MODELING OF THE WATER-QUALITY OF A PROPOSED IMPOUNDED LAKE OF A TIDALLY INFLUENCED RIVER

The unsatisfying water quality of especially the upper regions of the tidally influenced River Usk, Wales, UK, have resulted in a proposal for a barrage across the river in order to eliminate the tidal influence upstream. The area will then not be subject to high changes in water levels, which at low water leave the sediment banks dry and sometimes smelling and looking bad. The barrage project is supposed to increase the recreational and touristic value of the area. The water of the resulting impoundment upstream of the barrage would be almost stagnant and the impoundment would be receiving treated and untreated municipal waste water. Eutrophic conditions of this lake must be foreseen. A periodically two-layered structure of the water masses, due to the development of a thermocline, must also be foreseen. Modelling of the hydrodynamic conditions and of the water quality were performed in order to analyse the consequences of introducing a barrage. Several schemes that were meant to improve the water quality were analysed. The oxygen concentration in the bottom water was the key parameter in these analyses, since this parameter to a large extent is a result of the eutrophic conditions (algae growth) in the surface layer. A combination of a flushing scheme, physical constructions, and waste water treatment was found by which a predicted minimum oxygen concentration around 5 Mg O2 l-1 was achieved. The situation with no flushing and existing sewage discharge was predicted to result in minimum oxygen concentrations of 1-2 mg O2 l-1.