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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
which are particular to London, although some
will be found in other metropolitan areas. Some
are unique, others less so, but all will influence
the integration of SuDS into the public realm.
London is by far the UK’s biggest urban
area, occupying an area four times that
of Birmingham, and is experiencing an
intensification of use and development. As
mentioned in chapter 1, the Capital sits within
the Thames River Basin and contributes the
largest share to the 17% of the Basin’s area
which is urbanised. Urbanisation produces
significant surface water run-off and has
historically resulted in the modification of many
of the watercourses in order to collect and
move water rapidly away from built up areas.
The approach to surface water management
afforded by SuDS can have a significant
influence in this respect.
Hand in hand with urbanisation has come
population growth. London’s population
exceeded its pre-war peak of 8.6 million in
2015 and is forecast to grow by 100,000 per
year. Much of this growth is expected to be
accommodated in the existing built-up area,
putting significant and increasing pressure on
the available water and drainage infrastructure
in the Capital.
Like most UK cities, much of London’s
drainage infrastructure consists of piped
networks. Climate change, population increase
and densification will all contribute to surface
water run-off and increase the pressure on
the system. If our drainage network is not to
exceed capacity or need total replacement
at significant cost and disruption, then a
long-term approach to surface water runoff
management is needed. The Thames Tideway
Tunnel at a strategic level is addressing some
of these issues, particularly in relation to events
of intense rainfall, but more local interventions,
such as SuDS, will be needed to more
effectively manage the process.
The opportunities for SuDS, however, will vary
substantially depending on the context, both
above and below ground. For instance, in
conservation areas designated for their special
landscape, architectural and historic interest,
there may be more limitations than in an area
of redevelopment, where a comprehensive
and integrated approach to water resource
management will be more achievable.
Other factors, which are highlighted in this
chapter, include archaeology and geology,
both of which can define the scope and
appropriateness of the scheme and where,
with the former, there is over 2,000 years of
history in the heart of the city.
As London develops and grows, its public
realm needs to work much harder. Not only will
it be more intensively-used, it will also need to
fulfil multiple demands. Well-designed SuDS
interventions can make a major contribution
and can help reduce flood risk, improve water
quality, improve/create a sense of place and
contribute to the amenity value of the urban
realm. This guidance shows how this can be
done.
Chapter 2 - 27
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
2.2 London’s geological conditions
Greater London sits in the London Basin, a
geological depression that runs approximately
160 miles from the south east coast of
England in a roughly triangular shape west
to Marlborough. It is made up of layers of
deposits of chalk, clays, sand, and gravel.
General maps of the London Basin’s geology
can be found on the British Geological
Society’s website at http://www.bgs.ac.uk.
These show a relatively simple picture of
the London Basin’s geology, however, its
structure is actually complex due to the
processes involved in their formation. Some
of London’s geological formations, which are
not well mapped, may present risks such as
compressible deposits, collapsible deposits,
shrink-swell clays, running sand, soluble rocks
and landslides.
Understanding the geological condition of
the ground is vital to the implementation of
SuDS features, as different ground conditions
will dictate how SuDS will interact with their
local environment. This should be gathered
early as baseline information on geotechnical
properties such as permeability, porosity,
soakage. The British Geological Survey (BGS)
offers a wide range services that can provide
useful preliminary information. A geotechnical
survey will confirm site specific geology.
Geology of the London Basin
Table from the British Geological Survey Memoir Geology of London, 2004
SuDS In London: A Design Guide Chapter 2 - 28
Further information
CIRIA The SuDS Manual C753 Chapter 29
British Geological Survey: http://www.bgs.
ac.uk
Geology of London. (2012). Royse et al.
Engineering Geology of British Rocks and Soils
– Lambeth Group
http://news.bbc.co.uk/local/london/hi/people_
and_places/nature/newsid_8088000/8088779.
stm
Management of the London Basin Chalk
Aquifer: Status Report 2015. Environment
Agency: https://www.gov.uk/government/
uploads/system/uploads/attachment_data/
file/429468/2015_London_GWL_Report_
online.pdf
Hampstead Ridge
Hampstead Ridge
Lea Valley
Lea Valley
Finchley Ridge
Finchley Ridge
Barnet Plateau
Barnet Plateau
Barnet Plateau
Ruislip Plateau
Essex Plateau
Essex Plateau
Essex Plateau
ColneValley
ColneValley
Brent Valley
Hounslow Gravels
Hayes GravelsHayes Gravels
South LondonClays and Gravels
South LondonClays and Gravels
North Thames Terraces
North Thames Terraces
North Thames Terraces
Lower Thames Floodplain
Lower Thames Floodplain
RodingValley
WandleValley
RavensbourneRiver Valley
South London Pebbly Sands
South London Pebbly Sands
Lower North LondonDowns Dip Slope
Lower North LondonDowns Dip Slope
Lower North LondonDowns Dip Slope
Upper North LondonDowns Dip Slope
Upper North LondonDowns Dip Slope
South ThamesHeaths and Commons
Upper Thames
Upper ThamesRiverCrayValley
The geology of London, All London Green Grid, GiGL
Chalk soils
Gravel and Sandy hilltop
Clays
Loams
Low level Gravels
Flood plain soils
Chapter 2 - 29
2.3 London’s chalk aquifer
Beneath London is a major aquifer called the
Chalk aquifer. This was substantially depleted
during the 19th and 20th centuries due to
extraction by industrial activities. The removal
of ground water over many years resulted in
the aquifer being depleted to 88 metres below
sea level. However, in the last 60 years, as
industrial activities relocated away from central
London, the Chalk aquifer has started to
rebound by as much a three metres per year.
