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Geomorphological analysis

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Geomorphology is the study of the features that make up the earth’s surface and their relationship to the underlying geology. A geomorphological study will provides a conceptual picture of coastal processes and the potential behaviour of the coastal system. This includes taking into account changes in the bedrock composition that could affect the potential rate of future coastal evolution. The results tend to be qualitative, rather than quantitative. This section starts with a description of how a sediment budget may be used to provide a view about future beach levels in front of a coastal structure. The section then moves onto describe useful projects that have has a significant geomorphological component, namely Futurecoast and Eurosion and introduces the concept of the coastal tract as a way of approaching very long term coastal evolution.

An example of the geomorphological approach is given by Honeycutt and Krantz (2003)[1] who illustrated how the local geology affected shoreline change rates along the Delaware coast, using data from high-resolution seismic-reflection profiles, cores and historic shoreline positions. They believe that it may be possible to quantify the effect of large-scale changes in geology on shoreline erosion, but not small-scale ones. Honeycutt and Krantz (2003) provide a scientific basis for modifying calculations of past shoreline change rates to estimate future shoreline change rates.

Many geomorphology studies use a range of tools, including predictive numerical models. As such many geomorphology studies are effectively a composite of the different modelling techniques, as advocated by, for example, Cooper and Pilkey (2004).[2]


Sediment Budget

Sediment budgets are often constructed to assist with coastal management. A sediment budget allows an estimate to be made of the rate of accretion or erosion of sediment within a pre-defined area of the coastal zone (see Rosati 2005 [3], for a recent review). The main steps involved in constructing a sediment budget are:

  • Set appropriate boundaries for the sediment budget and for internal boundaries that separate sub-cells within the overall area to be considered;
  • Identify sources, pathways, stores and sinks of sediment within the budget area;
  • Calculate the rate of erosion from sources and stores and accretion in stores and sinks. These estimates may come from numerical models but are more likely to be derived from data;
  • Calculate the sediment transport rates at the boundaries of the subcells and estimate the uncertainty in each transport rate. The calculations of transport rate may come from data but are more likely to be derived from numerical models; and
  • Integrate the gains and losses within each section to obtain an overall sediment budget.

A good sediment budget will provide a useful indication of whether a beach in front of a coastal structure is likely to be subjected to beach lowering due to loss of sediment from the entire beach. Even if this is not the case and beach volumes have been constant or increasing, a coastal structure may be subject to beach lowering due to local effects.

Futurecoast

Futurecoast (Burgess et al., 2002 [4]) was commissioned by the UK Department for the Envoronment, Food and Rural Affairs (Defra) to improve the understanding of coastal evolution for the open coast of England and Wales. Futurecoast is the obvious starting point for any assessment of future coastline behaviour over decadal timescales for these coastlines. It contains:

  • Shoreline behaviour statements that give an improved understanding of coastal behaviour and qualitative predictions of future coastal evolution at both large and small scales;
  • Assessment of future behaviour for an unconstrained scenario (with no defences or management) and a managed scenario (where present management practices continue indefinitely); and
  • A ‘toolbox’ of supporting information and data including cliff behaviour statements, historical shoreline changes, wave modelling, an uncertainty assessment, morphological measurements including beach width, a coastal geomorphology reference manual and a thematic studies on onshore geology, offshore geology, coastal processes, climate change and estuaries.

Eurosion

Eurosion (European Commission, 2004[5]) was a European study into coastal erosion at a European scale. Its outputs were:

  • A map-based assessment of European coasts exposure to coastal erosion;
  • A review of existing practices and experience of coastal erosion management;
  • Guidelines to incorporate coastal erosion into environmental assessment, spatial planning and hazard prevention; and
  • Policy recommendations to improve coastal erosion management.

Eurosion’s maps can be used to assess the coastal typography, geology and coastal erosion trends of a region. The maps also include the location of engineering works (whether harbours, jetties groynes or breakwaters). There is an additional map for regional exposure to coastal erosion.

