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In the Fall of 1993, I took a sabbatical leave in Fortaleza, Brazil,
to study droughts at the famed
Departamento Nacional de Obras Contras Secas (DNOCS) (National Department of
Works Against Droughts). During my stay in Brazil,
I was associated with the Universidade Federal do Ceara (UFC).1
One day I was asked by a colleague to help review a doctoral thesis that was near completion.
The student explained that he was coupling remote sensing techniques
with a numerical model of flood propagation in a spatial grid context.
After reviewing the manuscript, I asked the student what equation was he using for the
routing. With a mix of surprise and confidence, he answered: "The Manning equation."
I countered that if he was routing floods, that the steady-flow
Manning equation did not suffice;
that he had to use an unsteady form of the open-channel flow equations,
one that would take into account
the well established--yet surprisingly little known--
Seddon's law.
Moreover, I said that it was also necessary to account for
the subsidence of the flood waves by means of
Hayami's hydraulic
diffusivity.
Thus, the moral of the story:
You cannot do flood routing just using a steady-flow equation such as Manning's.
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