GSD (grain size distribution), constitutes a paramount parameter for comprehending the behavior and dynamic mechanics of mass movements, such as debris flows, rock avalanches, sediment transport etc. Alongside traditional sieving methodologies, the past few decades have witnessed a growing interest in photo-sieving, the technique of deducing GSD directly from photographic data. Photo-sieving holds promise for augmenting the spatial and temporal resolution of superficial GSD analysis by virtue of its accessibility, reduced labor intensity, and non-invasive nature. Moreover, the integration of aerial photography within the discipline enables to include the coarse-grained fraction, expanding the scope of particle size analysis beyond the capabilities of traditional sieving. This study introduces a novel algorithm for extracting the coarse-grained fraction using UAV (unmanned aerial vehicle) photography. This novel approach enables us to analyze hectare-scale extents, probing tens of thousands of clasts - surpassing previous similar techniques by two orders of magnitude - and generates a detailed map of the position and dimensions of each particle within the sedimentary system. Furthermore, the algorithm exhibits remarkable resilience in navigating real-world complexities, effectively discerning clasts from vegetation, anthropogenic artifacts, and handling exceptionally large boulders, rendering it suitable for application in diverse field settings. We anticipate that this technique could become a valuable tool for advancing our understanding of debris flow and rock avalanche dynamics, sediment transport processes, and the stability of landslide dams.

Large earthquakes rapidly denude hillslopes by triggering thousands of coseismic landslides. The sediment produced by these landslides is initially quickly mobilised from the landscape by an interconnected cascade of processes. This cascade can dramatically but briefly enhance local erosion rates. Hillslope and channel processes, such as landsliding and debris flows, interact to influence the total mass, calibre, and rate of sediment transport through catchments. Calculating the sediment budget of an earthquake lends insight into the nature of these interactions. Using satellite imagery derived landslide inventories, channel surveys and a literature review combined with a Monte Carlo simulation approach we present a constrained sediment budget of the first decade after the 2008 Mw7.9 Wenchuan earthquake. With this sediment budget we demonstrate that debris flows are dominant process for delivering sediment into channels and that large volumes of sediment remain in the landscape. In our study area over 88% (469 Mega tonnes) of the coseismically generated sediment remains on the hillslopes in 2018. Of the 12% of the sediment that was mobilised, 69% (40.7 14 Mt) was mobilised by debris flows. Despite the large proportion of sediment remaining on the hillslope, the frequency of debris flows declined significantly over our observation period. The reduction in debris-flow frequency is not correlated to reductions in the frequency of triggering storms, suggesting changes in the mechanical properties of hillslope sediment may drive this observation. The stabilisation of coseismically generated sediment greatly extends its residence time and may influence catchment sediment yields for centuries or millennia.