1 2

Tracing sediment pulses in lahars using multi-parameter recording stations and serialised geophones. Lengkong River, Mt Semeru, East Java, Indonesia.

Jonathan Procter (1), Gert Lube (1), Shane Cronin (1), and Jean-Claude Thouret (2)

(1) Massey University, Institute of Natural Resources, Palmerston North, New Zealand,

(2) Universite Blaise Pascal, LMV, 5 Rue Kessler, 63068 Clermont-Ferrand, France.

Introduction

Monitoring Equipment

Lahar - February 26th 2010

Conclusions

Acknowledgements. This work is partly supported by France ANR Lahar Risk Project, NZ FRST/NHRP

contract MAUX0401 and the Marsden Fund. Terrestrial Laser Scanner Data supplied by JC Thouret; Sylvain Labbé, Maison de la Télédétection/Institute for Remote Sensing.

Data analysed at INR, Massey University.

ReferencesArattano M, Marchi L (2008) Systems and sensors for debris-flow monitoring and warning. Sensors 8: 2436–2452.

Beverage JP, Culberston, JK (1964) Hyperconcentrations of suspended sediment. Journal of the Hydraulics Division, American Society of Civil Engineers 90 (HY6): 117-128.

Cole SE, Cronin SJ, Sherburn S, Manville V (2009) Seismic signals of snow-slurry lahars in motion: 25 September 2007, Mt Ruapehu, New Zealand. Geophysical Research Letters 36, L09405, doi:10.1029/2009GL038030

Doyle EE, Cronin SJ, Cole SE, Thouret J-C (2009) The Challenges of Incorporating Temporal and Spatial Changes into Numerical Models

of Lahars. Proceedings of the 18th World IMACS/MODSIM Congress, Cairns, Australia 13-17 July 2009. Modelling and Simulation Society of Australia and New Zealand, Cairns, pp. 2675-2671.

Doyle EE, Cronin SJ, Cole SE, Thouret J-C (2010) The coalescence and organization of lahars at Semeru, Indonesia. Bulletin of Volcanology 72: 961-970.

Doyle EE, Cronin SJ, Thouret J-C (2011) Defining conditions for bulking and debulking in lahars. Geological Society of America Bulletin, doi:10.1130/B30227.1.

Dumaisnil C, Thouret J-C, Chambon G, Doyle EE, Cronin SJ (2010) Distinctive hydraulic characteristics and a frictional model apply to lahar flows at Semeru volcano (Indonesia). Earth Surface Processes and Landforms 35: 1573-1590.

Huang C-J, Shieh C-L, Yin H-Y (2004) Laboratory study of the underground sound generated by debris flows. Journal of Geophysical Research 109: F01,008.

Lavigne F et al. (2000) Instrumental lahar monitoring at Merapi volcano. Journal of Volcanology and Geothermal Research 100: 457–478.

Pierson TC, Scott KM (1985) Downstream dilution of a lahar:

Transition from debris flow to hyperconcentrated streamflow. Water Resources Research 21: 1511-1524.

Smith GA, Lowe DR (1991) Lahars: volcano-hydrologic events and deposition in the debris flow-hyperconcentrated flow continuum. In: Fisher RV, Smith GA (eds) Sedimentation in Volcanic Settings. SEPM Special Publication 45, Soc. Sedim. Geology, Tulsa, Oklahoma: 59-70.

Suwa H, Yamakoshi T, Sato K (2000) Relationship between debris-flow discharge and ground vibration. pp. 311–318. In: Wieczorek G, Naeser N (eds) Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment, Rotterdam.

Thouret J-C, Lavigne F, Suwa H, Sukajat B, Surono (2007) Assessment of Volcanic hazards at Mount Semeru, East Java

(Indonesia), with Emphasis on Lahars. Bulletin of Volcanology 70: 221-244.

Upstream (Lava) Monitoring Site.

AFM, Broadband seismometer, Video

Camera, 5min Sediment Sampling, Observers,

Pore-pressure, load-cell.

Figure 2. Indonesia, East Java. A, Semeru-Tengger volcanic massif covers 900 km2 including the Mt

Bromo-Tengger, Jambangan and Ajek-Ajek calderas, Mt Kepolo, and the Mt Mahameru-

Semeru (3696 m) cone complex. The currently active Jonggring-Seloko crater is now 400 m across and 200 m deep at 3444 m asl. B, Currah Lengkong

river channel draining the slopes of Mt. Semeru. The ~500 m long study area is immediately

upstream of the confluence of the Kali Koboan River.

Figure 1.A, Currah Lengkong river

channel under normal flow conditions.

