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 • Water quality: The filtration capacity of soils or previous projects. It should be bespoke to BS3882:2015 Specification for Topsoil influence water quality by, for example, the project in hand. A suitably experienced affecting the amount of elements such as soil consultant, engineer and environmental BS8601:2013 Specification for Subsoil and nutrients or contaminants, taken up by the consultant should be sought early in the design Requirements for Use soil or dissolved into the water or the input process. of sediment into the water from the soil • Amenity/biodiversity: The nature and There is an increasing demand for soils in new availability of soil affects plant species schemes to be ‘validated’ for contamination. selection. Many soil properties affect the The inclusion of soil procurement clauses biodiversity of a soft landscape SuDS and appropriate testing schedules are vital scheme, as different planting types have to ensure a robust soil specification. Soil varying demands such as nutrient status or procurement programming is frequently pH value. underestimated or missed from contractor’s work schedules. Setting out the requirements within the specification reduces such risks. Chapter 2 - 31

Soils and the design process Suitably qualified soil scientists, engineers and environmental consultants should be appointed at the feasibility stage to inform the design process

Consider key soil properties: - Geotechnical - Horticultural - Environmental Strategic Baseline investigation: - Assessment of existing ground conditions - Tests - Presence of underground services

Consider soil requirements for the scheme: - Are existing soils available and do they have potential Concept for re-use? - Are imported soils necessary? - Is a load bearing system required? - What landscape types are desirable / feasible?

Produce a soil strategy: - Utilising the site’s existing soils (if available) - The soil requirements of the scheme; including number of soils types required Outline Design - In soft landscape; the soil requirements of each planting type and species - Imported soil and drainage media requirements - Requirements and selection of load bearing systems in hard landsape - Management / maintenance requirements

Detailed Design Construction requirements; - Produce a detailed specification

Soils and the design process SuDS In London: A Design Guide Chapter 2 - 32

2.5 Contamination

Runoff from London’s streets contains varying not be constrained. A geo-environmental levels of airborne particles from exhausts, professional should be appointed early to fuels, oils and other engine fluids directly identify contamination risks and sources so spilled on surfaces. This is especially so an integrated remediation strategy can be during warm, dry seasons when pollutants implemented. The designers should consider: can build up on sun-warmed tarmac surfaces. Heavy showers wash the pollutants off the • The use of infiltration may be prohibited surfaces and surface water absorbs the heat due to the risk of mobilising contamination and pollutants. This creates a contaminated, • Risk of contamination entering SuDS low-oxygen water mix, which discharges into features and contaminating relatively clean watercourses and groundwater. rain water runoff, which could potentially have adverse effects on vegetation and Despite the requirements of water quantity materials used within SuDS components management, water pollution is often at • Excavation and disposal of contaminated its worst during smaller, everyday rainfalls; soils is likely to be expensive generally heavier downpours dilute the • SuDS should not compromise remediation pollutants. systems in place to protect users from the contamination In London, contaminated soils and groundwater are likely to be found when Further information installing SuDS components, as there are CIRIA C753 The SuDS Manual, Chapter 26 few places that have never been subjected to development or industrial activity.

However, through careful planning, communication, risk assessment and design, the implementation of SuDS should

Anthropogenic London soil Chapter 2 - 33

2.6 Streetscapes of London

London’s streetscape varies considerably Streetscape consists of natural and man-made The relationship between streetscape and across the city. This is a reflection both of its elements. Depending on individual conditions, SuDS elements is examined in more detail in size and the history of its development. It also streetscape can have measurable effects Chapter 4. derives from the considerable variety of land on footfall, local economic performance, air uses to be found in London. quality, public health and sense of place. As a streetscape feature, SuDS can contribute Streets are often the most resilient feature of positively to all of these, provided that the the urban fabric. While street patterns may design is appropriate for the context. remain unchanged for centuries, streetscapes can evolve and respond to new requirements, such as SuDS. However, in central London and largely due its historic development, many of the streets and spaces are relatively constrained and incorporating SuDs would be a significant challenge. The opportunities would tend to occur within building plots, especially where there are setbacks, where historical remnants exist and in open spaces which are scattered throughout central London. In outer London, the opportunities will tend to be greater.

