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Storm Dennis Update – Cilfynydd

Written by on 21st September 2021

Rhondda Cynon Taf Council has published its third Section 19 Flood Investigation Report following Storm Dennis – with the latest detailed report focusing on the flooding, preparedness and response within the community of Cilfynydd.

The Flood and Water Management Act 2010 (Section 19) requires the Lead Local Flood Authority to provide a factual account of what happened in major flooding events. Following its investigation into 28 locations affected by Storm Dennis (February 15-16, 2020), the Council will produce 19 reports in total.

During July 2021, the first two of those reports were published – an Overview Report which detailed and analysed the rainfall, watercourses and river levels experienced across Rhondda Cynon Taf in Storm Dennis, along with a separate report focusing specifically on the severe flooding throughout Pentre.

The latest report, published on Monday, September 20, focuses on Cilfynydd (Flood Investigation Area RCT 10). The report identifies that 23 residential and two commercial properties were flooded, along with highway flooding.

The report is available to view in full on the Council’s website, here.

The report has been informed by inspections made by the Council’s Flood Risk Management Team during the days following Storm Dennis, as well as information collated from residents, the Council’s Public Health Team, Natural Resources Wales and Dwr Cymru Welsh Water. The Council is the Lead Local Flood Authority and the Land Drainage Authority.

The report establishes that, from the evidence gathered, that the primary source of flooding in Cilfynydd during Storm Dennis was a result of significant overland runoff being generated from the steep hillsides above the village. Heavy rainfall drained to lower ground via a series of ordinary watercourses – many of which became overwhelmed with the volume of water and debris.

The performance of the three culvert inlets known to have caused flooding to properties was reviewed. It was confirmed that the Nant Cae Dudwg culvert inlet became hydraulically overloaded, while the Heol Mynydd and Ely Brook networks had standard protection of up to a 1 in 100-year storm event.

Both inlets had sufficient capacity to manage the expected flows, but their capacities were significantly reduced due to blockages – resulting in flooding to several properties. The poor structural condition of the Ely Brook network is also considered to have contributed to the flooding at Pontshonnorton Road.

The Council, as Lead Local Flood Authority, has undertaken 12 actions in response. These include clearing the identified culvert inlet structures; surveying, jetting and cleansing around 1,229 metres of the culvert network within the investigation area; leading on the development of a central Control Room to coordinate an emergency response to future flood events; and initiating a project to offer expandable flood gates to high-risk properties.

The Council has also proposed to undertake a further eight actions, and look to better understand the catchment above the investigation area through the development of a Strategic Outline Business Case. This will provide a series of recommendations to mitigate the flood risk in the community of Cilfynydd.

The report concludes that Storm Dennis was an extreme event, and that it is unlikely flooding from a similar event could be prevented entirely. It concludes that Risk Management Authorities satisfactorily carried out their functions, with further actions proposed to better address future preparedness and response.

Councillor Andrew Morgan, Leader of Rhondda Cynon Taf Council and Cabinet Member with responsibility for Highways and Transportation, said: “The Council has published its third Section 19 report as a result of Storm Dennis – following publication of an Overview Report and a report specific to Pentre during July 2021. The latest report focuses on Cilfynydd, and is once again a detailed and fully-accessible document for the public.

“The report focuses on a number of areas, from flooding history in Cilfynydd, to the rainfall in Storm Dennis and the performance of the Nant Cae Dudwg culvert inlet along with the Heol Mynydd and Ely Brook networks. It also lists the 12 actions taken by the Council since Storm Dennis and the eight actions it plans to take in the future. The report concludes that, as the Lead Local Authority, the Council undertook its duties in a satisfactory way.

“The Council remains absolutely committed to carrying out flood alleviation work right across Rhondda Cynon Taf, particularly targeting at-risk locations. We continue to invest in this area as a priority, seeking external funding from Welsh Government to help support our efforts – while also taking important steps to further increase our preparedness for future storm events.”