FOI request detail

Drainage system in low points of underpasses and tunnels.

Request ID: FOI-2893-1920
Date published: 02 January 2020

You asked

I need following information as per my FOI request. London had lots of underpass and tunnels as part of its road network. I just need general information how drainage system works in these low points of underpasses and tunnels. I am interested to know where the water is usually discharged? how it is being maintained? do you use electrical and mechanical pumps to discharge water from these low areas? ultimately where this surface water run off? where this water is being discharged? how you tackle this surface water during heaving rain. What is involve during its routine maintenance schedule when it is not raining? I am sure it is not possible to discharge this water due to gravity as it is very low points as compare to surrounding roads. I am very impress how these are designed wonderfully as I never saw any flooding inside these underpasses.

We answered

Our Ref:         FOI-2893-1920

Thank you for your request received on 23 December 2019 asking for information about underpasses and tunnels.

Your request has been considered in accordance with the requirements of the Freedom of Information Act and our information access policy. I can confirm we do hold the information you require.

We use different methods based on location depending on the conditions. As this is question is not about a specific location we have provided a summary of the different methods that we use across the Transport for London Road Network (TLRN).

Most of our underpasses and tunnels are drained using pumps, though in a few locations we are able to use gravity based drainage.

With pump stations the surface water is taken into the pump station via a system of gullies along the edge of the carriageway and then into a ‘wet well’ at the pump station. This is a large sump (low space that can collect liquids) that will then store the water until it reaches a set depth and once this is reached the pumps are automatically turned on and the water is pumped out. On our network this can be to one of three locations: a watercourse or lagoon, the highway drainage system, or the public sewer system.

The pumps themselves are all electrical and many are on a centrally controlled system that we can monitor from our offices to let us know how much water is in the wet well, if the pump is running etc. At some of our larger pump stations we have the ability to manually control the pumps from this central system remotely. This allows us to monitor the health of our most critical pumps and react speedily to any alarms that they may trigger.

In some locations we also have a storage area under the road surface that can hold large amounts of surface water prior to any pumping to prevent the carriageway from flooding.

For maintenance, all gullies on the TLRN are cleaned every year, or more often should the need arise. Pump stations are serviced twice a year as a minimum. This is part of our contract through the London Highway Alliance. We carry out preventative maintenance to clean, check and service equipment so that failures do not occur.

When bad weather is forecast we have a variety of measures in place in relation to flood prevention. This includes activating our Severe Weather Desk within our 24/7 Network Management Control Centre, this means that we have a dedicated resource to coordinate any actions required. We also constantly monitor the pump stations and work with our contractors to have tools such as gully suckers on stand by in vulnerable areas.

If this is not the information you are looking for please feel free to contact me.

Please see the attached information sheet for details of your right to appeal.

Yours sincerely

Gemma Jacob
Senior FOI Case Officer
FOI Case Management Team
General Counsel
Transport for London

[email protected]

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