Case study:Cannop catchment natural flood management

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Catchment

River basin district	Severn
River basin	Severn Vale

Subcatchment

River name	Cannop Bk - source to R Severn Estuary
Area category	10 - 100 km²
Area (km2)
Maximum altitude category	200 - 500 m
Maximum altitude (m)	243243 m <br />0.243 km <br />24,300 cm <br />
Dominant geology	Calcareous
Ecoregion	Great Britain
Dominant land cover	Coniferous Woodland
Waterbody ID	GB109054032640

Project overview

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Status	In progress
Project web site
Themes	Environmental flows and water resources, Flood risk management, Habitat and biodiversity, Hydromorphology, Land use management - forestry, Water quality
Country	England
Main contact forename	Joe
Main contact surname	Baker
Main contact user ID	User:Joe.Baker02
Contact organisation	Gloucestershire County Council
Contact organisation web site	http://Gloucestershire%20County%20Council
Partner organisations	Forestry England
Parent multi-site project
This is a parent project encompassing the following projects	No

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Project summary

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GCC Flood Risk Management Team have embarked on an ambitious and groundbreaking project in the Forest of Dean, working with natural processes to increase biodiversity, sequester carbon and reduce the impact of the climate emergency by reducing peak flood flows. Lydney, in the Cannop catchment, is one of GCC’s highest priority and most frequently flooded communities. In seeking to address the impact on homes and businesses, the FRM team carried out a wide-scale hydrological modelling study to identify flood risk mechanisms and test opportunities to mitigate the risk. As a result of the study, the most beneficial option was shown to be creation of upstream attenuation through the application of natural flood management (NFM) techniques. Forestry England (FE) own much of the land upstream of Lydney, including 25 of the 29 watercourses and tributaries that flow through the area into many at-risk communities including Lydney itself. Due to the hard work and commitment of key individuals within both organisations, GCC and FE have developed a regionally, if not nationally unprecedented partnership model to deliver NFM measures throughout the catchment and reduce downstream flood risk, building on and augmenting the objectives of FE’s Catchment Management Plan. These measures are diverse and numerous, but all seek to mimic and enhance the way nature works with water. They include approaches such as large ‘woody debris dams’, wetland creation, bankside buffer planting and river restoration. The project will slow the flow of water, reconnect floodplains, increase habitat connectivity and diversity and help build a more climate-resilient catchment. For example, the buffer planting will allow for the future reintroduction of beavers, a keystone species which will sustainably manage the catchment in years to come, building on the successful enclosed release of two pairs of beavers in the area which are already having a positive effect on habitat and peak flows. Key statistics include: • 8 hectares of wetland to be created • 5 hectares of wet-woodland habitat to be created • 15 reaches buffer-planted • 1,000 leaky dams created so far, with at least 4,000 more to come • 40,000 trees (native species) to be planted, and non-native species removed • 74.5 Km of river restoration • 4 years project lifespan remaining, with potential to extend • £888,000 funding bid submitted, augmenting existing GCC capital and revenue commitment.

Monitoring surveys and results

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The project monitoring comes in the form of: • Repeat aerial drone surveys • MoRPH surveys • Local volunteer surveying • E-fish surveys • River gauge reviews and assessments from flood events • Anecdotal reports from local residents and local flood groups

Data so far suggest that wetting of the wider valley floor has been particularly successful with numerous palaeo-features now active along the restored reach. The rapids now provide additional higher energy habitat contrasting with the large pool areas. Work to create a high terrace cliff face has been particularly successful and is now forming a home for several bee and spider species. The large wood has also introduced a local complexity to the system encouraging both erosion and deposition helping retain important smaller grained material in the bed which was previously flushed through the system. The works at the Lido have maintained the habitats that had previously developed in the low energy area with the riffle-rapids installed downstream helping maintain water and energy levels and the downstream works have colonised rapidly and now blend into the landscape. A viewing point across the works was also constructed to allow the public to see and understand the restoration and this has proved very popular. Wet woodland rejuvenation at the bottom of reach three has also worked well with permanent and seasonal flow routes established across the entire valley floor. Organic materials are accumulating locally forming small flow obstructions, further adding to the diversity of the system and larger wood accumulations are developing well. Public access through the sites continues to be encouraged with bridges and stepping stones allowing river crossing and usage by the public has been high with a generally positive response to the works.

Lessons learnt

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Use larger woody material for leaky dams. Ensure stone blockages are installed above flood level to prevent scour and the the clay covering the bund from being washed away.

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Location: 51° 48' 36.22" N, 2° 34' 5.12" W

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Site Edit site Name WFD water body codes WFD (national) typology WFD water body name Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body National/international site designation Local/regional site designations Protected species present Invasive species present Species of interest Dominant hydrology Dominant substrate River corridor land use 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 Edit project background Reach length directly affected (m) Project started Works started Works completed Project completed Total cost category Total cost (k€) Benefit to cost ratio Funding sources 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 Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Hydromorphology Biology Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Floodplain / River corridor Planform / Channel pattern Other Non-structural measures Management interventions Social measures (incl. engagement) Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Physico-chemical quality elements Edit 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 Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information