Debris Flow Following the 1984 Eruption with Pyroclastic Flows in Merapi Volcano, Indonesia Takashi Jitousono*1, Etsuro Shimokaw

Journal of the Japan Society of Erosion Control Engineering. Vol. 48, Special Issue, 109-116 (1996)

[Original article]

Debris flow following the 1984 eruption with

pyroclastic flows in Merapi volcano, Indonesia

Takashi Jitousono*1, Etsuro Shimokawa*1 and Satoshi Tsuchiya*z *1 Faculty of Agriculture,Kagoshima University, Korimoto, Kagoshima 890, Japan *2 Faculty of Agriculture,Shizuoka University, Ohya, Shizuoka 422, Japan

Abstract Merapi volcano which has often erupted with pyroclastic flows is one of the most active volcanoes in Indonesia. Recently, large-scale pyroclastic flows occurred in the southwestern flank of the volcano in June 1984. As a result, the hydrological and erosional regime of the hillslopes was radically altered and more than 203 debris flows and floods have occurred in the Putih river catchment. In this paper, using the records of debris flow in the Putih river catchment, the characteristics of debris flow following the 1984 pyroclastic flows were analyzed. The rainfall conditions not causing debris flow are definitely different between the 4-year period after the 1984 pyroclastic flows and since then. The rainfall intensity not causing debris flow is small just after the pyroclastic flows and then has increased with time. Also, the large-scale debris flows occurred within a 4-year period after the 1984 pyroclastic flows. The magnitudes of debris flows have decreased with time. The sediment outflow by debris flows had almost finished within a 4-year period after the pyroclastic flows.

Key words: Pyroclastic flow deposits, Debris flow, Merapi volcano, Indonesia

Introduction

Merapi volcano had an eruption with pyroclastic flows in June 1984. The pyroclastic flows were thickly deposited in the southwestern flank of Merapi volcano. At the same time the surroundings were widely covered with a fine volcaniclastic material originating from the pyroclastic clouds. After the 1984 pyroclastic flows, debris flows and floods have occurred frequently at the Putih river in the southwestern flank of Merapi volcano. The debris flows have been under observation since November 1985. In this paper, the characteristics of debris flow following the 1984 eruption with pyroclastic flows are described using the debris-flow records in the Putih river catchment.

An outline of the research area

Merapi volcano is located in central Java (Fig. 1). The study area is the Putih river catchment situ- ated in the southwestern flank of Merapi volcano (Fig. 2). Pyroclastic flows have often taken place in the southwestern flank of the volcano. Pyroclastic flows which occurred in June 1984, were thickly deposited in some area of the Putih river catchment and at the same time a wide area of the volcanic hillslopes was covered with fine pyroclastic airfall. As a result, the hydrological and erosional regime of the hillslopes was radically altered and much sediment was produced by the debris flows and floods.

(C)JapanSoceityofErosionControlEngineering 110 T. Jitousono et. al.

Fig. 2 Location and topography of study area.

■:Water-level gauge station

●:Rain-gauge station

■:1984pyroclastic flow

Fig. 1 Location of Mt. Merapi. □:Putih river basin

Analyzed data and methods

The observation of debris flows and floods has been carried out at the Mranggen check dam (640m above sea level) on the Putih river since November 1985 by the Volcanic Sabo Technical Centre (VSTC). The VSTC was renamed the Sabo Technical Centre (STC) in 1992. The observation system of debris flows and floods is composed of an ultrasonic water-level gauge which was installed at the right wing of Mranggen check dam (VSTC, 1990). The ultrasonic water-level gauge is a nocontact type apparatus in which the water stage is detected by a round time of ultrasonic waves sent from a transmitter and receiver to the water surface. This apparatus is effective in observing debris flow as well as floods with sediment, and it is possible to get the data on all the debris flows and floods including small scale ones throughout the year. The data are useful for analyzing runoff characteristics of debris flow and sediment yield in a basin. The water-level records measured by the ultrasonic water-level gauge are transmitted to the STC by the telemeter every ten minutes for rainy days and one hour for fine days. The observation of rainfall has been carried out at the rain-gauge station of Mt. Maron (961m above sea level) in the Putih river catchment since 1984 (VSTC, 1990). The rainfall records are also transmitted to the STC by the telemeter every ten minutes for rainy days. One hundred and ninety-three debris flows and floods were observed during a 2. 5-year period from November 1985 to May 1988 and 10 floods were observed during a 0. 5-year period from August 1989 to February 1990. The water-level gauge was inoperative during the period from June 1988 to July 1989 and no debris flow or flood have occurred since March 1990. Using the records of debris flow and rainfall in the Putih river catchment, the characteristics of debris flow following the 1984 eruption with pyroclastic flows were analyzed. Debris flow following the 1984 eruption with pyroclastic flows 111

