Living below sea level

About 60 percent of the Netherlands is below sea level, which means that the Dutch are pretty good at building dikes to protect polders from flooding.* The photo at right illustrates how Amsterdam's Schiphol airport is below water level.**

As part of that effort, the Dutch need to have accurate measurements of land levels, for planning roads and rails, for spotting weak points, and for monitoring changes in land and sea levels.

The key to accurate measurements is to maintain a base reference point of "zero" to which all other points can be compared.** That point -- known as the Normaal Amsterdams Peil (NAP) or Amsterdam Ordnance Datum -- is in Amsterdam's City Hall (the old one is under the pavement in the Dam).

So I went for a visit. The small brass hub benchmarking NAP is on top of a stone column driven into stable bedrock (maybe 20m down) that is not subsiding. The Germans (!) adopted the NAP baseline in the 19th century. Now it's used throughout Europe.

As a comparison, the small exhibit also shows the scale of the NAP+4.5m storm surge that inundated Zeeland in 1953 and flooded many inhabited areas; nearly 2,000 people died. (The photo at right shows the column of water that rises to 4.5m, to show visitors how that much water would flood the ground floor of City Hall.) That storm convinced the Dutch to embark on a massive Delta Works that took about 50 years to build (it was completed this year). The main feature of the Delta works is a progressive system of allowing water to spread on less valuable land (e.g., farms) to protect more valuable land (e.g., urban areas). The works are calibrated to allow flooding in varying frequencies, from 1-in-250 1-in-1,250 years [see post above] to 1-in-10,000 years.

I find it interesting that the Dutch are planning the next stage of protection -- from rising sea level and higher river discharges -- as a result of Katrina's devastation of New Orleans. Americans have not responded with anything like the same enthusiasm. Rebuilt levies around New Orleans are designed for 1-in-100 year floods (skeptics say they are not even that strong). Remember that Katrina was an "1-in-400 year storm," but 4 other 20th century Atlantic hurricanes were bigger. (We need some bookies to recalibrate these probabilities.)

Bottom Line: You can't know where you're going until you know where you are. The Dutch are good at both.

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* That's also why so many Dutch engineers are working on programs to rebuild New Orleans and the protect Sacramento Delta; the ones I've spoken to are flabbergasted at Americans inability to grasp the size of the task -- politically, socially and financially -- in terms of moving ahead.

** This point is not actually "sea level," but a reference point relevant to staying dry during high tides and storm surges.