is directed downward and under the bridge deck, creating an increase in flow velocity and a corresponding increase in bed scour. Most scour prediction equations only address non-pressure-flow situations. This investigation will study scour caused by pressure flow beneath a bridge without the localized effects of piers or abutments. A model bridge deck was tested under a variety of laboratory-controlled pressure-flow conditions. Approach velocity, discharge, depth of bridge deck submergence, and sediment size were all varied. Values for approach velocity, pressure-flow velocity under the bridge deck, and contraction scour were all recorded. A conceptual relationship between pressure-flow scour and the flow conditions was developed and analyzed with the use of the experimental data.