4:00 PM - 4:15 PM
[MIS16-02] Formation conditions and mechanical properties of aggregates produced in tephra-water-snow flows
Keywords:Lahar, Mixing experiment, Compression test
In active volcanic zones capped with snow and glaciers, snow and ice melts are produced during volcanic eruption and rain-on-snow events. The resulting meltwater stream incorporates pyroclastic materials called tephra to become a lahar. The dynamics of lahars have been investigated through both laboratory experiments and numerical models. Previous studies on lahars have typically focused on two-phase flow consisting of tephra and water; however, recent field surveys of lahars have also revealed the occurrence of snow and ice in the deposition areas [1, 2]. These findings suggest that lahars triggered by snowmelts consist of snow as well as tephra and water. The dynamics of lahars should be elucidated through experiments and numerical models on three-phase flow consisting of tephra, water, and snow. In a pioneering experiment flowing a mixture of tephra, water, and snow into a channel, aggregates consisting of tephra, water, and snow were formed in the flow [3]. Such aggregates decreased the flow mobility as resistance. At present, no evidences of the aggregates have been reported in field surveys on lahars. Therefore, to elucidate the formation conditions and mechanical properties of the aggregates in three-phase (tephra-water-snow) flows, we performed mixing experiments of the samples on a rotating table and compression tests of the aggregates using a universal compression machine in the low-temperature room.
In the mixing experiments, we enclosed the sieved tephra and snow, water in a capped cylindrical glass pot such that the total volume was constant; then, the pot rotated at a constant angular speed on the rotating table. After rotating the pot, we removed the mixture onto a sieve and checked for the formation of aggregates. When aggregates were observed, we measured the number of aggregates and the mass of each aggregate on a scale. To confirm the mass composition of tephra in the aggregate, we dried the aggregate with an electric drying oven. In the compression tests, we measured the force and the stroke with a constant compression speed. The relationship between the stress and strain was obtained using the effective area and height of the aggregate, where we approximated the aggregate by the ellipsoid. In addition, we estimated mechanical parameters of the aggregate by making the fitting curve of the stress change against the strain.
From the mixing experiments and the compression tests, the following results were obtained: (i) the aggregate grew rapidly and reached maturity after a mixing time of 5 min; (ii) the mass of aggregates increased with snow concentration of flows, exhibiting an approximately linear relationship; (iii) single aggregates formed at lower and higher tephra concentrations, whereas multiple aggregates were observed at intermediate concentrations; (iv) the compressive mechanical behaviour could be modeled by an empirical nonlinear model. Further, to discuss the formation and the sustention of aggregates in natural lahars, we attempted scaling analysis based on experimental results and observational data. The typical diameter of aggregates was estimated by the particle Reynolds number. The sustention of aggregates was evaluated by the non-dimensional parameter, which is defined as the ratio of the strength of the aggregates divided by the dynamic pressure of the fluid. As the result, we concluded that the aggregates cannot form in lahars. Our findings suggest that snow and ice in lahars are dispersed without such aggregates.
[1] Cronin SJ, Neall VE, Lecointre HA, Palmer AS (1996) Unusual ``snow slurry'' lahars from Ruapehu volcano, New Zealand, September 1995. Geology 24:1107–1110.
[2] Lube G, Cronin SJ, Procter JN (2009) Explaining the extreme mobility of volcanic ice-slurry flows, Ruapehu volcano, New Zealand. Geology 37:15–18.
[3] Okita R, Kawashima K. Matsumoto T, Kataoka KS, Watabe S (2018) Influence of snow on the fluidity of lahars triggered by snowmelt. In: Proceedings of cold region technology conference 34:I-007 (in Japanese).
[4] Niiya H, Oda K, Tsuji D, Katsuragi H (2020) Formation conditions and mechanical properties of aggregates produced in tephra-water-snow flows. Earth, Planets and Space 72:148.
In the mixing experiments, we enclosed the sieved tephra and snow, water in a capped cylindrical glass pot such that the total volume was constant; then, the pot rotated at a constant angular speed on the rotating table. After rotating the pot, we removed the mixture onto a sieve and checked for the formation of aggregates. When aggregates were observed, we measured the number of aggregates and the mass of each aggregate on a scale. To confirm the mass composition of tephra in the aggregate, we dried the aggregate with an electric drying oven. In the compression tests, we measured the force and the stroke with a constant compression speed. The relationship between the stress and strain was obtained using the effective area and height of the aggregate, where we approximated the aggregate by the ellipsoid. In addition, we estimated mechanical parameters of the aggregate by making the fitting curve of the stress change against the strain.
From the mixing experiments and the compression tests, the following results were obtained: (i) the aggregate grew rapidly and reached maturity after a mixing time of 5 min; (ii) the mass of aggregates increased with snow concentration of flows, exhibiting an approximately linear relationship; (iii) single aggregates formed at lower and higher tephra concentrations, whereas multiple aggregates were observed at intermediate concentrations; (iv) the compressive mechanical behaviour could be modeled by an empirical nonlinear model. Further, to discuss the formation and the sustention of aggregates in natural lahars, we attempted scaling analysis based on experimental results and observational data. The typical diameter of aggregates was estimated by the particle Reynolds number. The sustention of aggregates was evaluated by the non-dimensional parameter, which is defined as the ratio of the strength of the aggregates divided by the dynamic pressure of the fluid. As the result, we concluded that the aggregates cannot form in lahars. Our findings suggest that snow and ice in lahars are dispersed without such aggregates.
[1] Cronin SJ, Neall VE, Lecointre HA, Palmer AS (1996) Unusual ``snow slurry'' lahars from Ruapehu volcano, New Zealand, September 1995. Geology 24:1107–1110.
[2] Lube G, Cronin SJ, Procter JN (2009) Explaining the extreme mobility of volcanic ice-slurry flows, Ruapehu volcano, New Zealand. Geology 37:15–18.
[3] Okita R, Kawashima K. Matsumoto T, Kataoka KS, Watabe S (2018) Influence of snow on the fluidity of lahars triggered by snowmelt. In: Proceedings of cold region technology conference 34:I-007 (in Japanese).
[4] Niiya H, Oda K, Tsuji D, Katsuragi H (2020) Formation conditions and mechanical properties of aggregates produced in tephra-water-snow flows. Earth, Planets and Space 72:148.