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
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
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
© 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

The aim of the Holnicote project in Somerset (Map 1) is to provide evidence to demonstrate how Working with Natural Processes (WWNP), implementing a range of Natural Flood Measures (NFM) measures, at the catchment scale can contribute to a reduction in flood risk while producing a range of other environmental and social benefits. A hydrological monitoring network was installed in 2010 to provide high quality, high resolution rainfall, stage and flow data for assessing the impacts of the NFM measures. A range of NFM measures have been implemented since 2011 including upland drainage attenuation features, woody dams, woodland creation, leaky weirs and offline storage areas (Photo 1). Since the project began, there has been no flooding in the vulnerable downstream villages that have experienced regular flooding in the past, even during the extreme rainfall events of winter 2013 to 2014, where measured hydrological data clearly showed a significant reduction in flood peak. This was confirmed when the same data were run through 'before' and 'after' NFM implementation scenarios in the hydraulic flood model of the catchment. During an extreme rainfall event on an already saturated catchment in late December 2013, NFM interventions reduced the flood peak by 10%. With a combined insurance value of £30 million, none of the 98 properties at risk were affected by flooding then, or during any subsequent flood events. Th3e capital costs of constructing the offline storage bunds on the floodplain upstream of the vulnerable properties were £163,000, a small cost compared with the insured value of the properties at risk of flooding.

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

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

Image gallery


Catchment and subcatchment


River basin district South West
River basin South and West Somerset


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


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€) £1.22m
"£" is not declared as a valid unit of measurement for this property.
Benefit to cost ratio
Funding sources Defra, Environment Agency, National Trust

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
Physico-chemical Nutrient concentrations
Other reasons for the project Recreation, Amenity, Ecosystem services, Landscape enhancement


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)


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 An analysis of the impacts of rural land management change - A position paper Video: Going with the flow - how the National Trust manages water and flooding on the Holnicote estate Catchment change management hub - Holnicote case study Holnicote case study summary Holnicote project presentation Video: Investigating the influence of land management change on flood risk Video: Holnicote after the 2013/4 Floods

Supplementary Information

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