Direct estimate of the breach hydrograph of an overtopped earth dam

Abstract

The quantification of the breach hydrograph of an overtopped earth dam remains a challenge, even in controlled laboratory conditions. In this work, the breach hydrograph is calculated as the product of the velocity normal to a breach cross section and the estimated area of that cross section. Although theoretically simple, because it is based on the very definition of discharge, this direct approach is an absolute novelty in dam-breach studies. To illustrate the application of the method, a laboratory model of an earth dam, built with cohesive sediments, was closely monitored. Competent velocities were estimated from surface velocity maps measured with large-scale particle image velocimetry (LSPIV). The breach area was estimated by analyzing images of the traces of a laser sheet on the free surface and the breach bottom. The direct breach hydrograph was compared with an estimate based on the mass balance in the reservoir and with the discharge obtained through a rating curve of a downstream spillway. The novel method was shown to reproduce the main expected features of the breach hydrograph. The potential to provide estimates free from experimental artifacts caused by improper description of inertial effects is underlined. The advantages and the difficulties inherent to the method are discussed.