Thames Water sets out plans to tackle sewer flooding

London’s sewer network struggling to cope with demands

April 6th 2015

Thames Water has announced it has invested £346 million to reduce the risk of sewer flooding for 2,500 properties in the Greater London area by 2015. The investment is focused on reducing the risk of flooding to properties which have already suffered internal flooding and its plans include the building of underground tanks to collect and store heavy rainfall, increasing the capacity of its sewers, and offering flood mitigation to homes at risk.

London’s sewer network was built in the 1800s but the city has since trebled in size and the sewer network is struggling to cope. Thames Water says: “More intense storms resulting from climate change, increased housing development and the paving over of green spaces to provide off-road parking, are all increasing the amount of surface water entering our sewer network. This, combined with people disposing of inappropriate waste into our sewers, is contributing to an increased risk of sewers flooding homes and properties.”

Sewer Surveys

Ofwat has set two targets for Thames Water: firstly, for a 10% reduction in annual instances of properties being affected by internal sewer flooding; and secondly, for a 9% reduction in leakage levels. As part of its strategy to reach the first target, Thames Water is in the process of carrying out surveys in areas of sewer problems to check whether the cause is a blockage or a sewer collapse. The surveys will involve camera investigations, water jetting the sewers, and mapping their location. The water company says the surveys will help to improve the network of sewer pipes in its region: “This will help reduce blockages and the need for future repairs. This programme of work is one of the largest we have ever undertaken for our sewer network and will involve checking the condition of the private sewers that came under our ownership in 2011.”

Counters Creek Flood Alleviation Scheme

Thames Water has been working on a range of measures to further reduce the risk of sewer flooding. Since 2010, the company has fitted 700 non-return valves on properties to stop sewage backing up through their pipes. A further 600 will be installed over the next six years. Thames Water also held a consultation recently on plans for a flood alleviation scheme for the Royal Boroughs of Hammersmith & Fulham and Kensington & Chelsea. The company says the Counters Creek flood alleviation scheme aims to reduce the risk of sewer flooding to over 1,700 basement properties in the two boroughs. The scheme will involve the construction of a new storm relief sewer, local sewer improvements, and trials of sustainable drainage systems and other measures to protect individual properties, such as anti-flooding (FLIP) devices. The consultation focused on how and where the new storm relief sewer will be constructed.

Water Briefing says 1,700 homes and businesses have flooded in recent years across the two boroughs, and more than a thousand properties are threatened by heavy rain “which overwhelms drains and sewers, forcing sewage to back-up in the pipes and overspill out of toilets and sinks.” Water Briefing also says that Thames Water set up an Independent Advisory Group comprising three leading academics in the field of urban drainage to review and challenge its plans as they developed. A Thames Water study revealed the urbanisation of London has contributed to the area’s problems, with impermeable land, which stops rainwater filtering through soil, increasing in the Counters Creek area by around 17% since 1971.

Scrutiny

Thames Water came under scrutiny recently from the London Assembly’s Environment Committee, following an incident in which more than 1,000 trains were cancelled or delayed due to a burst water main near Farringdon station. Richard Aylard, External Affairs and Sustainability Director for Thames Water, told the Committee that the company was “pushing hard” for a national policy statement by the Government for water, saying that a national policy would give a boost to the sector to get infrastructure and resource projects “fast-tracked” through the planning system. He also said that Thames Water has a protocol in place with London Underground and that a similar principle with Network Rail should be set up. He was also asked about the likely increase in water bills due to the estimated £4 billion cost of the proposed ‘super sewer,’ the Thames Tideway Tunnel. Water Briefing reports that the cost is predicted to be around £34 per Londoner per year by 2020.

Groundwater Flood Risk & Sewer Flooding

Mark Fermor, Managing Director of the environmental consultancy ESI Ltd, is a hydrogeologist with a particular expertise in groundwater. He says that ESI’s ground-breaking Groundwater Flood Risk Map has helped water companies and waste water service providers reduce the risk of sewer flooding. “Although emergent groundwater tends to be clear and relatively clean compared to muddy fluvial flood waters, it has the potential to be contaminated by sewers and brownfield sites,” says Mark, “and it is the discharge of untreated sewage effluent during storm events that forms one of the primary mechanisms causing pollution of surface waters in England and Wales.” Mark explains that such discharges can occur when the flow entering works exceeds the treatment capacity or when storm overflows in the sewer system become active to prevent the network capacity being exceeded. “Under such conditions,” he says, “flows in the sewer networks can be significantly increased when the water table rises above the sewer invert level through defects in the system that allow the ingress of groundwater.”

ESI’s Groundwater Flood Risk Map has identified that the areas at risk from groundwater flooding are considerably less than previously flagged by others, and it allows the user to get a large-scale understanding of the groundwater flood risks of a region and an indication of the potential flood risks at a given site. The map has improved and expanded existing data by introducing the probability of a flood event, based on the 1 in 200 year groundwater levels – i.e. a 0.5% annual probability of a flood event – and on the severity of the flood event. The map is a powerful screening tool when planning major infrastructure projects, allowing water companies to better understand the areas of most pressing need for remediation works for instance. For more information, see the ESI website.

Advertisements