River Loddon – Weir by-pass channel

A nature-like bypass channel was constructed in a floodplain of the River Loddon to facilitate fish migration around four permanent weir impoundments. 


On the River Lodden at Arborfield, historic mill and weir structures prevented fish passage and had impounded the river for 4km upstream. A combination of civil engineering and soft engineering techniques were used to design and build a 200m weir bypass channel to reconnect 25km of the River Loddon.

The solution

Cain Bio Engineering created a step-down pool/riffle sequence using rock-ramps to manage the steeper gradient, they created gravel riffles to create streaming flows and fish spawning habitat. This created an immediate and easy passage for a wide variety of fish species while improving their habitat

Before – An upstream view of the channel excavation area. Wooden stakes to delineate the centre of the new channel
Before – A downstream view of channel excavation aera. Wooden stakes to delineate the centre of the new channel

This solution was selected as this is a sustainable alternative to hard engineered structures while creating durable nature like channels, encouraging the fish to pass freely in this steeper gradient.  It’s also erosion free and flood-resistant immediately following construction.

During work – A downstream view of channel during excavation
During works – The creation of the ripple and pool sequence using imported gravel deposits. Note the inclusion of WD to pool margins
After – The completed concrete off-take
After – The completed coirnet and step weir to manage torrential flows at confluence
After – An upstream view of completed fish pass riffle and pool sequence
After – A downstream view of completed fish pass riffle and pool sequence
After – An upstream view of completed fish pass riffle and pool sequence once vegetation has established
After – A downstream view of completed fish pass riffle and pool sequence once vegetation has established