Predicting hydromorphological responses to dam removal

Abstract

The alteration of flow regimes through abstraction and impoundment represents a major threat to aquatic species and ecosystems. Dam and weir removal have become attractive restoration measures for mitigating flow regime alteration, habitat fragmentation and other ecological impacts. Yet, large heterogeneity between study sites, a lack of historical data and complex underlying processes, mean predicting the hydromorphological response remains challenging. In this work, the novel application of Physically Based Spatially Distributed catchment modelling, Geomorphic Modelling and Level Pool Routing combined with extensive fieldwork is used to predict the hydromorphological consequences of weir removal at Ennerdale Water; a Special Area of Conservation in the Upper Ehen catchment, where impoundment and abstraction date back to the 1800s. The area is home to the last remaining stronghold of endangered freshwater pearl mussels in England and Wales and is subject to unique natural and managed conditions. Using historic flow regime data, a national-scale assessment of freshwater pearl mussel rivers is conducted. It shows that healthier mussel populations inhabit rivers with less extreme fluctuations between low and high flows and catchments with lower magnitude high flow events. Healthy populations, however, are found in both regulated and unregulated rivers. Abstraction has reduced mean flows in the Upper Ehen by 10% from 1995-2019. Without impounding infrastructure and flow regulation, minimum flows are predicted to fall by around 65% with no change to Q95 values. In the event of weir removal, mean lake levels are predicted to fall by 0.95m, resulting in a 5% reduction in the surface area of Ennerdale Water. The novel application of water-borne ground penetrating radar has shown evidence of historic lake outlets and previous lateral channel movement at the head of Ennerdale Water. Lakebed analysis identified small pockets of fine sediment, totalling an estimated 825 m3 , directly upstream of the weir. Chemical analysis identified no high levels of contamination. Following weir removal, sediment transport rates were predicted to vary between 0.25 m3 and 309m3 per year, averaging around 30 m3 /year. Transport was highly correlated with extreme high flows. Correctly managed weir removal represents an opportunity for United Utilities to align the interests of regulators, customers and wider stakeholders. The hydrological and geomorphic results presented, provide critical and unique evidence for achieving this outcome. Furthermore, this study presents a novel and thorough methodology for investigating weir and dam removal that can be applied to other sites requiring ecological restoration.