The Mayor's draft water strategy - london.gov.uk - Greater London ...

More documents

Recommendations

Info

4 Managing rainwater 4.1 This chapter is concerned with the drainage of rainwater away from homes and businesses in London. Following this, Chapter 5 examines the removal of wastewater. In large parts of inner London, a combined drainage network (called the combined sewer) takes both rainwater and wastewater away from buildings. The problems associated with the combined system are considered in Chapter 5. 4.2 Rainwater is either lost through evaporation, seeps into the ground to replenish groundwater levels, flows over the ground and returns to streams and rivers, or enters the drainage systems and then flows on to a sewage treatment works. In the parts of London not covered by the combined sewer, the surface water drains carry rainwater from pavements, road surfaces and rooftops into local rivers and streams (see Figure 2.9). 4.3 Over the past 40 years, the intensity of rainfall has increased during the winter months across the country. The occurrence of more than 15 millimetres of rain on a day (referred to as heavy rainfall days) has become more frequent in winter. Since 1989, heavy rainfall days have consistently contributed more than ten per cent of the seasonal total rainfall; this was not always the case prior to 1989. Heavy rainfall can quickly overload the drainage system, as well as carry debris and pollutants from paved areas, such as roads and car parks, and from gardens into otherwise clean rivers, stream and ponds. 4.4 In 2007 a submission to the Shadow Cabinet entitled A Blueprint for a Green Economy 60 notes that we need ‘Slow Water’. Slowing water down through the use of green roofs and sustainable urban drainage systems filters and removes pollutants. The Draft Flood and Water Management Bill now proposes significant changes to the way in which we manage surface water. Policy 2 – Drainage in London The Mayor proposes the following hierarchy for the drainage of rainwater: 1 Store rainwater for use later 2 Use porous surfaces to let rainwater to soak into the ground where soil conditions allow 3 Slow the runoff by directing rainwater into ponds or open water features for gradual release to a watercourse 4 Slow the runoff by directing rainwater into tanks or sealed water features for gradual release to a watercourse 5 Discharge rainwater direct to a watercourse 6 Discharge rainwater to a surface water drain 7 Discharge rainwater to the combined sewer, as a last resort. 4.5 The flood risk principles (from LIFE Handbook 61 ) provide a useful guide to understanding where application of Policy 2 is important, so as not to increase the risk of fluvial flooding from surface water runoff: • upper catchment – ‘let rain slow’ • middle catchment – ‘let rivers flow’ • lower catchment – ‘let tides go’
60 The Mayor’s draft water strategy The Environment Agency’s Thames Catchment Flood Management Plan supports this as it emphasises the importance of surface water management and greater attenuation in a number of London river catchments that are susceptible to rapid flooding from thunderstorms, for example, the Rivers Ingrebourne and Ravensbourne. Given the short lead-in times on a number of the London rivers emergency response and flood awareness are also particularly important. Rainwater use 4.6 Solving the problem of surface water flooding by enlarging the drainage system alone, even if technically feasible, would be prohibitively expensive. Using rainwater before it goes down the drain can help to relieve the pressures on the drainage system. Instead of using water from the mains, householders could use rainwater for toilet flushing, clothes washing and outdoor uses. Figure 3.1 shows that these uses account for over a third of all water used within a house. Correctly collected and stored, rainwater can meet all these requirements with little treatment. The appliances should still have a connection to the public mains supply to guarantee that water is available even at times of low rainfall. 4.7 The treatment of rainwater will depend on the source of rainwater and its intended use. For instance, rainwater collected from clean surfaces and filtered to remove nutrients is likely to be low in harmful bacteria and,
Page 1 and 2:
August 2009 The Mayor’s draft wat
Page 3 and 4:
2 The Mayor’s draft water strateg
Page 5 and 6:
5 Disposal of wastewater in London
Page 7 and 8:
6 The Mayor’s draft water strateg
Page 9 and 10: 8 The Mayor’s draft water strateg
Page 11 and 12: 10 The Mayor’s draft water strate
Page 59: 58 The Mayor’s draft water strate
Page 71 and 72: 70 Figure 5.1 The London sewer syst
Page 73 and 74: 72 periods the sewers quickly fill
Page 99: 98 The Mayor’s draft water strate
show all

The Mayor's draft water strategy - london.gov.uk - Greater London ...

Create successful ePaper yourself

Delete template?

Save as template?