Debris-flow surges of a very active alpine torrent: a field database

This paper presents a methodology to analyse debris flows focusing at the surge scale rather than the full scale of the debris-flow event, as well as its application to a French site. Providing bulk surge features like volume, peak discharge, front height, front velocity and Froude numbers allows for numerical and experimental debris-flow investigations to be designed with narrower physical ranges and thus for deeper scientific questions to be explored. We suggest a method to access such features at the surge scale that can be applied to a wide variety of monitoring stations. Requirements for monitoring stations for the methodology to be applicable include (i) flow height measurements, (ii) a cross-section assumption and (iii) a velocity estimation. Raw data from three monitoring stations on the Réal torrent (drainage area: 2 km2, southeastern France) are used to illustrate an application to 34 surges measured from 2011 to 2020 at three monitoring stations. Volumes of debris-flow surges on the Réal torrent are typically sized at a few thousand cubic metres. The peak flow height of surges ranges from 1 to 2 m. The peak discharge range is around a few dozen cubic metres per second. Finally, we show that Froude numbers of such surges are near critical.