Natural Resources Wales fish passage improvement measures

Client: Natural Resources Wales                                                                                   Location: Wales


In 2020 we were commissioned to design eel passage improvement measures at 11 gauging stations across North Wales, for Natural Resources Wales (NRW). The gauging infrastructure at the sites ranged from low head crump weirs to compound crump weirs with existing multi-species fish passage measures already installed.

Whilst minimising impacts on the hydrometric functioning and accuracy of the gauging weirs was a key design criterion at all sites, some locations required pumped solutions, as gauge sensitivity and structure meant the existing weirs could not be broken into.

The solutions had to take account of the needs of a number of client teams, each with varying requirements.


The design criteria defined by each of the client teams meant that this was a truly collaborative project. This meant JBA civil, structural and mechanical, and electrical engineers were able to work alongside fish ecologists to undertake extensive consultation with the client team. This was facilitated through collaborative design workshops and teleconferences involving hydrometry and telemetry, fisheries and project delivery teams.

As part of the range of gravity solutions provided, we made use of eel tiles and mussel rope climbing substrate at seven of the sites. Whilst at three others we proposed pumped solutions. To assist the client team in understanding the flow of water required to deliver a target depth of water on an eel tile with a lateral slope (and therefore facilitate optimisation of the size of the pump required) we undertook Computational Fluid Dynamics (CFD) modelling.


Natural Resources Wales fish passage improvement measures

The CFD modelling undertaken demonstrated that a pass with a longitudinal slope of 10° and a lateral slope of 7°, needed a flow of 2l/s to drown the large bosses at the toe of the lateral slope, whilst at the same time positioning the water surface half way up the narrower bosses, at a distance of 1m from the apex of the pass – as shown opposite.

Our collaborative approach facilitated the integration of all client team requirements into our design work. The delivered solutions, therefore, provided confidence in technical performance, and in a way that could be safely and easily maintained. One example of this was the bracket system developed to hold mussel rope climbing substrate in place, which is low profile (reducing the risk of debris trapping debris) and adjustable. This adjustability enables client team operatives to raise the rope climbing substrate out of the water for the purposes of maintenance and cleaning, without the need for feeling around below water level and risking cuts/abrasions and the effects of cold water immersion.

NRW have so far installed our designed solutions at four of the sites: Pencaenewydd on the Afon Erch, Peblic Mill on the Afon Seiont, Pont y Cim on the Afon Llyfni and Bowling Bank on the Afon Clywedog.

Solutions at a further six sites are programmed to be installed in 2023 and 2024.

Want to know more?

For more information on this project please contact Jon Whitmore.

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