Current Coastal Flood Defences From Around the World

Coastal flooding is not a new problem and so some defences are already in place to protect human settlements from inundation; this post documents two of the most interesting examples.

Thames barrier, London

Opened in 1982, the Thames barrier is one of the worlds largest movable flood barrier [1], the 520 meter barrier spans the River Thames and protects some 125 square kilometres of London from flooding [1]. 

Such flooding would be the result of a storm surge travelling up the river Thames, the type of which hit London, during the Great North Sea Flood of 1953 and was the actual cause of the construction of the Thames Barrier. The North Sea Flood killed a total of 307 people across the South East of the UK [2], a death toll that would have been much higher, had the force of the wave not been deflected by the Netherlands, on its way to the Thames Estuary. Please see the following video, for a BBC view on the matter.

The use of the barrier, which cost £0.5 billion pound to construct and consists of 10 steel gates that can be raised to prevent high tides and storm surges from inundating London [1], has been increasing on a annual basis, since it became operational [3] (figure 1).

Figure 1: Number of annual closures of the Thames Barrier to protect London [3]

As can be seen from figure 1, apart from a few years of lag, the number of closures have been generally increasing since the beginning of operations. Such a trend raises questions of the barriers ability to protect London in the future and what, if any changes should be made to the protection of the UK's capital. The next post will discuss such matters. 

The Netherlands

The Netherlands is in a unique and hugely compromising physical position, in relation to the sea. 26% of the country is below sea level, 70% of the country would be flooded without coastal defence and 70% of the GDP of the Netherlands is produced in areas below sea level [4]. It is because of this, that the Netherlands coast is lined with man made dykes, sea walls, coastal dams and flood gates, in an effort to stop the sea. 

One example of such defences can be seen in the village of Petten, which is currently protected from the sea by a 13 meter high and 46 meters thick sea wall [5]. This wall has been increased in height by the Dutch authorities, since its construction in 1976 [5] and is another example of the relentless and increasing threat of rising sea levels.

A second example of Dutch coastal flood defences can be seen in the city of Rotterdam; a city that has to compromise between maintaining a prosperous port and preventing the flooding of a vulnerable area. A resolution of this problem was found in a swinging gate system that can be closed across the shipping channels when a storm surge is approaching the city. 

Please see the following video, for a detailed explanation of the system and altered channel floor, I have included Dutch subtitles, for my international fans. 

As the video explains, the efforts that have been taken to prevent flooding of human settlements have advanced, both technologically and in terms of costs. With sea level rise potentially accelerating, we must now ask ourselves how much longer we can hold off the advancing seas.  

The examples given during this post document the extreme lengths governments are willing to go to to protect areas of land from inundation. There is however a growing train of though, promoting managed realignment of coasts to allow natural defences to reform. The next post will document such thoughts and ask what changes, if any, should be made to the current system.

Reference List

[1] The Environment Agency, 2011. The Thames Barrier [online]. Available at:  http://www.environment-agency.gov.uk/homeandleisure/floods/38353.aspx [26.4.2011]

[2] Henry, E., 2007. Sever Flooding 'Could Put Lives at Risk' [online]. Available at: http://www.martinfrost.ws/htmlfiles/nov2007/1953flood.html [26.4.2011]

[3] King, D.A. ,2004. Climate change Science: Adapt, Mitigate or Ignore? Science. 303. 176-177

[4] Waterland Information Network, (n/d). Dutch Flood Control and Protection [online]. Available at: http://www.waterland.net/index.cfm/site/Water%20in%20the%20Netherlands/pageid/E3B3B416-FB4E-0AB8-2FB6E2B271F1BD6E/index.cfm [26.4.2011]

[5] Woodard, C., 2001. Netherlands Battens Its Ramparts Against Warming Climate [online]. Available at: http://news.nationalgeographic.com/news/2001/08/0829_wiredutch.html [26.4.2011]

Is Sea Level Rise Accelerating?

