In this chapter we have discussed the topic of sediment transport in quite general terms. In the next three chapters sediment transport is considered for specific situations as is the resulting morphodynamic response of the system.
Chapters 7 and 8 are devoted to wave-dominated coastal systems: uninterrupted coastal stretches, pocket beaches and wave-dominated deltaic coastlines. We make a distinction between cross-shore (Ch. 7) and longshore (Ch. 8) sediment transport (Fig. 6.19).
Such a distinction may seem artificial at first glance, but is often made because of the following differences:
- Long-term changes in the coastline are often the result of gradients in the long-shore transport. They are easily observed near disturbances, such as breakwaters or river mouths. Cross-shore transport is responsible for short-term variations, such as changes in position and size of breaker bars and dune erosion during storms. Long-term changes due to cross-shore transport (for instance a long-term loss to deeper water) may also occur, but are harder to detect (and may be difficult to distinguish from long-term changes due to alongshore processes);
- Where coastline change as a result of human-induced changes is concerned, alongshore and cross-shore effects are either of equal importance (on low wave energy coasts, e.g. the Mediterranean) or alongshore effects dominate (on high-wave energy coasts, e.g. the Dutch North Sea coast). This may explain why cross-shore impacting structures – such as offshore breakwaters and perched beaches (see Ch. 10) – perform better on low-energy coasts than on high energy coasts;
- The wave orbital motion is very important in transporting material in the cross-shore direction but not in the alongshore direction. The wave orbital motion is approximately cross-shore directed in the nearshore. Since every wave in principle moves sand back and forth, gross cross-shore transports (per m width) are large and much larger than longshore transport rates (per m width). For wave-dominated coasts not influenced or interrupted by coastal inlets, wave-induced surf zone longshore flow is the main driving agent. The direction and magnitude of the wave-induced longshore transport is determined by the wave conditions (wave height, period and direction). Net cross-shore transport rates however, are generally an order of magnitude smaller than longshore transport rates.
In Ch. 9, the specifics of tide-dominated systems (tidal basins) and tide- and wave-dominated coastal inlets are considered. The sediment exchange between coast and basins is also discussed. Dynamics of fine sediment transport are very important in tidal basins; coarser sediment (quartz and carbonate sands) is predominantly found in the seaside regions, while the finer sediment (silt and clay) settles in the more protected landward regions.