The measurement of flows and recording of storm events at combined sewer overflows are a key requirement in effective urban pollution management. Staff of Thames Water Utilities have developed a low‐cost system based on monitoring the behaviour of flap valves on numerous major river outfalls. The paper presents the development of a modelling approach based on the principle of conservation of angular momentum and its application to experimental data for circular gates. Flow‐prediction accuracies, based on the measurement of gate inclination and water depths, in the range ± 20–30%, are found to be achievable for wider gate openings (<20°) – the recommended calibration offering conservative flow predictions at lower angles.
The recent popularity of esturial barrage construction in the UK has been driven largely by commercial and amenity interests arising from the need to rehabilitate tracts of derelict urban land. Formation of a permanent water body to replace the tidal regime hides unsightly mud banks and facilities ready access for water based recreation. Unfortunately, in most cases, and presumably arising from upstream extreme flood risk assessment, the barrier is designed to overtop under the highest (spring) tidal peaks so allowing a substantial influx of heavier saltwater into the impoundment on a regular basis. This might well substantially exceed influx via any navigational lock operations. The problem created by the salt water is that its greater density causes stratification in the impounded water body. In times of low freshwater flow little mixing takes place and the normal action of the wind, in circulating and aerating the water body, will only extend within the lighter and often thin surface freshwater layer. The consequence is that little reoxygenation of the lower layers takes place leading to depressed dissolved oxygen levels and possible anoxia. Sluice provision in the barrage may be ineffective in expelling the saline layers, so this water remains near stagnant and deteriorates in quality due to sediment oxygen demands until a major fluvial flood, or new saline influx, is able to effectively mix the waters. Conventional continuous surface releases through fish pass and/or by crest overspill ensure the preferential release of the freshwater rather than the problematic saltwater. Retrofitting of novel low cost floating boom/skirt baffle system is proposed here for alleviation of the problem.
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