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

10

40

60

20

30

50

Time (s)

Dis

tanc

e (c

m)

r2 = 0.37

0

2

4

6

8

10

12

0.2 0.3 0.4 0.5 0.6 0.7 0.8

Wav

e he

ight

(mm

)

Velocity @ 25 cm (mm/s)

0 0.5 1 1.5 2 2.50

2

4

6

8

10

12

14

16

18

11

12

22

33

34

44

5

5

5

6

6

6

6

7

7

7

88

8

99

10 1011

SplitNo splitTransitionalFlow height (mm)

Flux (mL/s)

Slop

e (°

)Calculated flow heights

0 0.5 1 1.5 2 2.50

2

4

6

8

10

12

14

16

18

Flux (mL/s)

Slop

e (°

)

0.2

0.20.2 0.2

0.4

0.4

0.40.4

0.6

0.6

0.60.6

0.8

0.8

0.8

1

1

1

1.2

1.2

1.4

1.4

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

155°23'0"W

155°23'0"W

155°24'0"W

155°24'0"W

155°25'0"W

155°25'0"W

19°3

2'0"

N

19°3

2'0"

N

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

155°0'0"W

155°0'0"W

155°3'0"W

155°3'0"W

19°2

1'0"

N

19°2

1'0"

N

Flow extent on:Sep 4

Sep 16

Sep 23

Oct 6

Oct 9

Oct 21

Oct 28

Nov 9

Nov 25

Dec 15

Dec 18

300

400

200

100

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

155°17'30"W

155°17'30"W

155°20'0"W

155°20'0"W

19°2

2'30

"N19

°20'

0"N

19°1

7'30

"N

19°1

7'30

"N19

°20'

0"N

19°2

2'30

"N

155°20'0"W

155°20'0"W

155°22'30"W

155°22'30"W

155°25'0"W

155°25'0"W

155°27'30"W

155°27'30"W

19°3

7'30

"N19

°35'

0"N

19°3

2'30

"N

19°3

2'30

"N19

°35'

0"N

19°3

7'30

"N

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?

0 10 20 30 40 50 600

10

20

30

40

50

60

70

80

90

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

60

90

120

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 (hrd@uoregon.edu)1 and Katharine V. Cashman2 1University of Oregon 2University of Bristol