Refinement of the wetted perimeter breakpoint method for setting cease‐to‐pump limits or minimum environmental flows

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Abstract
Analysis of inflections or breakpoints apparent in relationships between measures of wetted perimeter and discharge can be used to assist in the determination of minimum environmental flows for perennial rivers. This paper suggests refinements and provides an example application of the wetted perimeter method for the determination of cease‐to‐pump limits in a perennial, unregulated gravel‐bed river subject to increasing levels of surface water extraction. HEC‐GeoRAS modelling outputs of riffle wetted area are used to illustrate that the magnitude of the discharge selected to represent 100% habitat availability is of crucial importance to the breakpoint method. Because of the dependence of the technique on this assumption, we suggest that it is prudent to use an upper and lower limiting discharge based on an assessment of the degree of flow variability to develop a flow range around the zone of diminishing return in the wetted perimeter to discharge relationship. For rivers exhibiting a low degree of flow variability, the mean and median daily flows are likely to provide appropriate discharges for representation of 100% habitat availability. For perennial rivers with a higher degree of flow variability and considerable differences between the mean and median daily flows we suggest use of the 50th and 80th flow duration percentiles. Wetted perimeter breakpoint results are also influenced by the degree to which areas of non‐riffle habitat are included in the analysis. Inclusion of excessive pool areas can lead to significant reductions in resultant recommendations for cease‐to‐pump limits or minimum environmental flows. Integration of hydraulic model outputs with GIS for wetted perimeter analysis of riffles provides a useful, rapid, field‐based approach that can assist with determination of cease‐to‐pump limits or minimum environmental flows in gravel‐bed rivers. However, care is needed in its application and interpretation as the technique is prone to numerous subjective choices that have a substantial influence on results. Copyright © 2004 John Wiley & Sons, Ltd.

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