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Are Sustainable Drainage Systems(SuDS) enough for Londons river?The role o SuDS in tackling combined sewer overows (CSOs):Case studies rom across the Atlantic

Philadelphia: Just like London,

Philadelphia in Pennsylvania has a

combined sewerage system, collecting

both rainwater and sewage. There the

comparisons end.

Philadelphia has indicated that SuDS

will, in an average year, control 19 to 25million cubic metres o CSO discharges

out o the total estimated at 38 to 49

million cubic metres. Thats a level o

control o 50 per cent, ar less than the

required minimum o 85 per cent.

The City o Philadelphia covers an area

o 276km2 with 60 per cent contributing

to the combined sewerage network.

In London the CSO service area is

550km2.

Philadelphia has a much smaller

population, housing just 1.5 million

people (according to the 2010 US

census), compared to Londons overall

population o around eight million.

Philadelphia is projected to serve an

additional 74,000 people by 2031.

In the same time rame London is

projected to have an additional one

million people in the area served by

CSOs.

Philadelphia has a much higherproportion o open space available or

implementing SuDS measures.

The population density o Philadelphia

is 5,400 people per km2. In London the

fgure in the total area served by CSOs

is almost double, at 10,200 people per

km2.

While London is built on top o a layer

o thick clay and saturated gravels that

do not easily soak up water, Philadelphia

sits on much more permeable soils.

By design, Philadelphias existing systemhas 164 discharge points. Londons

discharges are concentrated to ar ewer

points (57).

Each o the London CSOs has on

average an area connected to it

10 times larger than is the case in

Philadelphia. This means that per CSO,

the average volume discharged in

London is twice as much compared to

Philadelphia. This is despite the act that

rainall in Philadelphia is double that o

London.

Unlike London, Philadelphia has

received a disposition (exemption) rom

wastewater quality standards. The city

is projected to meet a 50 per cent level

o control (with ull implementation o

their SuDS plan), rather than the

85 per cent minimum required

elsewhere in the United States. This

ollows the US Environmental Protection

Agencys selection o the city to pilot

SuDS implementation over an extended

period o time.

Philadelphia aims totackle 85 per cent o CSOdischarges through SuDSmeasures over a 25 yearperiod. This work hasstarted, but cannot yetbe deemed successul. Atunnel solution akin to theThames Tideway Tunnelmay still prove necessaryto meet the requiredenvironmental standards.

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Portland: In the 1990s Portland in

Oregon embarked on an ambitious

programme aiming to tackle CSOs via

SuDS. They set out with the specifc

aim o removing the need or a tunnel.

The catchment area o the CSOs in

Portland is 108km2, with a population

density o about 3,000 people per km.

Thats much less than either London or

Philadelphia. London is over three timesas densely populated and has much less

space or implementation o SuDS.

Unlike Philadelphia, Portland is legally

required to achieve a 96 per cent or

higher level o CSO control, a target

comparable to the one London would

meet with the Thames Tideway Tunnel.

Like Philadelphia, and unlike London,

the geology o Portland is more suitable

or SuDS; the soils underlying the SuDS

areas o the city are more porous and

more able to soak up excess rainwater. Like Philadelphia, and unlike London,

Portland has relatively plentiul open

space available or collecting rainall

runo.

Portland has removedapproximately eightmillion m3 o storm watera year rom the systemusing SuDS (35 per cento total CSO volume), buttunnels still proved to be

a necessity to meet therequired levels o control.

London: SuDS measures alone would

be ineective in London and ail to

control CSO discharges to the required

level in the required timescale.

Unlike Portland and Philadelphia,

London is a heavily urbanised city, with

relatively little open land available or

SuDS measures.

Within London, re-assigning street

parking spaces and prime open land,such as parks, or SuDS, on a scale

large enough to have an impact on the

amount o water entering the sewerage

system, would be all but impossible.

London is built mostly on clay and

saturated gravels. The citys water table

is also high. A good analogy is to think

o London sitting on top o a sponge

that is already ull o water. Even i there

were space or SuDS, the water would

drain away very slowly, certainly not

rapidly enough to guarantee that SuDSmeasures, such as swales, would be

empty in time to be able to tackle the

next rain event.

Implementing SuDS would be

expensive, time consuming and hugely

disruptive throughout London, adversely

eecting nearly every road, home and

open space in nearly every borough.

The maximum practical level o retroft

SuDS would take over 30 years to

implement. It would cost a projected

13 billion and still not provide the level

o CSO control needed. Just within West Putney, one o the smallerCSO catchment areas in London, the

optimum implementation o retroftting

SuDS measures would:

Require the reconstruction o eight

hectares o roadway, park and driveway

suraces.

Need 22 hectares (the equivalent o

44 ootball felds) to be turned into

rainwater detention basins;

Impact approximately 2,500 homes;

These measures would still not meet therequired levels o CSO control at West

Putney.

The citys specifccircumstances dictatethat the proposedThames Tideway Tunnelis essential to tackle thescale o the discharges inthe timescale required.

SuDS could and shouldplay an important role inextending the lie o theThames Tideway Tunnel,but can in no way replacethe need or it.

A typical street in Portland A typical street in London