Case study:River Alt at Knowsley

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Location: 53° 25' 39.05" N, 2° 51' 16.79" W

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Project overview

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Status	Complete
Project web site
Themes	Fisheries, Flood risk management, Habitat and biodiversity, Water quality
Country	England
Main contact forename	Sue
Main contact surname	Slamon
Main contact user ID
Contact organisation	Environment Agency
Contact organisation web site	http://www.environment-agency.gov.uk/
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

Cross section of new floodplain and braided channels

Project summary

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The Alt is a small, low energy urban river which had been historically re-sectioned, straightened and over deepened. The river flows beside a road and was constrained within a trapezoidal channel. Improved water quality had seen fish return to parts of the river but further improvements to the riverine habitat were desired. Consultation with local authorities, community groups and local schools took place during the design and construction phases. Options were limited by planning permissions on parts of the site and the existence of a wildflower meadow. Improving public access to the river was a very important element of the scheme. The 1.5m, 30 degree banks were excavated back on either side of the existing course and up to 30m width of 'floodplain' was created. This comprises a mix of a main and braided channels separated by marginal berms. The new course was approximately 25% larger and deeper than the braided channels to encourage the majority of low flows along this route. Initial earthworks have delivered access and recreation improvements along this stretch of the Alt. Reedbed habitat has established reinstating a locally rare habitat. However, this has also has some negative impacts in that biodiversity has been limited by reedbed monoculture and encroachment of the reedbed has reduced velocities increasing siltation and reducing flow variation as a result. Minimal management has been undertaken since the project work and as a result the site has become overgrown with vegetation and some of the braided side channels have become choked. Tree work is underway as is work to re-open the braided channels (part of Knowsley 2020 Project). The project would have benefited from a tailored long term management plan. Water quality appears to have improved with the number and density of pollution sensitive invertebrates increasing since the works were carried out. Anecdotal evidence suggests that people are happy with the scheme. Knowsley MBC is undertaking further works to open up the riparian corridor and is currently installing a cycle path.

Monitoring surveys and results

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Lessons learnt

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Catchment and subcatchment

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Site

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Name	River Alt, Knowsley
WFD water body codes	GB112069060580
WFD (national) typology
WFD water body name
Pre-project morphology	Artificial channel, Straightened, Over-widened, resectioned
Reference morphology	Braided
Desired post project morphology
Heavily modified water body	No
National/international site designation
Local/regional site designations
Protected species present	No
Invasive species present	No
Species of interest
Dominant hydrology
Dominant substrate	Clay
River corridor land use	Urban
Average bankfull channel width category
Average bankfull channel width (m)
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Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m³/s)
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Average unit stream power (W/m²)

Project background

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Reach length directly affected (m)	140140 m <br />0.14 km <br />14,000 cm <br />
Project started	2006/02/01
Works started
Works completed
Project completed	2006/10/01
Total cost category	10 - 50 k€
Total cost (k€)	46.346.3 k€ <br />46,300 € <br />
Benefit to cost ratio
Funding sources

Cost for project phases

Reasons for river restoration

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Mitigation of a pressure
Hydromorphology
Biology	Biodiversity
Physico-chemical	Water Quality
Other reasons for the project	Improving public access

Measures

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Structural measures
Bank/bed modifications
Floodplain / River corridor
Planform / Channel pattern
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Non-structural measures
Management interventions
Social measures (incl. engagement)
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