Equipment setup; B, installation of pore-

pressure and load-cell into lava outcrop at channel base on the upstream monitoring site; C & D,

Geophone, AFM/Datalogger setup.

Downstream (Sabo) Monitoring Site.

AFM, Video Camera, 2 min Sediment sampling,

Observers, Pore-pressure, load-cell.

AFM Site 1

AFM Site 2

AFM Site 3

AFM Site 4

AFM Site 5

AFM Site 6

AFM Site 7

7. Channel - Confined, lava channel base; meandering flow under normal flow conditions

6. Channel - Confined, lava and sediment channel base; meandering flow under normal flow conditions

5. Channel - Confined, sediment (large boulder) channel base; wider sinuous meandering flow under normal flow conditions

4. Channel - Unconfined, sediment channel base;wider sinuous flow path under normal flow conditions

3. Channel - Unconfined, sediment channel base;wider dual flow path under normal flow conditions

2. Channel - Unconfined, sediment channel base; wider dual flow path under normal flow conditions

1. Channel - Unconfined, sediment/concrete Sabo Dam channel base; wider multiple flow path under normal flow conditions

A.AFM Data

B.Channel

Cross-sections

C.Channel DEM

(VEx1.5)D.

Monitoring Stations Comparisons

BA

A

B

C

D

Figure 5.Comparison of the pore-pressure derived stage and AFM (total) signals normalised between sites 1 and 7. Note how

the AFM signals do not always correlate with flow depth or sediment peak concentration. Figure 4. 26/02/10 Lahar, raw records. A, 7 AFM stations data showing a wide range in signal patterns, with overall increasing signal strength;

B, Corresponding channel cross-sections for each AFM site; C, Terrestrial Laser Scanner derived DEM of the ~500 m of channel studied;

D, comparison of upstream and downstream load-cell and pore-pressure data with sediment dip samples. Shaded blue areas indicated two major pulses or packets (c.f. Doyle et al., 2010).

A B

Figure 3.Normal and peak

lahar flow channel conditions; A,

Downstream sabo dam site; B,

Upstream lava site.

Mt. Semeru(G. Mahameru)

3676 m

Mt. Bromo-Tengger(G. Bromo)

2392 m

Study Area

Sabo DamDownstream Monitoring Site

Lava OutcropUpstream Monitoring Site

Kali Koboan

Currah Lengkong

100m5000m

AFM location

The Lengkong channel is a c. 30 m wide box-shaped valley with a base of alternating gravel and lava bedrock (fig. 2). Two recording sites were established ~510 m apart in the channel: an upstream ‘lava’ site (15-20 m wide, lava bedrock base), and a downstream ‘sabo’ site (25-30 m wide, concrete sabo dam) (fig. 2) . The two recording stations contained the following:

1. Pore pressure sensors (Hobo U20 Water and Solinst Levelogger) installed mid-channel, drilled into the bedrock or concrete base, recording at 1 Hz to determine stage height of the lahar through hydrostatic pressure (fig. 1);2. Fixed 25 fps video cameras on the true left bank of both instrument sites;3. Load cells – Geokon fluid-filled earth pressure cells, drilled into bedrock or concrete, recording at 1 Hz;4. Direct suspended sediment sampling (5 min intervals upstream; 2 min intervals downstream) by weighing a 10 L dip sample to provide estimates of particle concentration, grain size distribution and rheological properties;5. A 3-component Guralp CMG-6TD broadband seismometer was located 10 m downstream of the upstream ‘lava’ site, on the true left bank.

Also, 7 standalone, low-cost geophone/accoustic flow meters (AFMs) were tested at several locations between the 2 main sites (fig. 1), recording 25 Hz bulk (z-axis) vibrational energy to an inbuilt data-logger. Pre and post event TLS and RTK-GPS cross-sectional surveys were also carried out (fig. 4) .

Geophysical observations of lahars have traditionally focused on using seismic signals (Lavigne et al., 2000; Arattano and Marchi, 2008; Cole et al., 2009). Seismic signals of lahars are commonly analysed within the 10-100 Hz range (Huang et al., 2004) with peak flows exhibiting signals in the 50 Hz range (Suwa et al., 2000; Cole et al., 2009). Doyle et al. (2009; 2010; 2011) studied rain-induced lahars in the Lengkong River, Mt Semeru, with a broadband seismometer, pore pressure and video-velocimetry records. The greatest seismic energies were associated with highest sediment concentrations and/or flow turbulence and peak wetted area, with most lahars comprising several distinct peaks or pulses detectable in multiple localities. Pulses were attributed to individual tributary flows being combined in the observation sites. Occurence of standing waves and roll waves at detection sites could also be determined by increases in seismic signals.

