All semi-enclosed tidal waters (estuaries, harbours, marinas, etc) silt up naturally as a matter of course. On the tidal flood, seawater flows in, and there is a type of flocculation that deposits the suspended solids in the water as silt, and less silt than is deposited is removed when the water flows out with the ebb tide. So, estuaries and harbours have to be dredged to keep them operational. Marinas such as Conwy with no 'flushing' action have to be dredged on a virtually continual basis to maintain any depth. Newquay (mid-Wales) Harbour silted up rapidly and virtually killed off trade for the town. The other point to be noted is that any construction, gryones, harbour walls, anti-flood measures, etc will have an effect to a greater or lesser extent in the erosion or deposition of material in areas other than that of the construction.