Fallen tree and debris caused Rhondda Fach to overflow at Ynyshir during Storm Dennis, report finds

Anthony Lewis, local democracy reporter

Flooding in Ynyshir during Storm Dennis in 2020 was caused by the Rhondda Fach river and two culverts, a council report has said.

The latest section 19 report into flooding caused in February two years ago identifies the overflowing of the Rhondda Fach River and the surcharging of two culvert inlets located at Heath Terrace as the two main sources.

The river overflowing was caused by a fallen tree and other debris obstructing river flow.

This took place at a highway bridge connecting Avon Terrace and Riverside Close, causing water to rise behind this obstruction.

There was internal flooding to 26 properties, including 24 houses and two businesses, along with significant highway flooding in the area, the report said.

The report said very high river levels contributed to the severity of the river overflowing with Natural Resource Wales’ station at Maerdy (around 7.5 km upstream) recording its second-highest peak during Storm Dennis, which was 1.371m.

The report added 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 water flowing overland causing scouring on the hillside, with debris being taken down the hillside towards the culvert inlets.

Surface water was included as a source of flooding to one property at Standard Terrace, following accumulation on a low point of the road.

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The report found that Storm Dennis was an extreme event, and it is unlikely flooding could be prevented entirely in a similar event.

It added that the authorities satisfactorily carried out their roles in response to the flooding, but further measures to improve preparedness have been proposed.

The report said Natural Resources Wales (NRW) has carried out its own analysis to understand the flooding in Ynyshir.

It has commissioned the Rhondda Flood Modelling Project to feed into 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.

Rhondda Cynon Taf Council is responsible for managing ordinary watercourse and surface water flooding. In response to the flooding in Ynyshir, the report said the council has carried out 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 inspection and maintenance schedule to include the culvert inlets which surcharged, and carried out significant upgrades of the “culvert inlet two” and sections of the ordinary watercourse at Heath Terrace, to reduce the risk of scour and potential blockages.

This is the 11th report under Section 19 of the Flood and Water Management Act 2010 published by the council in relation to Storm Dennis.

Under the 2010 Act, the council as lead local flood authority must provide a factual account of what happened during significant flood events.

Following an initial investigation of 28 areas affected by Storm Dennis, the council will publish a total of 19 reports focusing on specific communities.