Some geology in London is susceptible
to shrink-swell movement caused by the
presence or absence of water. This can have a
devastating effect on underground structures
and foundations. To protect London’s
infrastructure from rebounding groundwater
levels, the General Aquifer Research
Development and Investigation Team (GARDIT)
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.
The designer should take account of the
groundwater because:
• In areas with a high levels of groundwater,
water can enter the SuDS component and
reduce the storage capacity
• There is a risk of flotation and increased
loads imposed by groundwater
• High levels of groundwater can reduce the
infiltration rate of SuDS features
• Groundwater can change the stability of
underground structures and foundations
Further informationCIRIA The SuDS Manual, Chapter 26
Management of the London Basin Chalk
Aquifer: Status Report 2015. Environment
Agency
London clay
SuDS In London: A Design Guide Chapter 2 - 30
2.4 London’s soils
Urbanisation has significantly altered London’s
conditions, adding another level of complexity
to the local context. Even so, there is great
scope to optimise the use of soils within
SuDS. Consideration needs to be given to
the availability and properties of existing soils,
the surrounding ground and the requirements
for imported soils. Soil properties typically
influence:
• Water quantity: The physical properties
of soil affects the attenuation capacity as
they dictate its drainage and water-holding
properties. In this capacity, soil is an
important element of bioretention schemes
to slow water runoff
• Water quality: The filtration capacity of soils
influence water quality by, for example,
affecting the amount of elements such as
nutrients or contaminants, taken up by the
soil or dissolved into the water or the input
of sediment into the water from the soil
• Amenity/biodiversity: The nature and
availability of soil affects plant species
selection. Many soil properties affect the
biodiversity of a soft landscape SuDS
scheme, as different planting types have
varying demands such as nutrient status or
pH value.
SuDS design should specify the procurement
of imported soils, if required, and soil
management is fundamental to the successful
functioning of SuDS components. There are
several British Standard guidance documents
available that are commonly referenced
for landscape specifications. While these
standards provide useful guidance on testing
and sampling soils, they should be used with
caution in relation to their application to SuDS
schemes. The documents should not be used
for grading, classification or standardisation of
topsoil or subsoil already present on site.
Soil specification should not be a ‘cut and
paste’ exercise from guidance documents
or previous projects. It should be bespoke to
the project in hand. A suitably experienced
soil consultant, engineer and environmental
consultant should be sought early in the design
process.
There is an increasing demand for soils in new
schemes to be ‘validated’ for contamination.
The inclusion of soil procurement clauses
and appropriate testing schedules are vital
to ensure a robust soil specification. Soil
procurement programming is frequently
underestimated or missed from contractor’s
work schedules. Setting out the requirements
within the specification reduces such risks.
Testing schedules should include parameters
from the groups listed below (as appropriate):
• Geotechnical; permeability; bulk density;
porosity; plastic/liquid limit; shear strength;
California bearing ratio
• Potential contaminants; heavy metals;
hydrocarbons; asbestos
• Horticultural; soil texture; pH value; fertility
status; salinity, phytotoxic (toxic to plants)
elements for SuDS schemes with planting
Further informationCIRIA C753 The SuDS Manual, Chapter 29
BS3882:2015 Specification for Topsoil
BS8601:2013 Specification for Subsoil and
Requirements for Use
Chapter 2 - 31
Strategic
Concept
Outline Design
Detailed Design
Consider key soil properties:
- Geotechnical
- Horticultural
- Environmental
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
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
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
- 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
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
levels of airborne particles from exhausts,
fuels, oils and other engine fluids directly
spilled on surfaces. This is especially so
during warm, dry seasons when pollutants
can build up on sun-warmed tarmac surfaces.
Heavy showers wash the pollutants off the
surfaces and surface water absorbs the heat
and pollutants. This creates a contaminated,
low-oxygen water mix, which discharges into
watercourses and groundwater.
Despite the requirements of water quantity
management, water pollution is often at
its worst during smaller, everyday rainfalls;
generally heavier downpours dilute the
pollutants.
