Controls on the breach geometry and flood hydrograph during overtopping of noncohesive earthen dams

Abstract

Overtopping failure of noncohesive earthen dams was investigated in 13 large-scale experiments with dams built of compacted, damp, fine-grained sand. Breaching was initiated by cutting a notch across the dam crest and allowing water escaping from a finite upstream reservoir to form its own channel. The channel developed a stepped profile, and upstream migration of the steps, which coalesced into a headcut, led to the establishment of hydraulic control (critical flow) at the channel head, or breach crest, an arcuate erosional feature that functions hydraulically as a weir. Novel photogrammetric methods, along with underwater videography, revealed that the retreating headcut maintained a slope near the angle of friction of the sand, while the cross section at the breach crest maintained a geometrically similar shape through time. That cross-sectional shape was nearly unaffected by slope failures, contrary to the assumption in many models of dam breaching. Flood hydrographs were quite reproducible-for sets of dams ranging in height from 0.55 m to 0.98 mwhen the time datum was chosen as the time that the migrating headcut intersected the breach crest. Peak discharge increased almost linearly as a function of initial dam height. Early-time variability between flood hydrographs for nominally identical dams is probably a reflection of subtle experiment-to-experiment differences in groundwater hydrology and the interaction between surface water and groundwater.

Outburst floods provide erodability estimates consistent with long-term landscape evolution

“…2b ) the 2012 analogue Experiment 4 by Walder et al . 25 where a 1-m-high compacted-sand barrier impounded a 23.7 m 2 lake. We use an hypsometric curve that linearly reduces the water surface to 0 m 2 when the lake is empty (at z = 0 m).…”

“…2b is now satisfactorily fit with a much higher erodability k e of 4.1 10 2 m yr −1 Pa −1.5 , in agreement with the weaker nature of the impounding dam relative to Bonneville’s.

Figure 2 Evolution of two model runs inspired by the Lake Bonneville megaflood (left) and an analogue physical experiment 25 (right). Model results: water velocity and discharge ( a , b ) and the levels of the lake water and the outlet sill ( c , d ); the area shaded in pink shows the effect of a 10% change in erodability; circles indicate field and experimental measurements of flow.

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“… Evolution of two model runs inspired by the Lake Bonneville megaflood (left) and an analogue physical experiment 25 (right). Model results: water velocity and discharge ( a , b ) and the levels of the lake water and the outlet sill ( c , d ); the area shaded in pink shows the effect of a 10% change in erodability; circles indicate field and experimental measurements of flow.…”

A Guide to Breach Prediction

“…Figure 10 -Smaller laboratory experiments are often used in conjunction with larger field studies when validating breach models. Image source: Walder et al (2015).…”

Laboratory Experiments on Breaching Characteristics of Natural Dams on Sloping Beds

“…e size of the breach and the magnitude of the resulting flood are controlled by many factors, which include geomorphological factors, e.g., bed slope and dam's geometry, sediment characteristics, and so on. Many researchers have studied the influence of dam's geometry on failure process and discharge hydrograph and found a steeper downstream slope of dam could result in a larger peak discharge and shorter duration [17][18][19][20]. e breaching process may differ notably due to different compositions of fine and coarse particles.…”