Case study:Holnicote multi-objective flood risk management demonstration project

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Location: 51° 12' 21", -3° 33' 43"
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Project overview

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Status In progress
Project web site http://www.nationaltrust.org.uk/holnicote-estate/
Themes Economic aspects, Environmental flows and water resources, Flood risk management, Habitat and biodiversity, Hydromorphology, Land use management - agriculture, Land use management - forestry, Monitoring, Social benefits, Water quality
Country England
Main contact forename Nigel
Main contact surname Hester
Main contact user ID
Contact organisation National Trust
Contact organisation web site http://www.nationaltrust.org.uk/
Partner organisations Penny Anderson Associates Ltd, University of Exeter, Environment Agency, Defra, JBA Consulting, JBA Trust
Parent multi-site project
This is a parent project
encompassing the following
projects
No
© Gene Hammond, Penny Anderson Associates Ltd

Project summary

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The National Trust Holnicote Estate is situated adjacent to the uplands of Exmoor and comprises around 40km2 of land draining the catchments of the Aller and Horner Water from Exmoor northwards through woodland, grassland and arable areas towards Porlock Bay. The key flood risk receptors in the catchments are the villages of Allerford, West Lynch and Bossington. There is nearly 100 properties in these villages at risk of flooding from the watercourses, which are influenced by a legacy of flow constrictions within the drainage networks, such as narrow historic stone bridges, and the lack of undeveloped channel and floodplain capacity through the built-up areas.

Driven by Defra, supported by the Environment Agency and managed by the National Trust, this project hopes to demonstrate that by looking at whole catchments and strategically targeting shifts in rural land management practices, sustainable support to flood management may be achieved. In addition, it is recognised that through rural land management change and intervention comes the opportunity to enhance the provision of a range of other ecosystem services within catchments. These include landscape quality, biodiversity, carbon stewardship, water quality, amenity and recreation.

The principal objectives of the Holnicote project, which is currently scheduled to run until 2015, are:
- To establish a robust hydrological monitoring programme across the study area
- To identify potential catchment (hillslope and floodplain) interventions that may contribute to managing flood risk
- To demonstrate the practical implementation of catchment interventions (e.g. changes to land use, land management practices, and hydrological connectivity)
- To assemble evidence, both from recorded datasets and hydrological/hydraulic modelling, about the impact of the catchment interventions on runoff and flood dynamics
- To assess what the evidence reveals about the potential or actual benefits, in terms of flood risk management and the delivery of a range of other ecosystem services

Monitoring surveys and results

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The ecosystem services assessment being undertaken for the project aims to provide an evaluation of the various goods and services provided by the existing ecosystems across the Holnicote Estate, and those anticipated following the range of expected habitat modifications scheduled as part of the catchment interventions. In addition, based on the most robust information available, the assessment will provide an evaluation of the value of these anticipated goods and services relative to the capital investment.

The National Trust is also co-funding a PhD student at Exeter University to establish whether the catchment management interventions being implemented can help to improve water quality. The research will complement the catchment-wide hydrological monitoring taking place with some additional chemical, biological and physical water quality monitoring to examine the effectiveness of the intervention measures to also meet water quality objectives.

For more information, refer to http://ccmhub.net/wp-content/uploads/2013/01/Holnicote-Position-Paper-v9.pdf

Lessons learnt

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- Modelling can assist in opportunity mapping, impact assessment and development of intervention design
- Demonstration events to show and discuss intervention approaches do work
- Early dialogue with stakeholders on land management or catchment interventions helps to collect local knowledge, identify issues and constraints
- Early dialogue with relevant regulatory, planning and consenting authorities on proposed interventions is essential
- Working through all the requirements of formal planning and consenting for interventions is time consuming - Need for clear guidance on the application of an NFM approach at a range of scales

Update from Autumn/Winter 2013 http://ccmhub.net/wp-content/uploads/2013/10/Holnicote-Flood-Meadows.pdf


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Catchment and subcatchment

Catchment

River basin district South West
River basin South and West Somerset

Subcatchment

River name Aller
Area category 10 - 100 km²
Area (km2)
Maximum altitude category 200 - 500 m
Maximum altitude (m) 426
426 m
0.426 km
42,600 cm
Dominant geology Calcareous
Ecoregion Great Britain
Dominant land cover Arable and Horticulture
Waterbody ID GB108051020230



Site

Name Holnicote estate
WFD water body codes GB108051020230
WFD (national) typology
WFD water body name Aller
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 Intensive agriculture (arable), Broadleaf/mixed woodland (semi natural), Grassland, Flood meadow
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)
Project started 2009/04/01
Works started
Works completed
Project completed 2015/03/31
Total cost category
Total cost (k€)
Benefit to cost ratio
Funding sources Defra

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 National Trust
Monitoring Exeter University



Reasons for river restoration

Mitigation of a pressure Flood risk management
Hydromorphology Channel pattern/planform
Biology
Physico-chemical Nutrient concentrations
Other reasons for the project Recreation, Amenity, Ecosystem services, Landscape enhancement


Measures

Structural measures
Bank/bed modifications Large woody derbis dams
Floodplain / River corridor Creation of a flood expansion area, Interventions in direct/rapid flow pathways on hillsopes & connectivity to watercourses
Planform / Channel pattern
Other Moorland restoration, Grip blocking, Woodland extension
Non-structural measures
Management interventions Implementation of best practice land & soil 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://ccmhub.net/wp-content/uploads/2013/01/Holnicote-Position-Paper-v9.pdf An analysis of the impacts of rural land management change - A position paper
http://www.youtube.com/watch?v=3MbkfVQs8Z8 Video: Going with the flow - how the National Trust manages water and flooding on the Holnicote estate
http://ccmhub.net/case-studies/holnicote-case-studies/holnicote/ Catchment change management hub - Holnicote case study
http://ccmhub.net/wp-content/uploads/2013/01/Holnicote-final.pdf Holnicote case study summary
http://www.crew.ac.uk/sites/www.crew.ac.uk/files/documents/Steve%20Rose%20-%20JBA.pdf Holnicote project presentation
http://www.youtube.com/watch?v=7rAc3fzwvus Video: Investigating the influence of land management change on flood risk
http://www.youtube.com/watch?v=BcuriB5q1Zc Video: Holnicote after the 2013/4 Floods

Supplementary Information

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