River responses to the 2010 major eruption of the Merapi volcano, central Java, Indonesia

Geomorphology 273 (2016) 244–257

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River responses to the 2010 major eruption of the Merapi volcano, central Java, Indonesia Frédéric Gob a,⁎, Emmanuèle Gautier a, Clément Virmoux a, Delphine Grancher a, Vincent Tamisier a, Kiki Widyaputra Primanda b, Sandy Budi Wibowo a, Caroline Sarrazin c, Edouard de Belizal a,d, Anouk Ville a, Franck Lavigne a a

Université Panthéon-Sorbonne (Paris 1), Laboratoire de Géographie Physique, CNRS UMR8591, 1 Place Aristide Briand, FR 92195 Meudon cedex, France Gadjah Mada University, Faculty of Geography, Yogyakarta, Indonesia Université Paris Ouest Nanterre La Défense, Centre d'Etudes Himalayennes - CNRS UPR 299, Nanterre, France d Université Paris-Ouest Nanterre La Défense, Département de Géographie, UFR SSA, Nanterre, France b c

a r t i c l e

i n f o

Article history: Received 11 May 2016 Received in revised form 17 August 2016 Accepted 17 August 2016 Available online 20 August 2016 Keywords: Fluvial readjustment Volcanic eruption Riverbed incision Sediment transport Merapi Opak River

a b s t r a c t This study examines the fluvial readjustment of a Javanese river impacted by the major eruption of the Merapi volcano (Indonesia) in October and November 2010. The basin of the Opak River, located on the southern flank of the Merapi, was subject to substantial sediment input related to massive pyroclastic deposits that were remobilized by numerous lahars during the year after the eruption. Two study sites were equipped in order to evaluate the morphodynamic evolution of the riverbed of the Opak River. Topographic surveys, bedload particle marking, and suspended sediment sampling revealed an important sediment mobilization during efficient flash floods. Surprisingly, no bed aggradation related to the progradation of a sediment wave was observed. Two years after the eruptive event, marked bed incision was observed. The Opak River readjustment differs from that of other fluvial systems affected by massive eruptions in two ways. Firstly, local population extracted the sand and blocks injected by the eruption as they represent a valuable economic resource. Secondly, several dams trapped the major part of the sediment load remobilized by lahars. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Since the 1980 eruption of Mount Saint Helens in the USA, numerous studies have focused on river adjustments after major explosive volcanic events (Gran and Montgomery, 2005). They mostly concern hydrological effects and the geomorphological dynamics of valley bottoms (Gran and Montgomery, 2005; Major and Mark, 2006; Tanarro et al., 2010; Gran, 2012; Gran et al., 2011, 2015; Pierson et al., 2011; Pierson and Major, 2014; Umazano et al., 2014; Zheng et al., 2014). They show how very large quantities of sediment brought through tephra fall, pyroclastic flows, or debris avalanches contribute to modifying the river system and sometimes even radically changing the drainage network. The eruptions of the 1980 Mount Saint Helens (USA), 1991 Pinatubo (The Philippines), and 2008/2009 Chaitén (Chile), produced tephra volumes as high as 2.5, 5 to 6 and 1 to 4 km3 respectively of extruded material. As a consequence, hillslopes and valley bottoms were buried by several meters of poorly sorted material over tens to hundreds of square kilometers (Major and Mark, 2006; Gran, 2012; Umazano et al., 2014). Pierson and Major (2014) in their synthesis on the hydromorphological ⁎ Corresponding author. E-mail address: [email protected] (F. Gob).

http://dx.doi.org/10.1016/j.geomorph.2016.08.025 0169-555X/© 2016 Elsevier B.V. All rights reserved.

effects of eruptions on drainage basins suggest two main series of consequences of this sudden and massive supply of sediment. The first one concerns runoff production and flood discharges; and the second, erosion mechanisms, sediment sources, and sediment transport. The hydrological disturbances correspond to a flashier response to storms because of a decrease in soil infiltration capacity owing to tephra deposits and a loss of vegetation (Gran and Montgomery, 2005; Pierson and Major, 2014; Umazano et al., 2014). Major and Mark (2006) – who studied the hydrological evolution of streams impacted by the Mount St Helens eruption – showed that flow peaks had larger magnitudes and generally rose more rapidly. They indicated that this increase in magnitude lasts between 5 and 10 years after the eruption and that it is not only owing to hillslope runoff but also to changes in channel hydraulics (reduction of flow resistance because of channel smoothening and straightening and to an increase of the sediment transport rate). The second major change concerns the channel geometry and the sediment transport rate. The main result of these large eruptions is a major increase in sediment yield directly after the volcanic crisis and for decades after its end. Sediment transport rates may reach levels amongst the highest in the world observed for hydrosystems, approaching 10 m3/ha/mm of rain (Manville et al., 2009). Lavigne (2004) calculated erosion rates of 1.5 to 2.7 × 105 m3/km2/y in two

F. Gob et al. / Geomorphology 273 (2016) 244–257

catchments on the Merapi and Semeru volcanoes. Sediment comes from two sources: (i) on hillslopes, abundant, fine, noncohesive sediments are subject to intense erosion through the development of rill networks; and (ii) in the river network, valley filling with pyroclastic materials or debris avalanche deposits are mobilized mainly by large mass movements such as lahars but also through vertical and lateral erosion during the high flow season (Lavigne and Thouret, 2000, 2002). Post-eruptive lahar sediment concentration may be 10 to 20 times higher than the sediment concentration of pre-eruptive floods and up to 100 times higher than that found in undisturbed volcanic catchments (Pierson and Major, 2014). Particularly in tropical regions, lahars are the main processes involved in reworking volcanoclastic material, as tens of them take place during the months following the eruption and they may occur for decades afterward. Once remobilized, the sediment inputs propagate downstream as a sediment wave modifying the channel geometry of the river. At the basin scale and considering a longer time span, Pierson et al. (2011) describe the migration of this sediment wave as a cycle with channel aggradation and then degradation. The elevation of the channel bed is coupled with widening, the development of mobile and braiding beds, and a fining and smoothening of the beds. The degrading phase corresponds to the reincision of the channel in the lahar and flood deposits. It leads to the return of a single-thread channel with progressive bed armoring and channel stability but also to the development of bank erosion and lateral migration (Pierson and Major, 2014). Riverbed stability and narrowing is accompanied by riparian vegetation growth (Gran et al., 2015). The duration and magnitude of this cyclical perturbation is described as very variable depending on the magnitude of the eruption, the climatic environment, and the local conditions. Considering the extreme nature of the volcanic events studied in the literature, the parameters that control the post-eruption evolution of the river system are generally described only as natural and that the

