Reallocation of compensation releases to restore river flows and improve instream habitat availability in the Upper Derwent Catchment, Derbyshire, UK

Abstract

The Upper Derwent catchment is situated in the Peak District National Park in North Derbyshire, England and includes the Derwent Valley Reservoir System. The natural inflows to the reservoir system are boosted by flow diversion schemes from the River Ashop and River Noe, leaving almost dry stretches in these rivers for long periods of time. Compensation releases are made into Jaggers Clough and the River Derwent. This study examined the possibility of altering the operation of the diversion scheme and compensation flow releases, both temporally and spatially to restore flows within these dry reaches. The overall intention was to minimize the ecological impacts of regulation in the four rivers whilst protecting the yield of this critical public water supply. The study utilized the Physical Habitat Simulation System (PHABSIM) to identify and compare feasible operational changes. This technique enables quantitative comparisons of the suitable habitat available under different flow regime scenarios. Brown Trout is the most abundant fish species in the Upper Derwent streams, with Grayling, Brook Lamprey and Bullhead also present. The invertebrate fauna is typical of upland streams with neutral to acid waters. The ecological data were assessed to identify suitable target species/life stages for use with PHABSIM. Brown Trout, Grayling and four invertebrate families (Rhyacophilidae, Leuctridae, Chloroperlidae and Heptageniidae) were selected. Habitat mapping along four stretches of river totalling 10 km was carried out in the summer of 1998, followed by PHABSIM fieldwork on 24 transects in the autumn. This information was utilized to examine the tradeoffs in habitat availability between reinstating flows in the dry stretches of river, and reducing compensation flows elsewhere to minimize the supply impact. Various operating scenarios were examined and two sets of compensation control rules proposed for normal and drought years. Each set included seasonal variability in the rules. The PHABSIM work described here is the first stage in the process of developing a more ecologically acceptable flow regime in the Upper Derwent catchment. The decision on the final implementation will be subject to further resource modelling and negotiation between the Environment Agency, the water company and local interested stakeholders.