Regent Street: Street life on the strategic road network SuDS In London: A Design Guide Chapter 2 - 34

2.7 Townscape

Townscape is the mix of physical and social Future SuDS interventions need to characteristics that make up the urban progressively complement and enhance the environment. This includes its buildings, townscape and become a fundamental part of landscapes, and the way those characteristics the character of London. are perceived. Townscape directly contributes to people’s sense of place and identity. Historic England has produced a useful guide called ‘Streets for All: A guide to the London has a complex townscape that management of London’s street’ which reviews reflects its rich and diverse history, culture many of these assets. and built form. London’s Roman origins are still visible in the City’s street pattern. Further When working on London’s streets there are a waves of expansion were created by trade, number of statutory consultees that need to be population growth, industrialisation and engaged. A list is contained within appendix A. transport infrastructure. Having absorbed formerly separate towns and villages, London’s Further information: character is inherently polycentric, with its Historic England. (2000). Streets for All: A many separate centres each having their own guide to the management of London’s streets. identities. Historic England. London, UK

London’s history and character is also reflected Jones, E. and Woodward, C. (1992). A Guide in its streetscape. Paving, materials, pillar to the Architecture of London: New boxes, street furniture, stone drinking troughs, Edition. Weidenfeld & Nicolson, London. telephone boxes, sculpture, memorials and other heritage assets all contribute to a strong sense of place. This is enhanced by the Capital’s green and blue infrastructure; its many parks, squares and gardens, the canals, the River Thames and its many tributaries.

Borough Road: Victorian street tree planting Chapter 2 - 35

St James’s Park: Parks provide natural storage, attenuation, infiltration, interception SuDS In London: A Design Guide Chapter 2 - 36

2.8 London’s Green Infrastructure

London is one of the greenest cities in the • providing amenity space Further information world with 47 per cent green space and 22 per • enhancing property values Greater London Authority. (2015). Natural cent tree canopy cover. The green (and blue) • creating a sense of place Capital: Investing in a Green Infrastructure for infrastructure of the capital, which includes the a Future London. Green Infrastructure Task River Thames and all its tributaries, defines Many of these benefits are the ultimate aim of Force. London, UK. the character of the city and is embedded in SuDS interventions which is why it is vital to London Plan policy as the All London Green protect London’s existing green infrastructure Landscape Institute. (2013). Green Grid (ALGG). when designing SuDS for London. Infrastructure: An integrated approach to land use. Position Statement. London, UK. London’s green infrastructure is made up of a London’s green infrastructure is not network of green spaces, that are ‘planned, fundamentally different from other urban Treeconomics London. (2015). Valuing designed and managed to deliver a range of contexts within the UK, except the Capital London’s Urban Forest: Results of the London benefits, including; healthy living; mitigating is larger. It is London’s unique geological i-Tree Eco Project. London, UK: https://www. flooding; improving air and water quality; conditions, landscapes, townscapes and london.gov.uk/sites/default/files/valuing_ cooling the urban environment; encouraging below ground infrastructure that tends to play londons_urban_forest_i-tree_report_final.pdf walking and cycling; and enhancing biodiversity a major role in shaping what is achievable with and ecological resilience’ (GLA, 2015). Green SuDS interventions. All London Green Grid SPG 2012: https:// infrastructure includes commons, parks, www.london.gov.uk/what-we-do/environment/ gardens, fields, street trees, woodlands, green London benefits from a legacy of Victorian tree parks-green-spaces-and-biodiversity/all- roofs, green walls and water bodies. planting that contributes significantly to the london-green-grid canopy cover of the Capital and interception of Existing green infrastructure provides many rainfall. These trees were established in much environmental, economic and social benefits more favourable, less engineered, conditions such as: than today’s high-performing pavements. Early collaboration is therefore needed to ensure • intercepting rainfall both soft and hard engineering components • providing attenuation for SuDS are seamlessly integrated in today’s • maintaining soil permeability public realm. • reducing urban heat island effect • improving air quality Chapter 2 - 37

2.9 Archaeology

London’s infrastructure is growing fast, Further information: with major below and above ground works Historic England. (2015). Guidelines for ongoing. London’s history covers millennia Archaeological Projects in Greater London. of settlement, represented as layers of significant archaeology that are discovered as Greater London Archaeological Advisory excavations occur. Service. London, UK.

When working in Greater London, it is Communities and Local Government. (1990). important to contact Historic England’s Planning Policy Guidance 16: Archaeology and Greater London Archaeology Advisory Service Planning. UK. (GLAAS) – or in the case of Southwark or the City of London their own borough National Planning Policy Framework, archaeology officers – as early as possible Department for Communities and Local to determine what policies and consent Government, March 2012 requirements are in place for the protection, enhancement and preservation of sites of archaeological interest and their settings, and designated archaeological priority areas. All local authorities maintain a record of their archaeological priority areas. Refer to Historic England’s ‘Guidelines for Archaeological Projects in Greater London’ or contact direction Historic England’s Archaeology Advisory Service for further information. SuDS In London: A Design Guide Chapter 2 - 38

2.10 Working with London’s utilities

Footway and carriageway space in London is limited and highly contested not just above ground by pedestrians, cyclists, and motor traffic, but also below ground by utilities that supply London’s gas electricity, water, sewerage and telecommunications. Much of this infrastructure, which was installed in the late 19th and early to mid-20th century, is aging, and poorly maintained and documented.