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role played by human activities seems negligible. This study investigates the morphologic adjustment of rivers following the 2010 October– November Merapi (central Java, Indonesia) eruption, in particular with regard to the role played by local populations. The Merapi is indeed a very active but densely populated volcano that experienced its largest eruption in N100 years in autumn 2010 (Surono et al., 2012; Cronin et al., 2013; Jousset et al., 2013; Pallister et al., 2013). The populations of the Merapi are highly dependent on the volcano. Rice and others crops rely on the very fertile properties of the volcanic soils and on a dense and complex irrigating system directly connected to the river network (Lavigne et al., 2015). Moreover, sandy volcanic material is widely exploited for the construction sector locally and for exportation. This close interaction between the population and the very active volcanic environment induces tremendous hazards but may also contribute to deeply modify the post-eruptive adjustments of the system on which the population depends. To follow the channel adjustments of the river to the 2010 eruption and the role played by human activities, two study sites were selected on the middle part of the Opak River. Those sites are located at the very end of the reach affected by the strongest lahars. Between September 2011 and March 2015, the morphometric evolution of the stream, the hydrological regime, and the sediment transport were surveyed. Our objectives were to evaluate the impact of a major eruptive event on the sediment reload of the middle part of a very anthropized river system and to better understand the role played by floods in the propagation of the sediment wave produced by hypercontracted flows and lahars in the upper part of the course. 2. Study area The Merapi is a volcano located in central Java (Indonesia) known for its frequent activity (Fig. 1). Its eruptions occur every 4 years on average with an interval ranging from 2 to 15 years. Merapi eruptions are

Fig. 1. Location of the Merapi volcano and the Opak River basin, Java, Indonesia.

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characterized by the formation and destruction of lava domes. Gravity and explosive processes cause lava dome collapses and trigger pyroclastic flows, known as Merapi-type pyroclastic density currents (PDCs; Beauducel, 1998; Jousset et al., 2013). Twenty out of the 61 known eruptions since the mid-1500s included tephra fall, pyroclastic surges, and flows (Voight et al., 2000; Lavigne and Thouret, 2002). The Merapi is one of the most dangerous volcanoes on earth, not only because of its activity but also because of the high density of the population living on its flanks (ranging between 900 and 4000 people per km2; Lavigne et al., 2015). Since the fourteenth century, about 7000 people have been killed, mostly by explosive events (Mei et al., 2013). Out of 1.1 million of people living on the volcano, about 440,000 remain in areas prone to PDCs and lahars (Lavigne and Thouret, 2002; Mei et al., 2013). The October–November 2010 eruption was a large explosive event (VEI 4; Newhall and Self, 1987). N400 people died, mainly because of basal avalanches and PDCs that reached up to 16 km from the top of the volcano (Fig. 2; Cronin et al., 2013; Jenkins et al., 2013; Komorowski et al., 2013; Pallister et al., 2013; Surono et al., 2012).

During the four phases of the eruption, up to 0.06 km3 of pyroclastic material was deposited on the western and southern flanks (Charbonnier et al., 2013). A fieldwork examination was conducted by Cronin et al. (2013) in January 2011 on the southern flank in an area that was roughly centered on the upper catchment of the Opak River. It revealed that the pyroclastic deposits affected an area of 34 km2 with a total volume of 49 Mm3, amongst which 38 Mm3 corresponded to valley and channel fills. Solikhin et al. (2015) calculated from satellite images that on the same zone (Opak catchment), tephra and PDC deposits covered an area of 26 km2 with 45 Mm3 of pyroclastic deposits and 18 Mm3 of tephra fall deposits. The Austral tropical climate of the Merapi region usually induces very heavy rainfall between October and April. The first monsoon season following the eruption in 2010 and 2011 recorded particularly high total precipitation owing to La Niña conditions. Between October 2010 and May 2011, monthly rainfall averaged 2000 mm and exceed 3000 mm in February 2011 (de Bélizal, 2012). It triggered over 240 lahars in the main rivers of the disturbed zone of the volcano. Over

Fig. 2. The upper Opak catchment and the longitudinal extension of the pyroclastic density currents (PDCs) and the lahar fronts (de Bélizal, 2012; Cronin et al., 2013).

F. Gob et al. / Geomorphology 273 (2016) 244–257

Area (km2)

Width (m)

Slope (m/m)

D50 (mm)

river in Dalem is a typical pebble-bed stream, 22 m wide with a local slope of 0.022 m/m and an average particle size (D50) of 75 mm. The river in Prambanan is 15 m wide, the slope is 0.01 m/m, and the D50 is 69 mm.

59.8 64

22 15

0.022 0.01

75 69

3. Methods

Table 1 Main characteristics of the river channel at the two study sites, Dalem (upstream) and Prambanan (downstream).

