Skip to main content

Flood Investigation Report following Storm Dennis - Treherbert

The Council has published its fourth Section 19 Flood Investigation Report following Storm Dennis last year – in order to identify the mechanism for the flood event, focusing on the community of Treherbert. 

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.

Today’s publication follows two initial reports published in July 2021 – an Overview Report which detailed and analysed the rainfall, watercourses and river levels experienced across Rhondda Cynon Taf in Storm Dennis, and a separate report focusing on flooding in Pentre. During September 2021, the Council published its third report, investigating the flooding in Cilfynydd.

The latest report, which has been published on Tuesday, November 16, has focused on Treherbert in the Rhondda Fawr Valley (Flood Investigation Area RCT 27). It outlines that 21 residential properties were flooded within the local community, along with two commercial properties and flooding to the highway.

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. As noted in the report, the Council is the Lead Local Flood Authority and the Land Drainage Authority.

The report says that the main source of flooding was significant overland runoff generated from the steep hillsides above the village. Rainfall drained to lower ground via ordinary watercourses, many of which became overwhelmed with water and debris – eventually over-topping and impacting properties.

Five culvert inlets were identified as sources of flooding to properties – all five of which are in private ownership. Two of the inlets, within the Abertonllwyd Street network, became hydraulically overloaded in the storm event – and they were identified as being below current design standards. The remaining three culverts had adequate protection of up to a 1 in 1,000-yr storm event with sufficient capacity to manage flows – but this was reduced by blockages.

The Council, as Lead Local Flood Authority, has undertaken 13 actions in response to the flooding, and have proposed to undertake a further six. These include clearance work carried out to the culvert inlet structures identified as sources of flooding, along with survey work, cleansing and jetting of around 800 metres of the ordinary watercourse network within the investigation area.

It has also started an interim Property Flood Resistance project offering expandable flood gates to those properties deemed at high risk of ordinary watercourse and surface water flooding – and is developing a flood routing scheme along the A4061 (Abertonllwyd Street) to manage overland flows.

The Council has also led on the development of an Emergency Control Room, bringing together its Call Centre and CCTV operations to provide a multi-agency, comprehensive and informed response in future storm events.

It will also look to better understand the catchment around Treherbert by developing a Strategic Outline Business Case to provide recommendations for suitable management mechanisms. This will aim to mitigate the future risk of ordinary watercourse, surface water and groundwater flooding locally.

The report concluded that the weather of Storm Dennis was extreme, and it is unlikely that flooding could be prevented entirely if a similar event was to occur. It concludes that the Risk Management Authorities satisfactorily carried out their functions in response to the flooding, with further measures proposed to better address preparedness and response in future flood events.

Councillor Andrew Morgan, Leader of Rhondda Cynon Taf Council and Cabinet Member with responsibility for Highways and Transportation, said: “The Council has now published its fourth Section 19 report following the flooding in Storm Dennis. This follows an Overview Report and a separate report relating to Pentre in July 2021, as well as a further report focusing on Cilfynydd which followed in September. The latest report for Treherbert is once again a detailed, fully-accessible document on the Council’s website.

“The report has investigated a number of factors – including the cause of flooding, the performance of infrastructure, the response and actions of the Council, and planned activities to increase our preparedness if a similar storm event was to happen in the future. The report concludes that, as the Lead Local Authority, the Council undertook its duties in a satisfactory way.

“Carrying out flood alleviation projects across the County Borough remains a Council priority, particularly targeting areas that are at greater risk. We’ll continue to seek external funding to complement our significant investment in this area, to protect our communities from the effects of climate change.”

Posted on 16/11/2021