The purpose of this chapter is to illustrate how alongshore sediment transport contributes to shaping our coasts. The insight gained in the previous chapters, describing the water and sediment movements along coasts will now be combined to provide the necessary explanations.
The main principle behind coastal change has already been indicated in Chs. 1 and 7: coastal change occurs where there are spatial sediment transport gradients and/or sediment sinks or sources. In this chapter we specifically look at gradients in mainly wave-driven longshore transport or littoral transport. Consider an infinitely long, straight sandy coast having parallel depth contours. Such a coast was sketched in Fig. 5.4. If waves approach this coast at a uniform angle along its entire length, and there are no other current driving forces such as tides and wind, then there will be a constant, uniform transport of sand along this coast. What, then, causes erosion or deposition? This is caused by changes (gradients) in transport rates along a coast. This change may result from changing any of the factors influencing the longshore sediment transport rate, such as nearshore wave height and angle of wave incidence.
Various methods to compute longshore sediment transport are treated in Sect. 8.2. The calculation of shoreline position is described in Sect. 8.3. Section 8.4 discusses the various coastal shapes and characteristics that can be explained from longshore sediment transport gradients.
In Sect. 6.9.2 it was mentioned that coastline change is dominated by alongshore effects in the case of human-induced changes on high-wave energy coasts (e.g. the Dutch North Sea coast). In Sects. 8.3. Section 8.4, we therefore mainly focus on typical time and spatial scales for engineering or coastal maintenance (years to decades and 1 km to 100 km, see Sect. 1.5.3) and implicitly assume dominance of alongshore transport processes.