8.4.1: Introduction

The knowledge acquired in the previous sections will be used in this section to explain a variety of coastal features and coastline changes as observed in nature. The basis of the explanations is the gradients in littoral transport rates. The littoral transport rates are dependent on the nearshore wave exposure and wave incidence angle as explained in Sect. 8.2. Hence, for a given offshore wave climate, coastal changes are determined by changes in the depth contours and shoreline orientation and the degree to which waves refract, shoal and diffract along the shoreline.

Assuming the offshore wave conditions do not vary along the shore (which is a reasonable assumption on a scale of order 10 km), a curved coastline will always be subject to a longshore sediment transport gradient. This is because:

1. The changes in coastline orientation are reflected in changes in the angle of wave incidence. The changes in shallow water depth contours and coastline orientation are generally more important than directional changes in offshore wave climate along the coast;
2. Due to refraction convergence and divergence of wave energy occurs (Fig. 5.6).

Figure 8.17: Longshore sediment transport along a coast for small deep water wave angles. Zones with positive transport gradients (transport divergence, erosion) are indicated with ‘−’ and zones with negative transport gradients (transport convergence, accretion) are indicated with ‘+’. The ‘bump’ in the shoreline erodes and the ‘dent’ accretes.

As long as the wave angles are relatively small, viz. a deep water wave angle with respect to the shoreline smaller than about \(45^{\circ}\), the transport rates increase with increasing wave angle (Fig. 8.4). This implies that convex depth contours result in sediment transport divergence along the coast and thus in erosion, whereas concave depth contours lead to sedimentation (see Fig. 8.17). Note that convex and concave are here defined as seen from the sea.

As a consequence, a shoreline has a tendency to flatten out bumps and dents until the straight, equilibrium coastline is restored (hence a negative feedback process, see Sect. 1.5.2). The specific case of high-angle waves is discussed in Sect. 8.4.4.

In this section, we consider the structural coastal response to large disturbances or interruptions (natural and human-induced) that can be explained based solely on long-shore transport gradients. The onshore and offshore movements of sediment that take place on short timescales, such as storm-induced erosion and seasonal variations, are assumed to cancel on the longer timescales of shoreline change. Any net gains or losses of sediment in the cross-shore direction (e.g. due to offshore canyons or inland aeolian transport) can be considered as sinks or sources. Natural disturbances are for instance coastline interruptions by tidal basins or rivers, the presence of an offshore island and river sediment supply. Human-induced disturbances are for instance harbour moles, shore protection structures and river regulation works, and nourishment schemes and maintenance dredging.