Slit structures: Fundamental mechanisms of mechanical trapping of granular flows

Slit-structures are rigid structures with one or more slits. To retard steep-creek hazards, slit-structures are sometimes installed to reduce the peak discharge, as well as to mechanically trap grains for frictional flows. However, designs are empirical. It is not possible to assess whether boulders will be trapped, or if smaller discharge of harmless flow material can pass downstream. This study starts by reviewing the inherent difficulties of characterising mechanical trapping processes for granular flows. A calibrated Discrete Element Method (DEM) model is then used to conduct a parametric study of the number of slits, the slit spacing, the Froude number and the ratio between grain and slit size. Results from the DEM study allow three interaction regions to be identified: 'self-cleaning', 'unstable' and 'trapping'. These regimes are extremely sensitive to the slit number, width and spacing, as well as the Froude number. This study shows that existing recommendations may coincide with the unstable region. However, the unstable region should be avoided because of unpredictable trapping behaviour. Additionally, increasing the slit spacing can increase material trapping because of an increase in the duration of binary grain contacts. This increase in the slit spacing tends to prevent shear transmission from slit to slit. Narrowly-spaced double slits may be most effective at the controlling outflow rate whilst enabling 'self-cleaning'.