THE CAUSES of flooding in Treorchy during Storm Dennis two years ago have been revealed.

The latest section 19 report from Rhondda Cynon Taf Council said that the main source of flooding was “significant overland run-off” being generated from the steep hillsides above Treorchy.

This drained to lower ground via a series of ordinary watercourses many of which became overwhelmed with water and debris and eventually overflowed, the report said.

Storm Dennis resulted in 44 properties in Treorchy flooding internally, including four commercial properties, and flooding to the highway.

The condition and performance of four culvert inlets identified as sources of flooding to properties were assessed. and it was judged that three inlets associated to the Nant Tyle-du network “do not provide adequate standards of protection” the report said.

While blockages caused by debris is judged to be the main cause of the culverts surcharging, the report says they would have become overloaded in the storm in any case, based on a capacity assessment.

An assessment for a fourth inlet, associated with the Nant Coly watercourse, could not be done but based on the structure’s poor condition the report concluded that the inlet became “hydraulically overloaded” in the storm.

The report said that the council as the lead local flood authority has carried out 15 actions in response with three more proposed.

These include clearance works to the culvert inlet structures identified as sources of flooding and the surveying, jetting and cleansing of an estimated 1673 meters of watercourse network in the area.

It has also led on the development of a central control room to provide a “comprehensive and informed” response to residents during storm events and started an interim property flood resistance project offering expandable flood gates to properties deemed at high risk of ordinary watercourse and surface water flooding.

It has also installed remote monitoring devices at key culvert structures to ensure the drainage systems in the area  are operating effectively.

The report said the council will look to better understand the area around Treorchy by developing a business case to provide recommendations for suitable management mechanisms so it can mitigate the future risk of ordinary watercourse, surface water and groundwater flooding locally and phase one of this is due to start in early spring.

The report concluded that the weather of Storm Dennis was “extreme” adding that it is “unlikely flooding from a similar event could be prevented entirely.”

It said the risk management authorities (in this case the council) “satisfactorily carried out their flood risk management functions in response to the flood event” but added that further measures haven been proposed to “better address preparedness and response to future surface water flood events.”

This is the 10th such report published by the council as part of its duties under Section 19 of the Flood and Water Management Act 2010.

It follows reports covering Hirwaun, Nantgarw, Pontypridd, Treforest, Glyntaff and Taffs Well last month and previous reports on Pentre, Cilfynydd and Treherbert as well as an overview report for all of Rhondda Cynon Taf.

The lead local flood authority must provide a factual account of what happened in significant flood events.

The council said it will provide a total of 19 reports for locations affected by Storm Dennis following initial investigation of 28 areas.