The Importance of Coastal Defence for Flood Risk

The importance of choosing an appropriate coastal defence to prevent risk of flooding.

Coastal erosion and flooding risk.

Ria Formosa, Praia do Faro

Ria Formosa, Praia do Faro (Portugal). Coastal system exposed to erosion tends to disappear without the appropriate coastal defence. Picture taken by Cristina Ortega.

The coast is a dynamic environment located where land, sea and air meet. This area offers a large number of resources, so it has always been a place of high population density. Furthermore, the coastal systems provide a wide range of functions including absorption of wave energies, nesting and hatching of fauna, protection of fresh water or siting for recreational activities. However, migration of the human population to the coast has turned coastal erosion into a problem of growing intensity. (1)

Coastal erosion can be defined as the removal of material from the coast by wave action, tidal currents and/or the activities of man, typically causing a landward retreat of the coastline (2)

The importance of reducing coastal erosion has been increased by the intensive use of the natural area for recreational, residential and tourism purposes. The direct impacts of human activities on the coastal zone have been more significant over the past century than impacts that can be directly attributed to observed climate change. (3)

The management of the risk of coastal erosion is really important. The changes in this natural area can increase the risk of coastal flooding. It can also result in land loss, which is an issue for urban and industrial areas, and nature reserves. It has been estimated that 23% of the world’s population lives within 100km distance of the coast and population densities in coastal regions are about three times higher than the global average. (4) The number of people in Europe subject to coastal erosion or flood risk in 2020 may exceed 158,000, while half of Europe’s coastal wetlands are expected to disappear as a result of sea-level rise. (1)

The most common problems that result from coastal erosion are the abrasion of the dune system as a result of a storm event, the collapse of properties located on the top of cliffs and dunes, the undermining of Tidal flood defences as a result of foreshore lowering and the loss of lands with economical value.(5)

Flooding and Erosion in UK

Seven Sisters

Property located on the top of cliff in Seven Sisters (UK). Picture taken by Cristina Ortega.

Of the 4,500km of coast in England, approximately 1,800km is at risk of coastal erosion (approximately 340km of which is defended). It has been estimated that approximately 200 properties are currently vulnerable to coastal erosion but by 2029, up to 2,000 residential properties and 15km of major road and railway may become vulnerable. (6) Almost all the coast in the UK, mainly on the east coast, has a 1 in 1,000 chance of flooding each year, without appropriate coastal defences. (7)

Seven sisters (East Sussex, UK) has been affected by erosion for thousands of years but the process is now speeding up. The cliffs are eroding by on average 0.7m per year. The erosion is unpredictable. For example, there were no significant cliff falls here for many years and then a 3m deep section was lost overnight in January of 2014. (8)

This means that it is becoming more important for councils and governments to manage their coastlines in order to protect them from increasing coastal erosion and flooding due to altering sea levels. Over the next six years, the UK government is planning to invest £2.3 billion in more than 1,500 projects across the country. It should be noted that these figures relate to all types of flooding, not just coastal, This figure is maintained in the Government’s recent Spending Review, even with the Environment Agency facing a 15% budget cut. This will reduce flood risk for more than 300,000 households, on top of the 230,000 homes already protected since 2010. (9)

Management strategies on coastal erosion­­­

It is not possible to prevent all flooding or coastal erosion, but there are actions that can be taken to manage these risks and r­­­­­educe the impacts on communities.

The traditional approach to any threat of coastal erosion or flooding has been the use of hard engineering sea defences. Those involve t­­­­­­­he building of artificial structures to reduce or stop the impact of coastal processes. The most common are sea walls: walls of concrete constructed parallel to the coastline as a barrier, supported by iron pilings dug into the underlying rock; groynes: these are wooden or concrete structures perpendicular to the shoreline. They work by blocking part of the littoral drift, whereby they trap or maintain sand on their upstream side; and breakwaters: which are offshore concrete walls that break waves out at sea so that their erosive power is reduced when they reach the coast. (10) In general, hard engineering techniques tend to be expensive, unsustainable and short-term with high impact on the landscape or environment, and they are not particularly pleasing to the eye.

An example of this practise that we can observe is at the Holderness coast, in the north east of England. This is one of the most vulnerable coastlines in the world and it retreats at a rate of one to two metres every year. The problem is caused by the strong prevailing winds creating longshore drift that moves material south along the coastline. Furthermore, the geology is clay which erodes quickly, especially when is saturated.  In 1991, the decision was taken to protect it. A coastal management scheme costing £2 million was introduced involving two types of hard engineering – placing rock armour along the base of the cliff and building two rock groynes. (11)