Dalem Prambanan

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the following years, N 180 lahars occurred. Several of them covered a distance of over 20 km and generated bank erosion, channel widening, and riverbed incision (de Bélizal, 2012; de Bélizal et al., 2013). Since October 2010, about 30% of the lahars have occurred on the main system draining the southern flank of the Merapi: the Opak River and its main tributary, the Gendol River (Fig. 2; de Bélizal et al., 2013). Our study was conducted on the downstream part of this river that was attained by the largest lahars of the rainy season 2010 to 2011 (note that rainy seasons are hereon referred to as 2010–2011). Two study sites, one in the village of Dalem and the other in the village of Prambanan, were equipped in October 2012 and monitored until March 2015 (Fig. 2). Prambanan is located 25 km from the top of the Merapi. The catchment has an area of 64 km2 and a mean slope of 0.099 m/m. Though only 2.6 km apart, the two study sites present different morphologies (Table 1; Fig. 3). While the upstream site presents manifest remains and erosion scars of the 2010–2011 lahars, the downstream reach presents no visible marks of recent lahars such as large deposits or fresh erosion scars. Fig. 3 clearly shows the widening of the valley consecutive to lahars between Dalem and the Prambanan dam. On this reach, the valley has an average width of 51 m, compared to only 23 m downstream of the dam. Here, the river is confined by terraces of ancient pyroclastic and volcanoclastic deposits. At several places, it flows on bedrock outcrops (pyroclastic and volcanoclastic tuff). The

3.1. Hydrologic data and analysis of floods This study is based on a combination of in situ observations and geophysical methods to document the hydrological features of the river and the sediment and morphological changes caused by floods. With regard to hydrological features, because of the violence of the floods and the different configurations of the two sites, several systems were used for security (Fig. 4). Hydrological data were collected from September 2012 to March 2015 on the two sites, associating two types of equipment: simple hydrological scales for direct observation of water levels, and two water level data-loggers for automatic measurements. Both data-loggers relied on two different types of sensors. Diver ©Schlumberger uses a water pressure sensor, while @Ijinus (LNU1000-2-80X) has an ultrasonic water level sensor. At Dalem, the steep gradient and very unstable banks did not allow the installation of a sensor directly in the stream. For this reason, an ultrasonic water level sensor was installed under a bridge. It measured the water height every 15 min during low-flow-level and every 5 min when the water column exceeded 80 cm. At Prambanan, two hydrologic scales were installed for direct observation on the upper section of the site and near the water level sensor. A water pressure sensor (Diver, Schlumberger) was installed in a tube directly in the river; it measured the water level every 15 min during low flow, every 5 min during the wet season, and every minute during ADCP (Acoustic Doppler Current Profiler) measurements.

Fig. 3. Longitudinal evolution of the valley width in Dalem (A) and Prambanan (B) in 2010. Note the narrowing of the valley corridor downstream of the limit reached by the 2010–2011 lahars.

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Fig. 4. Study sites and equipment used: Dalem (upstream, Fig. 3A) and Prambanan (downstream, Fig. 3B).

During several hydrologic events in February 2014 and February 2015, precise surveys of discharge and velocity were conducted with an ADCP (Suntek M9) on the Prambanan reach. The direct measurements of discharge helped us to determine the stage-discharge relation and to establish a rating curve for the Opak River. To ensure and multiply the velocity measurements during floods, an automatic camera (Ltl Acorn, Ltl-6210MC) was installed above the river at the Prambanan gauging site. During the rainy season, the camera took a picture every 5 s. With markers installed on each bank of the river, the flow velocity was calculated on separate images, thanks to floating debris (wood sticks, coconuts, plastic sandals, etc.). Because of the configuration of the upper section (Dalem), using the ADCP during floods was not possible. For this reason, discharge at Dalem (q in m3/s in Eq. (1)) was calculated using the water height curve and the discharge at Prambanan, with a formula that adapts the calculation to the basin area (Bravard and Petit, 1997): q ¼ Q ðA=aÞ0:8

ð1Þ

where Q: discharge at Prambanan (m3/s); A: basin area at Prambanan (km2); and a: basin area at Dalem (km2). To analyze the hydrological data, we adapted the Peaks-OverThreshold method (POT). The Peaks-Over-Thresholds method, developed by Lang et al. (1999), also called Partial Duration Series, allows flood maxima to be determined and flood duration to be calculated. The approach consists of retaining the discharge peaks that exceed a given threshold (or base level). The first step of the method entails verifying whether two successive events are separate or not. Two events must be at least five days apart or separated by a discharge between two peaks that is 75% below the lower of the two peaks. Because of the reduced area of our study catchment (64 km 2 ) and the steep gradient of the river, the hydrologic

reaction after two different storms can occur in less than five days. Thus, we decided to retain the value of 5 m 3 /s as the discharge threshold. The base level value was validated by our onsite observation. 3.2. Characterization of bedload mobility and fluvial bed readjustment In order to evaluate the remobilization of bed sediments injected by the 2010 volcanic events and the readjustment of the bed during floods, three methods were combined. First, owing to the practical impossibility of measuring sediment transport and morphologic changes in the bed during the floods, we adopted the morphologic (or inverse) method. This method helped to determine the processes controlling the lateral and vertical mobility of the active channels and to quantify the sediment movements (Martin and Church, 1995; Eaton and Lapointe, 2001; Fuller et al., 2003; Legleiter and Kyriakidis, 2008). Every September in 2012, 2013, and 2014, a dense lattice of topographic points was registered on the fluvial bed along the same cross sections: 12 and 10 cross sections were surveyed at Dalem and Prembanan, respectively (Fig. 4). Longitudinal profiles were also surveyed every year (Fig. 4). Topographic surveys were conducted with a total station (Trimble S6). Second, in order to assess the river capacity to rework its bed during fluvial events, tens of pebbles and boulders were marked with paint and precisely located with a total station on the two study sites a few weeks before the beginning of the rainy seasons in 2012–2013 and 2013–2014 (Fig. 4). The grain size of the painted particles needed to be as close as possible to the bed sediment (Haschenburger and Church, 1998). For this reason, cobbles and pebbles were measured on riffles and bar heads during the low-flow stage using Wolman's (1954) method. In Prambanan, boulder mobilization was surveyed on one zone for the entire period (zone 3, Fig. 4). In Dalem, two different zones were investigated: zone 1 in 2012–2013 and 2013–2014; zone 2 in 2013–2014 (Fig. 4).

