Sustainable drainage and the ‘Supersewer’ London needs both

December 1, 2012

Richard Aylard, Thames Water’s Director of External Affairs and Sustainability, explains in Thames Estuary Partnership’s magazine ‘Talk of the Thames’ why the proposed Thames Tideway Tunnel and Sustainable Urban Drainage Systems (SuDS) each have a vital role to play in tackling sewage discharges to the River Thames.

The Sir Joseph Bazalgette memorial, tucked away on The Victoria Embankment at the bottom of Northumberland Avenue, rewards close inspection. Sadly overlooked by the majority of passers-by, it provides an intriguing insight into the debate about whether SuDS could provide an alternative to the Thames Tideway Tunnel. Although he is justly credited as the founder of London’s modern sewer network, the great man’s memorial, like his own drawings, actually refers to ‘London’s main drainage system’.

Sir Joseph’s enduring achievement was to mastermind a solution that accommodated the practical reality that, by the 1850s, the city’s natural drainage had effectively long been requisitioned as an early sewer system, to facilitate Georgian and Victorian London’s property explosion. The accompanying vast increase in both population and impermeable surfaces in the north of the city led to much higher combined flows, causing serious problems for drainage, public health and the cleanliness of the Thames. Bazalgette’s visionary yet pragmatic drainage plan dealt with those problems and had benefits south of the river too, where development had been held back by the low-lying, marshy landscape.

Just like the team planning the Thames Tideway Tunnel today, Bazalgette did not have the luxury of a blank piece of paper. He had to deal with the fact that the city’s drainage and sewage systems had been evolving together over many years to keep pace with development. There was no practical way of reversing that situation. That is why he perpetuated the combined system, with Combined Sewer Overflows (CSOs) discharging to the river when volumes became too great. 150 years later, another six generations of development have made the situation even more intractable and the CSOs discharge ever more frequently.

The development of the Thames Tideway Tunnel will solve this problem. Many of the design team cut their professional teeth on successful schemes, combining tunnels and SuDS, which have been completed and provide models for London to follow, such as those in Portland and Milwaukee. In Portland, SuDS, are a key component of the ‘Cornerstone Projects’ that complement improvements to treatment works and ‘storage and transfer’ tunnels to achieve control of damaging discharges to their rivers. Other cities, such as Philadelphia, have ambitious and well-publicised long-term plans for SuDS but may yet also need to incorporate similar tunnels in order to achieve the required degree of control over discharges.

At Thames Water we are enthusiastic about the potential of local or targeted SuDS, which is why they feature in our proposals to tackling sewer flooding to customers’ properties in the Counter’s Creek catchment in west London. We expect that SuDS will increasingly be a part of other retrofit schemes and will play a key role in ensuring that new developments do not add to existing problems. However, the idea that SuDS might, on their own, somehow provide a solution to the modern day scandal of sewage discharges to the River Thames is a very different matter, and one that literally doesn’t hold water in modern London, any more than it did in the 1850s.

Nevertheless, the questions about whether London needs a tunnel at all require an answer. Surely, we are told, we should separate the sewage from the rainwater, or ensure they don’t mix in the first place, and then capture the rain water to meet growing demand for drinking water? Well, if a new city was being built, of course we would provide separate sewers and drainage. But we are dealing with a very old city and the opportunities for water sensitive urban design on a large scale are strictly limited.

The principal problem is one of sheer scale, with immense volumes of heavily polluted rainwater needing to be managed. I was inside the Lee Tunnel recently. This is the same diameter as the proposed Thames Tideway Tunnel and will connect to it. I can tell you it is awe-inspiring; wide enough to accommodate three London buses side by side. When the tunnels are combined they will stretch for 20 miles, collect the discharges from 34 CSOs and have a storage capacity of 1.5 million tonnes. What struck me most forcibly was the realization that this huge space would fill many times each year and from single storm events. How else could that immense volume be captured and managed in a densely developed city?

The capital self-evidently lacks sufficient areas of open land to capture such volumes of water. Even if that were not the case, the clay and saturated gravels that underlie much of London mean that any storage would take a very long time to drain. And storage that is full from one rainstorm is no use when the next one arrives. That is why the tunnel has been designed to be emptied, and its contents treated, in less than 48 hours, ready to absorb the next storm.

Yet surely this volume of water would be a huge benefit to our thirsty city? Well, let’s look at that. In a typical year, 18 million tonnes of sewage – or heavily polluted rainwater, if you prefer – enters the river through the CSOs that the Thames Tideway Tunnel will intercept. Assuming we could instead somehow capture and collect all of this with a new network of pipes, it could be pumped underground to replenish the aquifers, but it would first need to be treated – even if it were not contaminated with sewage – to comply with groundwater legislation. If we could do all that, and then re-abstract it from the aquifer and then re-treat and put it into supply, all in the centre of London, the volume of drinking water would amount to just under 50 million litres per day.

We currently supply around 2,000 million litres to London every day, so the benefit would be an increase of around 2.5 per cent. That is worth having, but achieving it would require sufficient space for collection, transfer, storage, treatment and pumping, distributed around London. That’s a lot of very expensive and disruptive new infrastructure. Replacing more Victorian water mains and retrofitting water efficient equipment would provide additional sustainable resource at a fraction of the cost.

London does not give his memorial due prominence, but our modern city owes Sir Joseph Bazalgette a huge debt. Without him it would not be the city it is today. Yet look back in the archives and you will see he was told his plans were unnecessary, not practical, too expensive and too disruptive. As his great-great-grandson, Sir Peter Bazalgette, observed recently, some things don’t change.

The problem of combined sewage discharges to the River Thames needs practical solutions that reflect London’s conditions, will achieve the required standards, and can be implemented quickly. The river and London as a whole urgently need the Thames Tideway Tunnel and also, over time, to implement SuDS to prevent the existing problems growing worse. The two approaches are complementary and we need to utilise both of them.

We are working hard to deliver the first half of the equation as fast as we can, with minimum cost and disruption. The SuDS dimension is more complex and will take longer. The Flood and Water Management Act was a big step forward. But subsequent progress has been slower than we would have liked, and the final pieces in the SuDS jigsaw of responsibilities are not yet in place.

As soon as they are, we will work with Councils, businesses and individual Londoners to achieve the benefits of SuDS, as a complement to the proposed Thames Tideway Tunnel.