If I were not so infernally busy, I think I would go to the banks of the Thames and cheer for David Walliams as he continues his epic downriver migration – like a lost whale or some vast confused salmon – from Gloucestershire to London. What a hero.

He is not only raising loads for charity. He is simultaneously drawing attention to one of the key environmental problems of our time; and he is thereby pointing the route to long-term economic prosperity for this country. He has only one thing to fear, as he porpoises along. There is one risk more dangerous than the cold, or the currents, or the rats or the pike or the snagging of underwater weeds or the churn of outboard motors or even the siren call of foxy Henley ladies as they lean from their launches in their leopardskin bikinis and invite him inside for a warming cup of tea.

The thing he has to worry about is rain. Yes, rain is the danger, even for a man in a wetsuit and immersed in the Thames. Because as soon as it rains more than 2mm, the sewers of London are no longer able to cope with their burden and pwooosh – the Bazalgette interceptors are discharged into the Thames, with consequences that simply cannot be ignored. It is not just that poor David Walliams will be at yet graver risk of an upset stomach, or that the rowers of Putney start to find the water full of unmentionable items. Nor is it just the massacre of fish and other aquatic life-forms.

We are facing the long-term deterioration in Thames water quality, and unless we act now I am afraid no one in their right mind will be swimming this river in 10 years’ time – certainly not beyond Teddington. The sewers of London are already so full, and so much rainfall now sluices into them off the concrete and tarmac rather than sinking into the turf, that these Bazalgette interceptors are already exploding into the Thames about 50 times a year, and the discharge rate is increasing the whole time. When Joseph Bazalgette built his remarkable system, he thought big. His sewers are still robust, and they are impressive feats of architecture and engineering. But they were designed for a city of 2.5 million people; and the population of London is now pushing eight million, and heading for nine. In one of the crimes for which we are truly all guilty, society is now discharging an awful 50 million tons of raw sewage into the river in London alone, and unless we are bold in our plans, that figure will rise to 70 million tons in 10 years; and no matter how gutsy David Walliams may be, his future swims could well be banned by elf ’n’ safety.

When Bazalgette designed his interceptors, in response to the Great Stink of 1858, he assumed that they would only kick into action in emergencies – truly torrential downpours of a kind that happen once or twice a year. That is why it is time to recognise that we can no longer rely on Victorian capital, and why Thames Water is right to be consulting on its proposed super-sewer, known as the Thames Tideway Tunnel. Of course, it must construct this cloaca maxima in a way that minimises hassle for local people and avoids damage to riparian beauty spots. But the basic idea is excellent, and essential.

At a depth of 75 metres – below the Tube and other excavations – and with a bore the width of three buses, this huge tunnel will run winding beneath the course of the Thames from Richmond to a series of vastly improved and upgraded East End sewage works. A separate leg of the tunnel is proposed to run from Abbey Mills to the Beckton sewage plant, to end or greatly reduce the discharge into the Lee. It is a breathtakingly ambitious project, on a scale that would have attracted the approval of Brunel and Bazalgette themselves. We have the prospect of protecting Walliams and other migratory forms of river life, such as sea-trout and salmon, and of ensuring that a much cleaner and sweeter river flows through the heart of the city. If we fail to act, we face smells and pestilence and a serious reduction in our quality of life. This new super-sewer is the right thing to do for the environment – and it is above all the right kind of thing to do for a country still struggling to get back to growth.

Never mind the supermasticated arguments about the 50p tax rate (which seem to be moving in the right direction). I know that George Osborne is also thinking about the economic stimulus that can be provided by infrastructure projects – and he is right. Big construction projects such as a supersewer generate myriad forms of employment – not just builders, but designers, architects, engineers, planners, and the list goes on. They create long-term competitiveness by making the city more pleasant to live in and move around. And it is a mistake to think that these projects always need to be funded by the taxpayer. There are plenty of investors and wealth funds around the world who can see the potential long-term revenue streams that can be generated by investing in a significant and beneficial piece of infrastructure. In other words, it is largely a question of vision, and of political will. It is becoming clear that this downturn could go on for so long that we need to think not just about projects that are “shovel-ready” now, but ones which could be “shovel-ready” in two, three, or five years’ time.

We are massively expanding Tube capacity, we are putting in Crossrail; but we need to go further. Even as these improvements come on stream, we will be struggling to catch up with the growth in demand. Commuter networks are jammed; Heathrow is running at 99 per cent capacity. We need Crossrail 2, and a new airport. We can’t afford to keep muddling along and relying on historic investment. On sewers, rail, river crossings, ports and airports it is time for neo-Victorian boldness.

It is the right thing for jobs now, and the right thing for this country’s long-term competitiveness.