Will it flood?

This article was published in 2011, in Newsletter 98.

There has been much discussion of the probability of the new cycle route alongside the Busway flooding between Swavesey and St. Ives, specifically at the ‘Dutch culvert’ described in the last Newsletter, which is meant to reduce the likelihood and impact of any flooding.

Many of the sections that flooded last year will be raised significantly as part of the works that will also result in this section receiving ‘blacktop’, but there is still that one outstanding issue.

We’ve tried to get definite answers about the Dutch culvert from both the County Council and the Environment Agency. This is an impossible task, not because of the nature of these public bodies, but more probably because of the nature of nature. The only answer we’ve had seemed extremely pessimistic, perhaps on the ‘precautionary’ principle.

After arriving in Cambridge some 25 years ago I spent ten years working on computing, instrumentation and statistics for a now long-closed Ministry of Agriculture unit looking at drainage and small-catchment hydrology. So I know that once in fifty year floods can occur in successive years, and that changing climate and catchment changes, such as housing development, can play hell with the best predictions.

I’ve asked some questions, and not got straight answers, but here is my interpretation with some very limited data.

If the Great Ouse overtops the banks in this area because of really heavy rainfall then the route will flood, but so may the Busway and many houses, including ones in Cambridge (it isn’t on the Ouse but the Cam is part of the system). The normal flow for the Ouse in this area is around 25m3/s. Peak storm rates have reached over six times this figure, and that would be enough to flood the whole of ‘The Lakes’ RSPB reserve area to about a metre in few hours!

Allowing this area to flood is what flood plains should be for, rather than housing! You could predict that the route would flood because parts of Bedford would be flooded and I’m told it takes over a day for the peak water flow to then reach St. Ives. Such a flood would obstruct the route, and even when the main river drops it could take days for excess water to drain away.

A local storm is apparently a more likely cause of flooding where the Dutch culvert is. The area behind the main river defences is divided into ‘cells’. A major storm over the A14 in the same cell could dump over 50mm of rain. That water has to reach the Ouse, preferably without flooding properties in Fen Drayton or Fenstanton. The Busway embankment may prove a barrier to the floodwater, or there may be flooding because of inadequate pipe capacity below the Dutch culvert.

I’ve seen an appalling design, funded by a First World government, for a main road in Tanzania, which was built on an embankment across the flood plain of a valley at three locations. These acted as dams in the first major flood, and three expensive bridges (still not replaced some ten years later) were washed away due to the hydraulic gradient undermining their foundations. This design is not that bad, but I still don’t understand the reason for it. The capacity of the box culverts beneath the Busway must be greater, and the maintenance track would not significantly reduce the storage capacity for storm water if raised to the level of the bus route.

I’ve put it in print. I don’t understand the design but I don’t think the cycleway will be closed for 20 or so days each year. You can cycle through water without too much difficulty as long as the water doesn’t reach your pedals. Of course it might happen this winter. If it does, AND then it floods the following year, members can search me out and dunk me in the water!

Jim Chisholm