Coarse grains accumulate in geophysical flow fronts and have high solid fractions. Such fronts may arch in slit structures such as baffles and slit dams, leading to the rapid trapping of particles and potentially high-energy overflow. Existing empirical slit-structure design recommendations are limited and inadequate since they only focus on the slit size to particle diameter ratio (s/δ) and neglect the pileup height and pre-impact flow energy. Flume modelling was thus adopted to study coarse flow fronts impacting a slit structure. The characteristic Froude conditions, flow particle diameter and the ratio s/δ were varied. Results have shown the pileup height, and hence the confining stress is dependent on Froude conditions, but is not strongly influenced by s/δ. The flow particle diameter influences collisional and frictional stresses and hence the mean outflow rate, which is correlated with pileup height. Grain-trapping efficiency depends on both s/δ and Froude conditions. In contrast to existing continuum-based theory for slit-structure interaction, frictional contacts should be considered for coarse-grained flow fronts. High-energy supercritical flows lead to low trapping efficiency since stable arches cannot form at high shear rates. This implies that multiple slit structures may be more appropriate for attenuating high-energy supercritical flows.

Multiple barriers are commonly installed along predicted geophysical flow paths to intercept large flow volumes. The main criterion for multiple-barrier design is volume retained. The velocity of the incoming (far-field) undisturbed flow is also sometimes used, although this neglects the influence of other obstacles on the flow characteristics. This study investigates the influence of upstream flowbarrier interaction on downstream runup and impact mechanisms of a dual rigid barrier system. Four physical flume tests were performed using dry sand to investigate flow interaction with dual barriers. Moreover, three-dimensional finite-element simulations were conducted to back-analyse the flume tests and to investigate the effects of upstream barrier height and barrier spacing on downstream impact characteristics. Two key interaction mechanisms that alter downstream flow are identified: (a) flow momentum redirection (i.e. runup) at the upstream barrier, reducing pre-impact momentum at the downstream; and (b) downstream flow-thinning. Runup mechanisms at the upstream barrier and flowthinning between the two successive barriers have profound effects on dynamic impact pressures at the downstream barrier. When the upstream barrier height is taller than twice the maximum flow thickness, flow energy can be dissipated effectively by momentum redirection. The downstream barrier height and design impact pressure can be reduced up to 17% and 35% for dry sand flows, respectively.

One of the challenges associated with small-scale flume modelling is achieving dynamic similarity. Froude (Fr) scaling is commonly adopted to capture the bulk characteristics of a flowing medium. Although the Fr number cannot capture the microinteractions of a flow medium, it is commonly adopted by engineers, due to its simplistic nature for characterising mass-wasting processes. Given the prevalence of Fr scaling, an improved understanding of the development of Fr characteristics for channelized surge flows is certainly warranted. A 5-m long rectangular flume model was adopted to carry out experiments using dense uniform dry granular and water flows, separately. Laser and photoconductive sensors, and high speed imagery were used to estimate flow velocity and thickness. Results reveal that the Fr behaviour of uniform dry sand and water flows is dependent on its energydissipation mechanism. The initial volume has a greater influence on suppression of Fr conditions compared to shallower channel inclinations for both granular and water flows. The major limitation of small-scale flume modelling lies in limited initial volumes. Limited initial volumes lead to shallow flow depths and results in the flow velocity controlling Fr development of the flow mass with transportation. Frictional materials are not favourable for developing low Fr numbers in flume modelling.