A theoretical model for the maximum scour depth under submerged bridge flow was proposed based on the mass and energy conservation laws as well as the recent flume experimental data. It is shown that the maximum scour depth can be described by a scour number and an inundation index. In general, for submerged flow the scour number increases with the inundation index, which is equivalent to the maximum scour depth increases with deck inundation level, decreases with increasing sediment size, and is independent of bridge girders. The proposed method is expected to be applicable to prototype flows without scaling effects since it was derived from the conservation laws. An application procedure was also suggested for bridge foundation design or field scour evaluation.