Spatial patterns of exposure to sediment-laden flows on an experimental alluvial fan

Sediment-laden flows with significant amounts of large wood can impact alluvial fans and seriously damage infrastructure and property which makes a profound knowledge of exposure an essential requisite for risk mitigation. To investigate its spatial variability, we executed experiments on a simplified physical model and assessed the observed spatial process and exposure patterns by quantifying synthetic indexes and geostatistically analysing spatial probabilities. We systematically varied the loading conditions, i.e. total flow volume, solid fraction and the tank opening controlling water release, and repeated each experimental configuration eight times. Two alluvial fan layouts were considered, one equipped with a guiding channel and a bridge and one with a guiding channel only. First, we tested the hypothesis, that water released though the fully opened tank outflow valve induces a sediment-laden flow which is associated with higher exposure and lower spatial uncertainty of exposure if compared to flows generated by a half-opened tank outflow valve. Second, we tested whether a higher flow volume is associated with higher exposure on the alluvial fan and with lower spatial uncertainty. It turned out that neither of the aforementioned hypotheses is verified for the whole set of tested flow conditions. The first hypothesis is rejected in the majority of the considered conditions. The second hypothesis is prevailingly corroborated when the exposed areas due to both sediment and water were considered. Instead, when only the areas of sediment deposition are considered, this hypothesis is prevailingly corroborated on the alluvial fan featuring the presence of the bridge. We provided exposure probability maps for all experimental conditions and presented the variability of exposure by standard deviation ellipses. Although solely indicative for the adopted alluvial fan layouts, a variation of the loading parameters led to remarkable changes in the patterns of exposure probability and the parameters of the standard deviation ellipses. Our results urge decision makers to fully acknowledge the potential variability of geomorphic responses on alluvial fans in their risk management practices as to avoid underestimating the impacts for the built environment.