Peter,

Not so much a different idea of risks.. a different idea of economics. All decent engineers know how to size something for various likelihood events. The decision what size to build is based off how much it costs.

I grew up near the Mississippi River in Missouri. The river floods every spring because of snow melt runoff. The levees there were all built based on a once-in-a-hundred-years flood estimate. There have been like 2 hundred year flood events in the last few years.

Can the levees be built taller and thicker to withstand larger floods? Sure they can. Is money available to do it? Generally, no it's not.

>From an article in my Dutch newspaper I understand that the American Army Corps of Engineers builds levees with an average chance of breakthrough once in 230 years. In the Netherlands the norm is once in 1250 years for the river areas and once in 10,000 years for the cities. According to a Dutch professor the Americans have a different idea about risks.