Results and discussion

Occurrence of debris flow and flood

Fig. 3 shows the monthly distribution of occurrence of debris flows and floods with the mean monthly rainfall at Mt. Maron. In the Merapi volcano area, approximately 86% of debris flows and floods occur during the rainy season from November to April. Similar analyses were carried out for the debris-flow records taken in the northern flank of Sakurajima volcano (Jitousono and Shimokawa, 1989). Sakurajima volcano is one of the most active volcanoes in Japan, and located at the center of Kagoshima Bay in southern Kyushu (Fig. 1). Sakurajima is a stratovolcano, whose flanks are covered with older and younger volcanic products. The present volcanic activity of Sakurajima volcano started in 1955 and ever since then eruptions have been continuous for more than 30 years. The hillslopes are largely susceptible to erosion, slope sliding and the occurrence of debris flow (Shimokawa and Jitousono, 1987a, b, c). In Sakurajima volcano, approximately 80% of debris flows and floods occur during the summer season from May to Septem- ber (Haruyama et al., 1984; Jitousono and Shimokawa, 1989).

Characteristics of debris flow and flood

The discharge of debris flow and flood at the Mranggen dam is calculated by multiplying the cross-sectional area of flow by the velocity of debris flow and flood, that is,

Q=AV (1) where, Q (m3/s) is the discharge of debris flow and flood, A (m2) is the cross-sectional area of flow, and V (m/s) is the velocity of debris flow and flood. The cross-sectional area of flow, A (m2) at the Mranggen check dam is calculated as

A=H(B+mH) (2) where, H (m) is the depth of flow measured by the ultrasonic water-level gauge, B (m) is the width of the spillway at the check dam, and m is the side-wall gradient of the check- dam spillway. B=30.4m and m=0.5 are given for the Mranggen check dam. The the velocity of flow, V (m/s) is calculated by the Manning's equation, that is, Fig. 3 (A) Distribution of mean monthly V=n1R2/3I112 (3) rainfall at Mt. Maron during a 4- year period (1985-1988). (B) Monthly where, V (m/s) is the mean velocity of flow, distribution of occurrence-number of n is the roughness coefficient, R is hydraulic debris flow and flood at the radius (approximately equal to mean depth Mranggen dam in the Putih river. 112 T. Jitousono et. al.

Fig. 5 Relationship between the peak dis- Fig. 4 Examples of observed hydrographs of charge and the total runoff of debris debris flows at the Mranggen dam in flows and floods at Mt. Merapi and the Putih river. Mt. Sakurajima. for wide channels), and I is the energy slope (equal to the channel slope for uniform flow). At the Mranggen check dam, n=0.06 (Koga and Agus, 1989) and I=0.048 are given. Fig. 4 shows the examples of the debris-flow hydrographs at the Mranggen dam and hyetographs at Mt. Maron. A certain correspondence between the occurrence of debris flow and that of the maximum rainfall per ten minutes is noted. Fig. 5 shows the relationship between the peak discharge and the total runoff of debris flows and floods at Merapi volcano and Sakurajima volcano. The catchment area at the observation station of debris flows and floods at the Putih river in Merapi volcano is 8. 22km2, and 1. 38km2 at the Saido river in Sakurajima volcano. The solid and broken lines in the figure show the regression curves at the Putih river and the Saido river, respectively. A nearly linear relation is obtained on logarithmic graph paper at both rivers. The equations at both rivers are obtained by a least squares method, that is,

Putih river: Qp=0.00558QT831 (r=0.977) (4)

Saido river; Qp=0.00135QTOS7o (r=0.902) (5) where, QP (m3/s) and QT (m3) are the peak discharge and the total runoff of debris flows and floods. A positive correlationship of high significant level is noted. The relationships between QP and QT at Merapi volcano and Sakuraj ima volcano shown in Fig. 5 resemble each other in shape. According to the motion pictures of debris flows taken at the Putih river and the Saido river, the debris flows are mostly mudflow including a great amount of volcaniclastic materials.