In London, contaminated soils and
groundwater are likely to be found when
installing SuDS components, as there are
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
not be constrained. A geo-environmental
professional should be appointed early to
identify contamination risks and sources so
an integrated remediation strategy can be
implemented. The designers should consider:
• The use of infiltration may be prohibited
due to the risk of mobilising contamination
• Risk of contamination entering SuDS
features and contaminating relatively clean
rain water runoff, which could potentially
have adverse effects on vegetation and
materials used within SuDS components
• Excavation and disposal of contaminated
soils is likely to be expensive
• SuDS should not compromise remediation
systems in place to protect users from the
contamination
Further informationCIRIA C753 The SuDS Manual, Chapter 26
Anthropogenic London soil
Chapter 2 - 33
London’s streetscape varies considerably
across the city. This is a reflection both of its
size and the history of its development. It also
derives from the considerable variety of land
uses to be found in London.
Streets are often the most resilient feature of
the urban fabric. While street patterns may
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.
2.6 Streetscapes of London
Regent Street: Street life on the strategic road network
Streetscape consists of natural and man-made
elements. Depending on individual conditions,
streetscape can have measurable effects
on footfall, local economic performance, air
quality, public health and sense of place. As
a streetscape feature, SuDS can contribute
positively to all of these, provided that the
design is appropriate for the context.
The relationship between streetscape and
SuDS elements is examined in more detail in
Chapter 4.
SuDS In London: A Design Guide Chapter 2 - 34
2.7 Townscape
Townscape is the mix of physical and social
characteristics that make up the urban
environment. This includes its buildings,
landscapes, and the way those characteristics
are perceived. Townscape directly contributes
to people’s sense of place and identity.
London has a complex townscape that
reflects its rich and diverse history, culture
and built form. London’s Roman origins are
still visible in the City’s street pattern. Further
waves of expansion were created by trade,
population growth, industrialisation and
transport infrastructure. Having absorbed
formerly separate towns and villages, London’s
character is inherently polycentric, with its
many separate centres each having their own
identities.
London’s history and character is also reflected
in its streetscape. Paving, materials, pillar
boxes, street furniture, stone drinking troughs,
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.
Future SuDS interventions need to
progressively complement and enhance the
townscape and become a fundamental part of
the character of London.
Historic England has produced a useful
guide called ‘Streets for All: A guide to the
management of London’s street’ which reviews
many of these assets.
When working on London’s streets there are a
number of statutory consultees that need to be
engaged. A list is contained within appendix A.
Further information:Historic England. (2000). Streets for All: A
guide to the management of London’s streets.
Historic England. London, UK
Jones, E. and Woodward, C. (1992). A Guide
to the Architecture of London: New
Edition. Weidenfeld & Nicolson, London.
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
world with 47 per cent green space and 22 per
cent tree canopy cover. The green (and blue)
infrastructure of the capital, which includes the
River Thames and all its tributaries, defines
the character of the city and is embedded in
London Plan policy as the All London Green
Grid (ALGG).
London’s green infrastructure is made up of a
network of green spaces, that are ‘planned,
designed and managed to deliver a range of
benefits, including; healthy living; mitigating
flooding; improving air and water quality;
cooling the urban environment; encouraging
walking and cycling; and enhancing biodiversity
and ecological resilience’ (GLA, 2015). Green
infrastructure includes commons, parks,
gardens, fields, street trees, woodlands, green
roofs, green walls and water bodies.
Existing green infrastructure provides many
environmental, economic and social benefits
such as:
• intercepting rainfall
• providing attenuation
• maintaining soil permeability
• reducing urban heat island effect
• improving air quality
• providing amenity space
• enhancing property values
• creating a sense of place
Many of these benefits are the ultimate aim of
SuDS interventions which is why it is vital to
protect London’s existing green infrastructure
when designing SuDS for London.
London’s green infrastructure is not
fundamentally different from other urban
contexts within the UK, except the Capital
is larger. It is London’s unique geological
conditions, landscapes, townscapes and
below ground infrastructure that tends to play
a major role in shaping what is achievable with
SuDS interventions.
London benefits from a legacy of Victorian tree
planting that contributes significantly to the
canopy cover of the Capital and interception of
rainfall. These trees were established in much
more favourable, less engineered, conditions
than today’s high-performing pavements. Early
collaboration is therefore needed to ensure
both soft and hard engineering components
for SuDS are seamlessly integrated in today’s
public realm.
Further informationGreater London Authority. (2015). Natural
Capital: Investing in a Green Infrastructure for
a Future London. Green Infrastructure Task
Force. London, UK.
Landscape Institute. (2013). Green
Infrastructure: An integrated approach to land
use. Position Statement. London, UK.
