Throughout 2022 we have learned more about what caused the devastating floods across RCT during Storm Dennis. Rhondda Cynon Taf Council has released several reports looking into the causes of flooding in February 2020 across the county borough.
In 2021 the council published an overview report for all of Rhondda Cynon Taf and section 19 reports focusing on the communities of Pentre, Cilfynydd and Treherbert. Here is what the reports which have been published during 2022 told us about the events of two years ago.
Treforest
In January, the reports covering Taff’s Well, Treforest and Glyntaff, Rhydyfelin and Hawthorn was published by the council. In Treforest, there was flooding to 50 properties in the area, including 43 houses, along with significant highway flooding.
This report said the primary source of flooding in Treforest was the overflowing of the River Taff following persistent and heavy rain. Natural Resources Wales’ (NRW) river level gauge recorded the river to be almost four times higher than it usually would be, peaking at 5.32m.
The investigation also identified that surface water accumulation on the highway contributed to the flooding and exacerbated existing river flooding.
Glyntaff, Hawthorn and Rhydyfelin
The report for Glyntaff, Hawthorn and Rhydyfelin said 27 properties (including 23 houses) flooded and there was significant highway flooding. The report said the primary source of this was the overflowing of the River Taff at several locations along its eastern and western embankments. NRW’s gauge recorded the river level to be almost four times higher than usual, also peaking at 5.32m.
Investigations also identified that surface water accumulation to the highway contributed to the flooding, as drainage infrastructure was overwhelmed. A manhole at Cardiff Road, associated to the Ilan Avenue culvert network, also surcharged.
Taff’s Well
And the report for Taff’s Well revealed that at least 36 properties, including 25 houses and 11 non-residential properties, flooded, along with significant highway flooding throughout the area. The report noted the primary source was again the overflowing of the River Taff. NRW’s gauge recorded the river level to be more than four times higher than its typical level – peaking at 5.49m.
A section of the northern riverbank was said to have no formal defence and along with accounts provided by residents, the information suggested the river initially overflowed at this location – allowing flood water to flow behind the formal defences towards Cardiff Road, the report said. It also said surface water accumulation to the highway also caused flooding and exacerbated river flooding in the area.
Pontypridd
In February, three more section 19 reports were released by the council which covered Pontypridd as well as Hirwaun and Nantgarw. The Pontypridd report said at least 158 properties in the area were flooded – including 80 homes and 78 non-residential properties.
It said that significant flooding to the highway also occurred and that investigations found that the primary source of flooding was the overflowing of the River Taff following persistent and heavy rainfall. NRW’s monitoring station at Pontypridd recorded river levels to be almost four times their typical level, peaking at 5.32 metres – the highest since the station’s opening in 1970.
The report added that as Storm Dennis was in excess of a one in 200 annual probability flood event according to NRW, the conclusion was that the River Taff flood defences became overwhelmed and were “overtopped” at several locations. Investigations identified that surface water accumulation on the highway also contributed to fluvial (river) flooding in the area.
It said surface water flooding was caused by the sheer intensity of rainfall during Storm Dennis and the settling of river deposits following the overflowing of the main river – which reduced the capacity of highway drainage infrastructure.
Hirwaun
The Hirwaun report said that Storm Dennis resulted in 30 homes and a commercial property being flooded internally. There was also extensive highway flooding. The primary cause is found to have been the River Cynon exceeding its banks to the rear of Cae Felin Parc – passing through privately-owned boundary walls and into properties.
Investigations have also found that surface water flooding was a primary source of flooding in the area – with the sheer volume of rainfall resulting in significant overland flows. Many roads acted as channels for the water and combined with the effects of main river flooding, drainage infrastructure was overwhelmed and this resulted in flooding to several local properties, the Hirwaun report said.
Nantgarw and Upper Boat
The report for the Nantgarw area, including Upper Boat, said 306 properties were flooded including 121 homes and 185 non-residential properties. Significant highway flooding also occurred throughout the area and the primary source of flooding was the overflowing of the River Taff at several locations, following persistent and heavy rainfall, the report said.
NRW’s Upper Boat monitoring station recorded river levels at more than four times higher than usual – peaking at 5.49 metres which is the highest level recorded since the station opened in 2001. It said the majority of the flood defences provide protection from a one in 100 annual probability flood event, but those at Upper Boat protect from a one in 20 year event.
Storm Dennis is estimated to be at least a one in 200 year event, and it was concluded that flood defences were overwhelmed and over topped at several locations. In addition, ordinary watercourse flooding was found to be a contributing source of flooding to Cardiff Road properties, after a culvert inlet associated with the Nant Garw ordinary watercourse became hydraulically overloaded and surcharged.
Surface water accumulation on the highway was identified as the primary cause of flooding to two residential properties, while it also contributed to the existing fluvial (river) flooding across the wider investigation area, the report said.
