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.

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Lessons learnt

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Image gallery

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River Alt prior to works

Vegetated channel threads

Diverse vegetation structure

Newly excavated floodplain area

Floodplain area - 2012

One of choked braided side channels - 2012

Issues with urban litter evident - 2012

Catchment and subcatchment Select a catchment/subcatchment Site Edit site 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) Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background 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 Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname Investigation and design Stakeholder engagement and communication Works and works supervision Post-project management and maintenance Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Hydromorphology Biology Biodiversity Physico-chemical Water Quality Other reasons for the project Improving public access Measures Edit Measures Structural measures Bank/bed modifications Lowering of embankments Floodplain / River corridor Floodplain re-connection Planform / Channel pattern Creation of braided channel, Allow future natural channel changes to occur Other Planting of riparian zone Non-structural measures Management interventions Tree management works and re-opening of choked side channels is planned as part of 'Knowsley 2020' project. The site would have benefited from a long term tailored management plan. Social measures (incl. engagement) Improving public access to the river was an important component of the scheme Other Consultation with local authorities, community groups and local schools took place during the design and construction phases Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Invertebrates No Yes No Yes No Improvement Physico-chemical quality elements Edit Physico-chemical quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative [[]] Yes Yes No No No Improvement Any other monitoring, e.g. social, economic Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Public perceptions No Yes Yes No No Awaiting results Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description http://www.therrc.co.uk/pdf/manual/MAN 3 4.pdf RRC Manual of River Restoration Techniques entry Supplementary Information Edit Supplementary Information