Steep-creek hazards are a hazardous phenomenon that afflict mountainous areas across the globe. To prevent such flows from claiming lives and destroying infrastructure, active countermeasures such as closed or open check-dams may be installed to intercept potential flows. Interaction between flows and check-dams is governed by the Froude conditions. Furthermore, the fronts of certain types of steep-creek hazards may be dominated by grain frictional and inertial stresses. These fronts may form stable arches after impacting open check-dams, which can potentially cause rapid trapping of grains and dangerous overflow. Generally, studies in the open literature have focused on inviscid flows interacting with slit-structures. This fundamentally affects arch formation around the mouth o...[ Read more ]

Steep-creek hazards are a hazardous phenomenon that afflict mountainous areas across the globe. To prevent such flows from claiming lives and destroying infrastructure, active countermeasures such as closed or open check-dams may be installed to intercept potential flows. Interaction between flows and check-dams is governed by the Froude conditions. Furthermore, the fronts of certain types of steep-creek hazards may be dominated by grain frictional and inertial stresses. These fronts may form stable arches after impacting open check-dams, which can potentially cause rapid trapping of grains and dangerous overflow. Generally, studies in the open literature have focused on inviscid flows interacting with slit-structures. This fundamentally affects arch formation around the mouth of open check-dams, because the effective stress between grains is much lower than for dry flows. The dry case, representing grain-stress-dominated geophysical flow fronts, also needs to be considered.

A dimensional analysis was performed to identify key variables. In excess of 100 physical experiments and 140 numerical simulations are then used to investigate interaction between dry granular flows and open-check dams. The ratio between slit size and grain diameter (s/δ), grain diameter (δ), the upstream Froude conditions (Fr, via the channel inclination) were varied for monodisperse flows. In addition, physical tests using bi-disperse flows were carried out investigating the effects of the proportion of larger grains (Ψ_L) and the ratio between grain sizes (δ_L/δ_s) (i.e. the particle size distribution). The effect of these parameters on the pileup and trapping at the slit-structure, as well as the outflow downstream, were investigated.

Results have revealed that contrary to existing design recommendations, the ratio s/δ is insufficient for robust design of the width of slit-structure slits. For example, the flow grain diameters affects the ratio of collisional and frictional stresses, and thence the mean outflow rate. Furthermore, high-energy flows cause fewer grains to be retained, for stable arches tend not to form if the shear rate is high. The pileup height increases as the Froude conditions increase, and decreases as the slit width increases; this trend is captured by the proposed analytic solution. Introducing a second size of grain into the flow causes pileup heights to tend to increase. Additionally, if there are relatively few grains (< 20%) that have a diameter close to the slit width, smaller grains will tend to cause them to flow out by disrupting the formation of arches between the larger ones. Since the upstream Froude conditions, the grain diameter and the particle size distribution all have significant effects on pileup and trapping, as well as the outflow rate, they should be considered together with the ratio s/δ. This contrasts with existing guidelines which (at best) consider the ratio s/δ independently of other flow and structure properties. Furthermore, also contrary to existing design recommendations, it is shown that existing theory and empirical guidelines developed for inviscid granular flows should not be applied to grain-stress-dominated ones interacting with slit-structures. Finally, design charts obtained from DEM simulations are also presented to allow engineers to link upstream flow Froude conditions with the trapping efficiency and pileup height.