Debris-flow generated tsunami waves
Sjoukje I. de Lange1*, Nikoleta Santa1, Shiva P. Pudasaini2, Tjalling de Haas1
1) Faculty of Geosciences, Utrecht University, Utrecht, Netherlands, 2) Institute of Geosciences and Meteorology, Geophysics Section, University of Bonn, Bonn, Germany *) Presenting author: [email protected]
3. Methods
Debris-flow generated impulse waves can be extremely dangerous for lakeside settlements, and prediction of their characteristics is of major
importance for hazard mitigation and management. However, the effects of debris-flow composition on wave generation and evolution are poorly understood.
2. Objectives
1. Introduction
We investigate the influence of multi-phase debris-flow volume, composi- tion (gravel, sand, clay, water) and subaerial outflow slope on wave celeri- ty and amplitude, in a small-scale 3D physical laboratory model. We focus on wave amplitude and celerity, being the two most important factors for hazard management.
Fig. 1. Experimental flume setup. A: the mixing tank, outflow slope and wave basin with various instruments (cam: camera. Lb/Lc: laser scanner). B: Planview and topview of the setup.
Physical experiments of debris-flow generated impulse waves and their dependence on debris-flow properties
Utrecht University Geosciences Department of Physical Geography
7. Predictor strength
A
A B B
debris-flow breaking wave wave being pushed
source
area source
area
debris-flow wave being pushed
steep wave front
B
θ
~ 3 m
0.12 m x = 1.85 m
0.90 m
2.0 m
0.5 m
0.3 m
outflow channel PLAN VIEW
CROSS-SECTIONAL VIEW
mixing tank mixing
tank
0.3 m
c
zSnapper
cam1 cam3
cam2
cam4
x = 2.0 m b water level
Lb2 Lb1 Lc2 Lc1
AA
Lb2 Lb2 Lb1 Lb1
cam2 cam2
cam1 cam1 Lc2
Lc2 Lc1 Lc1
wave basin mixing
mixing tank tank
a
0 1 2 3 4 5 6
experiment A 0
1 2 3 4 5 6
experiment B
A
average velocity (m/s)
thickness middle (cm)
thickness outlet (cm)
weight at load cell (*10-1 g)
0 0.5 1 1.5 2 2.5 3
experiment A 0
0.5 1 1.5 2 2.5
3 B
average crest celerity (m/s)
average trough celerity (m/s)
amplitude leading crest (cm)
at x = 0.50 m
amplitude max crest (cm)
at x = 0.50 m
1:1 line 1:1 line
R = 0.54 2
R = 0.79 2
R = 0.30 2
R = 0.61 2
R = 0.70 2
R = 0.77
R = 0.49 2
R = 0.56 2
2
1:2 line 1:2 line
b
b
Fig. 9. R
2 values (indicating linear correlation) of debris-flow characteristics and the corresponding wave char- acteristics. The darker the green color, the stronger the correlation.
Debris flow →
Waves ↓ Velocity Energy Momentum
Crest amplitude
(near field) 0.30 0.28 0.49
Crest amplitude
(far field) 0.50 0.58 0.60
Crest celerity
0.79 0.65 0.60
Wave energy
0.53 0.68 0.64
Detachment �me
0.30 0.40 0.47
Wavelength
0.64 0.50 0.45
Predictor strength
0.55 0.51 0.53
A
A B B
debris-flow breaking wave wave being pushed
source
area source
area
debris-flow wave being pushed
steep wave front
4. Experiments vs nature
Fig. 2. Comparison between debris-flow generated tsunami in nature, Sulawesi 2018 (A) and in our setup (B).
6. Natural variability 5.3 Debris-flow composition
Fig. 5. Wave celerity, period, wavelength and am- plitude over distance from impact area. The push- ing of the debris-flow over steepens and acceler- ates the wave, which increases its non-lineairity but does not result in wave breaking. After de- tachment (x = 0.80 m), the wave energy becomes dissipated.
t = 0.1 s
t = 0.4 s
t = 0.7 s
t = 1.0 s
Mass = 4.25 kg Mass =16 kg
Fig. 4. Wave profile of the ref- erence experiment (8 kg, 44 vol% water, 18 vol% gravel, 2 vol% clay, 30° outflow slope).
