Natural accretion along the borders of the basin, due to the deposition of silt is a very slow process. This process causes a gradual reduction of the tidal prism, see Fig. 9.36. Due to this reduction the channel volume is $$a\ m^3$$ too large and the sand volume of the ebb-tidal delta is $$b\ m^3$$ too big. This results in a sand demand from outside. Also in this case not all the sand can be supplied from the outer delta and this will be at the expense of the downdrift coast.