Appraisal of the Dinas Powys flood scheme options

The risk of flooding

We have identified a high flood risk in Dinas Powys from the Cadoxton River and its tributary the East Brook. We’ve reached this view through hydraulic modelling and examining previous flooding.

Flooding is caused by prolonged, heavy rainfall causing the watercourses to rise and the flow to exceed the channel capacities. This water spills on to the floodplain – which has been built on throughout much of Dinas Powys – to flood homes, businesses, roads and other infrastructure.

Records, dating back to 1903, show several instances of flooding, including on St Cadoc’s Avenue, Greenfield Avenue, Elm Grove Place and Cardiff Road. Many other “near-miss” events have occurred, when the river has been very high but not caused flooding.

We predict that 197 homes and 19 businesses have a 1% (1 in 100) chance of flooding each year. 94 of these homes are at high flood risk, having greater than 3.33% (1 in 30) chance of flooding each year. Due to climate change, we predict that this will rise to 368 homes and 50 businesses at a 1% (1 in 100) chance by 2117.

Possible options

We believe we’ve looked at all practicable options, such as natural flood management, improving river flow, upstream storage and flood defences, to manage the flood risk in Dinas Powys.

We have appraised them following published guidelines, to consider the extent and level of flood protection, economic viability, stakeholder and community opinion and the environmental and social impacts of each option.

Read more about all of the options.

Costs and benefits

An option must offer value for money to be considered economically viable. This means it must provide greater benefit than it costs to develop and build it. The benefits of a flood scheme are from avoiding flooding, which causes property damage and disruption, over a 100-year period.

Using this approach, we’ve identified options with the aim of delivering a scheme that reduces flood risk for as many homes and businesses as possible. The ideal target is to manage flood risk to a ‘low’ level. Low means that each year a property has less than a 1% (1 in 100) chance of flooding. So, we would ideally like to reduce the flood risk to all 197 homes and 19 businesses currently at greater than a 1% (1 in 100) chance of flooding each year, and if possible, future-proof this for possible effects of climate change.

Shortlisted options

From all the options we considered, we shortlisted the following for more detailed assessment and consideration of the extent and level of flood protection they could achieve, their economic viability, stakeholder and community opinion and the environmental and social impacts:

  • Natural flood management
  • East Brook channel storage
  • Cadoxton River upstream storage
  • Flood scheme through Dinas Powys
  • Combined Cadoxton River upstream storage and East Brook channel storage

Overall viability

In our assessment of these options, we considered whether they provided a community-wide management of flood risk in Dinas Powys, what issues (risks) could arise and the consequence of these risks materialising, and the possible environmental and social impacts.

  • The natural flood management option does not provide a community-wide solution to a low level of flood risk, as none of the 197 homes would be protected to a 1% (1 in 100) chance. It would reduce flood risk to 3.33% (1 in 30) for 44 of the homes currently at the greatest risk of flooding.
  • The East Brook channel storage is not a community-wide solution as 167 homes and six businesses would not be protected. It could reduce the risk to 31 homes and four businesses along the East Brook to a low level of risk.
  • The Cadoxton River upstream storage could reduce flood risk to 167 homes and six businesses in Dinas Powys to a low level of risk, but is still not a community-wide solution. It also has cost, environmental and social impacts and risks.
  • The flood scheme of walls and embankments through Dinas Powys could protect 197 homes and 19 businesses in the village to a low level of risk. However, it is not economically viable, as it would cost much more to deliver than the benefits it provides (through reduction in flood damages) and would impact property.
  • Combining upstream storage on the Cadoxton River and channel storage on the East Brook would provide the best community wide-solution as most (193) homes would be protected to a low level of flood risk. However, it is marginally economically viable (the benefits just outweigh the costs) and it has environmental and social risks and impacts.

The combined option above provides the highest level of protection to the greatest number of properties, with more benefits than costs, but we recognise that it has some significant potential environmental and social impacts. These include:

  • Loss of an important area of ancient woodland and broadleaved woodland.
  • Impact to a wider area of ancient woodland and wider areas of broadleaved woodland through occasional flooding.
  • Potential impacts to protected species (bats, dormice, kingfisher, otters, badgers, reptiles, amphibians) and licencing requirements.
  • Potential impacts to the river that would affect its Water Framework Directive status.
  • Landowner agreement to undertake the works.
  • Potential impacts or amendments to existing Public Right of Ways, walking routes and open space.
  • Given the above factors, it is possible that the planning application process and statutory Environmental Impact Assessment will not be straightforward.
  • Lack of broad community support.
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