Predicting the breach outflow hydrograph due to overtopped embankment dams is important for the development of flood inundation maps. Scaling the breach outflow characteristics from overtopped non-cohesive geometrically distorted dam models in the Hydraulic Laboratory at the University of Ottawa is investigated using the Froude criterion and substantiated by scale series tests. Another objective of this study is to examine the use of void ratio as an alternative scaling criterion for mobile bed models simulating dam breach flows, using a second scale series test. Photographic images of the breach channel captured during the test revealed an upstream-control section forming on the upstream face of the dam model in the shape of an arc, which migrates upstream as the breach channel slope retreats downwards due to erosion. The arc resembles an ogee-crest, defining the locus of Froude-critical points upstream the breach channel entrance. The discharge over the arched crest was approximated using weir equations and compared to the measured breach outflow. It was found that a correction factor for the estimated discharge must be accounted for constriction effects due to undercutting in the breach channel entrance and submergence effects due to side-slope collapses occurring in the downstream section of the breach channel. While the Froude criterion is substantiated for the distorted scale series for horizontal dimensions, scale effects did exist laterally. With respect to the compaction scale-series, scale effects in both the horizontal and lateral dimensions were minor.
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