July 24, 2021

Newyddion Cymru Ar-Lein : Wales News Online

Newyddion Lleol a Chenedlaethol Cymru – Local and National News for Wales

Main source of flooding in Pentre during Storm Dennis was “significant blockage” of culvert

THE main source of the initial flooding in Pentre during Storm Dennis last year was a “significant blockage” of a culvert, a report has revealed.

Rhondda Cynon Taf Council has investigated the causes of severe flooding in Pentre during the unprecedented weather of Storm Dennis last year. This found the inlet had sufficient capacity to deal with the storm if there wasn’t a blockage.

The Rhondda Fawr village of Pentre was hit by flooding on five separate
occasions in 2020.

Section 19 of the Flood and Water Management Act 2010 requires the lead local flood authority, in this case the council, to provide a factual account of what happened.

It focused on the initial flood event during Storm Dennis on February 15 and 16, but also took into consideration the four events that followed.

Unprecedented rainfall during Storm Dennis caused record river levels and flows as Rhondda Cynon Taf recorded its most significant flooding since the 1970s.

In total, 159 residential properties and 10 commercial properties were flooded
in the village of Pentre alone, along with significant flooding to local highways.

The report said the main source of flooding in the initial flood was due to a significant blockage by woody materials including parts of trees at the Pentre Road culvert inlet. It resulted in water flowing down Pentre Road, onto Elizabeth Street and Queen Street, and towards the lower streets of the village.

It found the culvert inlet, within the ownership of Welsh Government
Woodland Estates and managed by Natural Resources Wales (NRW), was blocked by woody debris washed off the mountainside, including an area where tree felling activity had been undertaken by NRW which severely reduced the hydraulic capacity of the inlet.

A review of the inlet itself found it had sufficient capacity to deal with
the storm event if there was no blockage. This also resulted in a significant amount of woody debris washing into the culvert network, contributing to repeat flooding in subsequent storms.

Mud and silt entered the highway drainage infrastructure, which substantially reduced its capacity too, the report found.

Councillor Andrew Morgan, leader of Rhondda Cynon Taf Council, said: “It is evident that blockage of the culvert inlet by woody material severely
impacted upon the ability of the drainage infrastructure to manage this
unprecedented level of rainfall.

“Despite the severe nature of Storm Dennis, technical assessment confirms that without this debris the culvert inlet would have provided sufficient capacity to manage the exceptional amount of water running from the mountain.

“Had the water been able to enter the culvert, the culvert would have significantly reduced the impact any flooding would have had upon the local community.

“The council is of the view that this evidence is irrefutable.

“As a council we recognise that Natural Resources Wales has managed the
land above the culvert and note their assertion that tree felling at this location
was undertaken in-line with national best practice.

“Whilst the council’s report does not comment on their land management practices, we do conclude that the primary cause of flooding was as a result of a blockage to the inlet with the significant contribution to the blockage being the presence of woody debris.

“The Section 19 report rightly states that the weather in Storm Dennis was
extreme and it is unlikely that flooding from a similar event could be prevented
entirely.

“It importantly concludes that the risk management authorities (the
council) satisfactorily carried out their functions in response.

“Within this report, as the local flood management authority, the council,
proposes a number of actions to reduce the risk of an occurrence such as this
happening again.”

Natural Resources Wales said: “We do not underestimate the impact that the February 2020 storms had on communities in Rhondda Cynon Taf.

“The scale of the structural and human impacts of the flood waters was
devastating and the grief felt in those communities palpable. Our thoughts continue to be with those whose houses were damaged, whose livelihoods were threatened and those still recovering and rebuilding today.

“The rainfall experienced in February 2020 was record breaking and exceptional. Wales has not seen a wetter February since records began in 1862.

“It was also the fifth wettest month on record resulting in some of the most significant flooding Wales had seen since the 1970s.

“Natural Resources Wales (NRW) agrees with Rhondda Cynon Taf County Borough Council’s (RCTCBC) report’s statement that the ‘event that occurred on 15 and 16th February 2020 was extreme, and it is unlikely flooding from a similar event could be prevented entirely.’

“NRW’s review into the February 2020 floods found that the working methods we adopted during the felling operation above Pentre were appropriate and in line with forestry standards, and that these operations were not likely to have been the primary cause of the flooding.

“We accept that woody material washed off the mountains above Pentre may have contributed to blockages to the culvert system, which also included a significant amount of soil and rock.

“However, a proportion of the woody debris was unrelated to NRW’s felling
operations and was washed down as a natural consequence of such an extreme event.

“NRW therefore disagrees with the report’s inference that its harvesting operations were the primary cause of the flooding during Storm Dennis.

“We know that the people impacted want to understand the reasons for the flooding and what can be done to help reduce the risk in the future.

“We have worked in partnership with RCTCBC to repair and upgrade the culvert inlet since the storms and carry out regular checks at key locations across the borough all year round. These increase in frequency
during the winter period and ahead of any heavy rainfall event.

“The report identifies the limited capacity in the culvert system at Pentre and as a result, RCTCBC is undertaking a programme of investigations and upgrade works.

“Our own reviews have underlined our commitment to learn lessons from the February storms and make improvements wherever possible. But there needs to be a fundamental consideration of the choices that we all have to make on how flood risks are managed and resourced on a national level in Wales.

“Where Pentre is concerned, we want to work constructively with RCTCBC and with other flood risk management authorities to keep under review what is practically possible to improve the situation in this community.

“We have to accept that we cannot stop the rain and that some flooding is inevitable. Climate change is leading to more extreme weather events, and we are certain to see more of the types of storms we saw in 2020 in the future.

“That is why the lessons learnt by every flood risk management authority involved in the response to February’s events should be the catalyst for the decisive action needed to adapt to the challenges of the future.”

So far, the council, NRW and Welsh Water have:

  • Carried out significant upgrades to the Pentre Road inlet to reduce potential
    blockages, with multiple overflow systems at the culvert should there ever be a future blockage to the main grill.
  • It is installing a monitored alarm system and CCTV at the culvert inlet to remotely alert the council of any problems 24/7
  • Developed and implemented a “super catchpit” capable of containing six tonnes of debris at Pleasant St to alleviate the risk of debris transportation to the flatter lower parts of the culvert system.
  • It is developing proposals for a flood routing scheme at Pleasant Street.
  • Worked with Dwr Cymru/Welsh Water to construct a high-level overflow to increase the capacity of the highway drainage network in Lewis Street.
  • Started developing proposals for significant efficiency upgrades to Volunteer Street pump station.
  • It is leading on the development of an outline business case for Pentre which seeks to mitigate flood risk in the community.

The council made the following recommendations:

  • To review NRW’sforest resource plans
  • To develop a forest resource plan for the Rhondda Fawr valley
  • To embed NRW’s Water Management Plans into their forestry
    management operations
  • To engage with residents in relation to forest resource planning and
    forest operations
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