Case study:Afon Clywdd

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Location: 53° 10' 57.73" N, 3° 22' 7.56" W

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Project overview

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Status	Planned
Project web site	http://www.therrc.co.uk/sites/default/files/projects/40_afonclwyd.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	AECOM
Parent multi-site project
This is a parent project encompassing the following projects	No

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Project summary

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The Clwyd (Photo 1 and Map 1) is a large catchment in north-east Wales, discharging north into the Irish Sea. The confluence of the Afon Clwyd with the Afon Elwy is downstream of St Asaph, which in November 2012 was subject to severe flooding resulting in a fatality. The impact of flooding from the Afon Elwy, which flows through the town, was exacerbated by the backing up of flow at the confluence with the Afon Clwyd, which was also in spate. In 2015, Natural Resources Wales commissioned AECOM Consultants to identify potential Working with Natural Processes (WWNP) in the Elwy catchment which would provide climate resilience to the proposed improvements to engineered flood defences in St Asaph (AECOM Consultants 2015). Following the study, AECOM wanted to develop techniques for rapid assessment of WWNP that could be applied to different types of catchments. AECOM used EU innovation funding to test techniques in a number of catchments, including this study of potential WWNP in the Vale of Clwyd. A systematic review of the aerial imagery, Ordnance Survey (OS) mapping and flood zone/flood extent information was carried out to identify the most important morphological features in the catchment and to identify locations where the WWNP measures could potentially be implemented across the catchment. In contrast to the Elwy, the identification process included an assessment of implementation and shortlisting to streamline the process. The overall impact of the modelled WWNP is a reduction in peak flow and an increase in time-to-peak (TP). Peak flow reduction was approximately 6% for the 5-year design event and approximately 1% for the 200-year design event.

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Catchment and subcatchment Select a catchment/subcatchment 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