Critical assesment of the Sustainable Urban Drainage component of the Church Street public realm...

Blue Infrastructure: critical Assessment of the Sustainable urban Drainage Systems (SuDS) Component

of the Church Street Public Realm Strategy

Achim von Malotki

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.

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.

…and then in an urban environment with and without SuDS

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.

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.

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.

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.”

Range of criteria to be considered when planning SuDS

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.

•

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

Pervious/permeable/porous pavements

Swales

Require soils that drain well

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.

Biorention different from raingardens, as excess water is not draining into soil, but

conveyed off by pipe underneath

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.

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.

Ponds, open and permanent water bodies

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.

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-t

anks.php• http://www.rainwaterharvesting.co.uk/graf-platin-5000-litr

e-rainwater-system-guk1.php

• http://www.alltexirrigation.com/rain-harvesting

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.

Example for larger developments: a 5000 litre tank

Another example: working with geotextiles

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.

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.

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….

Potential areas for green and open spaces for SUDS