The force exerted by granular flows on slit dams

Boulders often accumulate at the fronts of fast-flowing steep creek hazards. To trap these boulders, slit dams are commonly installed along steep creeks. An outstanding challenge when designing such dams lies in estimating the load exerted by a cluster of boulders: Clusters may exhibit discrete or continuum loading behavior, depending on their size. In this study, a physical flume test was carried out and the obtained results were used to calibrate a discrete element method (DEM) model. The DEM was then used to carry out a parametric study as a function of grain size, slit width and the channel slope. Results reveal that for continuum-like flows, the pressure near the slit of a slit dam can be almost double than elsewhere on the slit dam, as quantified using a newly proposed dimensionless ratio. For discrete flows, impact forces near the slit are more than double than elsewhere, thus representing a critical case for slit dam design. The concentration of forces near the slit necessitates designing for loads 2-3 times that of the rest of the slit dam to avoid wearing. (If wearing occurs, the slit width will increase, along with the volume of discharge, a potentially catastrophic scenario.) Furthermore, two dimensionless weighting coefficients that distinguish pressure from continuum-like flows and discrete impacts are proposed and evaluated. These coefficients can be incorporated into existing analytic expressions used by engineers.