（様式３）新潟大学災害・復興科学研究所
共同研究報告書

The Spatio-temporal control of pumice vesicularity on lahar disasterrisks (ラハール災害リスクにおける軽石発泡度の時空間変化による制約)
Christopher GOMEZ 1)
Kyoko KATAOKA 2)
1）Kobe University, Graduate School of Maritime Sciences
2）Niigata University, Research Institute for Natural Hazards and Disaster Recovery(Collaborating Partner)
研究要旨
The state of the art in debris flow, hyperconcentrated flow and their volcanic equivalent,lahars, has been developed from theory and fieldwork where the material is clastic (i.e.
non-volcanic) and with a higher density than the interstitial fluid. However, hydrologicalremobilization of pumiceous ignimbrites can generate flows that escape those traditionalmodels.
Using material produced by Numazawa Volcano, the present research aimed (1) to measure therate of water incorporation in the dry material, while at rest and when in movement, (2) andto measure abrasion rates in a flow.
The method relies on a series of benchtop experiments, measuring the mass change after various
period of immersion in either still water or turbulent water.
The result shows that water incorporation is asymptotic and after a rapid entry in the outerlayer, the incorporation rates slows exponentially. This is consistent with the physical modelof plume, contaminant, and groundwater through a porous media, where the progression follows
a model of the format 𝜕𝐷/𝜕𝑡 =∝. ∇2 . 𝐷 where D is the diffusion rate, ∇2 is the Laplace
operator, ∝ is the diffusion coefficient and t is the time. Results from material in movementshow that the relation does not hold through however. As the pumice clasts increase in mass
and gain momentum during flow transport, they start to loose mass by abrasion once a criticalthreshold of momentum is reached (either by velocity increase or by mass increase).
Ａ．研究目的total discharge of 1.6 km3 was derived from a

The research objective has been the measure

break of temporal dammed lake formed by theof the change in the “volcaniclastic sedimentignimbrite emplacement to the Tadami Riverload “having variable grain density such as

valley at Numazawa Volcano,pumice under a sediment-laden flow condition

research aims to (1) measure the waterwith an example of a gigantic outburst flood

absorption rate of the pumiceous material when

event that swept the Tadami River and Agano

it is at rest, representing the water absorbed

River (Kataoka et al., 2008).The flood with aby the temporary ignimbrite dam that barred theAlso, thisTadami River valley and (2) to estimate theweighing machine with a calibrated 0.001 mg

water absorption and pumice abrasion during

precision. After each plunge, the samples wereflow transport.

kept one to two seconds above the water toremove the capillary water at the surface. The
experiments were repeated for each sample 6Ｂ．研究方法

times to have results over the first minute ofThe methodology relies on laboratory testing

absorption. The work was then repeated overof pumiceous material of coarse sand size tolonger period of time, with a measure every 2

cobble size at different locations along thedays during a two-weeks period.

valley and at existing outcrop and quarries

In a second time, to simulate the incorporation

along the Tadami River and Agano River (fig.

of water in pumice during movements, a second1).

series of measure was performed using a mixer
in a 1-liter size beaker (Fig.2)
Fig.2 Mixer and beaker
used to test the waterincorporation in thepumice during the flow.
Themixer

has

aregulator that allows
thecontroloftheangular velocity. Theapparatus was plunged
in the upper part of thebeaker to avoid anycontact

withtherotating pumice clasts
during operation. In
the glass beaker, the boundary roughness has
been disregarded as a potential source ofFig. 1 General location of Numazawa Lake andabrasion.

the Tadami River and Agano River from where theoutburst flood travelled down to the Niigata

Ｃ．研究結果plain, about 150 km downstream.

The results show that the water incorporation
rate in the pumice is a function of theIn the laboratory, a first set of sample wasvesicularity and the size of the pumice. As thecut into rectangular shape for absorption rate

sampled chosen had a comparable vesicularity,

experiments: 1) by capillarity absorption andit yields that the wet mass of the pumice after2) kept as a whole for full size absorption,

one minute is a linear function of the dry massby plunging the samples in water. The samplesof the pumice, with a good correlation (Fig.

were plunged into the water or left in3).

capillarity contact for series of 10 seconds
periods and then weighed using a Metler Toledo

will not be reached due to a geomorphologic or

WET MASS - 10' (G)30rheological control but due to the time they = 1.3591x - 0.008
R² = 0.996

25
20

material property (the mass) changes.
Secondly, it also suggests that the material

15

size at the start of a given single flow will10

be very different from the one at the end, withabrasion occurring much faster than in clastic

5equivalent. This can thus explain the rapid

005101520

transport of material over long distance.

25

DRY MASS (G)Existing

modelsofdebris

flowsandFig. 3 Relation between dry and wet mass after

hyperconcentrated flows are therefore notone minute in water.

adapted and there is a need to develop new
simulation

methods

toaccountfortheThis correlation inverses for pumice that has

peculiarities of pumiceous sediment-laden

been plunged in flowing turbulent water forflows.

several minutes. The test on a 14.8 g pumice
at time 0, with water absorbed over a 2-weeks

Ｅ．結論

period, shows that its mass decreases to 14.5Pumiceous hyperconcentrated flows have ag at 80 minutes, 14.35 g at 140 minutes,

complex rheology with modification of the massreaching 14.2 g at 200 minutes (3h20 min). Itoffollows

meaning that for a given slope of energy, oralinearrelation,

showingathe“volcaniclasticsediment-load”,

progressive abrasion.

even a regular topography, the deposit and theThis abrasion of the pumice has been tied down

flowage of the material will display complex

to two different factors such as 1) thebehaviours.

presence of glass shards in the water that
“erode” the pumice clasts and 2) the contact
between the pumice clasts, “rubbing” oneagainst another.
Ｄ．考察

Ｆ．研究発表1.論文発表なし(掲載誌名･巻号･頁･発行年を記入し，2.学会発表（学会名・発表年月・開催地なども記入）

The present research indicates that (1) theGomez, C., Kataoka, K. 2018. Differentiatingwater incorporation in the pumice can beTransport

considered as a function of the size of theHyperconcentrated-flows

material. Therefore, the grain size andVolcano-Japan. Japanese Geophysical Union

sorting of the material will have an important

Meeting, Makuharimesse, Japan, May 20th-24th.

MechanismofPumices

frominNumazawa

role on how fast the water is being absorbed.
(2) The mass increase occurs when pumice is at
rest and/or when a threshold momentum is notreached. Passing this threshold, the massdecreases again by abrasion.
This phenomenon should therefore be considered
for hazard analysis, as the maximum momentum

Ｇ．知的財産権の出願・登録状況（予定を含む）
なし