Overtopping Breaching of Noncohesive Homogeneous Embankments

Homogeneous small-amplitude embankments were constructed in flumes from a range of uniform noncohesive materials and breached by overtopping flows under constant reservoir level conditions. Embankment erosion evolves from primarily vertical to predominantly lateral in nature. The breach channel initially erodes the downstream face of the embankment with an invert slope parallel to the face, the breach invert slope then progressively flattening to a terminal value by rotating about a fixed pivot point along the base of the embankment, the location of this pivot point being a function of the size of the embankment material. The breach channel is of a curved (hourglass) shape in plan. Below the water line, breach cross-section width B variation with elevation y above the breach invert is nondimensionally described by $B_{*}={\mathrm{2k}}_{*}{y}_{*}^{0.5}\text{,}$ where for the breach cross section at the embankment crest $k_{*}=-2.82\left[\ln \right(H_{b^{*}}\left)\right]+0.351\text{,}$ and $H_b$ is the centerline breach crest elevation. Breach discharge $Q_b$ can be nondimensionally expressed as a function of the head $h_b$ on the breach-crest centerline and the breach crest length in plan $L_b$ using $Q_{b^{*}}=0.242 L_{b^{*}}(h_{b^{*}}{)}^{1.5}\text{.}$ All expressions presented are applicable to full-width breach sections (double the half-breach section tested). The present findings enable prediction of the development with time of breach cross section, breach longitudinal profile, eroded volumes, and breach flows. The findings can be utilized for predictions of erosion and flooding occurring as the result of embankment failure, although in an engineering sense the quantitative findings of the present work await confirmation for larger embankments.