Eurosion concluded that a more strategic and proactive approach to coastal erosion is needed for the sustained development of vulnerable coastal zones. It developed the concept of coastal resilience: the inherent ability of the coast to accommodate changes induced by sea level rise, extreme events and occasional human impacts, whilst maintaining the functions fulfilled by the coastal system in the longer term. To promote coastal resilience, Eurosion introduced the concept of favourable sediment status: the situation where the availability of coastal sediments support the objective of promoting coastal resilience in general and of preserving dynamic coastlines in particular. This should be achieved for each coastal sediment cell by designating strategic sediment reservoirs: supplies of sediment of appropriate characteristics that are available for replenishment of the coastal zone, either temporarily (to compensate for losses due to extreme storms) or in the long term (at least 100 years). They can be identified offshore, in the coastal zone (both above and below low water) and in the hinterland.

A coastal sediment cell is a coastal compartment that contains a complete cycle of sedimentation including sources, transport paths, and sinks. The cell boundaries delineate the geographical area within which the budget of sediment is determined, providing the framework for the quantitative analysis of coastal erosion and accretion. Eurosion considered that coastal sediment cells constitute the most appropriate units for achieving the objective of favourable sediment status and hence coastal resilience (European Commission, 2004).

Coastal tract modelling

Coastal evolution over centuries to millennia requires a broader outlook than for shorter time and space scales, in that the shoreline evolution must be linked to the behaviour of the continental shelf and coastal plain (Cowell et al., 2003[6]). The coastal tract is the combination of lower shoreface, upper shoreface and back barrier that comprises the first-order system within a coastal tract cascade, whereby greater and greater levels of detail are needed to model coastal evolution at shorter and shorter time-scales and space-scales (Figure 3). The coastal tract cascade formalises concepts for separating coastal processes and behaviour into a scale hierarchy. An additional concept, that of coastal tract templating was also introduced by Cowell et al. (2003[7]) to provide a protocol for defining a site-specific problem by transforming data into a data model.

The aggregate dynamics of the coastal tract are modelled using behaviour-orientated coastal change models and constrained by sediment mass conservation (Cowell et al., 2003). The rate of coastal advance is governed by the balance between the change in sediment accommodation space caused by sea level rise and sediment availability.

References

  1. Honeycutt, M.G. and Krantz, D.E., 2003. ‘Influence of the geologic framework on spatial variability in longterm shoreline change, Cape Henlopen to Reheboth Beach, Delaware’, Journal of Coastal Research, Special Issue 38, 147-167.
  2. Cooper, J.A.G. and Pilkey, O.H., 2004. Alternatives to the mathematical modelling of beaches. Journal of Coastal Research, 20(3) 641 – 644.
  3. Rosati., J.D., 2005. Concepts in sediment budgets. Journal of Coastal Research, 21(2) 307 – 322.
  4. Burgess, K.A., Orford, J., Townend, I., Dyer, K. and Balson, P., 2002, ‘FUTURECOAST: the integration of knowledge to assess future coastal evolution at a national scale’, In Proceedings of the 28th International Conference, Coastal Engineering 2002. McKee Smith (Ed), World Scientific, pp 3221 – 3233.
  5. European Commission, 2004, ’Living with coastal erosion in Europe – Sediment and space for sustainability’, Luxembourg office for official publications of the European Commission. 40 pp ISBN 92-894-7496-3.
  6. Cowell, P.J., Stive, M.J.F., Niederoda, A.W., de Vriend, H.J., Swift, D.J.P., Kaminsky, G.M. and Capobianco, M., 2003. The coastal-tract (part 1): a conceptual approach to aggregated modelling of low-order coastal change. Journal of Coastal Research, 19(4): 812 – 827.
  7. Cowell, P.J., Stive, M.J.F., Niederoda, A.W., Swift, D.J.P., de Vriend, H.J., Buijsman, M.C., Nicholls, R.J., Roy, P.S., Kaminsky, G.M., Cleveringa, J., Reed, C.W. and de Boer, P.L., 2003. The coastal-tract (part 2): Applications of aggregated modelling of lower-order coastal change. Journal of Coastal Research, 19(4): 828 – 848.
The main author of this article is James, Sutherland
Please note that others may also have edited the contents of this article.