Trehafod report says overwhelmed culvert inlet was main source of flooding

Anthony Lewis, local democracy reporter

The causes of flooding in Trehafod during Storm Dennis two years ago have been revealed.

The latest section 19 report investigating the flooding caused by Storm Dennis in 2020 said that a surcharging culvert inlet, located above the railway line, was the main source of flooding in Trehafod.

A reported localised landslip is also considered to have contributed debris towards the culvert inlet and network reducing its capacity to manage the flow of water.

The report said that the culvert inlet surcharged over the railway, onto the A4058 and towards Trehafod Road.

It also said that intense rainfall and localised surface water accumulation are also considered to have exacerbated the flooding in Trehafod – especially in those areas identified as high risk of surface water flooding in NRW’s (Natural Resources Wales) flood risk mapping.

It said that extreme rainfall resulted in internal flooding to at least 68 properties (65 residential and three non-residential) and significant highway flooding locally.

Extreme event

The report said that Storm Dennis was an extreme event, and it is unlikely that flooding could be prevented entirely in a similar event. It adds that risk management authorities “satisfactorily carried out their functions in response to the flooding” but that further further measures have been proposed to better address preparedness and response to future flood events.

Rhondda Cynon Taf is the lead local flood authority, land drainage authority and highway authority in this case so it is responsible for managing the ordinary watercourse and the surface water flooding that occurred during Storm Dennis.

In response to the flooding experienced in Trehafod, the council said it has completed a total of 11 actions and has proposed to carry out a further six.

The council has done survey work, jetting and cleansing to local highway infrastructure and led on the development of a control room to complement its CCTV centre and contact centre during future flood events.

The council said it has also started an interim property flood resistance project offering expandable flood gates to those properties deemed at high risk, and is working closely with the highway authority and Dwr Cymru/Welsh Water to evaluate surface water management options to alleviate the future flood risk to properties.

This is the 12th report following the storm and since the new year. The council has published reports relating to Taffs Well, Glyntaff, Treforest, Pontypridd, Nantgarw, Hirwaun, Treorchy and, most recently, Ynyshir.

These have followed previous reports for Pentre, Cilfynydd, Treherbert, and an overview report for all of Rhondda Cynon Taf.

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.