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 dir ected downward and under the bridge deck, creating an increase in flow velocity and a corresponding increase in bed scour. Most scour predic tion equations only address non-pressure-flow situations. This investi gation will study scour caused by pressure flow beneath a bridge witho ut the localized effects of piers or abutments. A model bridge deck wa s tested under a variety of laboratory-controlled pressure-how conditi ons. Approach velocity, discharge, depth of bridge deck submergence, a nd sediment size were all varied. Values for approach velocity, pressu re-flow velocity under the bridge deck, and contraction scour were all recorded. A conceptual relationship between pressure-flow scour and t he flow conditions was developed and analyzed with the use of the expe rimental data.