How well can we measure peak discharge and volume? Instantaneous 3D LiDAR measurements of multiple debris flows at three locations along a channel

Debris flows represent significant landslide hazards and assessing their destructive potential requires understanding and predicting key parameters such as volume and peak discharge. However, a lack of detailed field data limits our understanding of the spatio-temporal evolution of these quantities. This study addresses these unknowns by utilizing a new suite of high-resolution measurements from 3D LiDAR scanners, which measure instantaneous velocity and flow depth fields. We analyze four debris flows at three monitoring stations along the Illgraben fan. Discharge and volume estimates derived from traditional methods are compared against a novel column-wise (CW) method, which calculates discharge by integrating area and velocity for discrete vertical columns, allowing the exclusion of deposited material (where v = 0 ). We classify the analyzed events into three types: a) fast front, b) depositional and c) wavy. For type a and b events, traditional methods overestimate event volume by >2× compared to the CW approach. This overestimation is attributed to assuming a constant or depth-dependent velocity that does not account for the decrease in velocity after the passage of the front or waves. The overestimation further results from the inclusion of deposited material (e.g. levees) in the cross-sectional area. For type c events, traditional methods underestimate peak discharge associated with wave arrivals, which reached up to 5× the front discharge. Our results align well with empirical relations between volume and peak discharge and are an important step towards obtaining more accurate values of these quantities, which are crucial for the design of check dams and retention basins. • Four debris flows were recorded at three monitoring stations along the Illgraben fan. • High-resolution 3D LiDARs enable a column-wise method to get discharge and volume. • Traditional methods overestimate volume >2× for fast front and depositional flows. • For events with surge waves, traditional approaches underestimate peak discharge. • The peak discharge of these waves can be up to 5× the front discharge. • Traditional methods fail to exclude deposited material and overestimate velocities.