F. Gob et al. / Geomorphology 273 (2016) 244–257

Because of the high water level, monitoring the displacement of boulders after each flood event was not possible. In order to evaluate their longitudinal progression, we conducted two position measurement campaigns in September 2013 and 2014. All sediment and topographic data were integrated into a GIS (ArcGis software). For each zone (1, 2, and 3), the largest floods of the rainy season were considered and the corresponding specific stream power was calculated. Finally, suspended sediments were sampled during several flood events in the course of the wet seasons of 2012–2013 and 2013–2014, and – for comparison – during low-flow stage. During the floods, sampling was conducted every 15 min from a bridge at the Prambanan site with a Bailer sampling tube, just a few decameters downstream of our study site. Because of the flow velocity, samples were taken at a 1m depth in the water column. For this reason, suspended sediment concentrations (SSC) are likely to have been underestimated. Some samplings were also punctually conducted in Dalem. Every 1-l sample was filtered (with a 0.45-μm filter) and weighed after having been dried at the Laboratory of Soil Science of Gadjah Mada University (Yogyakarta, Indonesia). The sampling was coupled with measurements of the pH

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and electric conductivity of the water. After filtration and drying, the samples helped to determine suspended sediment concentrations related to a precise discharge. 4. Results 4.1. Very rapid and efficient flash floods The Opak River has an Austral tropical pluvial regime characterized by a pronounced seasonal variability. During the study period, the mean annual discharge varied between 1.6 and 2.5 m3/s, inducing an average specific discharge of 25 to 40 l/s/km2. In fact, the low water stage represents the main part of the runoff as 75% of the water discharges are lower than 1.4 to 1.75 m3/s. In spite of the humidity of the climate, the lack of significant underground water storage and the elongated form of the catchment are responsible for very low discharges throughout the year. During the monsoon season, no considerable increase in the base discharge are observed. Short phases of floods interrupt a reduced base flow (Fig. 5).

Fig. 5. Water discharges of the Opak River recorded between 2012 and 2015 in Prambanan (Fig. 4).

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Fig. 6. Hydrograph and photographs of the flood on 22–23 February 2014 on CS-F in Prambanan, Opak (Fig. 4B).

The maximal discharges were recorded in January 2013 (184 m3/s) and in February 2015 (190 m3/s) (Figs. 5 and 6; Table 2). Floods that generally occur between November and April (and more rarely until June as in 2013) are highly irregular. The floods can be considered as flash floods showing a very steep rising limb with discharge increasing from a few cubic meters to hundreds of m3/s in 20 min (Figs. 5 and 6). The floods are triggered by a storm that generally begins in the afternoon on the volcano flanks. Flood peaks are generally registered at Prambanan between 4 p.m. and 9 p.m. Flood duration is very short, rarely exceeding 15 to 20 h (Fig. 6). The three study periods are unequal. The first monsoon season is characterized by numerous flood peaks, with 14 events exceeding 40 m3/s and three floods exceeding 100 m3/s. The duration of the rainy season was exceptional with floods until June. The second rainy season (2013–2014) had a shorter humid season and less intense floods: only seven peaks higher than 40 m3/s have been recorded. Finally, few floods occurred in 2014–2015 (partly as this is an incomplete year). During flood events, specific stream powers varied between 650 and 1600 W/m2 in the upper reach (Dalem) and between 170 and 1300 W/ m2 at the downstream site (Prambanan). Even if the specific power is high, Table 2 demonstrates that the duration of the efficient discharge is very short. In the case of the humid year (2012−2013), only 56 cumulated hours of discharge exceeding 40 m3/s were recorded. For the

Table 2 Main hydrologic parameters of the Opak River at Prambanan over the three rainy seasons of this study. Oct. 12-Nov. 13

Nov.13-Nov. Nov. 14 14-Feb.15

Mean discharge (m3/s) Maximal discharge (m3/s) Number of flood peaks N5 m3/s Cumulative duration of discharge N5

2.23 184 37 792/33

1.67 190 23 638/26.5

2.5 105 21 220/9

m3/s (Hours/days) Number of flood peaks N40 m3/s Cumulative duration of discharge N40

14 56

7 18

9 33.5

3

m /s (Hours)

two other study periods, the efficient discharges have a cumulative duration of a few hours. 4.2. Floods or lahars? In the two years following the 2010 eruption, tens of lahars were triggered in the upper Opak catchment. Some of them affected N15 km of the river course (Fig. 2). Since then the frequency of their occurrence has decreased in line with the depletion of the eruption deposits. However, 17 known lahars have been recorded or observed on the upper reach of the Gendol River (upper tributary of the Opak, Fig. 2) and on the upper Opak River since the beginning of this study (September 2012). Seven of them occurred between September 2012 and January 2014 (Table 3), while 10 lahars occurred between February and March 2014. Indeed on 13 February 2014, an explosive eruption of the nearby Kelud volcano (220 km to the east) led to a new deposition of 2 to 5 cm of fallout tephra on the flank of the Merapi, triggering a new series of lahars (Wibowo et al., 2015). The observations made by local populations, together with our measurements of sediment transport, indicate that lahars were always triggered in the upper catchment and none of them ever reached our study sites located 15 to 20 km downstream. In Java, lahars and floods have the same origin. They are generated by heavy rainfall cumulating several centimeters of rain in a short time interval (a few hours). In order to determine whether lahars and floods were related (i.e. whether lahars turned into floods when traveling downstream), the chronologies of these two types of event were investigated. The precise moment that the lahars were triggered and their duration are available from February 2014 (Table 3). Compared to our hydrological data, this shows that in most cases the flood events recorded in Prambanan are completely independent of the lahars that occurred 19 km upstream. Indeed five lahars could have induced a flood or a water level increase on the Opak River (Table 3) as the flood occurred only a few hours after the lahar was triggered. All of the lahars, apart from the last one, were followed by only a weak increase of the water level in Prambanan. Though the first lahar (28 February 2014) was a massive 7-m-deep event on the upper Gendol, it was only followed by a small flood on the Opak River, about 3 h after the lahar was triggered. We

