Surface Water flood risk has for a long time been poorly understood, recorded and mapped. For London with its combination of predominantly underlying clay soils and extensive impermeable surfaces, it is recognised that the risk of surface water flooding is high.
This risk is generally expected to be increasing due to two factors. Firstly London’s population is increasing within the existing built footprint. Therefore there are more people and we are living at higher densities, meaning that any flood is likely to have a greater impact. Secondly, climate change predictions are that storms will become more intense, therefore the type of storm that cause surface water flooding are becoming more likely. There is some evidence to show that these more intense storms have already increased in frequency over the past few decades compared with a longer time horizon.
The risk was identified initially through the Greater London Authority’s Regional Flood Risk Appraisal where the relatively well documented history and models of tidal and fluvial flooding illustrated the lack of any consistent records or models of surface water flood risk.
The Greater London Authority set up a partnership group with other key organisations responsible for managing surface water flood risk and drainage assets, called the Drain London Forum. Climate change was not the main driver, rather it was the lack of knowledge about an existing risk coupled with the expectation that the risk is likely to get worse with climate change.
Drain London Commissioned consultants to undertake a tiered approach to assessing the risks across London. This means that an intermediate assessment has been carried out for all of London and the areas showing up with high priority risks are then being investigated in more detail and detailed options for addressing the risks are being investigated for those areas. High priority areas can be identified either due to the number of properties affected, the impact of those properties being flooded – eg subway systems, major roads, power supplies or hospitals.
Four different consultant firms have been used so that we can compare different practices and we have built in a peer review process where consultants actively check each other’s work.
Adaptation options are currently being identified for the high priority risk areas. In some cases these will involve engineering works, and in others a more sustainable approach to rainwater management will be taken, by more closely mimicking natural systems.
In some cases it is recognised that there are no specific measures that can be taken to reduce the likelihood of flooding. In these cases the focus will be on preparing those at risk of flooding so that they can best cope and recover from the effects.
In parallel with the work to assess the risk, Drain London has also commissioned work to illustrate the impact of flooding on people have experienced it, setting up Community Flood Plans and a smartphone App to report flooding and drainage defects.