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Title: Development of Environmental Standards (Water Resources) Stage 3: Environmental Standards
Author: M C Acreman
Author: M J. Dunbar
Author: J Hannaford
Author: A R Black
Author: J S Rowan
Author: O M Bragg
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1354, Representation ID: 456, Object ID: 2527
To provide environmental standards in terms of water flows, allowing regulators to work towards licences in volumetric or flow terms, it was necessary to relate water level deviations to flows. This was possible using the assumption that flow over a sill or other outflow is related to level by a rating relationship with a stage exponent greater than unity. This is as indicated theoretically by the Chezy equation and has been confirmed empirically in this study by reference to data from a necessarily small number of sites at which levels and flows are available. Assuming the Chezy exponent of 1.5, permitted abstraction fluxes were found to be more lenient than their corresponding (and more ecologically relevant) permitted water level deviations. Level restrictions ranged from a mere 5% for some peat lakes to 20% for some brackish lakes, corresponding to abstraction restrictions of 7% to 28% respectively. This system of defining environmental thresholds provided for individual differences attributable to specific physical controls to be reflected through the concept of risk, but led to results with an unjustifiable level of apparent accuracy. As a means of addressing this concern, the final table of environmental standards, expressed in flow deviation terms, therefore introduces a rounding to the nearest 5%: the loss of unjustifiable minor differences in threshold is argued to compensate for possible exaggeration of threshold values in some cases where similar lakes fall just either side of a 5% boundary. The set of thresholds for the Good/Moderate Ecological Status boundary was then taken as a starting point to define threshold values for the other ecological status classes, while maintaining the assessments of relative difference in sensitivity. By ultimately defining standards in flow terms, it becomes possible to assess the possible effects of a water use proposal not only in relation to the adjacent rivers but also any lakes on the same river system, and so therefore be able to identify whether the river or the lake provides the more stringent environmental requirement. Given the assumption of the Chezy equation applying to outflow ratings, it is likely that lakes will often require less stringent provisions than rivers.
Publisher: Environment Agency
Subject Keywords: LakesRiversWater abstractionStandardsHydrologyClassification systemsFlow rate
Extent: 158
Total file downloads: 40

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