Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

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Zonation of the TIDE estuaries


by Lindsay Geerts, Kirsten Wolfstein, Sander Jacobs, Stefan van Damme & Wouter Vandenbruwaene

5. 4. Concluding remarks

As mentioned earlier, in every estuary already different approaches existed. It is not the aim of TIDE to replace them. However these existing zones were not easily comparable according to the aims of TIDE. Therefore the above described approach will be used in order to allow an interestuarine comparison between the different TIDE-estuaries. It has to be taken into account that not all zones have equal length and surface area, and that zones can move spatially depending on seasonal discharge and tidal phase. Furthermore, zones for different estuaries could differ in length because of differences in tidal range, river discharge etc. It can be concluded that this zonation has to be used and interpretated with care and can be used in a flexible way according to the research question to be adressed. Ideal would be to have a standardized normalization of the TIDE-km for all estuaries. On the longer term, by working with all the actual data available in TIDE, we might find a common parameter, other than chlorinity, to set out the data along the whole estuary gradient (also in the freshwaterzone). The latter approach can than be preferred for interestuarine comparison. Nevertheless, on short term, this zonation provides a good starting point for an objective interestuarine comparison for work packages 3 and 4 (conflict matrix, ecosystem survey, ecology and hydrogeomorphology).


In some cases existing zonation approaches, e.g. for the Water Framework Directive, may not be useful when comparing the features of several estuaries. According to the objective a different zonation might be necessary for a sufficient spatial distribution resolution and to be able to compare the estuaries. For a common basis, zones of equal properties have to be chosen. This zonation approach can be used in cases when it is more sensible to compare only certain areas of the estuaries instead of considering the whole estuary, e.g. for the different issues of ecological functioning, research questions, monitoring schemes, measures and management issues, etc… The zonation approach should be flexible enough to cover different issues and should allow direct comparison of certain areas of each estuary.
Therefore, the following agreements were set in TIDE:
  1. Setting km “0” of all estuaries at the inland tidal boundary on the longitudinal axis;
  2. Using the Venice system for a comparable zonation between the estuaries;
  3. Splitting the freshwater zone according to morphology, residence time and/or usage, based on expert judgment by each TIDE-partner.
Annual and seasonal calculations of averages, median, minimum and maximum values for 2004-2009, per measuring station were performed. A six yearly average for the period of investigation (2004-2009) can be assumed as a good basis estimation for the distribution of different ecological entities.

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Important to know

Reports / Measures / Tools

Tool: Zonation of the TIDE estuaries (Recommendation)

Management issues

What is important in establishing a zonation for estuaries?


SalinityVenice-systemZonation approach