Influence of water content on velocity of snowmelt due to pyroclastic material
Hiroaki IZUMIYAMA Public Works Research Institute, Japan
Takeshi SHIMIZU
Tasuku MIZUTANI
Kohei
NOIKE
(Now in National institute for land and Infrastructure management, MLIT, Japan)
Public Works Research Institute, Japan
Public Works Research Institute, Japan
Naoki
FUJIMURA Public Works Research Institute, Japan
Tadanori ISHIZUKA Rokko Sabo Office, MLIT, Japan
(Now in Nippon Koei Co.,Ltd. )
Public Works Research Institute, Japan
Daizo
TSUTSUMI Disaster Prevention Research Institute, Kyoto University, Japan
Introduction
Volcanic mud flow due to snow melt has caused disaster in large area( ex. Mt. Tokachi in 1926, Nevado del Ruiz in 1985).It is important to es mate an inunda on area, but it is difficult. →The mechanism is unclear. It seldom occurs and cannot be observed easily.
At first, to estimate the inundation area, we must assume hydrograph(Q ‐ t) as Boundary Condition. Murashige et al. (2015) developed a model with three basic assumptions. Actually, these assumptions cannot be accepted. Saturation ratio, temperature of soil particle and snow and snow density change drastically with time.
We conducted theoretical consideration, fundamental experiment and observation to investigate the influence of these property. Especially, we focused on the role of saturation ratio. Hiroaki IZUMIYAMA
Kohei NOIKE
Naoki FUJIMURA
What is the condition that the snow melt speed overcome infiltration speed?
It is difficult, but at the moment the pyroclastic material is
in contact with snow, we can estimate the condition theoretically as
E 1  rsl   Grsl  sn k  sn z 0 
Tt  T 0 z 0 
ks
Experiment condition
To confirm the influence of snow density, we set 3 cases.
Grain
Red dye
Stove
Cylinder
Where, Tt : threshold temperature of pyroclastic material, E : melting heat of snow, G : sublimation heat of snow, sn : wet snow density, k : permeability coefficient of snow, sn : pressure head, ks : heat conductivity, rsl : rate of heat loss of snow Run 2(loose)
Run 3(dense)
10000
1000
100
0
Mt. Yake
(active volcano)
S = 0.2
S = 0.15
S = 0.1
200
400
600
800
Dry density of snow [ kg / m3 ]
Theoretical relationship is in good agreement with experimental data.
The form of hydrograph will change whether T is greater than Tt or not.
Saturation ratio is most important( or the key) variable.
sn becomes 100 - 500 kg / m3 until Feb. 13 and
the depth distribution have not changed. sn
The condition of snow deposited in early winter differs from that deposited in early spring.
To confirm the changing trend of snow, we conducted a observation.
becomes larger than 500 kg / m3 in Mar. 18.
Period : Jan. 17, 2015 – Apr. 13, 2015
Snow height [ cm ]
（from GSI HP）
Run3
Run2
Tt line (Assuming S ≈ 0)
How the saturation ratio and snow density change with time?
（from GSI HP）
Run1
Run 3
295
349
≈ 0
Run 1(standard)
10
1
Run 2
188
289
≈ 0
Observation site
We measured snow density every 10 cm and saturation ratio at 5 , 30, 55, 75 cm above the ground.
Jan.16
Jan.22
Jan.23
Feb.13
Mar.18
0
200
400
600
800
Wet snow density sn [ kg / m3 ]
0.4
Saturation ratio
Snow is almost dry condition
until the end of Feb.
Unfreezing water exists in the
vicinity of the ground from the
beginning of Mar.
120
100
80
60
40
20
0
0.3
0.2
0.1
0.0
1/19
0
100
200
Snow depth [ cm ]
Assumption
Melt < Infiltration
Snow deposit
Wet
]
Temperature of pyroclastic material [ ˚C ]
Saturation ratio of snow Temperature of
pyroclastic material T [ ˚C ]
T > Tt : snow melt speed is faster than infiltration speed… High risk mud flow
T < Tt : snow melt speed is slower than infiltration speed… Low risk mud flow
Pyroclastic material
Run 1
202
242
≈ 0
snow density [ kg / m3
Snow depth
75 cm
55 cm
30 cm
5 cm
2/9
3/2
3/23
4/13
The form of hydrograph will change in long time scale.
Conclusion
The form of hydrograph will change whether T is greater than Tt or not.
In addition, the form of hydrograph will change in long time scale due to the existence of unfreezing water and the change in snow
density.
Hence, the potential of damages as a result of mud flow will be heavily dependent on the condition of snow.