Abstract

Debris flows affect people and infrastructure around the world, and as a result, many numerical models and modelling approaches have been developed to simulate their impacts. Observations from instrumented debris-flow channels show that variability in inflow depth, velocity and discharge in real debris flows is much higher than what is typically used in numerical simulations. However, the effect of this natural variability on numerical model outputs is not well known. In this study, we examine the effects of using complex inflow time series within a single-phase runout model utilizing a Voellmy flow-resistance model. The interactions between model topography and flow-resistance were studied first using a simple triangular hydrograph, which showed simulated discharges change because of local slopes and Voellmy parameters. Next, more complex inflows were tested using time series based on 24 real debris-flow hydrographs initiated from three locations. We described a simple method to scale inflow hydrographs by defining a target event volume and maximum allowable peak discharge. The results showed a large variation in simulated flow depths and velocities arising from the variable inflow. The effects of variable inflow conditions were demonstrated in simulations of two case histories of real debris flows, where the variation in inflow leads to significant variations in the simulation outputs. The real debris-flow hydrographs were used to provide an indication of the range of impacts that may result from the natural variability in inflow conditions. These results demonstrate variation in inflow conditions can lead to reasonable estimates of the potential variation in impacts.

Highlights

  • We modelled 210 each event using the 24 real input hydrographs scaled to the observed event volume, and selected Voellmy parameters based on calibrations not considering variable inflows

  • Future research using real hydrograph inputs with other single-phase numerical models, as well as with multi-phase numerical models that allow for 405 hydrograph inputs, could provide insight into how sensitive these behaviours are to varying inflow

  • We developed a modified version of the Dan3D runout model that allows for a hydrograph input

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Summary

Introduction

Debris flows are a common hazard in mountainous terrain. They are characterized by periodic, surging flows of water and debris in channelized paths that can affect people and infrastructure, with disproportionate effects on lower income countries 30 (Hungr et al, 2014; Dowling and Santi, 2014). Theoretical examinations of the development of surges have included solid-fluid mixture theory, with unsteady, coupled changes in fluid pressures and granular temperatures leading to the unsteady nature of the flow (Iverson, 1997), or variation in the basal resistance and pore pressure, with material segregation resulting in a drained, higher resistance flow front progressively transitioning to a fully fluid flow 45 (Hungr, 2000). Models employing these theories can reproduce surge formation, the simulated flows are not as complex as observed real debris flows.

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