Pontarddulais Flood Risk Management Scheme
The Scheme
The £6.1m scheme involved the construction of a flood storage reservoir upstream of the town of Pontarddulais and provides a 1 in 100 year standard of protection (1% chance of flooding in any year) to 224 residential and 22 non-residential properties in Pontarddulais.
The scheme was officially opened on 14 March 2019 by the First Minister and Minister for Environment, Energy and Rural Affairs.
Whilst the schemes primary objective was to reduce the risk of flooding to Pontarddulais, our flood risk management projects also look to deliver multiple social and environmental benefits.
Background
The town of Pontarddulais, north-west of Swansea, is situated on the banks of the River Dulais at its confluence with the River Loughor. The River Dulais is canalised through Pontarddulais, and the heavily modified watercourse passes through the centre of the town.
Historically, there has been a significant threat of fluvial flooding within the town due to the limited flow capacity of the channel. There are several low bridges that restrict the flow as well.
The largest flood in living memory occurred in 1947, and two other events in 1963 and 1979. More recent flooding happened in 2003, 2005 and 2008.
The town’s defences were considered to provide a 1 in 20 year level of protection, meaning there was a 5% chance of the defences overtopping in any year.
224 residential and 22 non-residential properties were at risk of flooding during a 1% (1 in 100) annual chance event, which would result in the entire town centre being flooded.
The Options
We looked at the whole catchment to establish the current flood risk, the causes and possible solutions to the flooding problem. We assessed each option for its benefits and risks; taking into account technical, environmental, social and cost factors of each one.
We considered options such as in line defences (flood walls) through the town, but we discounted this option as construction would’ve been extremely difficult working in a confined space, and it would have been very disruptive to the local area as major modifications to road crossings and bridges were required.
Other options we looked at, but discounted, included:
- looking at whether changes to land use within the Dulais valley could be implemented to reduce surface water runoff
- individual property protection - individual property owners would be responsible for implementing site specific flood protection measures
- demountable raised defences – temporary structures would be erected in response to flood warnings
Upstream Flood Storage Area
We concluded that the preferred option for Pontarddulais was upstream flood storage as it was less disruptive to people, had less visual impact and there was an opportunity to design ecological benefits into the scheme.
The flood storage area contains an earth embankment with a culvert running though it to restrict the amount of water flowing downstream during a storm, reducing the risk of flooding. Water will back up behind the embankment and flood the Dulais valley, which is sparsely developed, with scattered farmsteads and cottages dotted along the valley sides and upland areas.
The flood storage area can store 170,000m3 of flood water, which is almost the same as 70 Olympic size swimming pools.
Construction
We flew a drone over the scheme during construction and after it was completed to give you a bird’s eye view. Take a look at this video.
Wider social and environmental benefit
The scheme has also delivered wider benefits to local people and to wildlife.
- We’ve created a wetland area to the south of the embankment to compensate for loss of marshy grassland where the embankment was built. We also moved vegetation from the surrounding area to colonise the new wetland area
- Mine spoil/waste present at the site was used to construct section of the new embankment, this removed a need to use and import virgin soils
- We’ve made environmental improvements to the borrow area where the old mine waste material was taken from to construct the embankment, by creating a new wetland scrape (a shallow pond that usually forms in a natural low spot in the floodplain)
- We’ve also planted 56 native trees and almost 3,000 tree saplings and shrubs. Once established the trees and shrubs will not only soften the embankments visual impact but also provides biodiversity gain through improving habitat diversity and connectivity within the valley
- We’ve improved eel passage upstream by removing the upper section of the Graig Merthyr Colliery concrete dam, located immediately south of the new flood storage embankment
- We’ve capped an area of contaminated ground at the former Graig Merthyr Colliery site using material excavated from the embankment area. The capped area that was previously barren land has now been converted to acid grassland, which is consistent with grassland of the wider common
- We’ve also constructed ditches, bunds and block stone barriers around the area to try and deter fly-tipping and restrict access to off-road motorbiking on the adjacent common
- We worked in partnership with Swansea University and Common Vision to further investigate an innovative method (using biochar and compost) for soil restoration at past mining sites
History of Graig Merthyr Colliery
The flood storage area has been constructed near the site of Graig Merthyr Colliery.
The first commercial coal seam was worked in the Dulais valley in 1849 and between 1906 and 1919 typically 1,000 to 1,500 tonnes of coal was mined daily with records showing 495 men employed at Graig Merthyr Colliery in 1908.
The coal was transported from Graig Merthyr colliery to Pontarddulais. Initially this was done by horse and cart until a steam locomotive was purchased in 1890.
Graig Merthyr Colliery closed in June 1978.