Abstract: Taking a channel downstream the Sheyang sluice as a prototype, a two-dimensional generalized morphodynamic model is established to investigate the siltation mechanism of a typical channel downstream floodgate in muddy coasts, China. The influence of runoff on hydro-sediment dynamics, as well as the souring and silting evolution in the channel, has been explored with and without considering the change of annual runoff distribution, respectively. A semi-empirical equation describing the relation between the equilibrium discharge and the ratio of runoff volume in flood season, channel averaged cross-section, offshore tidal range, tidal wave deformation, channel length and other related factors is established. For a particular channel which is practically in dynamic equilibrium state of erosion-siltation, the above-mentioned equation can be simplified as a simple form of expression of equilibrium discharge and channel averaged cross-section. And the simplified form of this formula is validated by observational data.