Application: Detailed design and hydraulic study
Customer: The Environment Agency
Service: Physical Model Study

Problem: The Environment Agency proposed to construct a combined white water canoe run and fish pass (termed the 'multi-use' pass) within the existing channel of the River Calder, immediately upstream of Padiham Weir.

The pass was to allow the safe migration of fish upstream to spawn, together with the provision of a 'fish friendly' environment. Consequently, limitations on velocities through the multi-use pass would be a major consideration of the final design process. A detrimental effect on flood levels within the vicinity was to be avoided.

Objectives: Physical scale models were required to assist the Environment Agency in the detailed design of the combined white water canoe run and fish pass and in making an assessment of the hydraulic conditions within the river section following construction of the pass.

Approach: The detailed design, giving a complex series of rapids was studied with the aid of two physical models; one representing the existing river channel and multi-use pass (model 1) and the other representing the multi-use pass alone (model 2). Model 1 was constructed at a scale of 1:25, giving an overall length of 5.68m and a maximum reach of 600mm. The models were used to study the effect of the bifurcated channel on upstream and downstream river levels and to assess the hydraulic performance of the multi-use pass with respect to the migration of fish upstream together with the creation of white water conditions suitable for canoeists.

A programme of model testing was determined to assess the final design of the multi-use pass and adjoining river channel under specific flow conditions, including 1 in 100 year flood flows. This provided the levels and data required for construction of the full-scale multi-use pass. The output from the physical modelling was to be used to design the permanent works.

Outcome/Benefits: The final design of the multi-use pass provided conditions suitable for the safe passage of fish upstream combined with a challenging canoe run. The models also showed that under 1 in 100 year flood conditions the effect of the bifurcated channel was to reduce the effect of flooding on the adjacent flood plains.

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