Treorchy
Also in February, the report for Treorchy said 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 in February 2020 resulted in 44 properties in Treorchy flooding internally, including four commercial properties with 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.
Ynyshir
March saw the report for Ynyshir published and the overflowing of the Rhondda Fach River and the surcharging of two culvert inlets located at Heath Terrace were seen as the two main sources of flooding here. 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 river overflowing was caused by a fallen tree and other debris obstructing river flow which took place at a highway bridge connecting Avon Terrace and Riverside Close, causing water to rise behind this obstruction The report said very high river levels contributed to the severity of the river overflowing with Natural Resources Wales’s 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.
Trehafod
In April the council released details of how flooding happened in Trehafod and said that a surcharging culvert inlet, located above the railway line, was the main source of flooding there. 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 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.
Cwmbach
Also in April we heard more about what caused flooding during the storm in Cwmbach. Following a review, the judgement was that the infrastructure associated with the Cefnpennar Road culvert network provided inadequate standards of protection with free flowing water and when there was a blockage.
It added that the remaining three culvert networks identified as sources of flooding provided adequate protection in free-flowing conditions but despite having enough capacity to manage expected flows, the infrastructure was seen to be in a poor condition and their capacities were “significantly reduced” by debris blockages. The report said the main source of flooding was as a result of significant overland water run-off from the steep hillsides above the village.
It added that the rainwater then drained to lower ground through a series of ordinary watercourses – many of which became overwhelmed with water and debris and overflowed, affecting several properties. The report said the extreme rainfall during Storm Dennis saw internal flooding to 34 properties in Cwmbach, including 18 commercial properties.
Abercwmboi and Fernhill
June saw two reports published into flooding with one covering Abercwmboi and Fernhill in the Cynon Valley and the other covering Porth. In Abercwmboi and Fernhill, a total of 68 homes and five non-residential properties flooded and there was extensive flooding to the highway.
The report for these two areas said that flooding here was a result of significant overland run-off from the steep hillsides above, draining to lower ground through a series of ordinary watercourses many of which became “overwhelmed with water and debris” contributing to the surcharging of several culverts and the internal flooding of several properties. After reviewing the condition and hydraulic performance of the six culvert inlets identified as sources of flooding to properties, it was confirmed that culvert inlets linked with the Bronallt Terrace East and Mostyn Street networks “do not provide adequate standards of protection” in free-flowing condition and that this is further reduced when there’s a blockage.
The report also said that inlets associated with the Cwm Alarch Close, Fernhill and Glenboi networks did have adequate standards of protection but that all five of these inlets are thought to have surcharged due to blockages caused by debris from the hillside. Another culvert which is maintained by Natural Resources Wales (NRW) was judged as having inadequate capacity for the expected flows from the hillside during the storm event.
Surface water flooding associated to “sheeting run-off” from the hillsides and a collapsed section of highway drainage network were also identified as sources of flooding to properties. Dwr Cymru Welsh Water was identified as the relevant authority in relation to sewer flooding seen at Maple Terrace.
Porth
In Porth, there was internal flooding to 45 homes and 16 non-residential properties and there was significant flooding to the highway as well. There were three main causes of flooding in this area according to the report which were the overflowing of the two main rivers, the River Rhondda and the Rhondda Fach, the surcharging of two culvert inlets associated to the Nant Llwyncelyn ordinary watercourse and the build up of surface water.
The report for Porth said that the Rhondda Fach and Rhondda rivers overflowed their eastern and western embankments at several locations following persistent and heavy rainfall. It added that obstructions caused by floating debris at several bridges along the Rhondda Fach and Rhondda River channels are also considered to have contributed to the overflowing by causing water to rise behind the obstructions.
River level gauge data from NRW’s Trehafod monitoring station shows that the River Rhondda was over three times its typical level during Storm Dennis, reaching a peak level of 3.977 metres which is the highest river level recorded at the station since its opening in 2001. The surcharging of two culvert inlets linked to the Nant Llwyncelyn watercourse was caused by the accumulation of debris which had been displaced within the upper watercourse channels.
An “unconsented” culvert structure downstream of one of the culvert inlets is also considered to have contributed to the flooding due to the reduction in hydraulic capacity within the network. Surface water flooding is also said to have been a primary and contributing source of flooding to several properties within the area as a result of intense rainfall and the highway and surface water drainage infrastructure being overwhelmed.
Aberdare and Aberaman
In August we found out more about the causes of flooding in Aberdare and Aberaman in the Cynon Valley in one report and in Rhydyfelin and Hawthorn in another. 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. 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.
Rhydyfelin and Hawthorn
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.
Mountain Ash
October brought reports covering Mountain Ash in the Cynon Valley and Blaenllechau and Ferndale in the Rhondda Fach. For Mountain Ash, the primary source of flooding 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.
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.
Blaenllechau and Ferndale
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.
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