Temporal change of rainfall condition not causing debris flow or flood

From the records of debris flows and floods observed at the Mranggen dam and of rainfall at Mt. Maron, the temporal change of rainfall condition (not causing debris flow or flood) since the 1984 pyroclastic flows was examined (Fig. 6). In Fig. 6, Rio is the rainfall per ten minutes of continuous Debris flow following the 1984 eruption with pyroclastic flows 113 rainfall not causing debris flow or flood in the Putih river and . R is the cumulative rainfall from the beginning of the rain to the occurrence of R10. According to the Fig. 6, the rainfall conditions (not causing debris flow or flood) are definitely different between the 4-year period after the 1984 pyroclastic flows and since then. The rainfall intensity (not causing debris flow or flood) is small just after the pyroclastic flows and then has increased with time. On the other hand, Sakuraj ima volcano has erupted continuously with emissions of ash since 1955. The frequency of eruption has varied with time. Effects of the temporal variation of eruption on rainfall causing debris flows were examined (Jitousono and Shimokawa, 1991). The rainfall intensity causing debris flows is smaller during the more frequent volcanic eruptions than during the fewer eruptions. The rainfall conditions causing debris flows depend on the volcanic activity.

Temporal change of peak discharge and total runoff of debris flow and flood

Fig. 7 shows the temporal variations of the peak discharge and the total runoff of debris flows and floods at the Mranggen dam with the daily rainfall at Mt. Maron. The magnitudes of peak discharge and total runoff of debris flows and floods have decreased with time since the 1984 pyroclastic flows. The studies and investigations on soil texture, infiltration capacity and surface runoff on the hillslopes of the upper reaches are indispensable to the understanding of this decline of debris flow. However,

Fig. 6 Temporal change of the relationship between Rio and XR using the data of continuous rainfall not causing debris Fig. 7 (A) Variation of daily rainfall at Mt. flow and flood in the Putih river. Rio: Maron from November 1984 to June rainfall per ten minutes of continuous 1992. (B), (C) Variation of the peak rainfall, mm/lOmin; XR: cumulative discharge and the total runoff of debris rainfall from the beginning of the rain flows and floods at the Mranggen dam to the occurrence of R10, mm in the Putih river. 114 T. Jitousono et. at. these field studies have not been carried out in the Putih river catchment. It must be em-phasized that the large-scale debris flows occurred within a 4-year period after the 1984 pyroclastic flows.

Sediment yield by debris flow and flood

Based on the data of the Saido river and the Fukatani river in Sakurajima volcano (Jitousono et al., 1994), the Mizunashi river in Unzen volcano (Hirano et al., 1994), and the Boyong river in Merapi volcano (Lavigne et al., 1995), the relationship between the total runoff and the sediment runoff of debris flow and flood was analyzed. The catchment area is 1. 38km2 in the Saido river, 0. 38km2 in the Fukatani river, 15. 92km2 in the Mizunashi river, and 3. 52km2 in the Boyong river, respectively. Fig. 8 shows this relationship on logarithmic paper. The equation is obtained by a least squares method, that is,

Qs=0.00161QTi.34(y=0.913) (6) where, Qs (m3) is the sediment runoff and QT (m3) is the total runoff of debris flows and floods. A positive correlationship of high significant level is noted. Making use of Eq. (6) and the total runoff, QT of debris flows and floods obtained during the period from 1985 to 1990 at the Mranggen dam, the sediment runoff, Qs of debris flows and floods was calculated. Also, the ratio C of Qs against QT, representing the mean volumetric sediment concentration of debris flows and floods, was calculated. Fig. 9 shows the temporal variation of the mean volumetric sediment concentration, C of debris flows and floods at the Mranggen dam. The mean volumetric sediment concentration of debris flows and floods have decreased with the decrease of the magnitudes of peak discharge and total runoff since the 1984 pyroclastic flows. Using the total runoff and the sediment runoff of debris flows and floods obtained at Mranggen dam, the annual amount of sediment yield for a water year from November to October was calculated during the period from 1985 to 1990 (Table 1). In Table 1, the annual rainfall and the sediment yield per unit of rainfall are shown. Fig. 10 shows the annual sediment yield per unit of rainfall versus time since the 1984 pyroclastic flows. The annual sediment yield by debris flows and floods per unit of rainfall decreases with time. The regression equation is obtained by a least squares method, that is,

