Those semi-empirical exponential equations which can describe subsidence of flood peak discharges in channels during overbank flow periods are developed by using dimensional analysis to analyze the numerical results computed from various flood routing techniques based on unsteady free surface flow equations. The subsidence is expressed as a function of eight dimensionless parameters related to the geometry and Manning roughness coefficients of main channel and overbank sections, bed slopes, duration of flood wave, rising time and recession time of overbank flow hydrograph. Two different shapes of inflow flood hydrographs are considered.