Richard Ashley suggests that the £2bn Thames Tideway Sewage Tunnel is a white elephant and that alternatives should be considered (News last week).
The problem is that most of London has a combined sewer system - ie foul drainage and storm drainage are handled by the same sewers. This means that with as little as 2-3mm of rainfall, foul sewage is discharged into the Thames, in breach of the EU Urban Wastewater Treatment Directive.
The tunnel is an attempt to solve the problem. It's easy to be critical, but what, realistically, are the alternatives?
The combined sewer/over flows could be separated by installing a second pipe system.
This would cost about £12bn and result in congestion and seriously affect the local environment as all the streets of London are dug up.
SUDS infiltration systems are not viable as most of London is underlain by impermeable London clay.
Flood detention ponds? A good idea, but where would we put them in a city as built up as London? The idea of converting the Serpentine into such a pond with the inevitable residue is not likely to get public approval or, for that matter, Royal assent.
Underground storage tanks were dismissed as dispersed storage tanks act individually and only serve their local subcatchment. Therefore, over the whole contributing area much more storage would have to be constructed to provide the same level of service, even if there were available areas in each sub-catchment which, for many, there are not. Gates in the sewers to hold back the w wouldn't work either, because the limited head available means only very limited storage could be obtained.
Screens on the combined sewer over ws, while they would reduce the visible impact, would do little to stop the health hazard and would not satisfy the EU. Also, most of the over flows are along the river bank and there would be insuf ient space to construct the size of screens needed.
The great benefit of the tunnel, from roughly Hammersmith to Beckton/Crossness, is that it acts as both a conveyance and storage facility. Thus the total volume of storage in it is available wherever the storm occurs. The storm efficient in the tunnel will be pumped out and treated at Beckton sewage works.
As regards the cost and viability of the scheme, the upstream part of the tunnel is largely in London clay, an excellent tunnelling medium. The downstream part is largely in chalk. Similar size tunnels have been driven in chalk for the Channel Tunnel and its rail link.
The big cost overruns on the Channel Tunnel occurred on the revised specification of the rolling stock, not on the tunnelling, which was brought in near to budget and programme.
The Thames Tideway Sewage Tunnel is certainly big and it's certainly expensive. But a white elephant? A scheme that will protect the Thames from weekly sewage over flows and allow the UK to avoid large fines for breaching the European Directive can hardly be described as that.
Chris Binnie was chairman of the Thames Tideway Strategy Steering Group and sits on the ICE water board