Heavy rainstorms can put London at risk of flooding, which can result in costly disruptions and damage. For example, on 7 August 2002 heavy rain forced five key rail stations in London to shut for several hours during peak travel. Our climate is changing and the intensity of storms is expected to increase over the coming decades. Such storms will put us at greater risk of surface water flooding. To mitigate that risk today and in the future we need to first better understand it.
The Drain London project helps to predict and manage surface water flood risk in London. It does so by improving our knowledge of the surface water drainage system and identifying areas at greatest risk of flooding. As part of Drain London we will also demonstrate ways to reduce flood risk. The project is a direct response to the Mayor’s Regional Flood Risk Appraisal, which identified surface water flood risk as the most likely cause of flooding in London, rather than river, tidal or groundwater sources. The Appraisal highlighted that the risk of surface water flooding was not well understood, with no co-ordinated responses.
What is surface water flooding?
Surface water flooding is a problem in many urban areas that have a large amount of impermeable surface. It occurs when rainwater cannot soak into the ground and overwhelms the drainage system, forming puddles, pools and temporary flows. A small amount of this is normal but some storms completely overwhelm our drainage systems and we experience flooding that can affect properties and livelihoods. This can happen very quickly and can be followed by rivers and streams bursting their banks as they are inundated with water from the drainage system.
What will the Drain London project do?
Drain London is identifying areas of London at risk of surface water flooding and potential solutions to reduce or manage that risk. Boroughs are the Lead Local Flood Authorities (LLFAs) for London and have responsibility for managing flood risk from surface water, groundwater and watercourses. Each London borough will have a Surface Water Management Plan produced through Drain London, which contains a Preliminary Flood Risk Assessment and a Flood Risk Management Plan - both necessary under the Flood and Water Management Act (2010). The Act requires boroughs to investigate (by 2015) and address flood risk and maintain a public register of Flood Risk Management Assets. The Assessment and Plan will help them to do this.
Flood risk modelling through Drain London has been used to inform Environment Agency flood risk maps. It is also being used to inform the delivery of a number of projects to demonstrate how surface water can be managed in a more sustainable fashion. This includes increasing green cover in the city through programmes like Greening the BIDs.
Working across London
We are leading a partnership of 33 London boroughs, the Environment Agency, Thames Water, Transport for London and London Councils to deliver Drain London. We are also working with other agencies with drainage responsibilities in London. This partnership is called the Drain London Forum. The Forum helps boroughs meet their responsibilities for managing flood risk by providing guidance on asset registers, helping to form partnerships with a range of stakeholders and sharing good practice, knowledge and expertise.
The programme of work
Drain London has progressed in three stages. The first stage focused on data collection and establishing the standards and framework for flood risk modelling. Stage two undertook this modelling, identifying the areas at risk of flooding and producing a Surface Water Management Plan for each London Borough, with a prioritised list of areas needing attention. The final stage, still underway, involves detailed modelling of high priority flood risk areas such as schools and emergency services, as well as the installation of sustainable drainage demonstration projects in London.
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