Steady and unsteady pressure scour under bridges at clear-water conditions

Bridges are vital components of the transportation network and if they are damaged or destroyed during a flood, they isolate communities and limit movement of supplies and emergency services. Evaluating their constructional stability and structural response after a flood event is critical for bridge safety. Bridge studies are usually designed with an assumption of an open channel flow condition, but the flow regime can switch to pressure flow if the downstream edge of a bridge deck is partially or totally submerged during a large flood. The main goal of this paper is to study the pressurized flow scour under a bridge deck and downstream deposition that results from eroded sediment material governed by both steady and unsteady clear-water flow conditions. Experimental conditions used in this study involve clear-water scour of a sand bed of given median sediment size d(50) = 0.90 mm and sediment uniformity sigma(g) = 1.29, an approach flow characterized by a flow depth and velocity, a rectangular-shaped bridge deck, and a stepwise flood hydrograph defined by its time to peak and peak discharge. Different flow conditions were considered in confined flow under the bridge deck. Relationship between pressure-flow scour and flow conditions is presented and discussed under the obtained experimental data. Additionally, effects of single-peaked stepwise flow hydrographs (unsteady flow conditions) on bridge pier scour depth are investigated under clear-water pressure-flow conditions, whereas previous researches mainly focused on the equilibrium pressure scour under steady flow conditions.