Treeconomics London. (2015). Valuing
London’s Urban Forest: Results of the London
i-Tree Eco Project. London, UK: https://www.
london.gov.uk/sites/default/files/valuing_
londons_urban_forest_i-tree_report_final.pdf
All London Green Grid SPG 2012: https://
www.london.gov.uk/what-we-do/environment/
parks-green-spaces-and-biodiversity/all-
london-green-grid
Chapter 2 - 37
2.9 Archaeology
London’s infrastructure is growing fast,
with major below and above ground works
ongoing. London’s history covers millennia
of settlement, represented as layers of
significant archaeology that are discovered as
excavations occur.
When working in Greater London, it is
important to contact Historic England’s
Greater London Archaeology Advisory Service
(GLAAS) – or in the case of Southwark
or the City of London their own borough
archaeology officers – as early as possible
to determine what policies and consent
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.
Further information:Historic England. (2015). Guidelines for
Archaeological Projects in Greater London.
Greater London Archaeological Advisory
Service. London, UK.
Communities and Local Government. (1990).
Planning Policy Guidance 16: Archaeology and
Planning. UK.
National Planning Policy Framework,
Department for Communities and Local
Government, March 2012
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?
The implementation of works which affect
infrastructure below ground level are subject
to the New Roads and Street Works Act
1991, which sets out a code of practice for
the coordination of works. Under the Traffic
Management Act 2004, traffic authorities must
ensure road networks are managed effectively
to minimise congestion and disruption to
vehicles and pedestrians. They must also
plan and coordinate roadworks, taking into
consideration the impact on neighbouring
traffic authorities.
It is TfL’s responsibility to facilitate cooperation
between all of London’s traffic authorities
and utilities to improve conditions for all road
users. Each authority, however, decides how
and when work can be carried out on their
highway. Permits from the local street works
department for any planned works may
need to be applied for up to three months in
advance.
When working on the TLRN or on any borough
roads, see below for further information on
what road works are taking place, the code of
conduct, lane rental scheme, highway licences
and permitting:
https://tfl.gov.uk/info-for/urban-planning-and-
construction/roadworks-and-street-faults
https://tfl.gov.uk/info-for/urban-planning-
and-construction/highway-licences/traffic-
management-act
Each highway authority will have their own
restrictions (such as working hours, noisy
working etc), so it is advisable to visit their
individual websites.
If works are being planned near any of the
Underground or rail systems, refer to the
following website: https://tfl.gov.uk/info-for/
urban-planning-and-construction/urban-
planning-and-construction-contacts
If proposed measures affect apparatus already
within the highway, there is a legal duty to
mitigate the effect of the proposals. Common
measures include moving ducts to create clear
ground to build on, or moving pipes away from
the affected area. This can form a substantial
proportion of the overall costs, so must be
considered early.
As part of a feasibility study, it is advisable to
obtain high quality Ground Penetrating Radar
(GPR) surveys to identify services and avoid
abortive works later in the project.
2.12 Inclusive design
Any SuDS measure which influences the
public realm should be inclusively designed.
Each place must cater to the needs of all and
not restrict its use by any group or individuals.
The design process must consider the needs
of people under the Equality Act 2010.
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
secure environments, as outlined in s17
of the Crime and Disorder Act 1998. TfL’s
Transport Community Safety Managers
located in the Enforcement & On Street
Operations Directorate (EOS) provides advice
to design teams on meeting their duties
under the Act.
During design development it is advisable
to make early contact with a police Crime
Prevention Design Advisor (CPDA) to
understand existing crime patterns early on
in the design process and ensure risks are
mitigated. If police input is difficult to obtain,
the specialist team in EOS can help. Ensure
that routes designed exclusively for non-
motorised users are also well-lit, overlooked
(preferably by active frontages), well
signposted, well-connected, direct and wide
enough to avoid blind corners and ‘pinch
points’.
London’s streets are complex and layered
over time, above and below ground. Site-
specific constraints are one of the biggest
design drivers. Baseline data is vital, possibly
requiring surveys and investigations that
include, but are not limited to:
• Detailed topographical survey to show
the accurate position of source, pathways
and receptors
• Traffic and pedestrian movement surveys
• Survey through observation and
consultation of the existing use of space
(throughout the day/seasons); priorities
and hierarchy of uses
• Utilities mapping information to
understand the below ground constraints,
such as buried utilities, potential voids or
obstructions
• Below ground structures related to
London Underground, plus cellars and
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
• Intrusive investigations to provide
certainty as to the presence, line and level
of utilities, apparatus, existing pavement
build-ups, CBR and permeability of
subbase (infiltration testing to BRE 365)
• Soakage tests and soil surveys
including geotechnical, horticultural and
environmental characteristics
Further informationReference CIRIA The SuDS Manual, Chapter
7.3
Chapter 2 - 41
On site soil investigation