Flood Investigation Reports Hirwaun Pontypridd Nantgarw
An additional three Section 19 Flood Investigation Reports following Storm Dennis have been published by the Council today, bringing the total number to nine. The latest reports focus on Hirwaun, Pontypridd and Nantgarw.
Under the Flood and Water Management Act 2010 (Section 19) the Council, as the Lead Local Flood Authority, is required to provide factual accounts of what happened in any major flooding event. These reports must identify the Risk Management Authorities (RMAs), note the functions each authority has exercised so far, and outline what they propose to do in the future.
The Council initially investigated 28 locations affected by Storm Dennis from February 15-16, 2020 – and will produce a total of 19 reports. An Overview Report for all of Rhondda Cynon Taf has been published (July 2021), as well as Section 19 reports covering Pentre (July 2021), Cilfynydd (September 2021) and Treherbert (November 2021). Last week (January 25, 2022), reports were published for Treforest, Glyntaff and Hawthorn, and Taff’s Well.
The latest reports – published on Monday, January 31 – focus on Hirwaun, Pontypridd and Nantgarw. Each report was informed by RCT Flood Risk Management Team inspections in the days after Storm Dennis, as well as information from residents, RCT Public Health Team, Natural Resources Wales (NRW), Dwr Cymru Welsh Water and consultant led technical reports.
Hirwaun (Flood Investigation Area RCT 01)
Storm Dennis resulted in 30 homes and a commercial property being flooded internally in Hirwaun, and there was also extensive highway flooding. The primary cause is found to have been the River Cynon exceeding its banks to the rear of Cae Felin Parc – passing through privately-owned boundary walls and into properties. NRW mapping identifies these properties to be at ‘high risk’ of flooding from the main river, with no formal flood defences in place.
Investigations have also found that surface water flooding was a primary source of flooding in the RCT 01 area – with the sheer volume of rainfall resulting in significant overland flows. Many roads acted as channels for the water. Combined with the effects of main river flooding, drainage infrastructure was overwhelmed and this resulted in flooding to several local properties.
Pontypridd (Flood Investigation Area RCT 11)
At least 158 properties in this area were flooded – including 80 homes and 78 non-residential properties. Significant flooding to the highway also occurred. Investigations found that the primary source of flooding was the overtopping of the River Taf following persistent and heavy rainfall. NRW’s monitoring station at Pontypridd recorded river levels to be almost four times their typical level, peaking at 5.32 metres – the highest since the station’s opening in the 1970s.
NRW mapping indicates the properties impacted are at a ‘low risk’ of flooding from the main river, due to formal flood defences on sections of the eastern embankment at Sion Street, and the western embankment at Pontypridd Town Centre. However, Berw Road properties are at ‘medium risk’ with no formal flood defences in place. As Storm Dennis was in excess of a 1 in 200 annual probability flood event according to NRW, it is concluded that River Taf flood defences became overwhelmed and were overtopped at several locations.
Investigations identified that surface water accumulation on the highway also contributed to fluvial flooding in the area. Surface water flooding was caused by the sheer intensity of rainfall during Storm Dennis and the settling of fluvial deposits following the overtopping of the main river – which reduced the capacity of Highway drainage infrastructure within RCT11.
Nantgarw (Flood Investigation Area RCT 16)
This area, which includes Upper Boat, saw 306 properties flooded – including 121 homes and 185 non-residential properties. Significant highway flooding also occurred throughout the investigation area. The primary source of flooding was the overtopping of the River Taf at several locations, following persistent and heavy rainfall. NRW’s Upper Boat monitoring station recorded river levels at more than four times higher than usual – peaking at 5.49 metres. This is the highest level recorded since the station opened in 2001.
NRW mapping determines the impacted properties at ‘low risk’ of main river flooding, due to the formal flood defences along the eastern and western embankments. The majority of these provide protection from a 1 in 100 annual probability flood event, but those at Upper Boat protect from a 1 in 20 year event – with local properties at ‘medium risk’ in NRW’s determination. Storm Dennis is estimated to be at least a 1 in 200 year event, and it is concluded that flood defences were overwhelmed and overtopped at several locations.
In addition, ordinary watercourse flooding was found to be a contributing source of flooding to Cardiff Road properties, after a culvert inlet associated with the Nant Garw ordinary watercourse became hydraulically overloaded and surcharged. Surface water accumulation on the highway was identified as the primary cause of flooding to two residential properties, while it also contributed to the existing fluvial flooding across the wider investigation area.
Risk Management Authorities and their functions
For all three areas, the relevant RMA for managing main river flooding is Natural Resources Wales, and the reports detail that NRW has carried out investigative analyses to understand the mechanism of flooding – relating to the River Cynon at Hirwaun, and the River Taf at Pontypridd, Nantgarw and Upper Boat. It has also commissioned modelling projects to assess the viability of future flood risk management options. Specifically, a Cynon Flood Modelling Study is relevant to Hirwaun and will be completed by March 2022, while a Lower Taf Modelling Project will cover the Pontypridd, Nantgarw and Upper Boat areas. A Pontypridd-specific in-house project is also taking place.
NRW has also developed recommendations to address improvement areas – including its Flood Warning Service and incident management response.
The Council is the RMA for surface water flooding at all three locations. Each report details how the Council has carried out significant survey, jetting and cleaning works to infrastructure and has led on the development of a Control Room to provide a comprehensive response during future flood events. The Council has exercised its powers to engage with NRW and Welsh Water over their responsibilities as RMAs, and worked with NRW to expand the interim Property Flood Resistance project, offering expandable barriers to high risk properties.
Each report concludes that Storm Dennis was an extreme event, and it is unlikely that flooding could be prevented entirely in a similar event. It adds that RMAs satisfactorily carried out their functions in response to the flooding, but further measures to improve preparedness have been proposed by all RMAs.
Councillor Andrew Morgan, Leader of Rhondda Cynon Taf Council and Cabinet Member with responsibility for Highways and Transportation, said: “The Council has published six further Section 19 Flood Investigation Reports in the past week, with those covering Hirwaun, Pontypridd and Nantgarw today following reports relating to Treforest, Hawthorn and Glyntaff, and Taff’s Well last Tuesday. This brings the total number to nine so far, as well as an Overview Report – and they are all fully-accessible on the Council’s website.
“As the Lead Local Flood Authority, the Council is responsible for investigating each flooding event, and identifying the Risk Management Authorities. The Section 19 reports are legal and factual documents outlining what happened during Storm Dennis, based on collected evidence from several sources – and they report on what functions the RMAs have completed, and what they intend to do in the future. For Hirwaun, Pontypridd and Nantgarw, NRW is the RMA for main river flooding, and the Council is the RMA for surface water flooding.
“Investigating what happened in Storm Dennis and delivering flood alleviation work in our communities is absolutely a Council priority – with significant time, resources and funding invested over the past two years. More than 50km of underground watercourse has been surveyed, a Drainage Management team has been created, an emergency Control Centre was opened, and 26 extra cameras have been installed so we can better monitor higher risk locations.
“More than 400 sets of flood resistant equipment have also been given to households, while around 50 flood alleviation projects are being developed on top the more than 50 already delivered. In terms of investment, more than £13m will have been spent on flood alleviation by the end of March 2022 – with many further funding bids being submitted for the next financial year.”