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Flooding from Sewers


This document aims to give a brief outline of the background to flooding from public sewers in England and Wales with special emphasis on the internal flooding of property. The sewers operated by the water companies are legally known as 'public sewers'. This document has been written for the lay person who has no specialist knowledge of the water industry. The document will examine the problem of internal property flooding, water company procedures including the prioritisation of flooding problems and finally, the immediate outlook for the alleviation of flooding problems. Much flooding is due to watercourses and the reader is directed to the Environment Agency web site for information regarding this.

The water companies, who operate the sewerage systems in England and Wales, are regulated by the Water Services Regulation Authority (OFWAT) . Prior to 2006 this function was undertaken by the Director General of Water Services who's staff were known as the Office of Water Services, hence the use of the term 'OFWAT'. OFWAT approve the water company's level of charges to customers together with the companies' spending plans in five year long asset management plans. The current and fifth such plan (AMP5) runs from April 2010 to March 2015. As part of the regulation process the companies report on specific aspects of their performance. One of these is the DG5 (DG standing for Director General) which records instances of internal and external flooding of property.

Other relevant schemes and organisations include the Guaranteed Standards Scheme (GSS) which is provided for by regulations made under the Water Industry Act, 1991 and applies to all water companies. The GSS covers various aspects of the water companies services including internal property flooding. The Consumer Council for Water (CCW) is a non-departmental public body (quango) which represents water and sewerage customers in England and Wales. CCW will take up consumers complaints if they have tried and failed to have their problems resolved by the water company.

DG5 Flooding

Water companies are required to record all instances of internal flooding of properties. These are categorised on their cause, either hydraulic overloading of the sewers (the sewer pipe is too small or at too shallow a gradient) or other causes (blocked or collapsed sewers, pumping station failure, etc). In addition the companies are required to maintain a register of properties which are at risk of internal flooding due to hydraulic overloading and this is usually known as the DG5 at risk register. This register further categorises properties into the likely frequency of the flooding, the categories being 1 in 20 years, 1 in 10 years and 2 in 10 years return periods. (A 1 in 10 year return period means that the property is likely, on average, to be flooded once every ten years). To put these figures into context, new sewers are currently designed not to flood for a 1 in 30 years return period storm. OFWAT consider that flooding resulting from rainfall more severe than 1 in 20 years are 'exceptional' and are not sufficient to warrant a property being put on the DG5 at risk register. OFWAT recently commissioned consultants to review the operation of the flooding risk registers and their report contains much useful background information.

An important recent legal case regarding sewerage flooding was that of Marcic v Thames Water Utilities Limited. Mr Marcic's property in the London Borough of Harrow suffered flooding from the adjacent sewers and he took the water company to court claiming that his Human Rights were being infringed. The case went all the way to the House of Lords and although Mr Marcic ultimately lost, the attention brought to bear on the problem of flooding from sewers resulted in an increase in the level of importance given to it by OFWAT. Incidentally, Mr Marcic's flooding problem was eventually addressed by Thames Water and the scheme to alleviate the problem was completed in June 2003. The Opinions of the Lords of Appeal for Judgment in the case , while lengthy, cover most of the legal and engineering problems involved in this type of problem.

A question as to whether a property is on the DG5 at risk register is contained within the Residential Drainage and Water Search (CON29DW) which is recommended by the Law Society to be undertaken prior to the purchase of a residential property. The search should give some reassurance to people buying a new house with regards to the risk of flooding. It should be borne in mind, however, that properties on the DG5 at risk register are only a small fraction of properties suffering some form of flooding from sewers. In addition the presence of this search is thought to be deterring some people from reporting the flooding of their property to the water company in case the property becomes blighted and difficult or impossible to sell. This in turn has an effect on the accuracy of the DG5 at risk register itself.

Flooding Problem Prioritisation and process for alleviation of flooding

Unfortunately, since for the foreseeable future there will always be more problems than funding available to address them, some way of selecting the worst problems is called for. The water companies operate prioritisation systems to ensure that, in theory, the most benefit is made of the limited budget available. For instance, take the case of two schemes to address two different flooding problems with very similar characteristics, such as number of properties affected and cost. If one scheme would protect properties from flooding three times every ten years while the other would protect properties from flooding only twice every ten years, the best value for money is probably the first scheme. The details of the prioritisation system will vary from company to company but assuming that OFWAT have set the goal-posts correctly it should be reasonably fair. However there is likely to some measure of 'gaming' where the managers aim to play exactly to the wording of the rules but give the maximum benefit to the company (or the individual manager) rather than the customers.

