Case study:Nant Barrog

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Location: 53° 11' 58.17" N, 3° 36' 58.13" W

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Project overview

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Status	In progress
Project web site	http://www.therrc.co.uk/sites/default/files/projects/41_nantbarrog.pdf
Themes	Flood risk management, Habitat and biodiversity, Land use management - agriculture, Land use management - forestry, Water quality
Country	Wales
Main contact forename	Jacques
Main contact surname	Sisson
Main contact user ID
Contact organisation	Natural Resources Wales
Contact organisation web site	http://naturalresources.wales/?lang=en
Partner organisations	Coed Cymru, Bangor University, Woodland Trust
Parent multi-site project
This is a parent project encompassing the following projects	No

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In 2012, the city of St Asaph – located on a downstream reach of the Afon Elwy (Map 1) – suffered severe flooding which resulted in a fatality. Natural Resources Wales began an investigation into how traditional hard engineered approaches, such as those proposed for the St Asaph flood alleviation scheme, could be supported by Working with Natural Processes (WWNP) measures. The investigation also looked at how these measures could have multiple ecosystem benefits. Llanfair Talhaiarn is a small village in the Elwy catchment, located at the confluence of Nant Barrog with the main river Elwy. The Nant Barrog is conveyed beneath the main street to the Elwy through the Water Street culvert. In periods of high rainfall the culvert is overwhelmed, resulting in flooding of properties in the village. Natural Resources Wales is reviewing the refurbishment of the culvert. WWNP measures recommended for the Barrog in the Elwy Natural Flood Risk Management and Ecosystem Services Study (2015) were further reviewed at the subcatchment scale. WWNP measures for the Nant Barrog catchment were recommended in the Llanfair Talhaiarn Natural Flood Management Study (2015). A revision of the shortlisted options was further reviewed as part of the Llanfair Talhaiarn project appraisal report (2016) and the cost benefits of the different options were considered. The resulting recommended WWNP measures are listed in Table 1.. Coed Cymru, in partnership with Natural Resources Wales, the Woodland Trust and other organisations, will submit a bid in 2017 to the Welsh Government Rural Development Plan Sustainable Management Scheme for funding to implement the measures recommended in the Llanfair Talhaiarn report. The Water Street culvert has an inlet capacity of 4.2m3s-1. Flooding will occur at a return period of a 1 in 50 year event with a peak flow of 4.8m3s-1. Modelling of 4 WWNP packages showed that, when considered separately, sufficient reductions would occur during the 1 in 50 year event for flow to remain within the channel at the culvert.

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Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name Nant Barrog WFD water body codes WFD (national) typology WFD water body name 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 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 2017 Works started Works completed Project completed Total cost category Total cost (k€) Benefit to cost ratio Funding sources Rural Development Plan Sustainable Management Scheme 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 Flood risk management Hydromorphology Biology Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Floodplain / River corridor Planform / Channel pattern Other Runoff pathway management 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