# 8.3.1: Introduction

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On high energy coasts, long-term (years to decades) shoreline changes are predominantly due to human-induced longshore effects. Cross-shore movement of sediment typically occurs on short timescales (days) and has little direct influence on the longer term changes in beach position, unless material is permanently lost from or introduced to the system. Possible sediment sinks are offshore canyons from where any deposited sediment cannot return, the hinterland (through aeolian transport) or sand mining from the beaches (for instance for construction purposes). Examples of sediment sources are dredge disposal or sediment input from rivers. Often however, longshore effects are the most important cause of shoreline changes on the longer timescales. Coastal change due to longshore processes will be the focus in the following.

## Note

Please note that the coastline is aligned with the $$x$$-axis in the remainder of this chapter. This is different from the other parts of the lecture notes.

The long-term coastline changes are governed by net, yearly-averaged longshore sediment transport rates averaged over several wave conditions with varying magnitudes and directions. In computations this is taken into account by a schematised wave climate with a limited number of conditions (as discussed in Sect. 8.2.5). The very presence of sediment transport does not lead to either erosion or deposition. Indeed, if one considers a portion of beach as sketched in plan in Fig. 8.6, the coastline will remain stable as long as $$S_{\text{in}}$$ in is equal to $$S_{\text{out}}$$. On the other hand if $$S_{\text{out}}$$ is greater than $$S_{\text{in}}$$ ($$S$$ increases as we move along the beach, $$\tfrac{dS}{dx} > 0$$, or sediment transport divergence) material must be eroded in the area under consideration in order to maintain a sediment mass balance. The coastal profile including the shoreline will recede. The shoreline will move seawards (accretion) if the longshore sediment transport decreases in the transport direction ($$\tfrac{dS}{dx} < 0$$ or sediment transport convergence). A general form of the sediment mass balance was given in Eq. 1.5.2.1.