River Granta Babraham

Location: 10.2km of the River Granta at Babraham, Cambridgeshire.

Issue: The river had been moved to the edge of the floodplain and constrained within levees. Gravel carried excess sediment and there was limited bankside vegetation. 

Objective: Reconnect the river to its floodplain, increase floodwater storage and infiltration during high flows. Improved habitat for trout, especially clean spawning gravels, and increased bankside vegetation for water voles. Reduced flood risk to local houses.

Method: Levee removal, gravel redistribution to create pools and spawning riffles, bank regrading, large woody material and tree hinging.

WTT role: Lead partner — project design and delivery.

Partners: Babraham Research Campus (landowner), Environment Agency (funding through Flood and Coastal Risk Management).

Project date: April 2021. 

WTT Conservation Officer: Rob Mungovan

April 2021 saw major work completed along ~1km of the River Granta at the Babraham Research Campus (Cambridgeshire). The work has improved habitat for brown trout, of course, and the river’s increasing population of water voles. 

One of the main drivers for the work was floodplain re-connection. Typical of rivers that have been realigned for milling or water meadows, the river has been moved to the edge of the floodplain and an area of woodland is now growing at the lowest point of the valley, where the channel should be. The removal of levees together with careful bank re-grading and the creation of a few runnels now allows the river to spill onto its floodplain. More space is available to hold flood water, reducing the flood risk to houses. It also reduces the wash-out effect on juvenile fish associated with high flows, and once the water is on the floodplain it has the chance to soak into the ground, aiding aquifer recharge. The River Granta is badly affected by low flows as a result of many different licensed abstractions, so any chance to put excess water to better use is a great outcome.

The work followed a WTT Advisory Visit after the Babraham Research Campus sought help on how it could optimise river habitat. A plan was conceived, and the EA found funding through its Flood and Coastal Risk Management function. The local EA Fisheries Officer smoothed the path of the permit which meant that from the point of refining the plan in Dec 2020, machinery was on the ground by April 2021. 

River bed gravel redistribution. The river bed was carefully dug to lift gravel, which was re-distributed and topped with clean gravel to provide new spawning riffles for trout and minnows. Unfortunately, more loam was encountered than gravel in some areas, so it was not possible to put as much sinuosity back as had been hoped (loam is friable and does not hold its shape like a firm gravel seam).

Levee removal. The mounds of previously dredged material thrown up on to the river bank were removed to make space for flood water. Where levees contained a high volume of dredged gravel, it was simply put back into the river for it to sort during high flows. The Granta can experience quite energetic flow for a chalk river, due to its gradient.

Bank re-grading. Sections of the river bank were reshaped where the river was deeply incised. Vertical banks and tunnels of brambles were hiding the great potential of this unseen chalk river. Bank re-grading at a popular bridge has improved access and has made the river more resilient to people (and dogs), and hopefully steers them away from more sensitive parts of the river, such as spawning shallows.

Placement of very large woody material – felled trees were fixed across the river not only to initiate bed scour, but also to obstruct the flood flows and encourage the river to come out of bank and spill onto the floodplain.

Tree hinging. A technique to provide cover for trout, to deflect flows and protect banks from erosion. In this case, folding hawthorn trees into the channel in line with the bank also prevented dogs creating ​‘slides’, helping reduce the amount of silt entering the river which was smothering the gravel bed.