Project Details - Bypass channel design

Client: Ribble Rivers Trust

Categories: Detailed Design, Geomorphology

When: March-April 2043

Location: Lancashire, UK

The River Darwen is a tributary to the Ribble flowing south-east of Preston toward the Ribble Estuary on the north west coast of England. Edenvale Young. were appointed by the Ribble Rivers Trust to undertake hydraulic modelling and detailed design of a bypass channel at Holland Wood weir in order to improve fish passage along the River
Darwen.

The initial proposal of detailed bypass channel design subsequently led to an additional requirement for the development of a rating curve at Blue Bridge gauging station, 350m upstream of Holland Wood Weir in order to assess the impact on that rating relationship due to construction of the bypass channel. The rating curve denoting the relationship between stage (level) and discharge at gauging stations.

The key project requirements;

  • Merge existing 1D models of the River Darwen and scale to the reach surrounding Holland Wood weir.
  • Convert the merged 1D model of the Darwen into a 2D only model.
  • Analysis of river gauge data within the catchment to ensure sufficient quality for calibration.
  • Creation of baseline and design option scenarios and run through a range of flow scenarios to inform design criteria at both high and low.
  • Comparison of modelled and observed rating relationships.

Outcomes and challenges

Key challenges encountered during the modelling process include the fitting of our model rating curve to the NRFA’s observed rating, particularly at low flows, which required a site visit to investigate the gauging structure and the accuracy of its peak flow output. Multiple model simulations we’re required to alter model channel and floodplain characteristics so that the model and its rating relationship output was representative of that observed in real life.

The other challenging aspect was to produce and model a bypass channel structure alongside the design team that would not only achieve the design criteria set out by the client but most importantly achieve the desired low flow capacity so that fish could use the channel all year round.

Holland Wood Weir - Fish Pass

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