Submitted by Rupak Thapaliya on Mon, 2014-04-28 13:32
There is a clear need for identifying simple relations between changes in flow and ecosystem response for the improved management of rivers and the implementation of the EU WFD. Despite this, it appears very difficult to identify widely applicable relations between hydrological/hydraulic parameters and ecosystem response, and one specific water flow target/methodology could hardly be found nor recommended. The review of management practices of setting environmental/minimum flows in selected European countries, rather revealed that a number of approaches are used, most of them, however, ending up in minimum flow/environmental flows in the range of 5-10% of mean annual flow. The authors would propose to use the building block methodology (BBM) as a conceptual framework for setting flow targets in regulated rivers. This would support the overall idea of the EU WFD of introducing ecosystem-based management with stakeholder involvement. This is in line with recommendations given to authorities in e.g. the UK where trials are currently undertaken. The authors believe the use of hydrological/hydraulic analysis still have an unleashed potential in the environmental management of regulated rivers in general and related to the EU WFD in particular, possibly supporting rapid assessment ecological status in rivers in the future.
As part of the Glen Canyon Environmental Studies, we have developed a discharge model that routes daily discharge waves released from Glen Canyon Dam to Diamond Creek, 386 km downstream. Owing to the length of the diurnal discharge wave and the sparseness of the available topographic data, the latter were averaged over the entire length of the system. Terms too small to be significant in the momentum equation were identified by scaling arguments based on data from past dam releases and on channel hydraulic geometry. Channel friction results primarily from form drag on large topographic elements and from variations in cross-sectional area and flow depth, rather than bed roughness, producing a stage-dependent friction that is not well represented by a constant value of standard channel roughness parameters, such as Manning's n. Channel friction as a function of stage was determined from field data available at high discharge and intermediate discharge and by using simple kinematic wave theory together with wave speed measurements to determine channel friction at low discharge. Model predictions of wave speed and shape agree well with data from five streamflow gaging stations and 42 stage gaging stations located along this segment of the Colorado River.
One of the most popular models for simulating the effect of regulated streamflow on fish habitat is the Instream Flow Incremental Methodology (IFIM). This model was developed by the Instream Flow Group (IFG) and is composed of components which simulate water temperature, water quality, and physical habitat. The physical habitat component (PHABSIM), however, is so frequently the only p art of the method used that PHABSIM and IFIM are often confused.
PHABSIM (and therefore IFIM) is based on several assumptions which are not always met. Tests relating fish biomass to amounts of habitat based on these assumptions have given inconsistent results. Because IFIM has become so widely used for water management decisions (it is now a legal requirement in Idaho, California, Colorado, Washington, and Oregon and is receiving increasing use in Canada and New Zealand) there is a danger of extensive damage to natural resources if its predictions are inaccurate. The purpose of our paper is to examine the validity of PHABSIM's assumptions, to review the accuracy of PHABSIM's predictions, and to discuss other methods that have been used for determining instream flow needs in New Zealand.
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