Why does beach erosion occur

The tool enables historic trends of coastal erosion and progradation growth to be seen at both a local and continental scale.

The Smartline national dataset maps record the location of those coastal substrates and landforms that have greater or lesser sensitivity to potential coastal impacts of climate change and sea-level rise IPCC , such as accelerated erosion and shoreline recession, increased slumping or rock fall hazards, changing dune mobility, and other hazards. Qin, G. Plattner, M. Tignor, S.

Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. Midgley eds. Other larger scale techniques like beach nourishment may have better track records, at least in terms of slowing or delaying beach erosion, but are expensive enough as to warrant massive taxpayer expenditures.

Not only did the effort stave off erosion, it helped revitalize the tony South Beach neighborhood and rescue hotels, restaurants and shops there that cater to the rich and famous.

Box , Westport, CT ; submit it at: www. Read past columns at: www. Already a subscriber? Sign in. Thanks for reading Scientific American. Reduction of fluvial sand supply can result from different human interventions:. Thousands of dams have been constructed on rivers worldwide, creating reservoirs which retain a large part of the sediment discharge from the catchment areas Fig. The promontory propagated until and then began to erode.

Sand mining in rivers is a major cause of coastal erosion in many countries. Sand mining in a river lowers the river bed, causes bank erosion and reduces the supply of sand to the coast. There are five components in the sediment balance for a degrading river section, schematically represented in Fig.

Many rivers consist of a steep upper part, the mountain part, and a gently sloping lower part, where the river crosses the coastal plain. Sand extraction in the upper part of the river causes lowering of the river bed and a similar lowering of the water level, hence no changes in the sediment transport capacity.

Thus the sand extraction in the upper part of the river is almost entirely balanced by local bed degradation, and has hardly any immediate impact on the supply of sand to the coast.

Sand mining in the lower part of the river at some distance from the river mouth causes a local lowering of the river bed. However, the water level will not lower as much, which results in a local decrease in the flow velocity and in the sediment transport capacity. The river bed depression will gradually be filled in from upstream supply and will travel towards the coast. When the impact of sand mining reaches the coast there may be an accumulated deficit in available river bed material corresponding to several decades sediment supply from the catchment.

This means that an immediate halt in the sand mining will have hardly any remedial effect on the supply of sand to the coast, as the entire river bed has to rebuild before the original supply is re-established. Sand mining close to the river mouth causes an immediate decrease in the supply of sand to the coast, and halt of the sand mining in this situation will quickly cause recover of the supply of sand to the coast.

These impacts of sand mining on coastal sediment supply are observed in many rivers, for example for rivers in Sri Lanka [14]. Coastal erosion is not the only impact of river sand mining. Other impacts also have to be taken into consideration:. Hence, river sand mining requires an integrated approach taking into account all the impacts.

This calls for close collaboration between river authorities and coastal authorities. Small sandy bays enclosed between headlands have in general a crescentic shape, which is due to wave diffraction at the headlands and wave refraction in nearshore shallow water see Shallow-water wave theory.

However, the shape and position of the shoreline depends not only on the wave climate, but also on sand supply to the bay. There are two possible sources see Fig. The overall transport mechanisms in a crescent-shaped bay can be characterised as follows. The supply of sand from the upstream bay Q B will pass the headland and cross the bay via a bar. If a river also contributes Q R to the littoral budget, this material will be transported downdrift into the bay, partly along the shoreline and partly onto the bar.

The shape of the crescent-shaped bay is stable, apart from seasonal variations, as long as the supply of material to the bay Q S1 is not changed. This means that human interventions, which cause changes in one bay will gradually penetrate into the downdrift bays. Hence, crescent-shaped bays, although they appear fairly stable, are actually very sensitive to interventions that modify the supply of sand.

Wakes from fast ferries cause shore degradation in sheltered coastal environments [15] [16]. The special wake generated by fast ferries is characterised by a series of approximately 10 relatively low waves significant wave height below 1 m , but relatively long waves. These wake waves are very similar to swell waves and they are exposed to considerable shoaling when approaching the coast. They often break as plunging breakers.

If a fast ferry navigates through protected waters, the wake waves are very different from the natural waves along the navigation route. The wake waves caused by fast ferries may influence coastal conditions in the following ways:. A precondition for approval of a new fast ferry route is therefore to perform an environmental impact assessment study [17]. In undeveloped areas, these high recession rates are not likely to cause significant concern, but in heavily populated locations, one or two feet of coastal erosion may be considered catastrophic.

Sea level rise will cause an increase in coastal erosion and the human response will be critical. If we choose to build hard structures in an attempt to keep the shoreline position stable, we will lose beach area due to scour. If we let the shoreline migrate naturally, we can expect to see erosion rates increase, especially in regions of the coast that are already dealing with starved sediment budgets and rapid shoreline migration. Increases in storm frequency and intensity in the future will also cause increased coastal erosion.

The U. Geological Survey's Coastal Change Hazards Portal offers a Coastal Vulnerability Index that can help identify locations where coastal erosion may occur along undeveloped coastlines. Information is also available on various forms of Storm-Induced Coastal Change. These photos show an area near Drew Point, along Alaska's northern coast. Taken on August 9, , the photo on the left shows how ocean waves have undercut the land nearest the shore. Grassy turf extends out over a wave-cut notch.

Taken on June 20, , the photo on the right shows what often follows such undercutting: chunks of coastline tumbling into the sea. In the past, protecting the coast often meant "hardening" the shoreline with structures such as seawalls, groins, rip-rap, and levees.