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Table 3 Correlation between the lahars occurring on the upstream part of the course and the flood events occurring in Prambanan, 18 km downstream (‘?’ indicates that the exact moment of the lahar triggering or its maximum height is unknown). Upper basin lahars

Prambanan hydrology

Date

River

Duration (start-end)

Max. Lahar Height (m)

Discharge (m3/s)

Time of the peak flow

Theoretical speed of the event (m/s)

28/02/2014 11/03/2014 12/03/2014 13/03/2014 15/03/2014 17/03/2014 18/03/2014

Gendol Gendol Gendol Gendol Gendol Gendol Gendol Gendol

14:28–16:22 ? - 17:00 (N2 h) 16:04–17:31 14:22–16:40 15:55–16:49 11:26–13:59 Morning - Noon 15:24–17:29

7m N1,5 m 1m 1m 0,5 m 2,5 m ? 6m

6.2 8.4 0.25 1 0.21 3.65

17:45 18:15 15:15 20:45 14:45 16:15

1.5 1.5 – 0.78 – 1.05

23.8

16:45

3.75

can suppose that the two events are independent: if this small peak is the consequence of the arrival of the lahar, its front would have had a mean velocity below 1.5 m/s while covering the 19 km between the place where lahars were observed and Prambanan. In fact, the mean lahar velocity is known to be usually much higher. Wibowo et al. (2015), for instance, recorded a mean velocity of 4.1 m/s for the lahar reaching their Gendol upstream site on 28 February 2014. Consequently, only one flood event may be related to a lahar. On 18March 2014 a lahar reached the Gendol upstream site at 15:24 in the upper catchment, and a corresponding peak flow was recorded 1 h and 20 min later in Prambanan. The flow would have travelled through the upper valley with a mean velocity of 3.2 m/s. This is much more coherent with the mean velocities of lahars and floods recorded in the catchment. Thus this upstream lahar and the downstream flood could be considered as a single event. 4.3. Sediment transport The two campaigns of particle marking in Dalem and Prambanan show that a large majority of the coarsest particles of the bed were

transported by the stream over relatively long distances (Fig. 7; Table 4). The painted blocks were classified into 4 four categories according to their displacement: (i) nontransported block; (ii) moved block: i.e., block that was turned or slightly moved; (iii) transported block: i.e., block displaced several meters from its original position; and (iv) missing block: block that has not been found either because it travelled out of the surveyed area (i.e., hundreds of meters downstream), it had been buried or because the paint had been completely erased. With regard to the missing blocks, note that the last two hypotheses are unlikely because of the general incision of the bed (see below) and because the paint was rather well preserved on the other blocks (i, ii, iii). Between 49 and 88% of the marked boulders were transported downstream over several tens of meters, or completely disappeared, except in zone 1 in Dalem (Table 4). In all zones, the transported and missing blocks were initially located at the center of the channel where the flow velocity is the highest. The blocks that have moved slightly and those that have not moved at all are generally located on the margins of the riverbed where the water level and shear stress are the lowest (Fig. 7). Most of the blocks that were not transported still moved slightly, generally because of the undermining that results from a general

Fig. 7. Displacement of the marked blocks in (A) Zone 2 of the Dalem reach (2013–2014) and (B) Zone3, Prambanan (2012–2014).

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Table 4 Characteristics of the displacement of the marked particles in Dalem (zones 1 and 2) and Prambanan (zone 3). Zone (rainy season)

Dmin - Dmax (mm)

Max discharge (m3/s)

Rate of transport (%)

Dmax missing (mm)

Dmax transported (mm)

Dmax moved (mm)

Max specific stream power (W/m2)

1 (2012–2013) 1 (2013–2014) 2 (2013–2014) 3 (2012–2013) 3 (2013–2014)

300–1800 410–1800 230–1200 300–1800 200–1450

175 180 180 184 190

73 30 78 88 49

780 1300 900 1300 650

1300 880 950 – 1000

1800 1800 1200 1800 1450

1371 1416 1618 1289 1331

riverbed incision (see below). Blocks up to 1300 mm were transported over several meters or travelled out of the studied zone. The blocks with a larger diameter (from 1500 up to N 3000 mm) were not transported. They moved only slightly because of the incision of the bed or they stayed in place. The flow velocity recorded during high flow exceeded 6 m/s in Prambanan, leading to a very high transport capacity in the stream. Fig. 8A presents a plot of the maximum specific stream power against the largest missing boulder, the largest transported boulder, and the largest moved boulder. Some relationships from the literature have been added to the graph for comparison (Lenzi et al., 2006; Gob et al., 2010). These relationships have been established using different methodologies. They are relative to high energy mountainous streams but not located in a volcanic environment. They all indicate a threshold above which sediment transport occurs depending on the size of the transported particles. They demonstrate that in these environments sediment transport is selective as it is defined by fluvial processes. But they also show that this critical threshold of incipient motion is extremely variable from one river to another. Our data shows that particle entrainment in the Opak corresponds to what has been measured elsewhere in nonvolcanic environments and that sediment transport is selective. In other words, morphological events were driven by floods characterized by medium sediment concentrations and not by mass movement such as in both hyperconcentrated flow phases or debris flow phases of lahars, where the transport rates are not related to sediment-size. It also shows that the Opak data is in an intermediate position on the graph, between the Rio Cordon and the Vecchio River: specific stream power exceeds 1000 to 1500 W/m2 and allows

