Publication of the latest two Section 19 reports following Storm Dennis

Written by on 6th October 2022

Two further reports under Section 19 of the Flood and Water Management Act 2010, relating to Storm Dennis, were published today. Covering Mountain Ash and Blaenllechau/Ferndale, they complete the 19 publications in total.

As the Lead Local Flood Authority, Rhondda Cynon Taf Council must provide a factual account of what happened after each significant flood event. Storm Dennis (February 15-16, 2020) was an unprecedented event that brought widespread flooding to communities – and following initial investigation of 28 areas, the Council has been preparing 19 reports in total under the 2010 Act.

The first report in summer 2021 covered Pentre, and was followed by reports for Cilfynydd and Treherbert in the remainder of the year. Further reports for Aberdare/Aberaman, Rhydyfelin/Hawthorn, Abercwmboi/Fernhill, Porth, Taffs Well, Glyntaff, Treforest, Pontypridd, Nantgarw, Hirwaun, Treorchy, Ynyshir, Trehafod and Cwmbach were subsequently published throughout 2022.

Section 19 reports identify the Risk Management Authorities (RMAs), note the functions that they have exercised, and outline what actions they propose to take in the future. They have been informed by inspections and data collection carried out by the Council’s Flood Risk Management Team after the storm – as well as information collated from local residents, the Public Health Team, Natural Resources Wales (NRW) and Dwr Cymru Welsh Water (DCWW).

The latest publications on Thursday, October 6, cover Mountain Ash in the Cynon Valley (Flood Investigation Area RCT 06), and Blaenllechau and Ferndale in the Rhondda Fach Valley (Flood Investigation Area RCT 21).

Both reports are available to view in full on the Council’s website here. 

Mountain Ash (Flood Investigation Area RCT 06)

The report says that 67 properties received internal flooding (44 residential and 23 non-residential buildings) – adding that the primary source of flooding in the Mountain Ash catchment was as a result of significant overland runoff being generated from the steep hillsides above the area. Extreme rain in the storm resulted in rainwater draining to lower ground via a series of ordinary watercourses, many of which became overwhelmed with water and debris.

Two culvert inlets identified as sources of flooding to properties were found to provide inadequate protection in free-flowing and blockage conditions. Four separate networks identified as sources of flooding had adequate standards of protection but were in poor condition, with their capacities reduced due to blockages caused by debris mobilised from upper catchments in the storm.

The overtopping of the River Cynon was also identified as a primary source of flooding in lower areas of Mountain Ash West, due to the unprecedently high river levels. The NRW monitoring station at Aberdare is located around 5km upstream, and recorded its highest peak during the event at 2.125-metres.

The report added that surface water flooding, associated to sheeting runoff from the hillsides, overwhelmed the local highway drainage network. It was also an identified source of flooding to properties in the Mountain Ash area.

The Council – as Lead Local Flood Authority, Land Drainage Authority and Highway Authority – has undertaken 21 actions locally following the storm, and proposed a further eight. These include carrying out survey, jetting and cleansing to around 3,110-metres of ordinary watercourse drainage area, and progressed local flood alleviation schemes to upgrade several culvert inlets.

Remote telemetry monitoring devices have been installed at key structures, enabling operators to ensure local drainage systems are operating effectively – while the Council has led on the development of a central control room to provide a comprehensive, multi-agency response during future storm events.

As the RMA for main river flooding, NRW’s post-event investigations will aim to understand the mechanism of flooding from the River Cynon at Mountain Ash. NRW has also commissioned the Cynon Flood Modelling Project to assess the viability of potential flood risk management options, and developed an action plan to address improvement areas – including enhancement and expansion of its Flood Warning Service and incident management response.

Blaenllechau and Ferndale (Flood Investigation Area RCT 21)

The report notes that 25 properties received internal flooding (including one commercial property), while there was also flooding to the highway. This was caused by significant flows of overland runoff and groundwater that had been generated from the steep hillside above Blaenllechau – reaching the rear of some of the affected properties and then flowing towards lower ground.

An assessment by consultants Redstart concluded that a complex flow from the hillside caused water to find its own route to the catchment area, rather than using the existing drainage system. The hillside west of the investigation area is owned by Welsh Government and managed by NRW, while the hillside to the east is privately-owned. Storm Dennis was a 1-in-200-year event which exceeds the normal design criteria for culverts, but no flooding was observed from the ordinary watercourse and culvert networks in the investigation area.

This finding, along with Redstart’s hydraulic modelling, support the conclusion that surface water was unable to reach the drainage system in Storm Dennis.

The Council – as Lead Local Flood Authority, Land Drainage Authority and Highway Authority – has undertaken 13 actions following the storm, and proposed a further six. These have included survey, jetting and cleansing to 1,284-metres of culvert network, leading on the establishment of a central control room, and expanding its inspection and maintenance schedule to include further local assets when preparing for forecast extreme weather.

The Council has exercised its powers to engage with NRW in relation to its responsibilities as land manager of the local hillside areas. The Council will also look to better understand the upper catchment above Blaenllechau and Ferndale by developing a Strategic Outline Business Case, that will provide recommendations to mitigate the wider risk of flooding in the community.

Both reports concluded that Storm Dennis was an extreme event, and that it is unlikely flooding could be prevented entirely in a similar event. They add that all RMAs satisfactorily carried out their functions in response to the flooding, but have each proposed further functions to better prepare for future storms.

Councillor Andrew Morgan OBE, Leader of Rhondda Cynon Taf Council and Cabinet Member for Infrastructure and Investment, said:

“Today’s publication of two Section 19 flood investigation reports for Storm Dennis – covering Mountain Ash, Blaenllechau and Ferndale – complete the 19 reports the Council will publish in total. Each one provides conclusions of thorough investigations into how the flooding occurred, and outline what the relevant authorities have done, and plan to do in the future, to increase preparedness.

“The reports will remain a matter of public record and are available to view in full on the Council’s website, going back to the first publication for Pentre last summer, as well as an Overview Report covering all of Rhondda Cynon Taf. We know that climate change will bring more frequent storm events, and the reports intend to reassure residents that the relevant authorities are continuing to carry out important work to upgrade local infrastructure, and improve their ongoing monitoring and maintenance, to alleviate flood risk in communities.

“While the Council has taken the necessary time to compile each report, the progression of local flood alleviation schemes has in no way been delayed. Our accelerated capital programme includes over 100 schemes across the County Borough, of which more than half are delivered. Around £12m has been spent on infrastructure upgrades in recent years, and £15m on repairs since Storm Dennis. The Council has also secured multi-million pound funding across several Welsh Government programmes that are earmarked for flood alleviation, with work scheduled to take place in the current financial year.”


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