Chapter 2 - 25 2.0 The London Context Derbyshire Street Pocket Park SuDS In London: A Design Guide Chapter 2 - 26 2.1 What is unique about London? This chapter explains some of the conditions Like most UK cities, much of London’s As London develops and grows, its public which are particular to London, although some drainage infrastructure consists of piped realm needs to work much harder. Not only will will be found in other metropolitan areas. Some networks. Climate change, population increase it be more intensively-used, it will also need to are unique, others less so, but all will influence and densification will all contribute to surface fulfil multiple demands. Well-designed SuDS the integration of SuDS into the public realm. water run-off and increase the pressure on interventions can make a major contribution the system. If our drainage network is not to and can help reduce flood risk, improve water London is by far the UK’s biggest urban exceed capacity or need total replacement quality, improve/create a sense of place and area, occupying an area four times that at significant cost and disruption, then a contribute to the amenity value of the urban of Birmingham, and is experiencing an long-term approach to surface water runoff realm. This guidance shows how this can be intensification of use and development. As management is needed. The Thames Tideway done. mentioned in chapter 1, the Capital sits within Tunnel at a strategic level is addressing some the Thames River Basin and contributes the of these issues, particularly in relation to events largest share to the 17% of the Basin’s area of intense rainfall, but more local interventions, which is urbanised. Urbanisation produces such as SuDS, will be needed to more significant surface water run-off and has effectively manage the process. historically resulted in the modification of many of the watercourses in order to collect and The opportunities for SuDS, however, will vary move water rapidly away from built up areas. substantially depending on the context, both The approach to surface water management above and below ground. For instance, in afforded by SuDS can have a significant conservation areas designated for their special influence in this respect. landscape, architectural and historic interest, there may be more limitations than in an area Hand in hand with urbanisation has come of redevelopment, where a comprehensive population growth. London’s population and integrated approach to water resource exceeded its pre-war peak of 8.6 million in management will be more achievable. 2015 and is forecast to grow by 100,000 per Other factors, which are highlighted in this year. Much of this growth is expected to be chapter, include archaeology and geology, accommodated in the existing built-up area, both of which can define the scope and putting significant and increasing pressure on appropriateness of the scheme and where, the available water and drainage infrastructure with the former, there is over 2,000 years of in the Capital. history in the heart of the city. Chapter 2 - 27 2.2 London’s geological conditions Greater London sits in the London Basin, a General maps of the London Basin’s geology Understanding the geological condition of geological depression that runs approximately can be found on the British Geological the ground is vital to the implementation of 160 miles from the south east coast of Society’s website at http://www.bgs.ac.uk. SuDS features, as different ground conditions England in a roughly triangular shape west These show a relatively simple picture of will dictate how SuDS will interact with their to Marlborough. It is made up of layers of the London Basin’s geology, however, its local environment. This should be gathered deposits of chalk, clays, sand, and gravel. structure is actually complex due to the early as baseline information on geotechnical processes involved in their formation. Some properties such as permeability, porosity, of London’s geological formations, which are soakage. The British Geological Survey (BGS) not well mapped, may present risks such as offers a wide range services that can provide compressible deposits, collapsible deposits, useful preliminary information. A geotechnical shrink-swell clays, running sand, soluble rocks survey will confirm site specific geology. and landslides. Table from the British Geological Survey Memoir Geology of London, 2004 Era Group Formation Thickness (m) Palaeogene Thames Bagshot Formation 10-25 Claygate Member London Clay 30-90 Harwich Formation 0-10 Lambeth Woolwich and Reading Beds 10-20 Upnor Formation 5-7 Thanet Sands 0-30 Cretaceous Chalk 180-245 Geology of the London Basin SuDS In London: A Design Guide Chapter 2 - 28 Further information Finchley Ridge CIRIA The SuDS Manual