Tidal asymmetry analysis
Summary of key issues:
|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||
|Necessary software tools / skills||
|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.|
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.