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Section 19 report, Flood and Water Management Act 2010 – Ynyshir

The Council has published its eleventh report under Section 19 of the Flood and Water Management Act 2010 following Storm Dennis. The report focuses on the causes of flooding in the community of Ynyshir. 

Under the 2010 Act, the Lead Local Flood Authority (in this case Rhondda Cynon Taf Council) must provide a factual account of what happened during significant flood events. Following an initial investigation of 28 areas affected by the unprecedented weather of Storm Dennis (February 15-16, 2020), the Council will publish a total of 19 reports focusing on specific communities.

Today’s publication follows the most-recent report relating to Treorchy in February 2022, as well as those covering Hirwaun, Nantgarw, Pontypridd, Treforest, Glyntaff and Taffs Well in January 2022. All of these reports – along with previous publications for Pentre (July 2021), Cilfynydd (September 2021), Treherbert (November 2021) and an Overview Report for all of Rhondda Cynon Taf (July 2021) – are available to view in full on the Council’s website.

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

The latest report on Thursday, March 24, focuses on Ynyshir in the Rhondda Fach valley (Flood Investigation Area RCT 20). It notes that the extreme rainfall event of Storm Dennis resulted in internal flooding to 26 properties (including 24 residential and two commercial) along with significant highway flooding in the area.

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

The report has identified that there were two primary sources of internal flooding within the investigation area – the overtopping of the Rhondda Fach River and the surcharging of two culvert inlets located at Heath Terrace.

The river overtopping was caused by a fallen tree and other woody debris obstructing river flow. This took place at a highway bridge connecting Avon Terrace and Riverside Close, causing water to rise behind this obstruction. The reports establishes that very high river levels contributed to the severity of the river overtopping, with NRW’s station at Maerdy (around 7.5 kilometres upstream) recording its second-highest peak during Storm Dennis.

The report adds that surcharging of two culvert inlets at Heath Terrace was caused by a significant accumulation of debris on both debris screens. This came about due to overland flows causing excessive scouring on the hillside, with debris material being transported down the hillside towards the culvert inlets. Surface water was noted as a source of flooding to one property at Standard Terrace, following accumulation on a localised low point of the road.

As the RMA for managing main river flooding, NRW has carried out its own investigative analysis to understand the mechanism of flooding in Ynyshir. It has commissioned the Rhondda Flood Modelling Project to inform potential flood risk management options. NRW has also developed a series of recommendations to address areas of future improvement in storm events, including its Flood Warning Service and incident management response.

The Council, as Lead Local Flood Authority, is the RMA responsible for managing ordinary watercourse and surface water flooding. In response to the flooding in Ynyshir, the Council has undertaken 16 actions and proposed a further six. Some of the completed actions include survey work, cleansing and jetting to highway infrastructure, and leading on the development of a Control Room to provide a comprehensive response during future flood events.

It has also expanded its asset inspection and maintenance schedule to include the culvert inlets which surcharged, and carried out significant upgrades of the ‘Culvert Inlet 2’ and sections of the ordinary watercourse at Heath Terrace, to reduce the risk of scour and potential blockages.

The 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: “Today’s publication focusing on the flooding in Ynyshir during Storm Dennis is the Council’s eleventh Section 19 report, towards the 19 that it will publish in total. This is a requirement following the significant flooding in February 2020 – and today’s report follows those for Treorchy, Hirwaun, Nantgarw, Pontypridd, Treforest, Glyntaff and Taffs Well since the New Year.

“Investigating the causes of flooding in Storm Dennis remains a Council priority, and it is important to note that while officers take the time to compile these Section 19 reports, a major programme of work is being progressed in our communities. An accelerated capital programme of more than 100 flood alleviation schemes is continuing – with more than 50 of those complete in an investment of more than £13m so far, since February 2020.

“The Council has also recently been successful in securing more than £3.8m from Welsh Government to progress targeted flood alleviation work in 2022/23 – across the Flood and Coastal Erosion Risk Management and the Small Scale Works Grant programmes. Council contributions to complement this brings the total investment to £4.35m, for the benefit of our communities. 

“Today’s Section 19 report for Ynyshir identifies the causes of flooding locally during Storm Dennis. It also outlines what the RMAs – the Council and NRW respectively – have done and propose to do in the future, in order to increase preparedness for future storm events and the effects of Climate Change. The full report is available for the public to view on the Council’s website, and concludes that both RMAs undertook their duties in a satisfactory way.”