Coastal morphodynamics and society
It has long been obvious that our beaches have evolved with time in response to factors including the supply of sediment, the action of waves and currents, geological constraints, man made structures and sea level rise. The study of coastal morphodynamics involves investigating the changes to the physical features and environments in the coastal region, which occur over a broad range of time and length scales, as detailed below and illustrated in Figure 1.
- Toe scour – the short term and relatively small scale removal of sediment from the toe (i.e. base) of coastal structures. This often occurs and recovers completely during the course of a single tide (if in the intertidal zone, at least). Occurs over a cross-shore lengthscale of a few metres but may extend considerably further in the longshore direction;
- Storm response - lasting for a few tides and causing toe scour, beach lowering and recovery over cross-shore scales of up to a few hundred metres and rather longer distances in the longshore direction;
- Recovery between storms - the beach will respond to the changing forcing conditions and variations in beach level can be observed. Recovery from a storm can take 10s of tides and will affect a similar area to the storm;
- Seasonal variation – it is commonly observed that beach levels are draw-down (i.e. the beach level lowers at the top of the beach) in winter due to storm-induced erosion and build up during summer, leading to a seasonal variation in beach levels at the toe of a structure, for example;
- Inter-annual variability in climate – this will have a net effect on the coastline by generating erosion or accretion and there are considerable variations in the wind and wave climate between years. Changes in the tidal regime are less dramatic, although the nodal tide, which is an 18.6 year variation in tidal range, causes a measurable effect. The annual wave climate affects the whole coastline so its effects are felt over the scale of the sediment cell, say 10s of km alongshore by of the order of 1 or 2 km cross-shore; and
- Coastal evolution and sea level rise – changes are driven by sea level rise and dominated by longshore transport. Occurs over longer timescales and even larger spatial scales than beach changes due to variations in annual conditions.
In general, the spatial scale increases with the timescale and longshore sediment transport processes increase in importance compared to cross-shore transport processes as the timescale increases. These changes are important as they affect society in a range of different ways, some of which are illustrated in Figure 2, where the time scale represents the duration between events, not the duration of an event.
Changes to the morphology have effects, including:
- Altering currents and waves, with consequences for the safety of recreational beach users. Included in this category is the effect of rip-currents.
- The lowering of beaches leads to greater wave heights at coastal defences, which can cause greater overtopping rates (i.e. greater volumes of water being thrown onto and over the coastal structure) which in turn can cause damage to pedestrians, cars and even structures. People can be swept into the sea during such an event.
- The erosion of a beach can cause its width to diminish, lessening the potential for tourism.
- Sedimentation reduced the available water depth in channels, ports and harbours. In time this can become a risk to shipping, which is commonly alleviated by dredging.
- Cliff falls are dramatic events that can move a cliff edge landwards by several metres in the space of a few seconds. This can be dangerous and leads to the erosion of land. If this land is used for agriculture, industry, business, housing, transport etc. it will have a value that is almost always permanently lost. If the land is unused it will still commonly have a value as scenery, as our coastlines include some of the most beautiful landscapes there are.
- The failure of coastal defences fortunately happens rarely but can have dramatic consequences as large scale flooding or a high rate of coastal erosion can follow.
- Shoreline retreat is one of the symptoms of an eroding coastline, which can eventually lead to the loss of settlements, such as villages and towns. As an example, the approximately 63km long Holderness coast of Lincolnshire on the east coast of England has lost over 30 settlements to the sea since Roman times [link].
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