Mid-term – Green Infrastructure

Philadelphia’s Green City, Clean Waters, a future without CSO spills, droughts nor floods, we seriously risk missing out on…

“the main lack of space for retrofitting Green Infrastructure into existing cityscapes is in the head of the people who should be doing it”.
Foundation for Water Research

Today’s sewer overflows are chiefly the consequence of rainwater running off cities’ increasingly impervious surfaces and gushing down combined sewers[1].  This is actually the only problem that Thames Water’s proposed £4.3bn Thames Tideway Tunnel aims to solve[2].

Today a vast range of storm water management solutions based on “Green Infrastructure” (GI) exist – also sometimes referred to as Sustainable urban Drainage Systems (SuDS).  Roughly speaking, the aim of GI is to restore the natural water cycle degraded by urbanisation, as much as practical[3].

Two good examples of eminently feasible GI measures that require little to no additional space are green roofs[4] and porous asphalt[5]. Implementing these would be far less disruptive than Thames Water’s proposed Thames Tideway Tunnel (TTT) and our calculations show that they would save about £1bn in capital cost compared to the TTT[6], whilst delivering a comprehensive set of ancillary benefits such as flood, drought, air pollution and climate change mitigation.

Other ancillary amenity, such as harvesting of rainwater to be used for functions that don’t require treated water (irrigation, flushing toilets, etc), mitigating many issues – including floods and droughts – in one stone, and also alleviating the need for further unsustainable sewerage extension[7].

“If I were in London’s situation, I would learn from Milwaukee…”
Alderman Willie Hines, president of Milwaukee Common Council

Increasingly, enlightened cities, such as Philadelphia, New York City or Hamburg are leading the way, demonstrating that with a little political courage (and perhaps without the headwind of a privatized monopoly water industry), Green Infrastructure can be successfully leveraged as a foundation for surface water management and thus to tackle sewer overflows. The highly recommended three minutes video shown above depicts what we risk missing out on; the opportunity of our generation to foster a sustainable and resilient future for our City…

Officials in Milwaukee, which enjoys a sewage tunnel upon which the one proposed for London has been modelled, have recently stated that it was oversold and that with the benefit of hindsight, Green Infrastructure would come first on their shopping list to solve their storm-water management issues[8]

Benefits of the mid-term solution


Green Infrastructure works with nature instead of against it, mitigating carbon emissions rather than exacerbating them. Contrast this with the Thames Tunnel, a major industrial scheme, which would be a significant contributor to climate change, even when its raison d’être is to mitigate its effects – sewer overflows caused by increasingly degenerate rainwater patterns…[9].


Unlike the Tunnel, SuDS don’t require upfront capital, which is proving very tricky (and expensive!) to raise in today’s climate. If the 14m households due to be taxed £80 p.a. for the Tunnel were asked to spend it on SuDS instead, they would see direct benefits and deliver a safer future for everyone.


SuDS would create a sustainable local industry of green jobs, unlike those touted around the Thames Tunnel, which would be of limited duration, and for which Thames Water have set an “ambitious” quota of 20% local labour.


“Each dollar of investment in green infrastructure delivers other benefits that conventional infrastructure cannot…”
Prof Richard Ashley, University of Sheffield, acclaimed SuDS expert

Beyond those listed above, Green Infrastructure provides lots of ancillary amenities which enhance the city scape such as:

  • Aforementioned flood and drought prevention
  • Improved air quality through greening
  • Better living environment for all: visual amenities; stress reduction; positive impact on property value; recreation opportunities
  • Energy savings afforded by green roofs
  1. [1] Combined Sewers are named after the principle which combines sewage and rainwater in a single pipe.  This design is now well recognised to be flawed – and is even strictly outlawed in many countries – because sewers typically cannot accommodate all the rainwater that falls during intense storms and are therefore designed to overflow (into the Thames in London’s case).  Yet, Thames Water’s proposed Thames Tideway Tunnel (TTT) would essentially perpetuate this outdated principle by constructing a bigger combined sewer to capture the overflows from the current system.  Despite its exorbitant price tag, the TTT therefore merely defers the existing CSO issue rather than tackle it; and worse yet, it does so with a substantial carbon footprint.  Also, because it is simply impossible to build a pipe big enough to capture the rainwater that can fall in increasingly frequent extreme weather events, TW acknowledge that the TTT would still spill raw sewage into the Thames four times a year when commissioned, and increasingly more frequently with the effects of climate change
  2. [2] If TW or anyone else says otherwise, refer to our TTT mythbuster to understand the complete picture
  3. [3] Before cities and impervious surfaces, rainwater would simply infiltrate into the ground.
  4. [4] A 2008 GLA audit found that there are 10 million m<sup>2</sup> of roofs suitable for retrofitting within a 6km radius centered on Trafalgar Square.
  5. [5] In the 10 years it would take to construct the TTT, roughly 50% of London’s primary roads and 33% of the secondary ones would get resurfaced anyway.  Concerted resurfacing with porous asphalt, which typically allows rainwater to infiltrate back into the water table would therefore bring little in way of additional disruption and would be the opportunity to deliver significant ancillary amenity, such as a network of dedicated cycling lanes – all for about a 25% discount in capital cost compared to the TTT.
  6. [6] Please get in touch if you’d like to discuss this with one of our engineers.
  7. [7] Combining sewage and storm water in a single pipe has been outlawed in many countries for a number of years, for good reasons!
  8. [8] Milwaukee built a ‘deep sewage tunnel’ (implemented by the same US engineering firm, which is pushing the Thames Tunnel) under 20 years ago. Yet that tunnel is already so overwhelmed that by mid-2011, the Milwaukee Metropolitan Sewerage District had to dump over 24 billion gallons of untreated sewage and storm water into Lake Michigan. Tunnel or not, the same fate awaits London if we don’t seriously start investing in SuDS.
  9. [9] Just for the Thames Tunnel’s 5 main drive sites, the total number of lorry movements is estimated to be in excess of 580,000 (source: Thames Water’s consultation material). This number could rise sharply if contractors make reduced use of the option to transport spoils by barges – which is more expensive to them (guess what will happen?)… The carbon footprint of operating the tunnel would also be very significant; just considering the massive 15 GWh pumps that will be required to empty the tunnel in Beckton. Even if one doesn’t mind the ecological damage, this will add to operation costs, doubly so with the carbon levy headed towards £100/ton.