Pluvial (rain-related) flooding in urban areas: the invisible hazard

But where flood risk assessments are usually based on the number of properties at risk, this report pays attention to the number of people at risk. This emphasises the impact on people and particularly the impact on vulnerable groups, such as the elderly.

The impact of flooding varies according to economic and social factors. For example, there are systematic processes that lead to vulnerable groups being over-exposed to the hazard. This is likely due to older, smaller terraced housing being more prevalent in low lying flat areas, close to a river. This is because these areas tended to be developed first during industrialization in the 19th and early 20th centuries. As towns and cities expand outwards, development is more likely to be on elevated land and now tends to be characterised by larger, more expensive housing.

Furthermore, many private renters are less affected by small-scale localized floods because they can find alternative accommodation relatively easily. However, the same private renters can be severely affected by large-scale flooding if they cannot find suitable accommodation in the same town.

What measures can be taken to mitigate against pluvial flooding?

Suggestions to mitigate risk

• More accurate estimates of future rainfall

• Widespread affordable insurance against flooding

• Investment in drainage systems (including maintenance)

• Buildings designed to be flood proof

• Innovative management of surface water

Existing drainage systems can cause considerable problems in cases of flooding. Sustainable Urban Drainage Systems (SUDS) can be implemented to deal with run-off water, whilst also ensuring that any pollution or damage to the environment is minimized. A well-designed SUDS can help to:

• Reduce run-off rates

• Reduce the risk of flooding

• Encourage natural ground water re-charge

• Reduce pollutant concentrations in storm water

• Reduce volume of surface water run-off

• Provide habitat for wildlife

Source: Luton Borough Council (2012)

SUDS can be most effective when deployed in combination with surface water management plans and flood proofing of developments. This often involves separating storm water and foul water to increase drainage capacity and reduce the likelihood of sewage mixing with pluvial flood water. However, existing drainage systems will remain a part of the urban fabric for many decades to come. SUDS may not provide the most feasible option in all areas.

The report therefore makes different recommendations vary according to type of area, of which three are identified according to their relative levels of development (the density of buildings and infrastructure in an area).

Developed areas – Should ‘retro-fit’ (replace older equipment with new parts) drainage systems where possible. It is also important to identify and improve ‘pinch points’ (which struggle to deal with large flows of water) in the drainage system. The local landscape also requires attention to create safe routes for ground water to flow overland.

Undeveloped areas with pressure for development –SUDS (Sustainable Drainage Systems) should be implemented to effectively drain areas, whilst minimising pollution and managing water quality. Flood-proof building design is also important.

Undeveloped areas with less pressure for development – ‘Green’ (non-built-up vegetated areas) and ‘blue’ (urban spaces for storing water) spaces should be designed to effectively manage high rainfall. This should be supported through land use planning to set land aside for water management, especially to direct surface water away from properties.

Case study: Luton and the Ickley Close development

Luton, 30 miles north of London, is a large industrial urban centre which has rapidly expanded since 1945. The post-war infrastructure differs from the Victorian industrial urban forms of other towns and cities analysed in this study, which have older water management systems. With a recent history of pluvial flooding, Luton also has a surface water management plan.

The centre of the town is characterized by small Victorian terraced housing with little open space. This high density development means that it is more susceptible to flooding. However, further out more recent suburban developments are of lower density.

In line with the national urban average, 5% of the population of Luton is at risk from a 1 in 200-year pluvial flood. However, Luton would be more exposed to the hazard of pluvial flooding if climate change increases rainfall intensity. For example, a 20% increase in rainfall due to climate change would put 33% more people at risk. This is possibly due to the town’s flat topography, which would allow flood water to spread more widely than in other towns.