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Effect of climate change on coastline evolution

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Versie door Tasomerville (Overleg | bijdragen) op 18 sep 2008 om 15:50

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Global warming causes sea-level rise as oceans expand, and makes storm patterns more energetic. Consequently it will affect most of the world’s coastlines through inundation and increased erosion. Sound predictions of the development of these hazards over the next century are needed in order to manage the resulting risks. Coastal flooding is somewhat easier to predict than erosion since inundation can be estimated using coastal contours. However its prediction is not trivial since inundation may be followed by rapid reshaping of the shoreline by, amongst other things, waves, tidal currents and human interventions.

Understanding of coastal morphological response to climate change and sea-level rise is quite underdeveloped. This is partly because the timescales over which concern of its effects are greatest (annual to centennial) falls between the small scales addressed by most numerical models and the large sales described in the conceptual models of geomorphologists. An additional problem is that the type of model often used to bridge this gap, which is based on extrapolation of historic behaviour, is inappropriate if the climate changes.

Coastline response to accelerated sea-level rise

The most widely cited method of quantifying the response of a shore to rising sea-levels is known as Bruun’s rule (see Parametric equilibrium models). This was developed to describe the behaviour of sandy coasts with no cliff or shore platform. It assumes that the wave climate is steady and consequently the (average equilibrium) beach profile does not change, but does translate up with the sea-level. This rise in beach surface requires sand, which is assumed to be eroded from the upper beach and deposited on the lower beach. Thus as the profile rises with sea level it also translates landward, causing shoreline retreat. Note that despite the erosion of the upper beach no sand is actually lost; it simply translates a small distance down the profile. The Bruun rule has been the subject of some debate and criticism, but is still generally supported (e.g. Stive, 2004[1]) and a recent observational study by Zhang et al. (2004)[2] lends weight to it. They found that the Bruun rule modelled retreat of eastern USA shorelines well, although they recognised that it does not represent long-shore transport, and restricted their study to sites where this could be neglected.

Another constraint on the range of applicability of the Bruun rule results from its assumptions that the shore profile is entirely beach and loses no sediment. Along most coastlines the beach is a surface deposit that can only be eroded by a limited amount before the land underlying it is exposed and attacked. Here the shore profile is composed of both beach and rock. The rock element of such composite shores complicates its behaviour because it can only erode (not accrete) and it is likely to contain material that is lost as fine sediment. In addition, being purely erosive and relatively hard, it will have a different equilibrium profile to that of the beach and will take longer to achieve it.

Modifications to the Bruun rule can be used to account for the loss of fine sediment (cfi Bray & Hooke 1997[3]) but not changes in profile form. Relatively little work has been done on the relationship between sea-level rise and the profiles of composite beach/rock shores. Recent results indicate that such profiles do change, becoming steeper as the rate of sea-level rise increases (Walkden & Hall, 2005[4]).

The Bruun rule predicts that rates of increase of sea-level rise and shoreline recession will be the same, i.e. R2/R1 = S2/S1 where R and S are the rates of equilibrium recession and sea-level rise respectively and 1 and 2 indicate historic and future conditions. Walkden & Dickson (2006)[5] predicted that low beach volume composite shores are rather less sensitive and that, for them, R2/R1 = sqrt(S2/S1), although, like the Bruun rule, this equation does not account for longshore interactions.

Dickson at al (2007) [6] modelled alongshore interactions along a 50 km stretch of composite beach/ rock coast under a range of sea-level rise scenarios. They demonstrated a marked increase in complexity of shore response to sea-level rise in areas where alongshore sediment transport was important, even observing some shoreline advance.

Shore wave heights are normally limited by water depth, so an increase in sea-level might be expected to increase waves at the shore. This appears to be true at composite beach/ rock shores, however it does not necessarily occur at beach shores. Bruun’s model describes beach profiles remaining constant as they translate up and landward. This means that although the sea-level rises the water depth across the surf zone does not increase, and so larger waves can not be accommodated.

Coastline response to changed storm patterns

The form of a shoreline depends strongly on the climate of wave conditions it is exposed to. Larger waves are better able to erode both beach and land. The angle at which waves arrive has a strong effect on the rate at which beach material is redistributed along the shore. A shoreline may therefore represent a dynamic balance between the wave climate, land erosion and the distribution of beach sediment. Changes to the wave climate, such as a shift in average direction or a general increase in height will disturb this balance, and a period of shoreline adjustment would be expected.

Interaction of neighbouring coasts makes such shoreline adjustment complex and difficult to predict. Fortunately One Line morphological models are able to represent alongshore beach movement at large spatial and temporal scales. Studies that have used this approach to predict shore response to wave climate change have found differing shoreline sensitivity. Slott et al. (2006)[7] found such shoreline change could be an order of magnitude greater than those caused by rising sea levels. Conversely Dickson et al. (2007) found both smaller overall sensitivity and that sea-level rise had a stronger effect. This difference is unsurprising because the two studies examined coasts that are different in many ways; Slott et al. dealt with sandy cuspate shores exposed to high angle waves, whereas Dickson et al. modelled composite beach/ rock shores. It appears that the high dependency of cuspate shores on wave angle strongly increases their sensitivity to changes in wave climate, relative to composite beach/ rock shores.

References

  1. Stive, M. 2004 How important is global warming for coastal erosion? Climatic change 64, 27-39
  2. Zhang, K., Douglas, B., and Leatherman, S. (2004). Global Warming and Coastal Erosion. Climatic Change 64, 41-58
  3. Bray MJ, Hooke JM (1997) Prediction of coastal cliff erosion with accelerating sea-level rise. J Coast Res 13, 453–467
  4. Walkden M.J. and Hall J.W. (2005) A predictive mesoscale model of the erosion and profile development of soft rock shores. Coast Engineering 52, 535–563
  5. Walkden M and Dickson M, (2008) Equilibrium erosion of soft rock shores with a shallow or absent beach under increased sea level rise. Marine Geology, Vol 251/1-2 pp 75-84 DOI: 10.1016/j.margeo.2008.02.003
  6. Dickson, M.E., Walkden, M.J., and Hall, J.W., (2007) Systemic impacts of climate change on an eroding coastal region over the twenty-first century. Climatic Change 84(2), PP.141-166. DOI 10.1007/s10584-006-9200-9
  7. Slott J.M. Murray, A.B., Ashton, A.D. and Crowley, T.J. (1996) Coastline responses to changing storm patterns. Geophysical Research Letters 33 (18)
The main author of this article is Mike, Walkden
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