Figure 1.7 shows in plan view a part of a sandy coast with a river outfall. Waves are assumed to approach perpendicular to the (initial straight) coastline. The river discharges a volume of water \(Q_r\ m^3/s\) to the sea; at the same time sediment is transported by the river; say \(S_r \ m^3/yr\). \(S_r\) is expressed in \(m^3/yr\) since we are looking at large morphological timescales.
Like many rivers all over the world, also the river of Fig. 1.7 acts as source of sediments to the coastal system. It is interesting to understand the interaction processes between river and sea and to be able to predict the morphological changes with time, as shown in Fig. 1.7. Such a clear delta coastline develops when the supply of riverine sediments to the coast is faster than they can be dispersed along the coast by tidal and wave-generated currents. (See Chs. 2 and 8 for a more detailed discussion of deltaic coastlines.) The stability of a deltaic coastline is very dependent on the river sediment supply. If the sediment supply is cut-off or reduced by for instance the construction of dams, sand mining or irrigation schemes, the system is deprived of its regular supply of sediments. This leads to erosion of the coastline on either side of the river mouth and is quite common for present-day deltaic coastlines.