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 objective of this study was to present a design method for time-dependent scour depth under bridge-submerged flow. To this end, a series of flume experiments on scour depth under bridge-submerged flow were conducted to collect scour data at different times. 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. 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.