The causes of flooding in Aberdare, Aberaman, Rhydyfelin and Hawthorn during Storm Dennis in 2020 have been revealed. Two more section 19 reports have been published by Rhondda Cynon Taf Council, one covering Aberdare and Aberaman and the other covering Rhydyfelin and Hawthorn, looking into the floods of February two years ago.
The Aberdare and Aberaman report said there was internal flooding to 45 homes, five commercial premises and that there was flooding to the highway. The report said the main source of flooding in the area was from the significant overland run-off generated from the steep hillsides above Aberdare and Aberaman North draining to lower ground through a series of ordinary watercourses, many of which became overwhelmed with water and debris and eventually overflowed.
After reviewing the condition and hydraulic performance of the culvert infrastructures which were the sources of flooding to properties, two culvert networks were judged to have provided inadequate standards of protection in with free flowing and blocked water. The other culvert network identified as a source of flooding was judged to have adequate standards of protection in free flowing water.
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Despite the ‘Nant Gwawr’ culvert network having sufficient capacity to manage the expected flows, the culvert infrastructure was obstructed and its capacity significantly reduced due to blockages caused by debris from upstream. The overflowing of the River Cynon was also identified as a primary source of flooding to properties at Wellington Street in Trecynon and influenced the overflowing of the watercourse to the rear of Gloucester Street.
The unprecedented high river levels on the River Cynon during Storm Dennis is considered to have contributed to the severity of the overflowing, with Natural Resources Wales’ Aberdare station recording its highest recorded peak during this period (2.125 metres). Surface water flooding associated to run-off from the hillsides and the overwhelmed highway drainage network were also identified as contributing sources of flooding to several properties within the area.
In Rhydyfelin and Hawthorn, the report said there was internal flooding to 28 homes, five commercial properties and extensive flooding to the highway. The primary source of flooding here was a significant overland run-off being generated from the steep hillsides above Rhydyfelin draining to lower ground through a series of watercourses, many of which became overwhelmed with water and debris and eventually overflowed.
After reviewing the hydraulic performance of the three culvert inlets identified as sources of flooding, it was confirmed that all three inlets became overwhelmed with debris, causing a reduction in hydraulic capacity. The investigation also noted the overflowing of the Nant Lonydd watercourse at several locations as a primary source of flooding during Storm Dennis.
This was heavily influenced by the unprecedented high river levels within the River Taf which caused a backing up of flow upstream. A review of NRW’s Flood Risk Assessment Wales maps showed that there are no formal flood defences currently in place here.
In response to the flooding in these areas, the council has carried out clearance works to the structures identified as sources of flooding, carried out survey, jetting and cleansing operations to an estimated 921 metres of ordinary watercourse drainage network in Aberdare and Aberaman and carried out repair works to the river retaining wall which falls on unregistered land to the rear of Hawthorn Crescent and was identified as damaged during the storm event.
It has also led on the development of a central control room to compliment the council’s contact centre and CCTV centre, started an interim Property Flood Resistance project offering expandable flood gates to properties deemed at high risk of ordinary watercourse and surface water flooding, looked to engage with NRW to work collaboratively to manage the flood risk caused by the influence of the River Taf upon the Nant Lonydd ordinary watercourse, carried out upgrades to structures and networks across Aberdare and Aberaman to reduce the risk of potential blockages and installed remote monitoring devices at key culvert structures to enable operators to ensure the drainage systems within the area are operating effectively.
NRW has carried out their own investigation work after the floods to understand the flooding from the River Cynon at Aberdare, commissioned a Cynon Flood Modelling Project which will include an assessment of the viability of potential flood risk management options, and developed a series of recommendations and an action plan to address areas of improvement for future storms including the performance and expansion of NRW’s flood warning service and incident management response. Both reports said: “The event that occurred on 15 and 16th February was extreme, and it is unlikely flooding from a similar event could be prevented entirely.”
They said that risk management authorities satisfactorily carried out their flood risk management functions in response but further measures have been proposed to improve preparedness and responses to future floods.
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