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Understanding, communicating and managing uncertainty and risk related to future changes in catchments.

Holnicote – A MULTI-OBJECTIVE FLOOD RISK MANAGEMENT DEMONSTRATION PROJECT

In 2009, in response to one of the recommendations of the Pitt Review of the Summer 2007 Floods, Defra commissioned three new projects as part of the Multi-Objective Flood Management Demonstration Scheme.  This scheme aims to generate evidence to demonstrate how integrated land management change, working with natural processes and partnership working, can contribute to reducing local flood risk while producing wider benefits for the environment and communities.

One of these multi-objective demonstration projects is the Holnicote Project being delivered by the National Trust, Penny Anderson Associates and JBA Consulting (including the JBA Trust) on the Holnicote Estate in Somerset.

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.

Allerford Bridge and flood hot spot

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 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.

The National Trust Holnicote Estate is situated adjacent to the uplands of Exmoor and comprises around 40kmof 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.  Properties in these villages are 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.

The catchment interventions that are under consideration at Holnicote are:

  • Moorland restoration in the headwaters – including heather restoration, grip blocking, surface drainage management (on tracks, paths and roads).
  • Woodland extension up onto the edge of Exmoor.
  • Encouragement of the development of in-channel woody debris dams.
  • Implementation of best practice in-bye grassland and associated soil management.
  • Implementation of best practice arable soil management and field-side buffer strips.
  • Intervention in direct hydrological flow pathways between hillslope runoff generation areas and receiving arterial watercourses.
  • Creation of flood meadows on the middle Aller floodplain upstream of Allerford.

If you would like to find out more about the Holnicote Project  for a more detailed description of the assessment, monitoring, intervention and modelling elements that are being undertaken.

Feel free to contact any of the project partners if would like to follow up on any aspect of the research at Holnicote:

Steve Rose (JBA Consulting) – 

Pete Worrall (Penny Anderson Associates) – 

Nigel Hester (National Trust) – 

This case study is also available to  as a pdf document.

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