Figure 1: IPCC sea level rise projections and more recent satellite observations [1] Figure 1 portrays the root of the debate; recent satellite observations have place observed sea level rise in the uppermost echelons of the IPCC sea level rise predictions, at a gradient that indicates acceleration in sea levels during the late 20th century and throughout the 21st century. However, the debate is ongoing, with conclusions ranging according to the type and length of study undertaken and some claims of the impossibility of a global prediction of present and future sea level change [2]. The reasons for the continuing debate and the varying conclusions of sea level rise are outlined below. Varying Lengths of Study  An example of the problems with varying lengths of study can be seen with the use of tide gauge data to predict sea level rise. Short term tide gauge data can be easily affected by short term changes, due to weather events, such as El Nino and isostatic sea level change, caused by tectonic movement or glacial rebound [2]. This issue has led researchers to study tide gauge data of 60 years or more in length [3], in an effort to avoid any short term changes obscuring the general trend. However, this increased length of observation created the need to average each data set, in an effort to remove the noise of short term, tidal changes. Such an averaging has been found to obscure any acceleration that may be evident [4] and has led to the general assumption that there was no acceleration in sea levels, during the 20th century [4].  As well as the accusations of the loss of any meaningful data, there have also been claims that the study of tide gauge data alone is not long enough to identify any acceleration that may have occurred [4 & 5], this has forced other researchers to combine tide gauge data and satellite observations, an effort that has produced more intriguing results (figure 2).  Figure 2: Sea levels between 1880 and 2001, showing an increase in the rate of rise after 1930 [4]   Figure 2 shows the merits of studying longer periods of time and combining methods of measurement; the sharpening of the incline after 1930 may have gone unnoticed if the length of study was shortened or the data of both tide gauges and satellites were not used.  Ice Sheet and Glacier Knowledge Many of the predictions of accelerated sea level rise are based around future ice sheet and glacier contributions. Claims have been made that half the present sea level rise can be attributed to the melting of terrestrial ice [6] and that glacier and ice cap melting makes up 60% of the ‘new water’ component of sea level rise (1.8 mm/year) [7]. A second point regularly made in the literature concerns the rise in sea levels, as a result of temperature rise and increased ice melt [7 & 8]; sea levels have been predicted to increase by 3.4 mm/ year per degree Celsius increase in temperature [8]. Such claims may make more of an impact, if the uncertainties of ice sheet dynamics and the response to warming were not so wide ranging and well publicised.  Such uncertainties include a lack of understanding of ice sheet dynamics, polar ice thickness, depth of bed rock, speed of movement towards the sea, the potential of mountain glacier disappearance and Antarctic contribution to sea level rise [6 & 8]. Such a spectrum of unknowns has prevented the creation of solid conclusions of the future increase in sea levels and the potential of acceleration (figure 3). Papers are forced to conclude with very open statements, covering the possibilities of no change, continued present day speed and rapid sea level rise [5, 6, 7 & 8].  Figure 3: Ranging predictions of sea level rise by 2100, upper most and lowest predictions include error bars, further increasing the range of possibilities [5]   The compounding factors tested in each scenario of figure three and the uncertainties of ice sheet reactions to heating have caused sea level predictions for 2100 to range between 0.1 and 0.7 meters. Despite the uncertainties and on going debates of future acceleration and sea levels, the current evidence does point towards 21st century acceleration in increase and generally higher sea levels across the globe (figure 1). With the advent of satellite observation, the global coverage, quality and temporal resolution of observations are only going to increase, making any future predictions and observation much more reliable.  After consideration of the findings and in full knowledge of the problems with any predictions, we must now decide how to react to the chance of sea levels rise, the steps we are willing to take to protect ourselves and cities and what, if any, change we should make now, in an  effort to prevent the potential sea level rise.  The remaining posts will focus on such matters. References  [1] Allison, I. , N.L. Bindoff, R.A. Bindschadler, P.M. Cox, N. de Noblet, M.H. England, J.E. Francis, N. Gruber, A.M. Haywood, D.J. Karoly, G. Kaser, C. Le Quéré, T.M. Lenton, M.E. Mann, B.I. McNeil, A.J. Pitman, S. Rahmstorf, E. Rignot, H.J. Schellnhuber, S.H. Schneider, S.C. Sherwood, R.C.J. Somerville, K. Steffen, E.J. Steig, M. Visbeck, A.J. Weaver, (2009) The Copenhagen Diagnosis, Updating the World on the Latest Climate Science. The University of New South Wales Climate Change Research Centre (CCRC), Sydney, Australia. [2] Douglas, D.C., M.S. Kearney, S.P. Leatherman eds. (2001) Sea Level Rise: History and Consequences, Volume 1 London: Academic [3] Douglas, B.C., (1997) Global sea rise: a redetermination. Surveys in Geophysics, 18, 279-292.  [4] Church, J.A., N.J. White (2006) A 20th Century Acceleration in Global Sea-level Rise. Geophysical Research Letters, 33, L01602 [5] Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. Van Der Linden, X. Dai, K. Maskell, C.A. Johnson eds. (2001) Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.  [6] Thomas, R., E. Rignot, G. Casassa, P. Kanagaratnam, C. Acun, T. Akins,H. Brecher, E. Frederick, P. Gogineni, W. Krabill, S. Manizade, H. Ramamoorthy, A. Rivera, R. Russell, J. Sonntag, R. Swift, J. Yungel, J. Zwally (2004) Accelerated Sea-Level Rise from West Antarctica. Science 306, 255  [7] Meier, M.F. M.B. Dyurgerov, U.K. Rick, S. O’Neel, W.T. Pfeffer, R.S. Anderson, S.P. Anderson,A.F. Glazovsky (2007) Glaciers Dominate Eustatic Sea-Level Rise in the 21st Century. Science 317, 1064    [8] Rahmstorf, S. (2007) A Semi-Empirical Approach to Projecting Future Sea-Level Rise. Science 315, 368