Mont Saint Michel Bay is a 30 km wide bay located on the French side of the English Channel. The tidal range reaches 14 m during spring tides, which leads, along with the gentle slope of the bathymetry, to a tidal flat up to 11 km wide. The sedimentary coverage exhibits strong longshore and cross-shore gradients, with purely muddy environments to the west of the domain, and pure sands to the east, where the natural channels of three incoming rivers induce rapid morphological changes. The contributions of tides and waves to sedimentary processes are analysed by means of numerical modelling and results of field observations. Maximum tidal bottom shear stresses are shown to account for the sediment distribution throughout the bay, while the longshore gradient in wave intensity seems to drive the amount of suspended sediment concentration. The bay has been a privileged ground for shellfish farming for over a century. Farming structures (oyster tables, mussel posts and wooden fences used as permanent fishing nets) have significantly hindered natural flow patterns, creating quiescent areas which significantly increase fine sediments deposits in farming areas. The paper focuses on introducing into a numerical model the effects of mussel farms on flow circulation and sediment dynamics.