Debris flows are water-laden masses of soil and rock, which are common geological hazards in mountainous regions worldwide (Iverson, 1997). Over the past decades the occurrence and hazardous effects of debris flows have increased as a result of population expansion in mountainous regions, climate change, severe wildfires, and earthquakes (Cannon & DeGraff, 2009;Stoffel et al., 2014). The magnitude of debris flows can increase substantially by basal and bank erosion while it traverses from initiation zone to valley floor (Frank et al., 2015) resulting in an increase in casualties and property loss (Dowling & Santi, 2014). In addition, debris flows are increasingly recognised as one of the fundamental physical processes that transport sediment and erode bedrock in mountainous topography (McCoy, 2015;Stock and Dietrich, 2003). However, limited understanding of the processes that control debris-flow erosion currently hampers (a) accurate estimation of debris-flow magnitude and effective hazard mitigation (De Haas et al., 2020;Dietrich & Krautblatter, 2019) and (b) understanding and modeling of landscape evolution (Penserini et al., 2017; Tucker & Hancock, 2010).Observations show that erosion volumes may strongly vary between debris-flow events: some flows increase >50 times their initial volume (Hungr et al., 2005), while others barely increase in size (Santi et al., 2008), and we currently lack the means to explain these contrasting pathways of development. Understanding debris-flow erosion is notoriously complicated for a number of reasons: (a) debris flows are complex hybrids between a fluid flow and a moving mass of colliding particles that may vary greatly in composition, such that both shear and

Debris flows are gravity-driven mass movements that are common natural hazards in mountain regions worldwide. Previous work has shown that measurements of ground vibrations are capable of detecting the timing, speed, and location of debris flows. A remaining question is to what extent additional flow properties, such as grain-size distribution and flow depth can be inferred reliably from seismic data.Here, we experimentally explore the relation of seismic vibrations and normal-force fluctuations with debris-flow composition and dynamics. We use a 5.4 m long and 0.3 m wide channel inclined at 20 , equipped with a geophone plate and force plate.We show that seismic vibrations and normal-force fluctuations induced by debris flows are strongly correlated, and that both are affected by debris-flow composition.We find that the effects of the large-particle distribution on seismic vibrations and normal-force fluctuations are substantially more pronounced than the effects of water fraction, clay fraction, and flow volume, especially when normalized by flow depth. We further show that for flows with similar coarse-particle distributions seismic vibrations and normal-force fluctuations can be reasonably well related to flow depth, even if total flow volume, water fraction, and the size distribution of fines varies. Our experimental results shed light on how changes in large-particle, clay, and water fractions affect the seismic and force-fluctuation signatures of debris flows, and provide important guidelines for their interpretation.

<p>Debris flows are water-laden masses of soil and rock, which are common geological hazards in mountainous regions worldwide. They can grow greatly in size and hazardous potential by eroding bed and bank materials. However, erosion mechanisms are poorly understood because debris flows are complex hybrids between a fluid flow and a moving mass of colliding particles, bed erodibility varies between events, and field measurements are hard to obtain. Here, we combine detailed flow measurements, rainfall data, and high-resolution UAV measurements of channel-bed erosion and deposition for 13 debris flows in the Illgraben (CH), to identify the key controls on debris-flow erosion and deposition. We show that flow conditions and bed wetness jointly control debris-flow erosion. Flow conditions that describe the cumulative forces exerted at the bed over the full event (flow volume, cumulative shear stress, and seismic energy) have the strongest correlations with measured erosion and deposition. However, we also find statistically significant correlations between erosion and deposition and frontal flow properties, including frontal velocity, flow depth, shear stress, and peak discharge. Antecedent rainfall over a period of 2-3 hours prior to the debris-flow events strongly correlates to erosion and deposition, while the correlation decreases in strength and diminishes towards shorter and longer time periods of antecedent moisture. Shear forces and particle-impact forces are strongly correlated and act in conjunction in the erosion process. A shear-stress approach accounting for bed erodibility may therefore be applicable for modelling and predicting debris-flow erosion.</p>

<p>Debris flows are a highly hazardous landslide type, and their impact forces, peak discharges and runout distances are dependent on the flow velocity. Knowledge of flow velocities is therefore often required for hazard planning and mitigation, as well as for validating numerical models. One commonly used method for post-hoc estimation of debris flow velocities uses the mudlines left behind by a passing flow as it travels through a bend. The surface inclination derived from these mudlines can be used to estimate velocity based on the forced vortex equation, originally developed for clear water flows and later adapted to debris flows using a correction factor <em>k</em> to back-calculate the flow velocity<sup>1,2</sup>:</p> <p><img src="" alt="" width="229" height="38" /></p> <p>where <em>R<sub>c</sub></em> is the radius of curvature of the bend, <em>g*</em> is the bed-normal component of acceleration due to gravity, <em>B</em> is the flow width, and <em>Δh</em> is the difference in elevation of the flow surface between the inner and outer bend.</p> <p>This approach involves some uncertainties, however, such as how best to define the radius of curvature, the influence of roll waves and splashing on the post-event mudlines used to measure the surface inclination, as well as the meaning and appropriate value of the correction factor <em>k</em>. In this study, we first derive a database of superelevation velocity estimates based on pre- and post event UAV data for seven events from the years 2019 to 2021 in the monitored Illgraben torrent in Switzerland. Analysis of this database firstly indicates that the placement of cross-sections for surface inclination measurements is more important than how the radius of curvature is defined due to the large influence of local topography on mudlines. Secondly, the data indicates that the correction factor <em>k</em> increases nonlinearly with decreasing Froude numbers, as has been previously suggested<sup>2,3</sup>. The correction factors were back-calculated using eq. 1 and reference velocities from geophone detections of the front arrival, and seemed to range between approximately 1 and 7. We next present a first comparison of these data to surface inclination and radius of curvature values derived from high-resolution 3D LiDAR scanners for one event in the summer of 2022. We use this unique dataset to directly derive the radius of curvature (based on surface velocity vectors) and surface inclination of the flow, as well as the appropriate correction factor.  We compare these values to those derived by the above-mentioned commonly used method based on bend topography and post-event mudlines to assess the efficacy of these methods. This preliminary study thus provides a validation of the superelevation approach and will provide a basis for more in-depth research on this topic.</p> <p> </p> <p><sup>1</sup> Hungr, O., Morgan, G.C. and Kellerhals, R., 1984. Quantitative analysis of debris torrent hazards for design of remedial measures. <em>Canadian Geotechnical Journal</em>, <em>21</em>(4), pp.663-677.</p> <p><sup>2</sup> Scheidl, C., McArdell, B., Nagl, G. and Rickenmann, D., 2019. Debris flow behavior in super-and subcritical conditions. <em>Association of Environmental and Engineering Geologists; special publication 28</em>.</p> <p><sup>3 </sup>Scheidl, C., McArdell, B.W. and Rickenmann, D., 2015. Debris-flow velocities and superelevation in a curved laboratory channel. <em>Canadian Geotechnical Journal</em>, <em>52</em>(3), pp.305-317.</p>