Debris flows contain mixtures of fluids and large boulders. These mixtures can pass through constrictions, either man-made or natural, for example, structures such as slit-dams, open check dams and sabō dams (hereafter all referred to using the umbrella term of "slit-dams"), or canyons and gullies (e.g., Hübl & Fiebiger, 2005;Rudolf-Miklau & Suda, 2013;Shima et al., 2016). In all cases, the flow rates of sediments and fluids are affected by both horizontal and vertical constrictions. Boulders that are larger than the constriction size are certainly jammed, whilst smaller boulders may become jammed if they approach the constriction as a group (Figure 1). This jamming can be used advantageously for slit-dams in areas where large-scale debris flows can occur. During small-scale routine events, where boulders are sparse, jamming ought not to occur. This minimizes the need for