A review of the extent to which deposition and accumulation of sediment behind the weir of hydro schemes impact fluvial dynamics, habitats and species, both on single developments and cumulatively within catchments, and what mitigation can be implemented to counter any impacts.

There are currently over 530 feed-in-tariff (<5 MW) scale hydro-electric schemes installed across Scotland, with a further c.30 in planning, consented (awaiting construction), or under construction.  The majority of installed small scale schemes are run-of-river designs consisting of a weir and screened offtake. SEPA guidance for developers of run-of-river hydropower schemes requests that proposals are assessed against particular criteria to ensure they comply with Water Environment (Controlled Activities) (Scotland) Regulations 2011 (CAR). One of the criteria covers the management of sediment accumulating upstream of the weir. SEPA, and SNH in their guidance on assessing impacts from hydro schemes on the natural heritage, clearly identify sediment accumulation and associated impacts as an issue to be considered in the Environment Impact Assessment (EIA), and CAR and planning applications.

Currently, there are many schemes where there is provision in the CAR licence for sediment management as suggested but no requirement, and few hydro schemes have been developed with an associated sediment management plan specifically aimed at maintaining natural sediment transport. A particularly problematic issue is the relative rates of transport and accumulation of fine and coarse sediment. Fine sediment is transported and accumulates more frequently. The result can often be an overload of fine sediment in the downstream channel when management is carried out.

The overall aim of this project is to enhance our understanding of what can be done to mitigate or prevent the impacts to the water environment and surrounding natural heritage associated with the accumulation of fine and coarse sediment behind run-of-river hydro scheme weirs, both now and in response to different climate change scenarios and the impacts these could have on fluxes of water and sediment supply and transport. 

This project will focus on identifying the most appropriate suite of sediment management techniques for use in Scotland, based on a review of existing techniques, the possible identification of new techniques, and a cost-benefit analysis of these techniques.