incipient motion of boulders with b-axes of up to 1300 mm; however, the specific power is not able to displace particles with b-axes of 1500 mm and over. By considering the relative grain size (Fig. 8B), Lenzi et al. (2006); Mao et al. (2008), and Gob et al. (2010) have shown the role played by the channel slope and the presence of coarse particles in the bed in inducing strong dissipation of energy that delays sediment transport. Only the largest missing or transported blocks on the Opak were considered (Fig. 8B). This shows that incipient motion in the Opak seems to work in a similar way to the Rio Cordon (Lenzi et al., 2006) and in several French streams studied by Gob et al. (2010). Those streams have slopes ranging from 0.03 to 0.13 m/m and a D50 from 110 to 340 mm, which is quite comparable to the Opak (0.01 to 0.02 m/m and 69– 75 mm). Based on this comparison, these relations may be applied to the Opak. Then a specific stream power of around 300 W/m2 is needed to set the D50 of the Opak River in motion. This corresponds to a discharge of about 40 m3/s that has been exceeded 14 times in 2012– 2013 and 7 times in 2013–2014, during periods of 56 h and 18 h, respectively (Table 2). This indicates that 30 to 40% of the flood events have transported the D50 of the bed. Despite the fact that suspended sediment concentration could not have been measured during the largest event, Fig. 12 confirms that SSC (Fig. 9) never presents very high values: maximal SSC values are 20 to 40 g/l, but as mentioned previously, the sampling method is likely to have underestimated the suspended sediment. Logically, SSC mainly depends on water discharge. However, during the first year of survey (2012–2013), SSC values were much higher than during the second year, even for low discharges. Several floods that were sampled during

Fig. 8. Relation between specific stream power and (A) grain-size of mobilized particles (largest blocks considered); (B) relative grain size of the mobilized particles (largest missing and transported blocks considered).

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excavated from the bed during the two rainy seasons. The vertical adjustments of the bed on this site have therefore been limited. The local configuration of the river bed and the human constructions (walls, bridges) indicate moreover that the river in Prambanan has recovered its pre-eruption position, suggesting that the bed has only experienced slight aggradation between October 2010 and October 2012.

5. Discussion

Fig. 9. Suspended sediment concentration (SSC) of the Opak in Prambanan.

the first monsoon season reached 10 to 30 g/l, whereas, for the second year SSC never reached these values. Thus, the SSC expresses the general sediment decrease that we observed in terms of bedload. 4.4. Morphological adjustments of the riverbed This study began two years after the 2010 eruption, in September 2012. Aerial images and field observations show that the plane geometry of the valley has been modified by the 2011 lahars only in Dalem (Fig. 3). The upper reach of our upstream study site underwent a massive erosion of the right bank of up to 70 m and the formation of a large bar composed of cobbles and boulders (Cross section CS-D on Fig. 4A and 10/2012 photo on Fig. 10). The general configuration of the valley at both study sites has experienced very weak modifications since then. The most notable evolution concerns the vegetation that developed extremely quickly on the margins of the active bed. Fig. 7 shows that the large bar deposited by the 2010–2011 lahars was almost bare of vegetation in October 2012 and covered by a dense bush, up to 3 to 4 m high, two years later. The rapid vegetation growth on this bar and on the river margins on both study sites suggests a disconnection of these areas from the active channel and thus a probable incision of the bed. The three surveys undertaken from September 2012 to September 2014 confirm this vertical evolution of the channel (Fig. 11). The comparison of cross sections and longitudinal profiles reveals a general incision of the river bed at both study sites. Between October 2012 and September 2014, the mean depth of the cross sections in Dalem increase by an average of 0.43 m with (in some places) an incision larger than 1.2 m. Fig. 8 shows that most of this adjustment happened during the first rainy season following the survey; indeed the incision never exceeded more than a few centimeters for the second year of survey. The trend is similar in Prambanan, though the incision is weaker and better distributed between the two interannual surveys. There, the average incision is 0.15 m with a maximum of 0.38 m. The weaker incision in Prambanan may be explained by the bedrock outcrops that have been progressively

Even if the 2010 eruption of the Merapi volcano injected an exceptional sediment load (Cronin et al., 2013; Ville et al., 2015) that in-filled the upper reach of the valleys, only limited sediment input was observed on the river bed of the Opak farther downstream. The readjustment of the Opak was particularly rapid: in less than five years, the downstream section of the river recovered a pre-eruptive situation; whereas, in previous studies outside of the tropics (Pierson et al., 2011; Pierson and Major, 2014), one to two decades were needed for the pre-eruption morphodynamics of rivers to be restored. In the case of the rivers of the southern flank of the Merapi, the bed aggradation associated with the pyroclastic deposits never really prograded downstream. The load that was deposited in the medium valley during the 2010–2011 rainy season was ephemeral as it was rapidly evacuated. Thus, the evolution model of rivers on the Merapi differs radically from the other rivers impacted by exceptional eruptions (Gran and Montgomery, 2005; Major and Mark, 2006; Gran, 2012; Pierson and Major, 2014; Umazano et al., 2014). We have demonstrated that the capacity of sediment transport of the stream is high because of high stream power because of steep gradients and efficient flash floods. The Opak data have critical parameters close to those measured in nonvolcanic environments presented by Lenzi et al. (2006); Mao et al. (2008), and Gob et al. (2010). Even if the mobilization of the largest of the Opak's boulders requires specific stream powers higher than 1000 W/m2, the D50 of the bed is very mobile as cobbles travelled downstream over distances exceeding several hundreds of meters after only a few flood events. During the two rainy seasons surveyed, this study has demonstrated that the majority of the particles (bD50) could have been transported by 21 events corresponding to a cumulative period of almost 75 h. This should have led to a massive sediment yield. But, surprisingly, bed aggradation seems very limited and only consecutive to the massive sediment supply owing to the huge lahars that occurred during the first rainy season following the eruption. The following years were characterized by fluvial bed incision. The maximum incision occurred during the first year of this study and decreased after 2014. The suspended sediment followed the same trend, as a strong decrease of SSC was observed between 2012 and 2014. These findings demonstrate that our study sites located in the middle valley of the Opak catchment finally received a very limited sediment supply from upstream, despite the massive sediment input consecutive to the 2010 eruption. Several factors specific to the Merapi may explain this aberrant situation.

