Dinas Powys flood scheme long-list options

Below is a summary of the 16 long-list options and our assessment, from which five were shortlisted

Undertaking no measures to reduce flood risk

We stop maintaining the river channel and clearing blockages.

  • Flood risk would increase over time as the river channel capacity is reduced.
  • No costs would be incurred, but there would be no financial benefit.
  • There would be no environmental or social impacts from any work.

Please be assured that walking away from Dinas Powys is not an option for us, subject to continued funding availability.

Continuing with current approaches

We continue our current approach of maintaining the river and clearing blockages, subject to continued funding availability. We continue to warn people of forecast flooding.

  • Flood risk would increase due to the effects of climate change.
  • Minimal costs would be incurred, but there would be minimal financial benefit.
  • There would be no environmental or social impact from any work.

Please be assured that we are committed to undertaking this work in the future irrespective of the outcome of the options appraisal.

Natural flood management

We implement nature-based measures across the upper Cadoxton catchment, such as building ponds, placing leaky dams across streams, planting trees and de-compacting soil. This would slow rainfall entering the river.

  • Measures are estimated to cost £839,000, and would provide an estimated benefit of £1.17 million. Note all costs and benefits are best estimates based on current information at appraisal stage.
  • This would reduce smaller flood flows by 10 – 15%, but would have much less of an effect at larger flood flows, as the measures would become drowned – around a 3.33% (1 in 30) chance flood event.
  • Benefits 44 homes, protecting them up to a 3.33% (1 in 30) chance flood event. Does not protect homes across community to a low level.
  • Habitat and water quality would be improved (e.g. slowing the flow of water can reduce the amount of suspended sediment within the river).
  • This option may be delivered through other partners or with the community.

East Brook channel storage

We use the natural oversized channels on the East Brook to store flood water to reduce peak flood flows downstream in Dinas Powys.

  • Work is estimated to cost £852,000 and would provide a benefit of £1.5 million.
  • Benefits 31 homes and four businesses, protecting them up to a 1% (1 in 100) chance flood event. 166 homes would not be protected and if the East Brook work was undertaken in isolation it would compromise any future business case to protect the 166 homes due to shared benefits.
  • Would improve water quality and potentially improve habitats.

Cadoxton River upstream storage

A grassed earth embankment (120 metres long, 60 metres wide, 3.5 metres high) with sloping sides is built in Cwm George so that during heavy rainfall up to 180,000m3 of river water is temporarily stored.

  • Work is estimated to cost £4.59 million and would provide an estimated benefit of £5.7 million. However, significant risks exist that could cause the cost to escalate and the option to become economically unviable.
  • Benefits 167 homes and six businesses, protecting them up to a 1% (1 in 100) chance flood event.
  • Approximately 1,000 square metres of ancient woodland and 4,400 square metres of other broadleaved woodland, that are designated Sites of Importance for Nature Conservation (SINC), would be lost.
  • The amenity use of the site would be affected during heavy rainfall.
  • There may be an impact on the natural and historic landscape.
  • There may be an impact on habitat and species.
  • The works present an opportunity to improve existing and create new habitat, such as wetland, and enhance the amenity value of wider area.

Flood scheme through Dinas Powys

We build 2 kilometres of walls and embankments along the river banks, in the gardens of properties and the verges in St Cadoc’s Avenue, to contain the river. Walls would typically be 1 metre high, built of metal sheet piles and concrete, and then clad in brick. Embankments would be made of earth and be grass-covered.

  • Work is estimated to cost £8.87 million and would provide estimated benefits of up to £3.54 million.
  • Benefits 197 homes and 19 businesses, protecting them up to a 1% (1 in 100) chance flood event.
  • There may be an impact on habitat and species (e.g. change to river corridor).
  • Would involve major construction works in the village and properties’ gardens.
  • There may be opportunities to deliver water body improvements, such as the removal of culverts.

Local flood wall on St Cadoc’s Avenue

We build a 120 metre long flood wall at the southern end of St Cadoc’s Avenue to locally protect six houses.

  • Work is estimated to cost £750,000 and would provide a benefit of £663,000.
  • Benefits six homes, protecting them up to a 3.33% (1 in 30) chance flood event. 191 homes would not be protected.
  • Disruptive construction work in the gardens of properties.

Improve conveyance at Cardiff Road (A4055) bridge

Working with the Vale of Glamorgan Council, we enlarge the road bridge to pass more flow through, lowering upstream water levels and flood risk.

  • Work is estimated to cost £2.5 million and provide a benefit of £217,000.
  • Does not reduce flood risk to properties at the 1% (1 in 100) chance event, so 197 homes and 19 businesses would remain at risk.
  • Would require the closure of the main road from Barry to Cardiff for up to six months, with limited diversion route available.

