Submerged-Flow Bridge Scour under Maximum Clear-Water Conditions (I): Experiment

Submerged-flow bridge scour at clear water threshold condition has been studied experimentally. The experiments were conducted in a self contained recirculating tilting flume where two uniform sediment sizes and two model bridge decks with eight different inundation levels were tested for scour morphology. The experiments showed that the longitudinal scour profiles before the ma,\:imum scour depth can be approximated by a 2-D similarity profile, while the scour morphology after the maximum scour depth is 3-D. Finally, two empirical similarity equations for scour profiles were proposed for design purpose, and the collected data set could be used for analytical studies of bridge scour. INTRODUCTION Bridges are a vital component of the transportation network. Evaluating their stability and structural response to hydrodynamic loading is critical to highway safety in design phase and after flooding. The studies of bridge scour usually assume an unsubmerged bridge flow, but the flow regime can switch to submerged flow when the downstream edge of a bridge deck is partially or totally inundated during large flood events. For example, a submerged bridge flow occurred in the Cedar River in Iowa after heavy rains in June 2008 (Figure 1), which interrupted traffic on 1-80. Submerged flow most likely creates a severe scouring capability because to pass a given discharge, the flow under a bridge can only scour the channel bed to dissipate its energy. Investigations on submerged-flow bridge scour have been reported by Arneson and Abt (1998), Umbrell et al. (1998), and Lyn (2008) . Arneson and Abt (1998) did a series of flume tests and proFigure 1: Bridge-submerged flow in Iowa in 2008 posed the following regression equation "!!.!... = -0.93 + 0.23 (hu) + 0.82 (Ys + hb) + 0.03 ( Vb) (1) hu hb hu Vue