Fig. 10. Evolution of a bar deposited in 2011 by a lahar on the Dalem study site between October 2012, October 2013, and September 2014.

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Fig. 11. Evolution of the longitudinal profile of the two study sites (Dalem and Prambanan) and box-plots of the elevation variation of the fluvial bed (maximum depth and mean depth) on the two study sites.

The first may be attributed to the revegetation of the river bed and the flanks of the Merapi. Indeed, the Opak River presents several features similar to the rivers that drain the east flank of the Pinatubo volcano. The 1991 eruption of Pinatubo injected 1.1 to 1.3 km3 of pyroclastic debris flows, mainly composed of sand. Over two decades, the river eroded half of the deposits of the upper basin (Gran et al., 2011). During the first decade, active braiding channels limited the vegetation growth, but perennial vegetation had developed by 2009 and recolonized the braid-plain (Gran, 2012; Gran et al., 2015). On the Opak, the revegetation was much more rapid than on the rivers crossing the Pinatubo hillslopes and the same observation can be made: when the lateral sediment mobility decreases, riparian vegetation is able to grow, and in turn the vegetation inhibits lateral migration and accelerates the narrowing of the channel and its incision (Gran et al., 2015). But a second factor, probably the most significant, explains the rapidity of the fluvial readjustment on the Opak. When comparing Pinatubo, Mount Saint Helens, or Chaitén with the Merapi environment, one must consider the very high density of the human population on the Javanese volcano. The upper basin of the Gendol and the Opak rivers

have low densities. But downstream, in the middle and low valleys, the population density exceeds 2000 inhabitants/km2, reaching 4000 inhabitants/km2 in some places (Lavigne et al., 2015). This very high density of human population leads to an overexploitation of two resources that are strongly associated with the river: water and sediment. The sediment injected by the frequent eruption of the Merapi represents readily available and profitable resources for the populations living on the volcano flanks. The very high quality of the deposits justifies the intensive extractions from the river bed of sand for the production of concrete and blocks for buildings and sculptures. This extraction activity has occurred for decades and leads to huge quantities of materiel being removed directly from riverbeds every year. The frequent eruptions of the volcano refill the sediment of the quarries and allow the extraction industry to develop (de Bélizal et al., 2013). Ville et al. (2015) studied the evolution of the morphology of a reach of the Gendol River located between 6 and 11 km from the top of the Merapi, following the October–November 2010 eruption. They showed that the PDC led to an aggradation of the river bed ranging from 30 to 50 m. Over the following years, the bed underwent a rapid incision

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resulting from the triggering of tens of lahars. They had an average incision of about 7 m/y between 2010 and 2014. But they observed a noticeable difference between the upstream and downstream part of their study zone. Indeed, sediment began to be extracted from quarries in the downstream section of their study reach from 2012. The incision rate consequently increased from 6.1 m/y before 2012, to 9.4 m/y between 2012 and 2014. Meanwhile in the upstream section where no extraction of material occurred, the incision rate decreased from 7.7 to 6.1 m/y over the same period, as the number and the magnitude of lahars decreased. Quarries are mainly located on the Gendol River and only a few of them are located in the upper Opak River (Fig. 12A). The number of quarries grew very quickly almost immediately after the 2010 eruption. de Bélizal (2012, updated) counted about 2200 trucks per day extracting sediment from the upper catchment of the Gendol River, leading to a volume of extracted material of 8800 m3/d. For the whole southern flank of the Merapi, they estimated that the volume of extracted sediment reached a total 3.9 Mm3 in 2013. Between 2010 and 2014 these estimations led to a total of about 15 Mm3 of sediment exported from the river network. When this value is compared to the 38 Mm3 of sediment injected in the valleys during the October to November 2010 eruption (Cronin et al., 2013), one may note that the river was deprived of almost 40% of the sediment input. A second element also played a major role in the disconnection between the upper catchment and the downstream part of the valley: the numerous sabo-dams and dams (Fig. 12B). Sabo-dams have been built to limit hazards related to lahars, and dams have been built to irrigate crop cultivation (i.e., rice fields) during the six or seven months of the dry season. Sabo-dams began to be built from 1969 following a major eruption of the Merapi. These dams were constructed on the model of the Japanese sabo-dams in order to slow down the lahar and reduce the sediment yield (Sarrazin, 2013). On the southern flank of the Merapi, sabo-dams were built in order to retain boulders and blocks in the upper course of the streams and gravel and sand in the lower

