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Project LeaderEnvironment AgencyProject PartnersGreen CorridorLondon Borough of Richmond-upon-ThamesLondon Borough of HounslowEnvironment AgencyLondon Wildlife TrustFriends of the River Crane Environment (FORCE)Kneller GardensMoormead ParkNorthcote Nature ReserveTidal Crane AssociationFunding SourcesEnvironment AgencyProject Budgetcirca £15,000Project TimescaleNov 2017 - November 2018

Lower River Crane Flow Improvements

The tilting weir at Mereway Road in Twickenham is designed to regulate the water level in the lower Duke of Northumberland's River (DNR), due to a historic arrangement that keeps the river at an almost constant level. As a result, water flowing down the lower River Crane is controlled as well, resulting in very little water being diverted during dry spells.

The Crane Valley Partnership’s catchment plan highlights the need for ‘Increased and more variable flow throughout the River Crane, Duke of Northumberland’s and Longford Rivers’, to be achieved in part by implementing an ‘improved water level management scheme for the lower River Crane involving all relevant stakeholders’. The impact of the diversion of flows down the DNR on flows in the River Crane is a particular concern.

Phase 1 - One day lowering test

As part of the Environment Agency's hydrological investigation in support of the partnership’s aims and in recognition of its concerns, Mereway Weir was lowered on 18 October 2016 for a one day test, to allow more water to flow down the River Crane. This was done in two stages: a first lowering at 11:00 and a second at 14:00. The weir was then returned to automatic operation at 16:00. See the photos below for before and after comparisons on the day.

The report discussing the results of the one-day test is available to download.

Mereway Weir before lowering
Mereway Weir after lowering
Mereway weir from downstream on River Crane before (photo on left) & during test (photo on right)

Mereway Weir before lowering
Mereway Weir after lowering
River Crane near Twickenham station before (photo on left) & during test (photo on right)

Phase 2 - One year lowering test

Based on studies there was no expectation that these changes in flows would negatively impact the Duke of Northumberland’s River. However, a one year test was undertaken to investigate this further, starting on 29 November 2017. The purpose of the test was to identify the impact of improving flows in the lower River Crane on local water abstractions, ecology of the Duke of Northumberland’s River and ecology of the lower River Crane. If the test proved to be successful the weir would subsequently remain at the lower operational level.

The Agency's hydrological investigation work supports the Crane Valley Partnership’s Vision for an improved landscape along the River Crane from Kneller Gardens to Cole Park Island.

What was monitored during the test?

Real time river level monitoring was carried out at the Environment Agency gauging stations and Mereway Weir. If the levels fell outside the set limits an automatic notification would be sent allowing the Agency to act accordingly. Real time river levels can be viewed at gauge map.

In addition to river level monitoring, observations of water depth and velocity (flow speed) at multiple sites along the Duke of Northumberland’s River and Lower River Crane were taken each month. The ecology was monitored through habitat, fish and invertebrate surveys and by fixed point photographs.

What effect would changing the weir settings have on flood risk?

The change of settings allowed more water down the lower River Crane at medium and low flows, diverting it from the Duke of Northumberland’s River. At very high flows the weir automatically lowers to allow as much water through as possible - this was not changed. Therefore, the test did not alter the flood risk on either river.

Initial indications were very positive; the DNR seemed to have a good year and no significant negative impacts from the reduced flow were identified. The Lower River Crane showed a significant improvement from the increased flow compared with Summer 2018. For example, in a 100 m reach of the River Crane next to Craneford Way Recreation Ground, only 5 fish were found during the Autumn 2017 fish survey. Whereas in Autumn 2018 hundreds of fish including one Pike and 6 Eels, were found.

Weir replacement

Some EA resources had to be diverted to work on planning the replacement of Mereway Weir which was approaching the end of its operational life. This planning work took priority as the replacement weir has the potential to significantly improve the ability to manage the split of flows between the DNR and the River Crane. The replacement weir will enter operation in early 2020. The aim is also to install a fish pass on the new weir to ensure fish are able to travel from the Lower River Crane to the rest of the Crane and DNR system.

Weir lowering test report

The final report from the one year lowering test was published in December 2019. The investigation confirmed that the split of water down the DNR and River Crane significantly improved the Crane without adversely impacting on the DNR or its abstractions. The lowering of Mereway Weir was considered a success. The report therefore recommends that the revised flow split between the two rivers be maintained.

Read the executive summary here.

Read the full report here.

© The Crane Valley Partnership 2019