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In the winter of 1982, my friend Newton Carvalho
invited me to inspect the failure of the
cofferdam for Norte Dam, which was
under construction at that time on the Itajai river, in Santa Catarina, Brazil.
The dam was being built by the Departamento
Nacional de Obras Sanitarias (DNOS) [National Department of Sanitary Works].
The cofferdam had failed because an unusual storm event followed on the heels of a severe drought. Consequently, the storm
mobilized an extraordinary amount of debris, which clogged its two Morning Glory spillways in short order (see photo below).
On the day of our visit, a meeting took place
at the damsite between government officials and representatives from
a local Indian reservation. I attended the meeting as an observer.
The dam's spillway had been sized using the 10,000-yr design flood,
a return period roughly comparable to the Probable Maximum Flood in U.S.
hydrologic practice.
The people from the reservation contended that the design flood would place a portion of their lands under water.
They wanted the project changed so that it would not compromise their lands.
The government representatives
countered that the risk of flooding was real, but that it was extremely low;
and that it was a small price to pay for the flood control benefits to accrue to the downstream valley. Historically, the Itajai river is exceptionally prone to
extreme flooding events.
The dam was built anyway, and the 10,000-year flood is still waiting to happen.
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