This document evaluates sustainable urban drainage systems (SUDS) in the Church Street public realm strategy, emphasizing their role in managing stormwater effectively while enhancing urban environments. It highlights the importance of integrating SUDS features such as green roofs, permeable pavements, and bioretention systems to mitigate flooding and support biodiversity, especially in the context of climate change. The paper outlines the challenges and criteria for effective SUDS implementation, including maintenance, water quality management, and the potential for reducing runoff into the sewer system.

1. Blue Infrastructure: critical Assessment of the
Sustainable urban Drainage Systems (SuDS) Component
of the Church Street Public Realm Strategy
Achim von Malotki

2. What are SuDS?
• SuDS are an alternative way of urban drainage to
collect, clean, store and release stormwater slowly to
the environment in as natural a manner and as close
to its sources as possible.
• They are designed to minimise the amount of
rainwater on urban land to enter canalisation.
• They attenuate the velocity of runoff and slow down
its eventual discharge.
• They provide amenity benefits to the local and wider
community.
• Provide habitats for wildlife in urban areas.
New understanding of stormwater as a resource
rather than as a waste product, nuisance or hazard.

3. Why SuDS? – compare the hydrographs with those before urbanization…
The covering of the land surface by impervious materials means that a
much larger proportion of any rainfall forms immediate runoff.

4. …and then in an urban environment with and without SuDS

5. Severity of rainwater runoff-problem in London
• Historically, surface water runoff has been combined with sewage
flows through a single, combined sewer.
• Challenge to reduce volume and velocity of surface water runoff
particularly acute. Surface water from rainfall puts significant
burden on wastewater treatment works, triggering untreated
sewage via combined sewer overflows (approx. 39 million tonnes
of storm sewage annually) into the Thames, causing pollution.
• Apart from mainly the area between the goods yard walls, flood
risk in Church Street is not particularly high as the area is has a
gentle north-south slope.
• Environment Agency target for new developments: essential
standard of 50% attenuation of the undeveloped site’s surface
water runoff at peak times.
• Mayor’s preferred standard is for 100% attenuation of the
undeveloped site’s surface water runoff at peak times.

6. The additional challenge of climate change
• Cities are increasingly vulnerable to flooding
because of changes in the precipitation patterns
caused by anthropogenic climate change.
• Climate projections for the UK suggest that by the
2080s average annual temperatures may increase
by between 1°C and 5°C.
• Extreme weather events, including intense rainfall
events and droughts will become more frequent.
• For the associated increased flood risk the current
stormwater drainage is likely to be insufficient.

7. The challenge for urban planning
• Has to be geared towards offsetting development
consequences by driving relative changes from
impermeable to permeable areas, i.e. minimising hard
surfaces.
• Each surface ought to be considered as a potential SuDS
feature, particularly in dense urban developments.
• Minimise the impact of new hard surfaces and housing on
the canalisation.
• All design planning must assess the maintenance required
to ensure SuDS continuously work as intended - future
repair or replacement requirements and the questions of
governance and responsibility included.

8. The Public Realm strategy according to Feilden
Clegg Bradley / Grant Associates
• Central aim: “climate-change-adapted public realm
strategies.”
• “Blue infrastructure”: green roofs, pervious
pavements, water rills, swales and rain gardens,
biorention areas and retention ponds to attenuate
surface water runoff within the public realm.
• All should be integrated into a wider scheme of
sustainability with greening, particularly the
creation of a ‘green spine’.
• “Water can play an important role in influencing
urban climates through both irrigation and the
support of urban vegetation.”

9. Range of criteria to be considered when planning SuDS

11. Green roofs
• With lighter rainfall events often sufficient to retain almost all the rainwater.
• SuDS feature that requires no additional space.
• Runoff from green roofs is unlikely to carry significant pollution loads.
• Meeting several sustainability objectives: in addition to water attenuation also
thermal insulation of buildings, carbon sequestration, urban heat island
mitigation, improved air quality and a longer roof life.

