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Tidal asymmetry analysis

Method indicator
Bottom-Up Hybrid Top-Down
    YES

Summary of key issues:

Issue Description
Description Assessment of data from different periods in time in order to identify directional trends and possibly rates of change of morphological features or physical processes within an estuary.
Temporal applicability Short term (several tides) to long term (100 years).
Spatial applicability Whole estuary or upstream areas of the estuary.
Links with other tools
  • Complements longer-term geological analysis approaches.
  • Can provide useful data to inform ‘regime analyses’.
  • Provides key input to establishing a conceptual understanding of the longer-term estuary behaviour during synthesis of results or Expert Geomorphological Assessment.
Data sources
  • Bathymetry: maps and charts, aerial photography, topographic and bathymetric surveys, remote sensing imagery;
  • Tidal information: Admiralty tables, tidal gauge measurements, flow models;
  • Tidal current information: current measurements, flow models.
Necessary software tools / skills
  • Identifying, collating and reviewing relevant data and information sources;
  • GIS, image processing software, photogrammetry;
  • Cartography, digital ground modelling;
  • Geomorphological interpretation of output.
Typical analyses
  • Changes in ebb/flood tide duration as a proxy for changes in net sediment transport;
  • Identifying equilibrium morphology on the basis of tidal symmetry.
Limitations There is always uncertainty regarding the nature of equilibria, and because of this the method works best in a relative sense or qualitatively rather than as a quantitative assessment.
Example applications
  • Mersey Estuary
  • Stour/Orwell

Asymmetry relationships can be used as a means of evaluating historical changes in estuary functioning and to evaluate physical impact arising from development. Asymmetry relationships focus on the effect of estuary morphology on tidal propagation in order to identify trends in net sediment transport and thus to identify future morphological changes.

Estuaries or inlets, unlike rivers, experience a feedback relationship between their morphology and the current velocities generated inside them. In some situations it is useful to be able to characterise the nature of this feedback so that the implication of a change in the estuary, e.g. sea level rise, development etc, can be deduced in a broad overall sense. Of particular interest is how changes to estuary morphology change the asymmetry of current velocities through a given cross-section. In basic terms cross-sections with peak ebb and peak flood velocities of the same magnitude will not induce net transport while asymmetric tides produce higher ebb or higher flood velocities, leading to net sediment transport in one direction or the other. Thus changes to an estuary could cause erosion or deposition in a previously morphologically stable estuary or enhance or reverse the trend completely.

Read the full PDF document for more information on this methodPDF version

Analysis and modelling

Last Modified on: 19 June 2011
Printed from the Estuary Guide on 23/04/2014 19:36:00