Case study:Knepp Rewilding Project

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Location: 50° 58' 41", -0° 21' 33"
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Project overview

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Status Complete
Project web site http://knepp.co.uk
Themes Environmental flows and water resources, Flood risk management, Habitat and biodiversity, Hydromorphology, Land use management - agriculture, Social benefits, Water quality
Country England
Main contact forename Penny
Main contact surname Green
Main contact user ID User:Penny Green
Contact organisation Knepp Castle Estate
Contact organisation web site http://knepp.co.uk
Partner organisations Environment Agency, Sussex Wildlife Trust, Royal HaskoningDHV, West Sussex County Council and Natural England
Parent multi-site project
This is a parent project
encompassing the following
projects
No
View of the river from the tree platform

Project summary

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Jointly funded by Natural England and the Knepp Estate, the restoration of the upper reaches of the River Adur lies at the heart of the largest rewilding project in lowland UK.

The work, carried out by the Environment Agency, has involved removing 4 weirs, returning 2.4km of canalised river to its original meanders and linking it to 5.5kms of restored floodplain upstream.

It is part of a landscape-scale conservation enterprise aimed at restoring the full range of hydrological processes from the moment raindrops fall on the land, filtering through vegetation and the soil, to their passage into watercourses towards the sea.

This reach of the River Adur was enlarged and diverted to the edge of the floodplain at least two hundred years ago, in an attempt to improve land drainage and allow the land to be farmed more productively. This extensive modification created a deep, uniform trapezoidal channel that supported very little flow or habitat diversity and was largely cut off from its floodplain in all but the largest floods. The extent of channel enlargement meant that flows became very low during dry summer periods, with parts of the river resembling a series of connected ponds rather than a flowing channel. A sluice and two stepped weirs were installed in an attempt to maintain depths of flow, but these impounded flows, encouraged sedimentation and limited fish passage. As a result of these modifications, natural river processes were limited and the river did not support the range of habitats that would be expected, resulting in a failure to reach Good Ecological Status under the Water Framework Directive.

The main aim of the restoration project was therefore to physically modify the existing channel to restore natural river processes, reconnect the river to its floodplain, and create the conditions that are required to support a rich and varied range of habitats for plants, invertebrates and fish. The project formed part of the wider Knepp Castle Rewilding Project, which aims to produce a large scale minimal-intervention landscape.

Creation of 1 km of new meandering river channel with naturally variable bed and bank profiles. Re-naturalisation of an additional 1.5 km of channel through bank re-profiling. The uniform, high banks were “pushed” into the channel to create a low flow channel with natural bank profiles that support a range of emergent and marginal habitats.

The new channel has a much smaller capacity than the modified channel that it replaced, concentrating low flows and improving floodplain connectivity during periods of higher flow. Blocking and diversion of floodplain drains and creation of scrapes to retain water on the floodplain.

Works included: Realignment of 1km river channel to its former naturally meandering course. Removal of four obsolete in-channel structures to restore natural flow and sediment transport processes and allow the free passage of fish. Creation of two backwaters to provide valuable habitats for aquatic plants and invertebrates.

Monitoring surveys and results

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Lessons learnt

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Image gallery


Completed works
Pre-works – canelised river with former meanders visible in floodplain
Completed works with LWD
River restoration in progress
Completed works
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Catchment and subcatchment

Catchment

River basin district South East
River basin Adur and Ouse

Subcatchment

River name Adur
Area category 10 - 100 km²
Area (km2)
Maximum altitude category Less than 100 m
Maximum altitude (m) 69
69 m
0.069 km
6,900 cm
Dominant geology Calcareous
Ecoregion Great Britain
Dominant land cover Improved grassland
Waterbody ID GB107041012270



Site

Name
WFD water body codes GB107041012270
WFD (national) typology
WFD water body name Knepp Rewilding Project
Pre-project morphology
Reference morphology
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
River corridor land use
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) 1000
1,000 m
1 km
100,000 cm
Project started 2006/01/01
Works started 2011/09/01
Works completed 2013/12/31
Project completed 2013/12/31
Total cost category
Total cost (k€)
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 Barriers to fish migration, Land drainage, Impoundments (not hydropower)
Hydromorphology Freshwater flow regime, Continuity of sediment transport, Quantity & dynamics of flow, Structure & condition of riparian/lake shore zones, Substrate conditions
Biology Fish, Invertebrates
Physico-chemical
Other reasons for the project This reach of the River Adur was enlarged and diverted to the edge of the floodplain at least two hundred years ago, in an attempt to improve land drainage and allow the land to be farmed more productively. This extensive modification created a deep, uniform trapezoidal channel that supported very little flow or habitat diversity and was largely cut off from its floodplain in all but the largest floods. The extent of channel enlargement meant that flows became very low during dry summer periods, with parts of the river resembling a series of connected ponds rather than a flowing channel. A sluice and two stepped weirs were installed in an attempt to maintain depths of flow, but these impounded flows, encouraged sedimentation and limited fish passage. As a result of these modifications, natural river processes were limited and the river did not support the range of habitats that would be expected, resulting in a failure to reach Good Ecological Status under the Water Framework Directive. The main aim of the restoration project was therefore to physically modify the existing channel to restore natural river processes, reconnect the river to its floodplain, and create the conditions that are required to support a rich and varied range of habitats for plants, invertebrates and fish. The project formed part of the wider Knepp Castle Rewilding Project, which aims to produce a large scale minimal-intervention landscape.


Measures

Structural measures
Bank/bed modifications Bank modification, Re-profiling, Creation of low flow channel, Habitat restoration, Creation of low flow channel, Barrier removal, Creation of backwaters, Introducing large woody debris, Riverbed restoration
Floodplain / River corridor Floodplain reconnection, Scrapes, Increase water retention area, Wetland habitat
Planform / Channel pattern Channel realignment, Re-meandering, Enhancing flow diversity, Habitat restoration, Meandering channel
Other Selective removal of organic muds and sludge contaminated with toxic, Create more natural flow regime, Improvement of natural flows, Improving fish migration
Non-structural measures
Management interventions Flow management
Social measures (incl. engagement)
Other


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

Physico-chemical quality elements

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

Any other monitoring, e.g. social, economic

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


Monitoring documents



Additional documents and videos


Additional links and references

Link Description
http://https://www.youtube.com/watch?v=IHZr1FUCP8g&feature=player embedded&list=PL3eoaBdiC8XR5xuXn1VwVFxHwvn0HLCKo YouTube link

Supplementary Information

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