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course. On the downstream part of our study reach (from the GendolOpak confluence), another kind of dam may also be found. They were built between the 1920s and 1990s in order to provide water to the villages along the river course and to supply the dense irrigating system for the crop fields. They are larger structures (6 to 11 m high) with a capacity ranging from 14,000 to 60,000 m3. The 27 sabo-dams and dams are upstream of Prambanan: 20 on the Gendol River and 7 on the Opak River (Fig. 12B). The sabo-dams of the upper course of the Gendol were very efficient during the 2010 eruption and fully played their part as sediment trapping infrastructures. They were almost entirely buried under several meters of sediment during the eruption or because of the ensuing lahars. Farther downstream, the dams were subject to weaker lahars but were still infilled by the lahars, causing severe damage to the irrigation system and damaging crop production (Sarrazin, 2013). Aerial images prior to 2010 show that most of these dams were already rather full of sediment before the eruption, possibly because of the previous eruption that happened in 2006. One may however speculate that the sabo-dams and the dams still stopped a non-negligible part of the sediment load and accentuated the disconnection of the lower part of the basin from the upper part. Finally the numerous quarries, sabo-dams, and dams located along the upper course of the Gendol and Opak rivers seems to prevent direct interaction between lahars and floods, thus reinforcing the upstreamdownstream disconnection of the catchment. Indeed previous studies (Cronin et al., 1999; Lube et al., 2012; Vallance and Iverson, 2015) have shown that diluted lahar fronts (hyperconcentrated flow type) and boulder-rich lahar fronts (debris flow type) usually have two direct impacts on streamflow. First, by pushing the tail of normal stream flow, they form a bow wave of streamwater in front of the lahar (Lube et al., 2012). Then a water-bulking of the lahar front (Vallance and Iverson, 2015) leads to its progressive dilution and to the downstream changes in lahar dynamics. None of these phenomena were observed during our fieldwork in 2014, indicating that downstream dilution of rain-

Fig. 12. (A) Zone of main extractions in 2011 and 2014 and (B) Sabo-dams in the Opak basin (adapted from de Bélizal, 2012; de Bélizal et al., 2013; Sarrazin, 2013).

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triggered lahars by normal streamflow does not really occur on the Merapi; thus limiting the sediment supply to the downstream part of the catchment where this study was conducted. 6. Conclusion The morphological adjustments of the Opak consecutive to the 2010 eruption of the Merapi are uncommon in comparison to those that have been studied elsewhere on volcanoclastic aprons of active stratovolcanoes. Despite a massive input of the sediment from Pyroclastic Density Currents, the expected sediment wave propagating over tens of kilometers into the medium and lower valley never materialized. The upper part of the course experienced a classic aggradation during the eruption, followed by a rapid incision subsequent to lahars and floods over the following years. But the sediment injected into the fluvial system only travelled a few kilometers and never reached the medium course of the Opak. The 2010 sediment was trapped by numerous sabo-dams and dams that slowed down their progression. Furthermore, extractions massively and definitely exported gravel and sand from the river beds. The river beds of the medium and lower valleys show clear indications of a sediment deficit suggesting that they are disconnected from the upper part of the catchment where the sediment production occurred. If the positive role played by sabo-dams and extractions in disaster risk reduction (DRR) should be acknowledged, the lack of sediment supply to the lower part of the course cannot be ignored. An examination of all of the dams throughout the medium and lower valley reveals numerous signs of river bed entrenchment, often in excess of 2 m. At the Gendol-Opak junction, the incision excavated the substratum and the incision was estimated at around 3.70 m. Downstream, the incision varies between 2.3 and 3.6 m, leading to destabilization at the foot of several dams. Acknowledgements This work took place in the framework of the Sedimer Program supported by the Axa Research Fund. The authors would like to thank the colleagues and students from the University Gadja Madah for their help, particularly Pr Danang Sri Hadmoko. The authors would also like to thank the two anonymous reviewers and the editor for their constructive remarks, and Natasha Shields for assistance in translating. References Beauducel, F., 1998. Structures et comportement mécanique du volcan Merapi (Java): une approche méthodologique du champ de déformations. PhD thesis. Université Paris 7, IPGP (260p). Bravard, J.-P., Petit, F., 1997. Les cours d'eau. Dynamique du système fluvial. A. Colin, Paris 222p. Charbonnier, S.J., Germa, A., Connor, C.B., Gertisser, R., Preece, K., Komorowski, J.-C., Lavigne, F., Dixon, T., Connor, L., 2013. Evaluation of the impact of the 2010 pyroclastic density currents at Merapi volcano from high-resolution satellite imagery, field investigations and numerical simulations. J. Volcanol. Geotherm. Res. 261, 295–315. http://dx.doi.org/10.1016/j.jvolgeores.2012.12.021. Cronin, S.J., Neall, V.E., Lecointre, J.A., Palmer, A.S., 1999. Dynamic interactions between lahars and stream flow: a case study from Ruapehu volcano, New Zealand. Geol. Soc. Am. Bull. 111, 28–38. http://dx.doi.org/10.1130/0016-7606(1999)111b0028: DIBLASN2.3.CO;2. Cronin, S.J., Lube, G., Dayudi, D.S., Sumarti, S., Subrandiyo, S., Surono, 2013. Insights into the October–November 2010 Gunung Merapi eruption (Central Java, Indonesia) from the stratigraphy, volume and characteristics of its pyroclastic deposits. J. Volcanol. Geotherm. Res. 261, 244–259. http://dx.doi.org/10.1016/j.jvolgeores. 2013.01.005. de Bélizal, E., 2012. Les corridors de lahars du volcan Merapi (Java, Indonésie): des espaces entre risque et ressource. Contribution à la géographie des risques au Merapi. PhD Thesis. Université Pantheon-Sorbonne (Paris1), LGP (414p). de Bélizal, E., Lavigne, F., Hadmoko, D.S., Degeai, J.-P., Dipayana, G.A., Mutaqin, B.W., Marfai, M.A., Coquet, M., Le Mauff, B., Robin, A.-K., Vidal, C., Cholik, N., Aisyah, N., 2013. Rain-triggered lahars following the 2010 eruption of Merapi volcano, Indonesia: a major risk. J. Volcanol. Geotherm. Res. 261, 330–347. http://dx.doi.org/10. 1016/j.jvolgeores.2013.01.010.

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