12. Reduction in peak runoff from a green roof compared
with a traditional roof – typical cumulative run-off

13. Pervious/permeable/porous pavements

14. Swales
Require soils that drain well

15. Rain gardens
• Designed to convey and/or store runoff.
• Infiltrate the water into the ground.
• Emphasis on diverse vegetation, its importance for ecology and
amenity: the plants trap pollutants and also take them up
systemically, thereby improving water quality.

16. Biorention
different from raingardens, as excess water is not
draining into soil, but conveyed off by pipe underneath

17. Bioretention features planned for the Lisson Arches site
• Here the visual impact of SuDS is essential due to the exposed
location of this site.
• Wherever aesthetics are of key importance, this will add to
maintenance cost.

18. What is planned for the
Lisson Arches site is not
a raingarden.
Why? It is a
bioretention system,
because excess water is
not infiltrated into the
soil but conveyed away
by pipe.
Good functioning of the
drainage pipe will be
absolutely crucial.
If the bioretention area
is small it runs the risk
of overflowing as runoff
from green or brown
roofs will contain
sediment and detritus.

19. Ponds, open and permanent water bodies

20. What is a retention pond?
• A lined permanent or at least semi-permanent
water body.
• Serving as permanent freshwater refuge, mainly for
biodiversity and visual amenity.
• Allows excess water above the lining level to
discharge into the ground.
• Advantages:
o providing cost-effective storage for large volumes of
water,
o mimicking natural ponds,
o promoting biodiversity,
o visually pleasing.

21. Water harvesting and irrigation
• Double benefit of water storage and potentially reducing
potable water demand by rainwater re-use for irrigation.
• Irrigation may prove vital in times of expected prolonged
droughts to maintain healthy vegetation and accentuate
urban cooling.
• Various providers:
• http://www.rainwaterharvesting.co.uk/rainwater-storage-
tanks.php
• http://www.rainwaterharvesting.co.uk/graf-platin-5000-
litre-rainwater-system-guk1.php
• http://www.alltexirrigation.com/rain-harvesting

22. Water-sensitive urban design, rainwater
harvesting and the Luton Street site
• Due to the underground car park, most of the
vegetation will be in planters.
• These will have to be irrigated regularly, even more so
in periods of drought.
• Sustainability principle: substitute drinking water with
harvested rainwater for irrigation purposes.
• If you already have a water-collecting surface, i.e. the
roofs, you should use this for harvesting rainwater.
• Especially in periods of drought, hose-pipe bans may
come into place which will make watering planters with
tap water impossible.

23. Example for larger developments: a 5000 litre tank

24. Another example: working with geotextiles

25. A word about water quality
• Of concern is the attenuation of the so-called
‘first flush’, the volume of runoff that flows from
hard surfaces shortly after the beginning of
storms following periods of drought, carrying
with it accumulated silt and pollutants.
• Layouts and drainage systems shall ensure that
there is no cross-contamination of water systems,
particularly not from surfaces with vehicular
traffic.

26. Long-term management
• Management regimes for the new public realm areas should be
agreed on a cost-effective basis to ensure that these areas remain
fit for purpose.
• In particular:
o Sediment management.
o Litter/debris removal.
o Weed/invasive plant removal.
o Ensure that public realm SuDS perform as intended.
o Ensure they do not present any health risk to people.
o For management regimes that maximise biodiversity, ensure
that people with appropriate skills take care, e.g. of removing
sediment, cutting back and removal of planting.
o Factor in structure rehabilitation/repair costs over longer
periods of time.

27. In essence: what could be done with SuDS…
• Key objective: reduce runoff to avoid combined sewer overflow leading
to discharges of raw sewage into the Thames, ideally so that all surface
water naturally drains away and is prevented from entering the sewer
system.
• Disconnect rain downpipes from the mains network for the rooftop
water to be diverted into SuDS to ‘enliven’ previously passive green
space, e.g. turn what may be just lawns around estate buildings into
more varied plant communities.
• Sustainability objectives: minimise capital and operating costs, energy
use, land area; maximize overall performance, social acceptance, and
accessibility.
• If the Church Street/Paddington Green regeneration area really was to
make a difference, the following map, indicating the potential green
roof space that could be created, would easily be the most important
single item of the entire concept FCB/Grant Associates came up with….

28. Potential areas for green and open spaces for SUDS