C753 Chapter 29 Lea Valley Barnet Plateau British Geological Survey: http://www.bgs. Finchley Ridge Essex Plateau ac.uk Essex Plateau Ruislip Plateau Colne Roding Valley Hampstead Ridge Valley Barnet Plateau Geology of London. (2012). Royse et al. Barnet Plateau Essex Plateau Lea Valley North Thames Terraces North Thames Terraces Engineering Geology of British Rocks and Soils Hampstead Ridge North Thames Terraces – Lambeth Group Brent Valley Colne Valley Lower Thames Floodplain Hayes Gravels Hayes Gravels Lower Thames Floodplain http://news.bbc.co.uk/local/london/hi/people_ and_places/nature/newsid_8088000/8088779. Upper Thames stm South London Pebbly Sands Hounslow Gravels Wandle Valley South London South Thames Clays and Gravels Ravensbourne Heaths and Commons River Valley River Upper Thames Cray Management of the London Basin Chalk Valley Aquifer: Status Report 2015. Environment South London Pebbly Sands South London Agency: https://www.gov.uk/government/ Clays and Gravels uploads/system/uploads/attachment_data/ Lower North London file/429468/2015_London_GWL_Report_ Downs Dip Slope Lower North London Lower North London online.pdf Downs Dip Slope Downs Dip Slope Upper North London Chalk soils Downs Dip Slope Upper North London Downs Dip Slope Gravel and Sandy hilltop Clays Loams Low level Gravels Flood plain soils The geology of London, All London Green Grid, GiGL Chapter 2 - 29 2.3 London’s chalk aquifer Beneath London is a major aquifer called the The designer should take account of the Chalk aquifer. This was substantially depleted groundwater because: during the 19th and 20th centuries due to extraction by industrial activities. The removal • In areas with a high levels of groundwater, of ground water over many years resulted in water can enter the SuDS component and the aquifer being depleted to 88 metres below reduce the storage capacity sea level. However, in the last 60 years, as • There is a risk of flotation and increased industrial activities relocated away from central loads imposed by groundwater London, the Chalk aquifer has started to • High levels of groundwater can reduce the rebound by as much a three metres per year. infiltration rate of SuDS features • Groundwater can change the stability of Some geology in London is susceptible underground structures and foundations to shrink-swell movement caused by the presence or absence of water. This can have a Further information devastating effect on underground structures CIRIA The SuDS Manual, Chapter 26 and foundations. To protect London’s infrastructure from rebounding groundwater Management of the London Basin Chalk levels, the General Aquifer Research Aquifer: Status Report 2015. Environment Development and Investigation Team (GARDIT) Agency was established. Since 1992, GARDIT has licensed the removal of groundwater with the aim of controlling and eventually stabilising the rise in groundwater levels. Details of this work can be found in the London abstraction licensing strategy. London clay SuDS In London: A Design Guide Chapter 2 - 30 2.4 London’s soils Urbanisation has significantly altered London’s SuDS design should specify the procurement Testing schedules should include parameters conditions, adding another level of complexity of imported soils, if required, and soil from the groups listed below (as appropriate): to the local context. Even so, there is great management is fundamental to the successful scope to optimise the use of soils within functioning of SuDS components. There are • Geotechnical; permeability; bulk density; SuDS. Consideration needs to be given to several British Standard guidance documents porosity; plastic/liquid limit; shear strength; the availability and properties of existing soils, available that are commonly referenced California bearing ratio the surrounding ground and the requirements for landscape specifications. While these • Potential contaminants; heavy metals; for imported soils. Soil properties typically standards provide useful guidance on testing hydrocarbons; asbestos influence: and sampling soils, they should be used with • Horticultural; soil texture; pH value; fertility caution in relation to their application to SuDS status; salinity, phytotoxic (toxic to plants) • Water quantity: The physical properties schemes. The documents should not be used elements for SuDS schemes with planting of soil affects the attenuation capacity as for grading, classification or standardisation of they dictate its drainage and water-holding topsoil or subsoil already present on site. properties. In this capacity, soil is an Further information important element of bioretention schemes Soil specification should not be a ‘cut and CIRIA C753 The SuDS Manual, Chapter 29 to slow water runoff paste’ exercise from guidance documents