Experimental investigation on the influence of valley-constraining dam length on breach hydrodynamics

Five flume experiments employing an L-shaped glass channel were designed to systematically investigate how along-channel dimensional variations in landslide dams govern breach mechanisms. Results demonstrate that such dimensional changes steepen the gradient ratios of upstream and downstream slope surfaces for a given dam height. Specifically, these steeper slopes accelerate water-level drawdown rates, concurrently amplifying peak discharges for both clear-water and suspended sediment discharge. Critically, upstream dimensional variations predominantly modulate breach flood discharge, while downstream dimensional changes dominantly control sediment concentration in the outflow. Furthermore, the position of the retrogressive erosion point correlates not only with median sediment particle size but also with slope gradient ratios. Steeper upstream slopes critically enhance the erosional coupling intensity and synchronisation between longitudinal Ez and retrogressive Ex mechanisms, thereby maximising breach efficiency. In contrast, downstream slope variations exhibit negligible impacts on these erosional dynamics. These findings provide vital theoretical foundations for earth dam engineering design and landslide-dam breach disaster mitigation.