Fujita, Masaharu

Ikhsan, Jazaul

Sulaiman, Muhammad

English

Hydraulic Engineering Repository

Conference Paper, Published VersionIkhsan, Jazaul; Sulaiman, Muhammad; Fujita, MasaharuA Study on Sustainable Sediment Management in MerapiVolcanic AreaVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/100175Vorgeschlagene Zitierweise/Suggested citation:Ikhsan, Jazaul; Sulaiman, