A new method for evaluating stony debris flow rainfall thresholds: the Backward Dynamical Approach

Abstract

Debris flow rainfall thresholds aim to provide a level of rainfall duration and average intensity above which the probability of a debris flow occurrence is significant. Estimating reliable thresholds for use in early warning systems has proved to be a challenging task. In fact the methodologies currently available in the literature are not entirely satisfactory since they provide thresholds unlikely low. The goal of the present research is exploring new paths aimed at improving the reliability of rainfall thresholds. A possible weak point of the literature approaches is the way the duration and the average intensity pertaining to a debris flow is determined. Up to now, these values are evaluated using only the characteristics of the hyetograph associated to a debris-flow event. In the present paper, we propose a new methodology based on volumetric relations deriving from a simplified description of the dynamic of a stony debris flow: by using these relations, from a measure of the deposited volume it is possible to estimate backwardly the volume of rain that caused the deposition; then, from this last value and the knowledge of the relevant hyetograph, it is possible to reconstruct the duration and the average intensity. Application of this new technique to a sample study area allowed us to prove the feasibility of the method and, to some extent, its capabilities: with respect to a classical literature method, the new approach produces an higher threshold and a smaller characteristic duration scales. Finally, strengths and weakness of the method have been evaluated thoroughly.

Uncertainty analysis of the estimation of stony debris flow rainfall threshold: the application to the Backward Dynamical Approach

“…This work aims to assess the uncertainty related to the debris-flow threshold estimation based on the Backward Dynamical Approach (BDA), introduced by Rosatti et al (2019). Given a rainfall event related to a debris flow occurrence, the BDA allows to quantify the rainfall volume strictly pertaining to the surveyed debris flow.…”

“…Then, the effects of these outputs variability on threshold calibration are quantified with a further MC computation. Finally, the results are compared with the reference values obtained in Rosatti et al (2019) to assess their consistency.…”

“…The paper structure is the following. A brief description of the BDA method and the calibrated threshold obtained by Rosatti et al (2019) for a study area is presented in Sect. 2.…”

“…The Backward Dynamical Approach is an innovative method proposed by Rosatti et al (2019) that defines the rainfall volume strictly relevant to a stony debris flow in a physical-based way. As described in Rosatti et al (2019), the BDA starts from the knowledge of the deposited volume V dep occupied by the sediments, surveyed after a debris flow event.…”

“…The Backward Dynamical Approach is an innovative method proposed by Rosatti et al (2019) that defines the rainfall volume strictly relevant to a stony debris flow in a physical-based way. As described in Rosatti et al (2019), the BDA starts from the knowledge of the deposited volume V dep occupied by the sediments, surveyed after a debris flow event. Thanks to a simplified volumetric description of the phenomenon dynamic ( Fig.1), the rainfall volume pertaining to the debris flow V DF r , defined as the volume of water necessary to convey downstream as a mixture the volume of sediments V dep , can be express as a function of this latter volume:…”