10.1: Introduction

We have seen in the previous chapters how coasts can develop – can erode or accrete. Unfortunately, these processes may conflict with economic interests. A channel to a harbour may silt up making the entrance too shallow for shipping, or a valuable structure (such as a hotel or jetty) may be washed away as a beach erodes. Beach accretion is rarely felt as a problem, except when the beach is too wide and the walking distance to the waterline is too far from a recreational point of view. We will therefore mainly focus on erosion problems.

Large parts of sandy coasts all over the world suffer from structural erosion and/or dune and beach erosion during severe storm surges. In coastal engineering practice an important aim is the proper protection of these threatened coasts. Also newly reclaimed areas or upgraded beaches have to be protected from the attacks by the sea. Further, construction of harbours and other engineering works inevitably impacts the coastal system. Mitigation of such adverse impacts should be an integral part of the design of these works (see Sect. 10.5.2).

Strategies and methods for coastal protection are introduced in Sect. 10.2. Next, the nature of coastal erosion (permanent or structural versus temporary) is extensively described in Sect. 10.3. Subsequently, Sect. 10.4 discusses the principle of interference in longshore transport rates to defend a structurally eroding stretch of coast. Section 10.5 deals with structures that are designed to interfere in longshore transport rates. Section 10.6 is devoted to those structures that literally act as a barrier between the sea and the land. They prevent the storm-induced, temporary loss of material from the dunes or land as well as flooding of the hinterland. Nourishment, a so-called ‘soft’ protection method, is treated in Sect. 10.7.

In the sections on structures, we focus on the functional design aspects of these structures. We will therefore discuss their effectiveness in coping with certain coastal problems and (briefly) the appropriate choices regarding position, crest height, length, etc. No attention will be paid to constructive/technical design aspects like for example the required mass of stones of armour layers, thickness of various layers in the structures, etc. Other DUT courses like Bed, Bank and Shoreline Protection (CIE4310) and Break- waters and closure dams (CIE5308) deal with this. We have, however, highlighted some interesting similarities between sediment transport and breakwater damage in App. D. Also, a recent overview of these topics can be found in ‘River, Coastal and Shoreline Protection’ (1995) and in the Coastal Engineering Manual (CEM, see Sect. 1.7.3).

Although the term ‘coastal protection’ is usually not only used to mean protection against erosion, but against flooding as well, the determination of safety levels against flooding is not discussed in CIE4305, as yet. The same holds for measures against sedimentation in approach channels to harbours. Dredging, the ‘soft’ solution to the latter problem, is discussed in detail in the course CIE5300 on Dredging Technology.