"Basil Jet" wrote in message
...
Looking at a map of the Jubilee River (The Thames flood channel near
Windsor) it occurs to me that some of it would have made a useful road. So
I'm thinking that quite a few bypasses could have been built deliberately
low and could then operate as roads most of the time but as flood
alleviation channels when required.

I suspect if you built the road it low enough to be useful as a flood
channel it would then become prohibitively expensive to drain the road for
normal use. You would also be disrupting all the other (natural and
artificial) drainage features that would normally be routed under a new
road. Relatively cheap under-bridges would become expensive over-bridges,
junction designs would probably be an expensive compromise, and railway
bridges (to bring us vaguely on-topic) may well be impossible to accomodate.
The far more likely scenario would be to enclose the flood channel and build
the road on top of it.

The closest example I can think of is the Emabankment in London, which was a
combined road, Underground and sewer project, although I think they built
one of them first and then had to rip it all up and build it all over again.

--
DAS