Improving fish passage and river flow below Tongland Dam

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SEPA have been undertaking a review of the current CAR license which the Galloway Hydro Scheme operates under.

SEPA have been undertaking a review of the current CAR license which the Galloway Hydro Scheme operates under.  As part of this review, GFT have been working alongside the operations team at Drax, the SEPA Ecology and Fish Ecology team, Water Resources and Hydrology to investigate if a new compensation and freshet flow regime is required below Tongland Dam.    

You may recall, GFT began a similar process of investigating how a freshet scheme could be incorporated into the compensation flow plan at Tongland back in 2013, when we trialled a range of freshet flows and which resulted in an annual freshet release plan being delivered in addition to the licensed baseline discharge during times of year that we were able to advise fish would benefit most from increased flows to aid their upstream passage in the lower river (1km section of river between Tongland Power Station and Tongland Dam).

Using GFT's original study, we were able to assist SEPA in locating two bottlenecks where changes of flow would have the greatest impact on upstream migrating fish (including a series of natural falls where extreme low and high flows may inhibit fish movement). During a two-day trial, these bottlenecks were hypothetically scored for their fish passage potential and up to three sets of flow data was gathered by SEPA to quantify the discharge from Tongland Dam of a variety of flows in relation to the Water Framework Directive UKTAG guidance for acceptable compensation flow regimes within heavily modified waterbodies.

Following a final day of flow trials undertaken in August 2022, a new flow regime was considered across the winter and Drax began implementing this from April 2023.  The new baseline compensation flow, which runs April to December, has risen to 1.4 m3/s.  This is now accompanied by up to 20 freshets, which are delivered over a period of between 24 and 48 hours at a volume of 4.5 m3/s – to help draw fish into the river and/or ease fish passage at key bottlenecks within the lower river.

With the infrastructure of Tongland Dam now in its 88th year, Drax have been busy this year installing new apparatus that allows them to more accurately gauge the flow rates from the compensation and needle valve, in order to meet the demands of the new license, likely to conclude in Spring 2024. 

During the winter, GFT will be reporting to Drax and SEPA their understanding on how the freshets may have influenced adult salmon migration in the lower river during 2023.  We look forward to updating you on this exciting collaborative project with Drax and SEPA and thank the many people that have been involved so far.

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