Numerical Modeling of Infrasound Energy Radiation by Debris Flow Events

M Coco,O Morandi,E Marchetti

doi:10.1007/s00024-021-02759-2

M Coco, O Morandi + Show 1 more

Open Access

https://doi.org/10.1007/s00024-021-02759-2

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Abstract

Debris flows constitute a severe natural hazard in Alpine regions. Studies are performed to understand the event predictability and to identify early warning systems and procedures. These are based both on sensors deployed along the channels or on the amplitude of seismic and infrasound waves radiated by the flow and recorded far away. Despite being very promising, infrasound cannot be used to infer the source characteristics due to the lack of a physical model of the infrasound energy radiated by debris flows. Here the simulation of water flow along a simple channel is presented, experiencing the fall from a dam, performed within the open source simulation code OpenFOAM. The pressure perturbation within the atmosphere produced by the flow is extracted and the infrasound signature of the events as a function of the flow characteristics is defined. Numerical results suggest that infrasound is radiated immediately downstream of the dam with amplitude and period that scale with dam height and water level. Modeled infrasound waveform is interpreted as being produced mostly by waves at the water free surface developing downstream of the dam. Despite the effect of sediments is not considered in this first study and will be implemented in future investigations, numerical results obtained with this simple model are in general agreement with experimental results obtained from array analysis of infrasound data recorded at Illgraben, Switzerland. Results highlight how numerical modeling can provide critical information to define a source mechanism of infrasound energy radiation by debris-flow, that is required also to improve early warning systems.

Highlights

Debris flows are water/sediment mixtures typically mobilized in steep mountain torrents affecting highly erodible areas
The comparison between the modeled flow evolution, as a function of dam height (Fig. 2) and water depth (Fig. 4), and the related pressure time series (Figs. 3, 5) allows us to define some basic features of infrasound energy radiated by a flow
Modeling shows that the generation of waves at the water free surface appears as a consequence of the water drop and is enhanced for larger water depths

Summary

Introduction

Debris flows are water/sediment mixtures typically mobilized in steep mountain torrents affecting highly erodible areas. To boulder size and to flow velocity as well as to boulder snout extension This model was confirmed by the direct seismic observations of the Montecito debris flow occured on 9th January 2018 (Lai et al 2018), despite additional seismic sources that might be related to water flow interaction with channel bed and banks and standing waves (Schmandt et al 2013). While increasing number of experimental observations have been performed in the last decade and a large dataset of events is available, numerical physical models of the infrasound energy radiation by debris flows and rivers are required to link acoustic waves to the initial physical parameters of the phenomenon, that is, density, velocity, terrain’s characteristics. Numerical results are eventually compared with experimental measurements of the infrasound wavefield produced by debris flow events recorded in the spring of 2017 at Illgraben in Switzerland (Marchetti et al 2019)

The Mathematical and Simulation Model

D Dt o ot þ

Simulation Results

Discussion and Conclusions

Boundary Conditions