During feasibility and option appraisal stages of design, the SuDS design team should apply to each utility owner to provide existing information pertaining to their assets or associated assets. The design team is then responsible for validating the information and determining whether it is suitable for design purposes. Inspections and investigations should be carried out to help develop proposals. It can be expensive to divert utilities and in the interest of costs it would be prudent to avoid this. SuDS in the highway will need approval from the local authority, and there is an exclusion zone around structures such as sewers, where SuDS cannot be used.

Below ground infrastructure Chapter 2 - 39

2.11 Who do I contact? 2.12 Inclusive design

The implementation of works which affect https://tfl.gov.uk/info-for/urban-planning-and- Any SuDS measure which influences the infrastructure below ground level are subject construction/roadworks-and-street-faults public realm should be inclusively designed. to the New Roads and Street Works Act https://tfl.gov.uk/info-for/urban-planning- Each place must cater to the needs of all and 1991, which sets out a code of practice for and-construction/highway-licences/traffic- not restrict its use by any group or individuals. the coordination of works. Under the Traffic management-act The design process must consider the needs Management Act 2004, traffic authorities must of people under the Equality Act 2010. ensure road networks are managed effectively Each highway authority will have their own to minimise congestion and disruption to restrictions (such as working hours, noisy vehicles and pedestrians. They must also working etc), so it is advisable to visit their plan and coordinate roadworks, taking into individual websites. consideration the impact on neighbouring traffic authorities. If works are being planned near any of the Underground or rail systems, refer to the It is TfL’s responsibility to facilitate cooperation following website: https://tfl.gov.uk/info-for/ between all of London’s traffic authorities urban-planning-and-construction/urban- and utilities to improve conditions for all road planning-and-construction-contacts users. Each authority, however, decides how and when work can be carried out on their If proposed measures affect apparatus already highway. Permits from the local street works within the highway, there is a legal duty to department for any planned works may mitigate the effect of the proposals. Common need to be applied for up to three months in measures include moving ducts to create clear advance. ground to build on, or moving pipes away from the affected area. This can form a substantial When working on the TLRN or on any borough proportion of the overall costs, so must be roads, see below for further information on considered early. what road works are taking place, the code of conduct, lane rental scheme, highway licences As part of a feasibility study, it is advisable to and permitting: obtain high quality Ground Penetrating Radar (GPR) surveys to identify services and avoid abortive works later in the project. SuDS In London: A Design Guide Chapter 2 - 40

2.13 Crime and disorder 2.14 Bringing it all together

All design should seek to provide safe and London’s streets are complex and layered • Intrusive investigations to provide secure environments, as outlined in s17 over time, above and below ground. Site- certainty as to the presence, line and level of the Crime and Disorder Act 1998. TfL’s specific constraints are one of the biggest of utilities, apparatus, existing pavement Transport Community Safety Managers design drivers. Baseline data is vital, possibly build-ups, CBR and permeability of located in the Enforcement & On Street requiring surveys and investigations that subbase (infiltration testing to BRE 365) Operations Directorate (EOS) provides advice include, but are not limited to: • Soakage tests and soil surveys to design teams on meeting their duties including geotechnical, horticultural and under the Act. • Detailed topographical survey to show environmental characteristics the accurate position of source, pathways During design development it is advisable and receptors Further information to make early contact with a police Crime • Traffic and pedestrian movement surveys Reference CIRIA The SuDS Manual, Chapter Prevention Design Advisor (CPDA) to • Survey through observation and 7.3 understand existing crime patterns early on consultation of the existing use of space in the design process and ensure risks are (throughout the day/seasons); priorities mitigated. If police input is difficult to obtain, and hierarchy of uses the specialist team in EOS can help. Ensure • Utilities mapping information to that routes designed exclusively for non- understand the below ground constraints, motorised users are also well-lit, overlooked such as buried utilities, potential voids or (preferably by active frontages), well obstructions signposted, well-connected, direct and wide • Below ground structures related to enough to avoid blind corners and ‘pinch London Underground, plus cellars and points’. other chambers to understand the presence and extents of basement structures • Drainage CCTV survey to determine the line, level, size, type and condition of drainage runs and chambers Chapter 2 - 41

On site soil investigation