River Engineering & Restoration
Powys Flooding Pre-Feasibility Studies
Keywords: Flooding & Risk Assessment, Benefit Cost Analysis, Flood Alleviation Condition Survey.
Client, Architect & Other Organisations
Powys County Council
The Welsh Assembly Government
Knighton Mill Green
In March 2002 Powys County Council received funding from the Welsh Assembly Government to undertake pre-feasibility studies for 67 locations in the County subject to regular flooding from ordinary watercourses. The majority of sites considered during the study have had a long history of flooding with many seriously affected during recent flooding events of April 1998 and October 2000.
The project scope included urban flooding issues in: Knighton (River Teme), Welshpool (Leden Brook), Machynelleth (Urban Drainage), Crickhowell (River Usk), Talgarth (River Ennig) and Prestigne (Clatter Brook). Rural flooding locations were also investigated typically affecting one or two residential properties. At Llansilin in the north of the county overtopping from the Afon Ogau affects two cottages and at Cwm-y-Gaist, culvert blockage also causes flooding to two properties in the village.
Scope of Work
The objective of the work was to consider the technical, environmental and economic impact of flooding to local communities. The framework for the pre-feasibility assessment was the Treasury's Green Book and the Flood and Coastal Defence Project Appraisal Guidance documents published by DEFRA. The large number of sites and the volume of information warranted the development of two bespoke databases to allow information to be collected in the field and analysed in the office. The property database included over 325 residential and commercial properties recording details of property type, age, flooding history, flood depth profile and other information required to calculate flood related damages.
The second database contained information relating to the existing flood defence and the estimated costs associated with implementing engineering solutions to mitigating flooding. A spreadsheet linked to the databases was also developed to assess the information contained in the databases and evaluate average annual damages; NPVs and benefit cost ratios. The spreadsheet also produced the final report.
All the sites were subject to a preliminary site inspection to assess the property and the condition of the watercourses and structures. Where possible residents were interviewed in relation to their flooding experience and information gathered to populate the property and flood defence databases. In addition, archive records from Powys County Council, the Shire and the Fire Brigade were also assessed to determine the extent of problems associated with flooding.
As a result of this work, thirty six of the sites were recommended for grant aid funding. A further twenty three sites were recommended for monitoring or other action by Powys County Council with the remaining eight needing no further action. As a result of the work the Welsh Assembly Government made further grants to Powys County Council amounting to approximately £350,000 for project appraisal studies.