Amongst the many different types of obstructions that occur in watercourses, culverts represent the most difficult structures through which to allow upstream passage of migrating fish and eels. Whereas natural waterfalls and weirs can be easily engineered with a sequence of open pools allowing fish to jump between the pools and swim upstream, culverts present a difficult design challenge due to their confined dimensions and length. The challenge in the case of the Afon Rhyd culvert was not just to create a sequence of weirs and pools. There was also a requirement to create a wetted crawling surface in the same confined space, to allow slower swimming or crawling species, such as young eels, to pass up through the same culvert. A workable design to provide that combination in a narrow culvert has never been satisfactorily achieved in the UK or elsewhere.
Working with fisheries specialist, Dr Peter Fox, the design and construction teams developed a culvert detail incorporating a series of baffles. This creates pools of slack water, allowing fish to rest and regain energy between bursts of swimming upstream against the flow, whilst the spacing of the baffles within the culvert gradient creates a minimum depth. In addition, ‘elver mats’ were fixed to an angled shelf along one side, providing pegs through which the juvenile eels can move upstream in their annual migration. Careful on-site positioning of boulders and gravel at the culvert openings was integral to achieving effective access for fish and eels.
Monitor Report. Balfour Beatty. Wales. August 2020.