Storm rapid sedimentation of Maotou deep trough in the Sanmen Bay of Zhejiang

The Sanmen Bay is frequently influenced by the typhoon,sediment exchange between beach and channel is stronger when storms hit this bay.Tidal currents are forceful with strong bottom current due to the topographic effect and predominantebb occurs in the Maotou Channel.These features of tidal currents are a dynamic foundation to form deeper trough and to keep stability.The seabed is in the equilibrium of erosion and siltation with slight siltation in last 100 a.However,when the storms hit the bay,siltation occurs obviously in the trough and erosion in the broad tidal flat.The sediments which are eroded from tidal flat provide resources for rapid deposit in the Maotou Channel.An average depth of profile across deeper trough reduced about 142 cm measured on August 26,1994,a week after No.9417 typhoon passed the bay.T he data show that siltation streng thens from the slope to the bottom. Then the setting sediment by the storm can be resuspended and removed with the tidal current during normal weather.The depth in the trough is restored with slight erosion measured in January 1995,comparing the depth measured in June 1994.But some years,sedimentation occurred in the Maotou Trough.Three and 4 continued strong tropical storms hit the bay in 1989,1990 respectively,they resulted in storm sediment to retain the trough.It is considered that deposited strata have stored in information of storm sediments in the deeper trough.Some "black mud" layers which contain higher water content,high organic content,low solidity and no-bedding in the undisturbed core are recognized as a rapid deposit retained in the Maotou deeper trough by all previous storms.