Coastal erosion

The National Oceanography Centre (NOC) is involved in the study of coastal erosion as part of both UK and EU projects, a major aim of which is to enable research-based coastal management.

Erosion can be considered as a change in the coastline position or level, eg a cliff or dune system retreating inland or a beach becoming a lower level.  The changes are often in response to sea-level rise. This means that typical wave-tide conditions act more frequently on the coastline. Changes in storminess can also increase the intensity of one or a combination of wind, wave and surge levels at the coast.  Managing erosion is of great importance, because changes in natural defence can lead to an increased risk of coastal flooding. It can also result in land loss, which is an issue for urban and industrial areas, and nature reserves.

Coastal erosion is common around the UK, especially in soft sediment (sandy) coastlines.  However, the UK is not alone in experiencing coastal flooding and erosion: many low lying areas are subject to flood risk. If mean sea level remains as at present (2010), then regions shown as red in Figure 1a have a 1 in 1,000 chance of flooding each year, without appropriate coastal defenses.

At Holyhead (Fig. 1b) a probability of high water exceeding 7.1m now is 1 in 1,000. Even if mean sea level is just 35cm higher (quite possible by 2100), a high water of 6.75m will produce the same level, with an increased probability of 1 in 100.

a) Coastal areas at risk from flooding in England and Wales, b) Estimate (and confidence limits) of extreme high water return period levels for Holyhead (North Wales) relative to Ordnance Datum Newlyn.

 

 

 

 

 

 

 

 

 

 

Meanwhile, erosion on the east coast of England is a particular concern (Fig. 2) as mean sea level rises because the land movement is downwards. This, together with predicted increases in storm severity and frequency, means that erosion rates will likely increase. Coastal defenses need strengthening and building up to alleviate flooding and erosion. However, this can be extremely expensive; an alternative that has been considered is ‘managed retreat’.

The Sefton Coast (northwest England) is protected by both natural and man-made defenses (Fig. 3).  Around Formby point, the largest UK sand dune system provides natural protection for the low-lying ground behind.  However, during periods of continuous high tidal levels, the dune system is at risk of erosion. This is because the dune toe is submerged for a period of time either side of high water.  Extreme south-westerly storms also pose a threat to the system as water levels are increased by surge and wave setup combined with wave activity, which is able to reach the dune toe. The NOC has been working with the local council to try to understand the storm conditions that lead to extreme erosion events along this stretch of coast.   

At the NOC, a suit of nested computer models is able to hindcast wave-tide-surge conditions to improve understanding of the offshore storms that contribute to coastal erosion around the UK. This information is combined with observations collected by the NOC and local councils, who monitor coastal evolution in the areas of interest. 

Coastal erosion at Happisburgh, Norfolk. More than 40 meters of land was lost between 1998 and 2007.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For example, along the Sefton Coast beach profile surveys are undertaken pre- and post-storm. In the mouth of the Dee, X-band Radar is used to monitor the movement of sandbanks.  By observing sediment transport (concentration) throughout the water column using acoustic and optical instruments in different field and lab conditions, the NOC aim to develop further the capabilities of the coupled hydrodynamic-wave modeling system (POLCOMS-WAM) to incorporate both sediment transport and morphological evolution over coastal regions.  This is intended to improve understanding of sediment transport pathways to understand how the coastline may respond to the future climate

 

Defenses along the Sefton coast, northwest England. (Photos provided by Sefton Council)



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