Our study incorporated a range of sensors (fig. 1) to measure a number of parameters independently. A lahar on the 26 February 2010 at 1555 (08:55 GMT) (fig. 3) provides an example integrated dataset. This flow occurred 30 mins after intense rainfall in the upper catchment and summit area, with stage heights increasing from 0.3-3 m in depth within 10-15 mins and observed velocities of 2-5 m s -1. The flow exhibited several periods of standing waves, periodic episodes of rolling boulders and peak suspended sediment loads of 30-40 weight %; ie hyperconcentrated flow characteristics.

Flow depth, pore pressure, load and sediment concentration (fig. 4D) show two main flow pulses, with an increase in stage and sediment concentration between the two sites corresponding to flow bulking of over 100 % by incorporation of water and sediment (c.f., Doyle et al., 2009 estimated 50 % bulking between the sites in a series of larger lahars). Individual peaks in sediment concentration indicate that these pulses or packets of sediment were moving at 1-2 m s-1. The flow reorganised the gravel bed portions of the flow channel with ongoing migration of the flow thalweg during passage (fig. 3).

All AFMs recorded the progression of the flow downstream (fig. 4A-C), with increasing energy observed only in the lowermost two sites, associated with increasing flow volume. There are peaks in seismic activity corresponding with those in flow and sediment concentration, but additional peaks also occur, and appear to be related to periods of standing wave formation, which differed between sites.

Lahar is an Indonesian term describing a rapidly flowing mass of water, rock and debris from a volcano (Smith and Lowe, 1991). Lahars generally encompass the spectrum of sediment/water ratios from hyperconcentrated flows to debris flows (Pierson and Scott, 1985), with the flows commonly transforming from each phase both spatially and temporally. Hyperconcentrated flows generally have sediment concentrations of 20-60 % by volume and 40-80 % by weight (Beverage and Culberston, 1964). Mt. Semeru (East Java, Indonesia) is an active stratovolcano (fig. 2) with small-scale lahars (Q<400 m3 s−1 and V<100,000 m3) several times each season, medium-scale lahars (400<Q<600 m3 s−1 and 100,000 – 1,500,000 m3) at least once a year, and large-scale lahars (Q>600 m3 s−1 and V 1.5 – 6 million m3) every 6 years on average (Thouret et al., 2007). Rain triggered lahars consist of ~80 % of all lahars from Mt. Semeru with an annual sediment yield of 426 000 t km-2 yr-1 (Dumaisnil et al., 2010).

Lahar erosion, bulking and deposition, and de-bulking are characteristics that are difficult to quantify. Real-time recording of sediment motion within lahars has also proved difficult, but such data are needed to numerically characterise lahars and forecast their potential hazards. The Lengkong river channel, at 9.5 km from the summit of Mt. Semeru (fig. 2), has been used as a “life-sized flume” for the last 5 years to capture in real time the physical characteristics of passing lahars and hyperconcentrated flows.

The initial implications of this dataset are that AFM signals may not correlate very well with flow discharge or sediment concentration. While they respond to these factors, it appears they respond equally well to other flow conditions, such as formation of standing waves or periods when boulders are rolling within flow. In addition, there are strong variations in the signals, depending on the channel substrate (solid lava vs. gravel) and geometry. These changes locally influence the transfer of the energy detected by the AFMs, with the uppermost lava site and lowermost Sabo site showing the greatest amplitudes, probably due to this hard substrate. The higher energy signals at the final sites correlate with the flow measurements showing increased lahar size (by over 100 %) through incorporation of more water and sediment. Comparing AFM and flow depth/concentration/load, we see that the AFM signal increases dramatically with the onset of the flow to peak sediment concentration yet quickly decreases as sediment concentration drops, even though peak stage continues (fig. 5). Following the initial onset, AFM signals appear most strongly coupled to processes inducing bed-load sediment movement and channel migration, and other processes such as periods of standing wave formation. Based on a preliminary analysis of the load cell data, the peak loads of up to 20 kPa correspond to an equivalent load of c. 30 kg over the surface area of the sensor. This is 2-3 times that expected based on a static load with the measured sediment concentrations. This may indicate that the dynamic pressure on the channel base (even averaged over 1 s intervals) may greatly exceed an expected average static load. This may be due to accelerating particles impacting on the channel bed and/or developing granular networks that concentrate load in specific locations for short periods. Further work is needed to characterise these properties.

Pulse 1 Pulse 2

Peaks in Sediment Concentration

Trough in Sediment Concentration