PROJECT | The Holnicote Project is one of three multi-objective flood management demonstration projects commissioned in 2009 as a direct result of the recommendations from the Pitt Review of the Summer 2007 floods. These projects were all tasked with providing evidence to demonstrate how applying a Working with Natural Processes (WwNP) approach, to slow and store flood water in the landscape, can contribute to reducing flood risk to downstream receptors whilst also providing a range of other benefits to the environment and society. Two instrumented catchments on the NT Holnicote Estate – Horner Water (22km2) and the River Aller (18km2), a range of WwNP measures were implemented.

WwNP measuresFollowing the application of a range of detailed monitoring, assessment, spatial analysis and modelling approaches across • Offline storage ponds and wetlands: Five shallow earth bunded flood meadow areas (with outlet control pipes) and some surface water scrapes have been created on the middle Aller floodplain to increase floodplain storage capacity. • Instream structures: Instream woody debris dams and accumulations have been installed to slow flows in the Horner headwaters; the development of natural in-stream debris dams has also been encouraged in the existing historic woodland (Horner Wood).

• Soil management practices: Tenant farmers across both catchments have been advised on applying in-field soil management best practices in managed grassland and arable fields, including the use of a soil aerator in fields experiencing soil compaction. • Moorland runoff attenuation features: Surface runoff from the Horner headwaters has also been slowed and connectivity with the arterial drainage network has been reduced through the construction of 800 shallow earthen cross bunds along 20km of footpaths and tracks across the moorland.• Woodland creation/restoration: pockets of woodland have been created and wet woodland restored in the middle Aller catchment to intercept and slow floodwater, and improve infiltration of water into the soil profile.• Land use change: Reversion of arable land to grassland on moderately sloping ground in the Aller catchment to provide a more permanent and continuous vegetative surface that can slow surface runoff and/or improve inception and infiltration of direct rainfall.

Summary of Project Outcomes • Comparison of the basic characteristics of the ‘before change’ flood hydrographs with the ‘after change’ flood hydrographs are showing evidence of lowering and

From Source to Sea: The Holnicote Project

Client: Defra Partners: JBA Consulting, National Trust, Environment Agency, Penny Anderson Associates, Wessex Water, Exeter University, National Trust tenant farmersLocation: West Somerset, EnglandDate/Phase: 2009-ongoingFunding sources: Defra, Environment Agency, National Trust

Offline storage pond on Aller floodplain (with controlled pipe outlet)

flattening of the hydrograph shape and delay in the timing of the flood peak.• No property flooding was experienced in the vulnerable villages during the very wet winter 2013/14 in Somerset when local residents reported that they had expected some flooding to take place. • The combined effect of the five offline storage ponds (storing an additional 18,000m3 of floodwater) reduced the flood peak flow in the River Aller by 10% during the most severe flood event measured in winter 2013/14 (which had an estimated return period of in excess of 1 in 50 years).• On-going, regular and sensitive engagement with catchment stakeholders and the local communities, through a trusted catchment advocate (NT Project Manager), including the provision of appropriate monetary and non-monetary incentives for change, has greatly helped the continuing success of the project.• Sufficient time and resources are needed to explore and discuss WwNP aspirations with catchment stakeholders, and to engage with all the relevant regulatory authorities with respect to consents and approving the implementation of WwNP measures.

Project Benefits • The development, upgrade and maintenance of this demonstration and research platform, with the invaluable support of the National Trust, is helping to influence, shape and encourage the wider uptake of the WwNP approach both regionally and nationally in the UK.• A number of the spatial data analysis tools and hydrological/hydraulic modelling techniques developed and/or applied by JBA across the study catchments have been successfully transferred to several other WwNP initiatives in the UK.• The Holnicote project continues to provide evidence on the impact of WwNP on downstream flood risk, together with the provision of multiple benefits to the environment and society, including: improvements to biodiversity, landscape quality, carbon stewardship, water quality, and amenity and recreation.

Reference A Summary Report on the findings of the Holnicote Project covering the period 2009-2015 can be found on the National Trust web site at: http://www.nationaltrust.org.uk/features/living-with-change---managing-water-and-flooding. ContactSteve Rose: steve.rose@jbaconsulting.com for more information see: LInk

Modelled flood depths on Aller floodplain – pre and post-WwNP bund construction, severe Dec 2013 event

Moorland runoff attenuation features in Horner headwaters

Woody debris dam in Horner Wood