Malm Lock Fish Bypass Channel

Hampshire & IoW Wildlife Trust

River Itchen SSSI/SAC

Summer 2012

Malm Lock Fish Bypass Channel

The Malm Lock Bypass Channel formed part of the larger Itchen Navigation Heritage Trail Project (for which Aquascience were the Principle Designer and Principle Contractor). The Malm Lock posed a complete obstruction to the upstream migration of fish with a head-loss of over 1.5m. Aquascience designed and constructed an 80m long bypass channel to facilitate fish movement. The channel was designed as a long rock-ramp. The new channel is a high-energy environment due to the steep gradient. Robustness against erosion was achieved by incorporating synthetic geotextiles, the positioning of large rocks and the planting of margins with characteristic plants of local progeny.

This project took place on private land and required careful negotiation with the landowners and a finished product with appropriate landscaping features.

In addition, an obstacle to fish passage was addressed at College Mead Lock, situated 500m downstream. The two projects enabled migratory salmonids to access over 1km of channel from which they were previously excluded. The work surrounding both impoundments resulted in a lowering of water levels, and therefore an increase in long-profile gradients. Consequently, the condition of the chalk stream channel, sediment transport and conditions favouring characteristic chalk stream species were all enhanced. In addition, pressure on embankments that had previously led to overtopping and breaching was alleviated.

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