Case study:River Alt at Knowsley

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Location: 53° 25' 39", -2° 51' 17"
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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.

Community involvement - Consultation with local authorities, community groups and local schools took place during the design and construction phases.

Monitoring surveys and results

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Lessons learnt

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Image gallery


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
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Catchment and subcatchment

Catchment

River basin district North West
River basin Alt or Crossens

Subcatchment

River name River Alt
Area category 10 - 100 km²
Area (km2)
Maximum altitude category Less than 100 m
Maximum altitude (m) 19
19 m
0.019 km
1,900 cm
Dominant geology Calcareous
Ecoregion Great Britain
Dominant land cover Suburban
Waterbody ID GB112069060590



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

Reach length directly affected (m) 140
140 m
0.14 km
14,000 cm
Project started 2006/02/01
Works started
Works completed
Project completed 2006/10/01
Total cost category 10 - 50 k€
Total cost (k€) 46.3
46.3 k€
46,300 €
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

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


Measures

Structural measures
Bank/bed modifications Embankment lowering
Floodplain / River corridor Floodplain reconnection, Protect/restore riparian zone, Riparian planting
Planform / Channel pattern Creation of braided channel, Channel naturalisation
Other
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) Improved public access, Engagement with schools
Other Community engagement


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

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

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

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



Additional documents and videos


Additional 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

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