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Temporal and spatial changes of evolving and coalescing lahars at Semeru, Indonesia

Geolsoc 09, Oamuru. Temporal and spatial changes of evolving and coalescing lahars at Semeru, Indonesia. E. E. Doyle 1 , S. J. Cronin 1 , S. E. Cole 1 , J. -C. Thouret 2 . emmadoyle79@gmail.com 1 Inst. Natural Resources, Massey University, Private Bag 11 222, Palmerston North

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Temporal and spatial changes of evolving and coalescing lahars at Semeru, Indonesia

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  1. Geolsoc 09, Oamuru Temporal and spatial changes of evolving and coalescing lahars at Semeru, Indonesia E. E. Doyle1, S. J. Cronin1, S. E. Cole1, J. -C. Thouret2. emmadoyle79@gmail.com 1Inst. Natural Resources, Massey University, Private Bag 11 222, Palmerston North 2Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, France

  2. Semeru Survey at Lengkong Channel, Semeru . - Rain-triggered lahars Nov-April, mostly during the day - 20+ hyperconcentrated streamflows and debris flows per season

  3. Mt. Semeru Curah Lengkong Curah Kobokan Survey location Rain induced lahars & “Banjirs”: -Debris flows (φ>60%) -Hyperconcentrated flows (20%<φ<60%) Lengkong Channel, Semeru

  4. “Lava” Site Distance ≈ 510 m ΔZ ≈ 28 m Slope, θ ≈ 3° Gravel/Sand bed “Sabo” Site

  5. Upstream “lava” site Height (m) Distance (m) Channel Station • Pore Pressure sensor - 10 s • Load cell – 2 s True left river bank • Camera – 25 fps • 3 Component Broad band - North axis parallel to stream - 10 m downstream from station

  6. Downstream “sabo” site 4 3 Height (m) 2 1 0 0 10 20 30 40 Distance (m) Channel Station • Pore Pressure sensor - 2 s • Load cell – 2 s True left river bank • Barometer – 1 min • Camera – 25 fps • Sediment – bucket samples

  7. Recorded events 8 lahars - hyperconcentrated streamflows - coarse non-cohesive debris flows - rapid onset - surging and unsteady flow - several large pulses or waves • Flow depths: 0.5-2 m • Peak propagation velocities: 3 – 6 m/s • Maximum discharge (estimates): 25 – 250 m^3/s

  8. Ground velocity, m/s 50 hz 0.1 hz Seismicity: Upstream “lava” site (7th March) X-channel Seconds of day Seconds of day

  9. Stage/pore pressure at both sites (7th March) upstream (lava) downstream (sabo) Normalised to peak stage

  10. Stage/pore pressure at both sites (7th March) upstream (lava) downstream (sabo) Varriv = 1.8 m/s 3.6 m/s 1.9 m/s

  11. Packets due to: • - Upstream springs, • - Tributary inputs, • - damming, ponding, surging • Markers to examine evolution downstream Mt. Semeru Curah Lengkong Curah Kerobokan Survey location

  12. Seismics, upstream (lava): 5-20 Hz Ground velocity, m/s WA: upstream (lava) NB:BOTH aligned to sabo (+337 s) WA: downstream (sabo) % vol conc at sabo Sediment concentration (5th March) This region travels faster - bouldery flow fronts?

  13. - organisation and coalescence of lahars downstream • As larger, more concentrated regions catch up with the flow front • evolve: • from multiple discrete “packets” • to a single, stable, debris flow like wave.

  14. PIV from video → body velocities Upstream Downstream

  15. Bulking between sites

  16. Debulking between sites

  17. Cycles of Bulking & Debulking

  18. Cumulative volume at both sites (05/03/08) • Vol. increase less downstream • Debulking • sedimentation Downstream 40% 60% Upstream • Vol. increase greatest downstream • Entrainment • Channel collapse 48% 26% Volume and concentration changes linked

  19. Concluding remarks Concentration influences flow evolution - high concentrated, high stage → catching up with front - shortening & coalescing flows - one lahar have multiple parent sources? - shortening = stage & inundation increase Discharge and volume estimates - bulking and debulking behaviour in these lahars Controls on bulking or debulking? Quantify? - Size? erosive energy - concentration? Turbulence & sediment carrying capability - high concentration → sedimenting laminar flow. - volume, concentration, rheology, basal resistance changes

  20. Many thanks for your time! • Any questions? • Acknowledgments: • Céline Dumaisnil, Yves Bru, Emma Phillips, Kat Holt, Gert Lube, Jon Procter, Vern Manville, Frank Lavigne • Latif Usman (Bruno), Mahjum and the people of Curah Lengkong village • EED & SJC: Marsden Fund Research Project (MAUX0512) and NZ Foundation for Research Science and Technology Project (MAUX0401). • SEC: Commonwealth Scholarship Scheme and Massey University Graduate Research School. • JCT: INSU-CNRS “Hazards and global change” ACI research programme (2004-2007).

  21. - positive correlation between wetted area and seismic energy • Strong correlation between • sampled volume fraction • ratio of seismic energyperpendicular to channel and parallel to channel (averaged across each packet)

  22. Upstream Area Upstream Seismicity Upstream S. Energy Downstream Area % vol conc. Varriv (m/s) = 1.5±0.1 2.9±0.2 4.0±0.3 1.8±0.1

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