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.

1 Ponce, V. M. 1995. "Management of droughts and floods in the semiarid Brazilian Northeast - The case for conservation," Journal of Soil and Water Conservation, 50(5), September-October.

Meandering river in the flood plain of the Rio Meta, Colombia.

Meandering river in the flood plain of the Rio Meta, Colombia.