S=-0.653Y+3.839 (r=-0.988) (7) where, S (103m3/mm) is the annual sediment yield by debris flows and floods per unit of rainfall and Y (year) is time after the 1984 pyroclastic flows. In Table 1, the sediment yields per unit of rainfall during the periods from November 1984 to October 1985 and from November 1988 to October 1989 were calculated by Eq. (7). The annual sediment yield was calculated by multiplying the sediment yield per unit of rainfall by the rainfall amount (Table 1). The total amount of sediment yield by debris flows and floods during a 6-year period from No- vember 1984 to February 1990 is about 37.8x106m3 and the percentage for the water year from No- vember to October is 35.6%, 29.3%, 16.0%, 13.5%, 5.6% and 0.0%. Consequently, the outflow by debris flows and floods from the Putih river catchment almost finished within a 4-year period after the 1984 pyroclastic flows.

Conclusion

Merapi volcano is one of the most active volcanoes in Indonesia. Pyroclastic flows have often taken place in the southwestern flank of the Merapi volcano. Recently, large-scale pyroclastic flows occurred in June 1984. The pyroclastic flows were thickly deposited in some area of the Putih river Debris flow following the 1984 eruption with pyroclastic flows 115

Fig. 9 Variation of the mean volumetric sediment concentration of debris flows and floods at the Mranggen dam in the Putih river.

Fig. 8 Relationship between the sediment runoff and the total runoff of debris flows and floods.

Table 1 Total runoff and sediment yield of debris flows and floods at the Mranggen dam in the Putih river.

Fig. 10 Annual sediment yield by debris flows and floods per unit of rainfall at the Mranggen dam in the Putih

* Calculated using equation (7). river versus time since the 1984 pyroclastic flow. catchment and at the same time the surroundings were widely covered with the fine volcaniclastic materials originating from the pyroclastic clouds. As a result, the hydrological and erosional regime of the hillslopes was radically altered and debris flows and floods have occurred frequently in the Putih river catchment. On the basis of the data of debris flow observed by an ultrasonic water-level gauge in the Putih river catchment, the characteristics of debris flow following the 1984 eruption with pyroclastic flows are described. The results are summarized as follows: (1) More then 203 debris flows and floods have occurred during the period from November 1985 to February 1990 after the 1984 pyroclastic flows in the Putih river catchment. These debris flows and floods ranged from about 1 to about 2,OOOm3/s in peak discharge and from about 500 to about 5,000,OOOm3in total runoff. A positive correlationship of high significant level is noted with a nearly linear relation on logarithmic graph paper between the peak discharge and the total runoff of debris flows and floods. (2) The rainfall conditions not causing debris flow or flood are definitely different between the 4- year period after the 1984 pyroclastic flows and since then. The rainfall intensity not causing debris flow or flood is small just after the pyroclastic flows and then has increased with time. 116 T. Jitousono et. al.

(3) The large-scale debris flows occurred within a 4-year period after the 1984 pyroclastic flows. The magnitudes of peak discharge and total runoff of debris flows and floods have decreased with time. (4) The total amount of sediment yield by debris flows and floods during a 6-year period from No- vember 1984 to February 1990 is estimated at about 37. 8 x 106m3. The outflow by debris flows and floods from the Putih river catchment had almost finished within a 4-year period after the 1984 pyroclastic flows.

Acknowledgements

The authors are pleased to acknowledge the considerable assistance of Prof. Dr. Sumiji Kobashi of Kyoto University and Prof. Dr. Yoshihiro Fukushima of Nagoya University. The authors would like to express their appreciations to Dr. Suharyono, Mr. Agus Sumaryono, Mr. Haryanto, Mr. Hariyadi Dj amal and many other staffs of STC, Dr. Badruddin Machbub and Mr. Supardiyono Sobirin of Research Institute for Water Resources Development, Indonesia and Mr. Tomio Hirozumi and other staffs of JICA experts team at that time.

References

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