On the other hand soft engineering approaches have become a preferred alternative because they are less expensive, long term, attractive and sustainable. These approaches work with the natural processes of the area by making use of the natural materials, features and processes to absorb or reduce wave impact. Beach nourishment requires the addition of sand to an eroded beach in order to make it more ample (10) Having a more extended area increases the distance that the wave has to travel to reach the cliffs and therefore loses more energy reducing its erosive power when it reaches the cliffs. The sand has to be obtained from elsewhere, normally from offshore dredging. Furthermore, beach stabilisation involves planting vegetation in the sand to stabilise it and lower the profile of the beach. Transplanting vegetation will not prevent erosion, but it will accelerate natural recovery after storm damage creating a reservoir of sand within the foredunes that will enable the dunes to better withstand the next period of erosion. (12) Another useful technique is the creation of intertidal wetland that can be used to break up the waves and reduce their speed and power. The creation of saltmarshes and grazing marshes are highly important as they act as natural coastal defences, dissipating wave energy and allowing the coast to respond more naturally to changes in sea-level (13)

As an example of this technique we can observe Studland Bay in the Isle of Purbeck, Dorset. The nature reserve which is a vulnerable environment has been protected with marram grass to stabilise the dunes. In addition, the area is fenced off to limit access and man-made damage, and boardwalks have been laid through the dunes to focus tourists onto specific paths. Furthermore, information boards educate visitors about the environment and how they can help to protect it. (11). The costs for dune vegetation transplanting are dependent on labour, sources of transplants, extent of works, the need for management and the cost of ancillary works to help stabilise the dune face. According to the UK Biodiversity Action Plan the main costs for dune management can vary with it costing approximately £140/ha in England and £50/ha in Wales (14) (1 ha is the equivalent of the size of a football field). However a regular project could vary from small schemes implemented by volunteer labour using local transplants which may cost almost nothing, to extensive schemes using commercial nursery transplants and contracted labour may cost up to £20,000/km2, plus ongoing management costs. (12)

Regardless of which method is used, the importance of appropriate coastal management is vital. Besides the fact that it keeps coastal erosion under control, given its vulnerability and importance, we should carefully consider how we manage the coastline to ensure that we can make use of its resources and allow it to remain to be enjoyed by future generations.

 


 

References:

  1. Eurosion (2004).  Living with Coastal Erosion in Europe: Sediment and Space for Sustainability. Part-1 Major Findings and Policy Recommendations of the EUROSION Project. Guidelines for implementing local information systems dedicated to coastal erosion management. Service contract B4-3301/2001/329175/MAR/B3 “Coastal erosion Evaluation of the need for action”. Directorate General Environment, European Commission, 54 pp. Available at: http://ec.europa.eu/ourcoast/download.cfm?fileID=1233
  2. British Geological Survey. Coastal Erosion. Available at: https://www.bgs.ac.uk/downloads/start.cfm?id=2495
  3. Scavia, D., J.C. Field, D.F. Boesch, R. Buddemeier, D.R. Cayan, V. Burkett, M. Fogarty, M. Harwell and Co-authors, 2002: Climate change impacts on U.S. coastal and marine ecosystems. Estuaries, 25, 149-164.
  4. Small, C., and R.J. Nicholls, 2003: A global analysis of human settlement in coastal zones. J. Coastal Res., 19, 584-599.
  5. EUROSION (2004). A guide to coastal erosion management practices in Europe: lessons learned. Major Findings and Policy Recommendations of the EUROSION Project. Guidelines for implementing local information systems dedicated to coastal erosion management. Service contract B4-3301/2001/329175/MAR/B3 “Coastal erosion – Evaluation of the need for action”. Directorate General Environment, European Commission. Available at: http://www.eurosion.org/shoreline/lessons_learned.pdf
  6. Environment Agency (2011). Understanding the risks, empowering communities, building resilience:  the national flood and coastal erosion risk management strategy for England. Available at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/228898/9780108510366.pdf
  7. The National Oceanography Centre (NOC). Coastal erosion. Available at: http://noc.ac.uk/science-technology/marine-hazards/coastal-erosion
  8. National Trust. Birling Gap and the Seven Sisters. Managing change at Birling Gap. Available at: http://www.nationaltrust.org.uk/features/managing-change-at-birling-gap
  9. Environment Agency (2015) 2010 to 2015 government policy: flooding and coastal change. Part of: Flooding and coastal change.
  10. Masria, A., and Negm, A, 2015: Coastal protection measures, case study (Mediterranean zone, Egypt). Journal of Coastal Conservation (Impact Factor: 0.9). 05/2015; 19(3). DOI: 10.1007/s11852-015-0389-5.
  11. BBC. Geography. Coastal management. Available at: http://www.bbc.co.uk/schools/gcsebitesize/geography/coasts/coastal_management_rev1.shtml
  12. Scottish Natural Heritage: A guide to managing coastal erosion in beach/dune systems, Appendix 2 Monitoring erosion and change in dune systems.
  13. Devon City Council (2005) Coastal & Floodplain Grazing Marsh Project.
  14. Hudson, T et al. 2015. Delivering benefits through evidence: Cost estimation for coastal protection summary of evidence. Environment Agency.

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