The Significance of Residual Currents in the Interpretation of the EU Urban Wastewater Treatment Directive in Coastal Locations

Consideration is given to the problem of defining a mixing zone near an outfall in the case where the zone is to be used to examine eutrophication problems. Mixing zones have traditionally been defined in terms of the tidal excursion for problems relating to the dispersion of bacteria with fast decay rates. It is demonstrated here that long-term, and often non-tidal, coastal residual currents play an important role in flushing nutrients away from a long sea outfall, and can render such a definition irrelevant when a pollutant has a long decay time. Far field effects, such as the interaction of neighbouring outfalls, may be more important than is implied by the mixing zone concept. These ideas run counter to the guidelines published by the Comprehensive Studies Task Team for the application of the European Union Urban Waste Water Treatment Directive in UK waters, which emphasizes the use of a tidal mixing zone for eutrophication problems.