Tuesday, July 24, 2007

Building on Flood plains

One of the hot topics for debate is going to be "can we build housing on flood plains?"

The answer is going to be, depends.

If you imagine a river that is 20 feet deep (about 6 metres) from the bottom to the top of the river bank and 40 feet wide (about 12 metres) running from from a few feet deep in the middle of a long hot summer to almost near the top in a wet winter, can you build on the flood plains around it?

Well, in unusual weather you will easily top the river bank. You can put in flood barriers but the problem is that if you try to constrain the river at the same width, then the only option is to force the water to go high. There is only so high that you can build temporary flood barriers before the weight of water will just push them away. So the river may well need to expand sideways, and possibly by some margin.

If then we build on flood plains, but away from the river, so that we can erect the flood barriers 40 feet away from the river each side, then the river will have a great deal of room to expand reducing the height of the water.

It does involve sacrificing land to the potential of one of floods, but if this was done then at least people could get insurance for their houses and the risks would be mitigated.

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