We present results from flume experiments in which an 8% steep channel with longitudinal width variations and step‐pool morphology was subjected to sediment feed pulses of different magnitude and frequency under constant water discharge. The channel response to these pulses included (a) large bedload transport rates, (b) bed aggradation, (c) fining of the bed surface, and (d) continuous formation and collapse of steps. In between pulses, the bed surface coarsened, and bedload transport rates dropped by several orders of magnitude. Steps continuously formed and collapsed during and shortly after the pulses, but their stability increased when the sediment feed was turned off. High pulse magnitude enhanced step formation, while low pulse frequency (i.e., long interpulse period) enhanced step stability. We back‐calculated the threshold for motion based on measured bedload transport rates and bed shear stress. Changes in the threshold for motion were much larger than changes in bed surface slope. By accounting for energy dissipation through the effective slope based on flow resistance partitioning, a better prediction was obtained. The threshold for motion decreased following sediment pulses then increased immediately after and fluctuated until the next sediment pulse. Our results indicate that longitudinal width variations and episodic sediment supply are primary controls on the evolution of step‐pool channels. Sediment feed magnitude affects mostly morphological changes, while sediment feed frequency controls channel stability.

Once a landslide dam bursts, its reservoir discharges quickly in a flood which will cause catastrophic damage to life and property downstream. For a specific landslide dam, the peak flow rate and the evolution of downstream flood are influenced by the shape and size of the dike breach when dam‐break occurs. According to the general nature of landslide dams and field observations of dike‐breach development patterns, a dike‐breach propagation mode has been determined. By combining an improved empirical equation with knowledge of the dike‐breach propagation mode, a mathematical model for forecasting dam‐break flood routing has been developed and is presented here. Sensitivity analysis was then carried out based on the computed results for peak flow rate and the flood evolution curve under different parameters. The computed results showed that the width coefficient and the depth coefficient had similar effects on the dam‐break flood but that the impact of the depth coefficient was more significant than that of the width coefficient. Finally, the proposed model was used to calculate the flood evolution for the Tangjiashan landslide dam. The computed results showed that the error between the simulated result and the measured data was less than 5%.

Flexible grade control structures (GCSs) are increasingly popular alternative countermeasures consisting of conventional solid structures for protecting riverbeds from erosion. Among this structure typology, tetrahedron framed permeable weirs (TFPW) were proposed to stabilize riverbeds and protect cross-river structures. The bed morphology as well as the interactions among the structures, water flow and bed features have not been systematically analyzed. Therefore, based on the field data collected in the Jianjiang River, a series of physical model experiments were carried out in this study to investigate the river morphology in unstable gravel-bed rivers under five different structural conditions, including no GCSs, solid bed sills and flexible TFPW with three different layout types. Data on bedform features, topography changes and the volume of erosion or deposition were collected and analyzed. The results indicated that dramatic general erosion and bed armoring would occur without any protection. The bed would be protected from upstream erosion with bed sills, but, at the same time, in addition to general erosion and bed armoring downstream of the structures, deep local scour might pose a serious threat to structural safety. TFPW had good protective effects on riverbeds by preventing the bed from erosion and even inducing sediment deposition both upstream and downstream from the structures. Because the flexible structures have advantages in ecology, structural stability and construction, TFPW were suitable and optimal for riverbed stabilization in unstable gravel-bed rivers.