The causes of flooding in Mountain Ash, Ferndale and Blaenllechau during Storm Dennis two years ago have been revealed. The latest and final section 19 reports into the events of February 2020 in RCT show the primary source of flooding in Mountain Ash was significant run-off from the steep hillsides overwhelming watercourses, which overflowed.
In total, 67 properties, including 44 residential and 23 non-residential buildings were internally flooded. Two culvert inlets were found to have provided “inadequate standards of protection” both when water was free flowing and when there was a blockage.
The remaining four culvert networks identified as sources of flooding were judged as having adequate standards of protection. Despite these culvert networks having sufficient capacity to manage the expected flows, the culvert infrastructure was seen to be in “poor condition” and its capacities reduced due to blockages caused by moving debris.
Read more: Police warn '100mph Snapchat challenge' could lead to crashes on Welsh roads
The overflowing of the River Cynon was also said to be a primary source
of flooding to properties within the lower reaches of Mountain Ash West. The report said the unprecedented high level of the River Cynon during Storm Dennis contributed to the severity of the overflowing with Natural Resources Wales’ (NRW) Aberdare station, approximately 5km upstream of this area, recording its highest recorded peak of 2.125m during the storm.
Surface water flooding associated with run-off from the hillsides and the overwhelmed highway drainage network were also identified as sources of flooding. The main causes of flooding in Blaenllechau and Ferndale are said to have been a result of significant run-off and groundwater flow being generated from the steep hillside above Blaenllechau.
This reached the back of the impacted properties before continuing to flow towards lower ground, resulting in extensive surface water flooding to the highway and other properties. The area saw internal flooding to 25 properties, including one commercial property and flooding to the highway
It was found that the complex flow routing from the hillside caused water to find its own route rather than discharging into the existing drainage system. The area of hillside to the west is owned by the Welsh Government and managed by NRW and the area of hillside to the east falls under private land ownership, the report said.
It said the type of flood, a one in 200 years event, would have exceeded normal design criteria for culverts but no flooding was seen from the ordinary watercourse and culvert networks flowing through this area during the storm. Although flooding was not seen anywhere in the culvert system, the results from hydraulic modelling suggest there is a wider risk of ordinary watercourse flooding to the area based on the conditions and capacity of the culvert network.
The conclusion is that surface water was unable to reach the drainage system during the storm event. Both reports said Storm Dennis was extreme, and it is unlikely flooding from a similar event could be prevented entirely.
They said the authorities “satisfactorily carried out their flood risk management functions” in response and that all relevant authorities have proposed further measures to make them ready and improve responses to future floods. In terms of the response from the council, the reports said it has carried out survey, jetting and cleansing operations to an estimated 3,110m of ordinary watercourse drainage network in Mountain Ash and done clearance works to the culvert inlet structures identified as sources of flooding.
In Mountain Ash, it has carried out several flood alleviation schemes to help with the upgrade of culvert inlets across the area to improve capacity and structural condition and to reduce the risk of potential infrastructure damage and blockages. It has also installed remote telemetry monitoring devices at key culvert structures so operators can ensure the drainage systems within the area are working effectively.
It has done survey, jetting and cleansing operations with 1,284m of culvert network in Blaenllechau and Ferndale. It has set up a central control room, to compliment the council’s contact centre and CCTV centre, expanded it asset inspection and maintenance schedule to include several culvert structures within the area in its response to extreme weather event planning and has engaged with NRW in relation to their responsibilities as land manager of the Welsh Government Woodland Estate.
The council said it will also draw up a business case to provide recommendations for suitable ways to mitigate the wider risk of ordinary watercourse, surface water and groundwater flooding in the community. The reports said that NRW has carried out its own post-event investigative analysis work on the flooding from the River Cynon at Mountain Ash.
It has commissioned a Cynon Flood Modelling Project which assess the viability of potential flood risk management options and it has put forward recommendations and a plan to address areas of improvement for future storms, including the performance and expansion of its flood warning service and incident management response. These are the final two of the 19 reports the council planned to publish which have covered Pentre, Cilfynydd, Treherbert, Aberdare, Aberaman, Rhydyfelin, Hawthorn, Abercwmboi, Fernhill, Porth, Taffs Well, Glyntaff, Treforest, Pontypridd, Nantgarw, Hirwaun, Treorchy, Ynyshir, Trehafod and Cwmbach
To get the latest email updates from WalesOnline click here https://www.walesonline.co.uk/newsletter-preference-centre/
READ NEXT:
