The concept of flushing time (retention or residence time) is important in the consideration of the impact of polluting substances because it is one of the factors determining the exposure period to marine organisms. In relation to estuaries, flushing time is defined as the time required to replace the existing freshwater accumulated in the estuary by the river discharge. Flushing time is influenced primarily by freshwater flow from the river. McLusky (1981) states that the flushing time for the Forth estuary under mean river flow conditions is 12 days, but increases to 10 weeks in Summer with reduced river flow and decreases to 6 days following severe rainfall. Estuaries with shorter flushing times are better able to accept effluents because mixing of fresh and saltwater is greater and dilution of polluting substances is better (McLusky 1981). Models which may be used by the EA in predicting flushing time are usually used to define the retention time of an estuary, but the boundary conditions of the model and the position of the estuary/seawater boundary can make a large difference to the calculated retention time. Models take into account the fact that water is moved progressively out of the estuary over time but may move up and down over shorter distances with the ebb and flow of the tide. Retention times can also be calculated for sea lochs and coastal embayments where the complete turnover of the volume of water within the waterbody can be estimated. Flushing times can be influenced by stratification.

The importance of flushing time for non-toxic substances is that shorter flushing times reduce the risk of primary or secondary effects occurring. For example, the risk of deoxygenation of the water column is reduced if the oxygen depleting substances are better mixed in the water column and are washed out of an estuary quickly. Similarly, the risk of the secondary effects of eutrophication are reduced if the phytoplankton are rapidly removed from an estuary and do not die off causing oxygen depletion. For toxic substances, shorter flushing times result in reduced contamination of the water column and sediments and reduced exposure periods for organisms in the waterbody.

When setting consent conditions, it is important that the longest estimated flushing time is used (i.e. in an estuary, during minimum river flow and neap tides) in calculations to maximise the potential for protecting organisms in a European marine site.

In considering the effect of flushing time within a European marine site, it is important that the effect is considered over the entire site, e.g. including its effect on residence time within offshore/coastal parts of the site (where relevant) and not considering flushing time of the estuary alone.

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