River channel clearance

We remove obstructions from the river channel and dredge the river bed. Hydraulic modelling shows that this would only lower the river level by 4 centimetres at the 1% (1 in 100) chance flood event.

  • Work is estimated to cost £47,000 annually, so over 100 years this would total £1.4 million at present day prices, and would provide a benefit of £1.34 million.
  • Benefits nine homes, protecting them up to a 1% (1 in 100) chance of flood.
  • There would be significant and regular impact on the aquatic habitat and species.

Flood relief culvert on East Brook

We build a new culvert to increase the capacity of the East Brook and pass flood flows. 

  • Work is estimated to cost at least £370,000 and provide a benefit of £318,000.
  • Benefits only a handful of properties near the East Brook.
  • Construction work would disrupt the school and the railway line.

Property flood protection

Flood barriers are provided to each house individually to stop flood water entering.

  • The measures are estimated to cost £10,000 per property, and would have to be replaced every 20 years. Over 100 years this would total £4 million at present day prices and would provide a benefit of £3.54 million.
  • Could benefit 197 homes, protecting them up to a 1% (1 in 100) chance flood event, but unlikely to be effective on all houses and would not prevent disruption to roads.
  • There would be no environmental or social impacts.

Combined upstream storage and flood walls

A smaller flood storage area in Cwm George is combined with building lower walls along the river in Dinas Powys.

  • Together works would cost £11.8 million and provide an estimated benefit of £3.54 million.
  • Benefits 197 homes and 19 businesses, protecting them up to a 1% (1 in 100) chance flood event.
  • There is opportunity to improve existing and create new habitat, such as wetland, to make water body improvements and enhance the amenity value of the wider area.
  • Would involve major construction works in the village and properties’ gardens.
  • Loss of ancient woodland and other broadleaved woodland, that are designated Sites of Importance for Nature Conservation (SINC).
  • The amenity use of the site would be affected during heavy rainfall.
  • There may be an impact on the natural and historic landscape.
  • There may be an impact on habitat and species.

Combined flood walls and natural flood management

Nature-based measures are combined with building walls along the river in Dinas Powys.

  • Together works would cost at least an estimated £7.48 million and provide an estimated benefit of up to £3.54 million.
  • Benefits 197 homes and 19 businesses, protecting them up to a 1% (1 in 100) chance flood event.
  • Would involve major construction works in village and properties’ gardens.
  • Habitat and water quality would be improved by the nature-based measures.
  • There may be an impact on habitat and species in the river corridor.
  • There may be opportunities to deliver water body improvements.

Combined upstream storage and natural flood management

Nature-based measures are combined with upstream flood water storage.

  • Together works cost £5.43 million and provide a benefit of £5.70 million. However, significant risks exist that could cause the cost to escalate and the option to become economically unviable.
  • Benefits 167 homes and six businesses, protecting them up to a 1% (1 in 100) chance flood event.
  • Loss of ancient woodland and other broadleaf woodland, that are designated Sites of Importance for Nature Conservation (SINC).
  • The amenity use of the site would be affected during heavy rainfall.
  • There is opportunity to improve existing and create new habitat, such as wetland, to make water body improvements and enhance the amenity value of the wider area.
  • There may be an impact on the natural and historic landscape.
  • There may be an impact on habitat and species.

Combined Cadoxton River upstream storage and East Brook channel storage

We store flood water upstream of Dinas Powys in Cwm George from the Cadoxton River and in the East Brook river channel.

  • Together works cost £5.44 million and provide a benefit of £6.15 million. However, significant risks exist that could cause the cost to escalate and the option to become economically unviable.
  • Benefits 193 homes and eight businesses, protecting them up to a 1% (1 in 100) chance flood event.
  • Loss of ancient woodland and other broadleaf woodland at Cwm George, that are designated Sites of Importance for Nature Conservation (SINC).
  • The amenity use of the Cwm George site would be affected during heavy rainfall.
  • There is opportunity to improve existing and create new habitat, such as wetland, to make water body improvements and enhance the amenity value of the wider area.
  • There may be an impact on the natural and historic landscape.
  • There may be an impact on habitat and species.

Combined natural flood management and East Brook channel storage

We implement nature-based measures across the upper Cadoxton catchment and store flood water in the East Brook river channel.

  • Together works cost an estimated £1.69 million and provide an estimated benefit of £1.87 million. However, works could become economically unviable due to the sensitive nature of the costs and benefits.
  • Benefits 31 homes and four businesses on the East Brook, protecting them up to a 1% (1 in 100) chance flood event. 166 homes would not be protected and undertaking the East Brook work would compromise protecting these homes due to shared benefits.
  • Would improve water quality and potentially improve habitats.