Physical models have been constructed by BHRSolutions for a section of the River Calder and a proposed, combined fish and white water canoe pass. Termed a multi-use pass, the construction is proposed by the Environment Agency, within the existing channel of the river immediately upstream of Padiham Weir. The physical modelling forms part of an extensive feasibility study by the Environment Agency for environmental improvement at Padiham.

The multi-use pass must allow safe migration of fish upstream to spawn, and provide a fish-friendly environment. Consequently, limitations on water velocities were a major consideration of the final design process. Also, a detrimental effect on flood levels within the vicinity had to be avoided.

Two models, one at 1:25 and the other at 1:12 scale, were used to study the effect of the bifurcated channel on upstream and downstream river levels. They were also used to assess the hydraulic performance of the multi-use pass with respect to the migration of fish upstream, whilst simulating white water conditions suitable for canoeists. The output from the physical modelling has been used to design the permanent works.

Model testing indicated it was not possible to achieve the required flow split for the multi-use pass and river channel utilising the Environment Agency's proposed inlet design. Modelling also showed that at relatively low, river flow the maintenance berm separating the channel and pass became drowned. BHRSolutions modified the design of the inlet and berm to achieve the required performance.

The models have shown that under 1 in 100 year flood conditions the bifurcated channel will reduce the effect of flooding on the adjacent flood plains. The final design of this multi-use pass will provide conditions suitable for the safe passage of fish upstream, as well as a challenging canoe run.

PRW 19.10.01

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