Monday, September 05, 2005

Water water on the level 

So New Orleans is (largely) below sea level. Some sneering at the stupidity of building on such a spot (spoken lightly though if in the hearing of the Dutch). But flooding predictions aren't quite as simple as minus so many centimeters under Ordinance Datum or whatever.

One definition to de-mystify some jargon. Ordinance Datum Newlyn is the official Sea Level mark for the UK. This benchmark - a measure taken in Newlyn, Cornwall, is the basis for the 'spot heights' shown on detailed OS maps. If you ever have a flooding case in your area, the official flood height will be quoted as reaching a height of plus (so many) metres above OD Newlyn and not usually as so many metres or whatever over a local landmark.

On a world scale there is the Global Sea Level Observing System.

Sea levels are going to rise in the next few years, no question. And sure we do take more offical and media notice of this than does (say) the current U.S. administration. But we in the UK are also about to do things like invest hugely in expensive sporting facilities in an area of London at higher risks of flooding both from the sea (the Thames Barrier will be ending its planned useful life just about the time of the 2012 Olympics) and from the river Lea system. Get a map and look at the ODN spot heights for the development zone. This may not be an intelligent choice over the perspective of a half-century or so.

There is no ironclad criteria for makes a 'floodplain'. Most of the officially delimited floodplains, in planning maps in England anyway, are based on the areas of inundation of the 1947 floods adjusted for the very differennt experiences of the 1974 floods. Areas that did not flood then may actually be a high risk from flooding. Councillors who sit on Development Control Committees should be very sceptical of precise flood-risk boundaries in planning submissions.

Much of the historic data on floods was lost at the time of the privatisation of the old Water Authorities when people over 50 were compulsorily retired to 'slim down' the workforces and too late it was discovered that much flood information had been held as local lore and not written down... Even the Environmental Agency's Flood Risk Calculator is not the last word.

So good luck New Orleans, seriously, as the US looks to its Gulf Coast developments... and let's think more seriously about flood risks in our own development plans over here.

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