Case study:Holnicote multi-objective flood risk management demonstration project
Project overview
| 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 |
Project summary
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
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
- 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
Image gallery
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Reasons for river restoration
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MonitoringHydromorphological quality elements
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Supplementary InformationEdit Supplementary Information
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