Water Company Procedures

The involvement of the water company in a flooding problem will probably be split into two distinct parts. Firstly there are the operations staff, (who might actually be employed by a contractor who works for the water company) and then the people who work on capital schemes (who may be water company employees or may be employed by a civil engineering contractor or a firm of consulting engineers). The operations staff will visit the site of the flooding in response to the resident's telephone call. Depending on the situation they might attempt to solve the problem but in any event they should clear up the debris and disinfect. They should also report their findings to their management and a further visit by the operations staff may be made to briefly investigate the problem. It is understood that most water companies, as a matter of course, carry out a CCTV survey of the problem sewer to try to identify any obvious cause of the flooding such as a blockage or collapsed sewer. The information from these visits, if an obvious cause is not identified (and so hydraulic overloading is inferred), is then fed into the company's prioritisation system and a capital scheme to address the problem may result.

A capital scheme probably ultimately involves improving the sewer system by constructing new infrastructure. The exact process will vary from one water company to another, but it is likely that the first step will be prioritisation, see above. If the problem gains sufficient priority it is then likely to be investigated to determine the cause and an outline solution will be designed. The investigation will be tailored to the particular problem and will probably involve visiting the site, talking to the residents, further CCTV sewer surveys and taking sufficient data from site to enable hydraulic calculations to be undertaken. The latter are likely to be in the form of either constructing a computer hydraulic model or modifying an existing one. Computer modelling is used because the hydraulic calculations for sewer systems can be very complex. Since the input data into the model involves assessing the paved or roof area connected into the sewerage system over possibly a very wide area, it is open to error. For this reason the performance of the model is usually verified against the performance of the actual sewerage system during storms. This involves measuring at the same time both the rainfall and the subsequent flow response in the sewers. The measured rainfall profile is then run through the model and the predicted flow compared to the measured. Once a suitable level of agreement between the performance of the model and the actual sewer system has been reached the model is said to be 'verified'. Once the model is verified it can be used to design an outline solution which can be costed. The cost can then be used to update the prioritisation process and if the scheme is still good value it can be designed in detail and constructed. It will be appreciated that this process is very long winded but modelling the sewer system is better than assuming what the solution is and constructing costly civil engineering works only to find that the problem still remains or has been transfered elsewhere.

In addition to the complexities of the sewerage system itself there are often other drainage systems in urban areas which can contribute to flooding from the public sewer. These include highway drains, culverted watercourses and private sewers (the latter are currently planned to be transfered to be the responsibility of water companies ). These other systems can suffer flooding problems and it can be the case that this flood water finds its way into the public sewer system causing that system in turn to flood. In some cases a property is situated in a low point and receives flooding directly from both the sewer system and from another drainage system. It will be appreciated that to fully understand and address flooding problems can be complicated and the problem may ultimately prove not to be the water company's responsibility. In the latter case the water company should inform the other body of its findings in the hope that that body will address the problem.

There are sometimes low cost measures that the water company can take to mitigate the effects of the flooding problems. These will reduce the severity or frequency of the flooding but not reduce it to 'acceptable' levels. One such measure is to install a non-return valve in the pipe connecting the property suffering flooding to the public sewer. This should stop water from the public sewer flooding the property but will also stop the property draining to the sewer in times of high flows in the sewer. This can lead to flooding or a loss of the use of toilet facilities, etc. A non-return valve is by no means applicable in all cases and can lead to other problems such as blockages, so regular maintenance is called for.

Work planned for AMP5

In their final determination for AMP5 , (pages 45 -49) OFWAT detail the numbers of properties to be removed from the DG5 at risk register or are to be protected from external flooding, during the AMP5 period (2010 to 2015). The final determination is the plan which the water companies have to work to and is based on the companies' proposals adjusted to take into account other matters such as the level of customer's water bills. The numbers of properties to be protected from flooding is less than the companies themselves originally proposed but more than OFWAT's draft determination. Throughout England and Wales some 6,379 properties are anticipated to be removed from the DG5 at risk register with 5,277 of these currently being at risk of flooding at least once in ten years. In addition some 2,675 properties suffering from external flooding will be protected although some of these will be associated with DG5 flooding properties.

As already mentioned, the government plans to transfer the responsibility for existing private sewers to the water companies. This will mean a very large increase in the length of public sewers and presumably a corresponding increase in the flooding from public sewers. How water companies and OFWAT will respond to this in terms of flooding is not yet known.

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Copyright, 2011, Morton-Roberts Consulting Engineers Limited.
Last Modified 17 January 2011.