2.0 the london context · 2016-08-19 · 2.2 london’s geological conditions greater london sits...

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


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



Lower Thames Floodplain

Lower Thames Floodplain

RodingValley

WandleValley

RavensbourneRiver Valley

South London Pebbly Sands

South London Pebbly Sands

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


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


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.


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


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


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.


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

2.0 the london context · 2016-08-19 · 2.2 london’s geological conditions greater london sits...

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