Case study:River Irwell Restoration Project

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Location: 53° 29' 21.97" N, 2° 11' 31.15" W

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Project overview

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Status	In progress
Project web site
Themes	Flood risk management, Hydromorphology
Country	England
Main contact forename	Oliver
Main contact surname	Southgate
Main contact user ID
Contact organisation	Environment Agency
Contact organisation web site
Partner organisations	Groundwork, Natural England
This is a parent project encompassing the following projects	Castle Irwell Urban Wetland, Clayton Vale, Goshen Weir removal project, River Roch, Bury, Philips Park

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Project summary

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The River Irwell Restoratio Project plans to restore urban watercourses in an effort to achieve good ecological status for the watercourse. Such plans must be viewed against the considerable economic and physical constraints imposed on such rivers due to their setting, in particular the need to maintain or even enhance flood protection levels and to ensure infrastructure remains uncompromised. A restoration plan was developed for the heavily modified River Medlock at Clayton Vale and Philips Park, Manchester. The principal aims of the study were driven by opportunities to improve the hydromorphological and ecological status of the watercourse through naturalisation, working to develop a watercourse where the reinstated channel units function to temporarily store coarse sediment creating dynamic habitat within a restricted urban environment.

Known as the ‘Red River’ due to the brick lining along the study reach constructed in 1912, the present U-shaped 'flume' has created conditions with limited in-channel morphology (occasional berm top fine sediments) and flow diversity (a monotonous run due to the almost uniform width and depth along the reach). No significant sediment storage occurs, despite a strong coarse sediment supply, as a consequence of the unchanging steep gradient and immovable planform (the brick-lined banks have restricted any lateral movement of the watercourse locally) creating a uniform, high energy, transporting reach and preventing any sediment deposition on the channel bed. Under present flow conditions, the high velocities are considered to be a barrier to fish passage, due to velocities of >2m/s under low flow conditions and 3-4m/s during higher flow events within the channel.

The River Medlock at Clayton Vale and Philips Park is designated as a Heavily Modified Waterbody. At present the Water Framework Directive (WFD) defines the overall river status as Poor Ecological Potential, but with a target of reaching Good Ecological Potential by 2027. Restoration needed to be mindful of impacts locally, and upstream and downstream of Clayton Vale and Philips Park, including impacts on flood risk given the highly urbanised nature of the catchment. Removing the brick-lining and concrete base layer, without managing the steep gradient and high energy levels of the River Medlock could create uncontrolled destabilisation. Therefore, removal had be considered alongside morphological restoration and naturalisation through Clayton Vale and Philips Park to ensure a 'dynamically stable' restoration was implemented and that historic features lining the watercourse were not compromised.

Consultation throughout the restoration plan development highlighted the ‘stand-off’ between the objective of river naturalisation and the ‘need’ for stability. The River Medlock at Clayton Vale and Philips Park would be a dynamic, active single thread river if not constrained by the brick – lining flume. Upstream analogue features (including, rapids, riffles and pools) and hydraulic modelling were used to carefully design and size functional, dynamically ‘stable’ morphological features to manage the high energy system following removal of the concrete and brick lining. Engineering concerns remained over the potential for ongoing lateral erosion that could threaten local historic walls and public footpaths. Compromise was therefore necessary to satisfy the project board before works could be undertaken, however, the majority of the restoration objectives for naturalisation were approved and the results of the first stage of the project and initial river response is reported here.

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