Section 19 flood reports relating to Abercwmboi, Fernhill and Porth

Written by on 29th June 2022

Rhondda Cynon Taf Council has published two Section 19 reports under the Flood and Water Management Act 2010 following Storm Dennis, with 15 now published in total. The latest reports focus on Abercwmboi and Fernhill, and Porth, respectively.

As the Lead Local Flood Authority, Rhondda Cynon Taf Council must provide a factual account of what happened during any significant flood events. Under the 2010 Act, the Council will publish a total of 19 reports focusing on specific communities following the unprecedented weather of Storm Dennis (February 15-16, 2020). They follow initial investigation of 28 areas that were affected.

So far in 2022, the Council has published reports for Taffs Well, Glyntaff, Treforest, Pontypridd, Nantgarw, Hirwaun, Treorchy, Ynyshir, Trehafod and Cwmbach. Reports published in 2021 focused on Pentre, Cilfynydd and Treherbert, as well as an Overview Report covering the whole of Rhondda Cynon Taf. All of the reports are available to view in full online.

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

The latest two publications on Tuesday, June 28, focus on Abercwmboi and Fernhill in the Cynon Valley (Flood Investigation Area RCT 05) and Porth in the Rhondda valley (Flood Investigation Area RCT 19) respectively.

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

Abercwmboi and Fernhill (Flood Investigation Area RCT 05)

The report notes that, during the extreme rainfall of Storm Dennis, a total of 68 residential properties and five non-residential properties flooded, as well as extensive flooding to the highway. It adds that the cause of internal flooding was a result of significant overland run-off being generated from the steep hillsides above Abercwmboi and Fernhill, draining to lower ground via a series of watercourses – many of which became overwhelmed with water and debris.

A condition and hydraulic performance review of the culverts identified as sources of flooding found inlets associated with Bronallt Terrace East and Mostyn Street did not provide adequate standards of protection in free-flowing conditions. Inlets associated with the Cwm Alarch Close, Fernhill and Glenboi networks did provide adequate protection. All five are considered to have surcharged primarily due to blockages caused by debris from the hillside.

A sixth culvert, maintained by NRW as the land estate manager of the Welsh Government Woodland Estate, was assessed as having inadequate capacity for the expected flows from the hillside. Surface water flooding associated to runoff from the hillside, and a collapsed section of highway drainage network, were identified as sources of flooding to properties – while DCWW is identified as a relevant RMA in relation to sewer flooding observed at Maple Terrace.

As the Lead Local Flood Authority, Land Drainage Authority and Highway Authority, the Council has undertaken 16 actions and proposed a further six. It has undertaken clearance work to the identified culvert inlets, carried out survey, jetting and cleansing operations to 868m of ordinary watercourse and 749m of surface water drainage network in the investigation area. Significant upgrades have been made to culvert inlets identified as sources of flooding.

The report adds that the Council will look to better understand the catchment above the investigation area, through the development of a Strategic Flood Risk Assessment. It will work with NRW as the land estate manager to identify management mechanisms to reduce the risk of scour in upper catchments.

Porth (Flood Investigation Area RCT 19)

The report says that internal flooding occurred to 45 residential and 16 non-residential properties, alongside significant highway flooding. It identifies three primary sources – the overtopping of the River Rhondda and Rhondda Fach, the surcharging of two culvert inlets associated with the Nant Llwyncelyn ordinary watercourse, and the localised accumulation of surface water.

The rivers overtopped their eastern and western embankments at several locations following persistent heavy rainfall. Floating debris at bridges are also considered to have contributed to this, by causing water levels to rise behind obstructions. NRW gauge data at Trehafod showed the River Rhondda was more than three times its typical level in Storm Dennis, reaching 3.977 metres at its peak. NRW mapping identifies the majority of properties affected are at a high and medium risk of main river flooding, and are currently ‘undefended’.

As the RMA for main river flooding, NRW has carried out post-event analysis to understand the mechanism for flooding, and undertaken debris clearance to the main river channels. It has also commissioned a Rhondda Flood Modelling Project to assess flood risk management options, and developed a detailed action plan to address areas of improvement for future flood events. This includes the performance of NRW’s own Flood Warning Service.

The report adds that the surcharging of two culvert inlets associated with the Nant Llwyncelyn watercourse was caused by accumulation on debris screens, caused by displacement of debris within upper watercourses. An identified unconsented culvert structure is considered to have contributed to flooding, due to the reduction in hydraulic capacity in the network. Surface water collection due to heavy rainfall was also noted as a primary source of flooding.

The Council, as the Lead Local Flood Authority, Land Drainage Authority and Highway Authority, has carried out survey, jetting and cleansing operations to an estimated 445m of culvert network locally. Under relevant legislation, the Council has enforced the removal of the unconsented culvert structure and engaged with NRW and DCWW in relation to their responsibilities as RMAs.

Both of the reports in relation to Abercwmboi and Fernhill, and Porth, noted that the Council has led on developing a Control Room to complement its CCTV Centre and Contact Centre during future flood events. They also noted that the Council has initiated an interim Property Flood Resistance project offering expandable flood gates to those properties deemed at high risk.

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

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

“Today’s publication of Section 19 reports covering the areas of Abercwmboi, Fernhill and Porth, represents further progress by the Council to factually document what happened to cause the significant flood events in Storm Dennis. They are based on evidence collected by several organisations, and the Council has now completed 15 of the 19 reports relating to specific communities.

“The Council continues to heavily invest in targeted flood alleviation and drainage improvements as a priority. More than £6.4m Welsh Government funding is secured for Storm Dennis works this year, along with around £3.9m for flood alleviation across the Flood and Coastal Erosion Risk Management and Small Scale Works Grant programmes. In addition, £440,000 is allocated to progress a further 10 Resilient Roads schemes throughout this year.

“All published Section 19 reports are fully accessible on the Council’s website. The most recent reports for Abercwmboi, Fernhill and Porth have identified Risk Management Authorities, their actions since the event, and what actions they will carry out in the future. The Council also recently announced that work to deliver the major Upper Bronallt Flood Alleviation Scheme in Abercwmboi begins on June 27, to reduce the risk of flooding within the local community.”

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