22nd June 2016

Summer is always a busy time of year here at Fishtek and the relatively dry weather gives us the chance to carry out site surveys and installations.  Our team of Engineers have recently been installing Fishtek Tidal Gate Dampers (TGDs – patent pending) on several flood defence structures around the country, including two structures on the River Seven in Gloucester. Fishtek were commissioned by Birch Brothers Ltd on behalf of the Environment Agency to design and install bespoke TGDs for two top hinged flap gates, whose primary function is to limit flood risk. These structures are very effective at preventing tidal ingress, but unfortunately they also create a significant barrier to European eel migration.  

As juveniles, European eels undertake an impressive migration from the Sargasso sea into European rivers, where they typically spend 10 years before returning to the Sargasso sea to spawn. Barriers to migration, such as flood defense structures, limit access to freshwater habitat and are likely to be a primary cause of population decline. The Fishtek TGD is a mitigation device that delays tidal gate closure beyond level equalisation, thus allowing eels to migrate beyond the barrier on the rising tide. Glass eels are tiny and only require a small opening and flood risk is seldom a concern. The design aperture and opening duration are specific to each site and are dependent upon the fisheries requirements, flood risk and tidal levels.  

Images of the TGD’s recently installed on the River Seven are given below.  A single pair of dampers has been installed to maintain a small opening beyond level equalisation that allows for glass eel ingress. By adjusting the design or the positioning of the device on the gate it is possible to increase the aperture size to provide passage to other migratory fish species.

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