A) Near-field wave profile over the first 10 seconds after debris flow release. B) Lead- ing crest and trough at differ- ent longitudinal locations along the wave basin.
Fig. 8. Natural variabili- ty of debris flows (A) and the corresponding impulse waves (B).
The numbers on the axis correspond with the actual value of the plotted variables. This graph gives an idication of the expected natural variability in the result section.
8. Conclusion
Fig. 10. Debris-flow volume, water content and clay content determine the debris-flow velocity and momentum.
When the debris-flow debouches into the water, its momentum is transferred. The water mass is pushed away from the impact zone, until the wave celerity exceeds the debris flow velocity. The debris-flow velocity is the main driver for wave celerity and wavelength. Debris-flow momentum determines the far-field wave amplitude. The main driver of near-field wave amplitude is so far unknown and further research is warranted.
0.5 1 1.5
0.5 1 1.5 2 2.5 3
crest celerity (m/s)
0.5 1 1.5
0.5 0.6 0.7 0.8 0.9 1
period (s)
0.5 1 1.5
x (m)
0 0.5 1 1.5 2
wavelength (m)
0.5 1 1.5
x (m)
0 10 20 30
max. crest amplitude (mm)
√gh
gT/2π
mean
25-per cen tile 75-per cen tile
Fig. 6. Example of the influence of debris-flow composition (water, clay and gravel content) on debris-flow characteristics (velocity). Debris-flow velocity is enhanced with an increasing water and clay content (up to 22%) of the debris flow, which both have a lubricating effect. There is no significant relation with gravel content. Debris-flow thickness and thus effective mass, increase with increasing debris-flow volume (not shown).
Water content (vol%)
1.5 2 2.5 3
debris-flow velocity (m/s)
R 2 = 0.90
40 45 50 55 60 1.4 0 10 20 30
1.6 1.8 2 2.2 2.4 2.6
R 2 = 0.93
Clay content (vol%)
A B
0 20 40 60 80
Gravel content (vol%)
1.6 1.8 2 2.2
2.4 C
Fig. 3. Wave generation of two runs with varying debris-flow volume. When the debris-flow debouches into the water, it transfers a substantial portion of its energy (~15%) by pushing the water forward, until the wave celerity exceeds the subaqueous debris-flow velocity and the wave becomes ‘detached’ from the debris flow.
Fig. 7. Debris-flow velocity is the main driver for wave celerity (A) and wavelength (not shown). An increasing debris-flow velocity increases the momentum ex- erted on the water by the debris-flow, thereby increasing the wave celerity. It also increases duration of the pushing, thereby increasing the wave amplitude.
The pushing is also increased by a thicker, higher volume, debris-flow. Debris-flow momentum is the main driver for far-field wave amplitude (B), but is poorly related to near-field wave amplitude (C).
5.1 Wave generation
5.2 Wave evolution
5.4 Relation debris-flow & wave characteristics
0 2 4 6 8 10
-25 -20 -15 -10 -5 0 5 10 15
water level (mm)
A reference
0.5 1 1.5 2 2.5 3
-25 -20 -15 -10 -5 0 5 10
15 B reference
x
b = 0.15m x
b = 0.50m x
b = 0.85m x
b = 1.20m x
b = 1.55m
time (s) time (s)
1.5 2 2.5
debris-flow velocity (m/s)
0 0.5 1 1.5 2
average celerity (m/s)
R 2 = 0.79
R 2 = 0.83
A crest
trough
0 10 20
momentum (kg*m/s
0 5 10 15 20 25 30
far-field leading amplitude (mm)
R 2 = 0.57
B
0 10 20
momentum (kg*m/s
5 10 15 20 25 30
near-field leading amplitude (mm)
R 2 = 0.24
C
