SuDS in London - a guide

7.1 Cost benefit
The Greater London Authority’s (GLA’s)
document ‘Natural Capital Investing in a
Green Infrastructure for London’ highlights
two key challenges:
1. Unseen value (usually expressed
environmentally or socially, rather
than monetised, typically regarded
as intangible)
2. A lack of a revenue-raising
mechanism to offset management
and maintenance.
The emphasis of the London Sustainable
Drainage Action Plan (LSDAP) has been to
‘identify opportunities for implementing
sustainable drainage techniques that
have limited financial impact’. This focuses
on situations where other works are
likely to be undertaken. Integrating SuDS
would therefore be a component of a
wider project.
The LSDAP also notes that options to
increase London’s drainage system capacity
using conventional underground piped
networks, such as the Thames Tideway
Tunnel (under construction), are becoming
increasingly complex and prohibitively
expensive. This is due to the requirement
for large-scale and widespread excavations
in many streets, and the need to work in
and around other buried infrastructure.
Green infrastructure and sustainable
drainage (as opposed to hard-engineered
techniques) have many benefits, such as
reducing air pollution, reducing noise,
improving biodiversity, reducing summer
urban heat island effects and creating
places with identity and character.
These benefits can be challenging to
monetise. Some evaluation tools use
an ecosystem services framework as
a starting point to convert benefit
to monetised outcomes. The City of
Philadelphia, for instance, has identified
the net benefit of using surface drainage
techniques at almost $3bn compared
to $100m for the piped alternative. The
$3bn includes benefits such as changes
to property value, green job creation,
reduction in greenhouse gas
emissions and reduced crime through
an improved environment.
Four cost benefit references are:
• CIRIA’s SuDS Tool (BeST) which
provides monetised values to tangible
and intangible benefits applicable
to the UK’s drainage systems
• i-Tree Eco, a system related to valuing
trees in terms of ecosystem services.
This has estimated, for instance, the
economic value of London’s urban forest
at almost 3.5million m 3 /annum of storm
water alleviation, worth £2.8m/annum
• TfL’s Valuing the Urban Realm
Toolkit. This has identified a positive,
significant and consistent relationship
between the quality of streetscape
and benefits for users and property
owners. For further information,
contact urbandesign@tfl.gov.uk
• The Government’s Natural Capital
Committee which has developed
an accounting framework. This
is currently being trialled.
132 7 Cost benefit