complex channel networks in hawai‘i and the influence of ...kenhon/hawaiichapman/...models for...
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Testing the slope hypothesis of bifurcation formation
0 200 400 600 800 1000 1200
0.43 mm/s
0.55 m
m/s
Bifurcation
0.12 mL/s, 11°0
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Time (s)
Dis
tanc
e (c
m)
r2 = 0.37
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Wav
e he
ight
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)
Velocity @ 25 cm (mm/s)
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SplitNo splitTransitionalFlow height (mm)
Flux (mL/s)
Slop
e (°
)Calculated flow heights
0 0.5 1 1.5 2 2.50
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Flux (mL/s)
Slop
e (°
)
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0.20.2 0.2
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0.40.4
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0.60.6
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1.6SplitNo splitTransitionalVelocity (mm/s)
Calculated flow velocity
θ, slope
Q, flux Obstacle height6.25 mm
Using syrup as an analogue fluid, we can test the flux and slope conditions under which a flow splits around or overtops an obstacle
> Flow velocity, rather than height, appears to control flow splitting, seemingly due to bow wave formation upslope of the obstacle at high velocities
> This result is contrary to the channel network analysis results, suggesting that cooling and crust formation, rather than simple viscous flow are important for bifurcation formation from obstacle interactions
0.5 mL/s15°
2 mL/s11°
5 cm grid
Golden syrup properties:µ = 42 Pa sρ = 1450 kg/m3
Heights and velocities calculatedfrom equations in Kerr et al. 2006
Relationship between velocity and bow wave height?
Effect of bifurcation on flow advance?
Quantifying channel networks and their origins
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Braiding Index1 - 23 - 45 - 67 - 8Channels
Slope15
0
0 10.5 km
“Braiding index,” or the number of channels intersected by a flow cross-section, generally increases with steeper slopes
> Hypothesis for bifurcation formation: Flow height decreases with increasing slope, making the flow unable to overtop ostacles and therefore split more easily
From a pre-eruptive DEM
Flow margin in white
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Flow extent on:Sep 4
Sep 16
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Dec 18
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0 42 km
500
Sep 2, 2009 breakout from TEB tube
Splits on steeper slopes
from SAR coherence mapping, Dietterich et al., 2012
Time from the beginning of episode, hrs
Dis
tanc
e fro
m th
e ve
nt, k
m
00 25 50 75
3
6
SE flow
NE flow
NES
Middle
N
165 m/hr
77 m/hr
37 m/hr
151 m
/hr
45 m/hr
Pu‘u ‘Ō‘ō
NE flo
w
N lobe
NE lobe
S lobe
Middle lobe
SE flow
1 km
Episode 7
Wolfe et al., 1988
Pu‘u ‘Ō‘ō
Hawaiian lava flows display a range of channel network geometries. Heavily bifurcated, distributary flows (e.g., Pu‘u ‘Ō‘ō episode 7 and Mauna Loa 1984) seem to produce shorter, slower flows than confined, tributary flows (e.g., Kīlauea Dec 1974 and Mauna Loa 1859) for similar durations, slopes, and effusion rates.
100
10
1
100 10001 10Effusion Rate (m3/s)
Flow
Len
gth
(km
)
Walker’s lower limit
Walker’s upper limit
Mauna Loa
KilaueaMauna Loa (ocean limited)
1859 flow (confined)18-27 days 1984 flow
20 days
Dec 1974 flow(confined)6 hours
~ 25 km limit?
July 1974 flow3-5 hours
Pu‘u ‘Ō‘ō (e7)55 hours
Pu‘u ‘Ō‘ō (e12)(confined)36 hours
Branching and merging of active lava channels appearto profoundly influence flow emplacement
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0 2 km
Channels
Flow area
Kīlauea
Mauna
Loa
Hualālai
Mauna Kea
Kohala
Koa‘e Fault Zone
Fissure ve
nts
Flow
dire
ctio
n
N
Flow direction
Pāhoehoe
Confined Flow: Kīlauea December 1974
Bifurcating Flow: Mauna Loa March-April 1984
Vents
Flow direction
Flow direction
Confined Flow: Mauna Loa - 1859
N0 2 km
Flow confined aroundHualālai
Confluence intoone channel
> What causes the bifurcations and confluences that make up these complex channel networks? - Bifurcations form where the flow divides around obstacles (maybe also viscous fingering) - Confluences form from the merging of parallel channels through lateral spreading or confinement
Similar features are found on other planets, too - what can they tell us?
10 km 100 m
Venus
100 m
ML 1859
ML 1984
Summary- Hawaiian lava flows are rarely simple in planform, but instead develop complex channel networks- Flows that split are shorter and advance more slowly than flows that are confined - Flows split because they interact with topography (constraints require further investigation)- Split flows converge around topographic obstacles unless merging is prevented by lateral levee formation–this condition should be predictable using coupled thermo-rheological models for flow advance- Understanding the interaction of lava flows with topography is important not only for hazard assessment but also for hazard mitigation, such as design of lava flow barriers and other intervention tools
References and AcknowledgementsKerr, R. C., Griffiths, R. W., and Cashman, K. V., 2006, Formation of channelized lava flows on an unconfined slope. Journal of Geophysical Research, 111, doi:10.1029/2005JB004225.Wolfe, E. W., Garcia, M. O., Jackson, D. B., Koyanagi, R. Y., Neal, C. A., and Okamura, A. T., 1988, The Puu Oo eruption of Kilauea volcano, Hawaii: episodes 1 through 20, January 3, 1983, through June 8, 1984. USGS Prof. Pap. 1350.
We would like to thank the following people for their contributions of data, equipment, and assistance: Adam Soule, Ben Mackey, Alison Rust, Einat Lev, Jeff Karson and Robert Wysocki (Syracuse University Lava Project)
How does flow cooling affect confluence formation?
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Time (s)
Exte
nt (c
m) Bifurcates
A
B
Flow length
Flow width
Pre-obstacle max flow width: 30.8 cm
3.8 cm
/s
1.1 cm/s
1.98 x 10-4 m3/s, 120° bifurcation
50 cm
A
50 cm
11.0 s: Flow reaches stable width
56.0 s: Bifurcation formslobes that never merge
B
With cooling, lateral spreading will be restricted by crustal growth, preventing flow merging
To test this we simulate real lava with molten basalt poured from a furnace onto sloping dry sand
> The experiments confirm that conditions that increase crustal growth (low flux, high bifurcation angle) form parallel channels instead of merging. We also see a reduction in velocity at the bifurcation.
1065.8
979.7
903.1
823.8
678.4
348.2
207.3
100.0
°C
10 cm
FLIR at 56.0 s
3D model from photogrammetry
The experimental flows qualitatively fit the Kerr et al. (2006) model for a solidifying flow on an unconfined slope.
5 cm/s8.4cm/s
3.3 cm/s
3.5 cm/sSurface velocities Analysis by Einat Lev
0 0.5 1 1.5 2 2.5 330
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Approximate flux (x10-4 m3/s)
No confluenceConfluence
Bifu
rcat
ion
angl
e (°
)
10° slope
Complex channel networks in Hawai‘i and the influence of underlying topography on flow emplacementHannah R. Dietterich ([email protected])1 and Katharine V. Cashman2 1University of Oregon 2University of Bristol