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Failure of bridges due to local scour has motivated many investigators to explore the reasons of scouring and to give the prediction of the scour depth. But most scour prediction equations only address non-pressure-flow situations. Little research has been dedicated to another destructive scour, submerged-flow bridge scour (pressure flow scour) which can cause significant damages to bridges when partially or totally submerged during a large flood.
This thesis is specifically focused on the experimental study for time-dependent scour depth under bridge-submerged flow. The experiments were conducted in a self contained re-circulating tilting flume where two uniform sediment sizes and one model bridge deck with three different inundation levels were tested for scour morphology. To this end, a semi-empirical model for estimating time-dependent scour depth was then presented based on the mass conservation of sediment, which agrees very well with the collected data.
As current practice for determining the scour depth at a bridge crossing is based on the equilibrium scour depth of a design flood (e.g., 50-year, 100-year, and 500-year flood events), which is unnecessarily larger than a real maximum scour depth during a bridge life span since the peak flow period of a flood event is often much shorter than the corresponding scour equilibrium time. The proposed method can appropriately reduce the design depth of bridge scour according to design flow and a peak flow period, which can translate into significant savings in the construction of bridge foundations.