Option 5: Technical Larinier Fish Pass on West Weir
Solution
This option would consist of a technical fish pass (with super-active baffles i.e. Larinier) installed on the west weir, which as stated takes the dominant river flow with the entrance to its downstream culvert providing the greatest ‘lead’ for upstream migrating fish. A Larinier fish pass is proposed as it requires a relatively low flow (compared to other types of technical fish passes). The maximum gradient of a Larinier fish pass should be 15% with the length of any one flight limited to 10-12 m for migratory salmonids. Single flights can accommodate head differences up to a maximum of 1.5 m for migratory salmonids. As such, given the head drop at the weir is 1.75 m, a double flight Larinier with a resting pool will be required to meet the criteria for the successful operation of a Larinier. In addition, to ensure the successful migration of fish through the west weir culvert, fish friendly modifications to the culvert, such as baffles, could be implemented.
Concept Design
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Appraisal
Option 5 has the potential to allow for the safe passage of fish and eels through Shepley Mill, contribute to GEP under the WFD and comply with the Eels Regulations. It also predicted that the construction of a two flight Larinier, which may require a small notch of approximately 200 mm for operation, would maintain the upstream water levels and existing habitats of Wilderness Island, in particular during low flow conditions. Overall the suitability of this option to meet the project objectives is considered High, although would need additional habitat restoration solutions, such as channel narrowing, desilting and bed regrading, planting of silt deposition bars, flow deflectors and refuge areas.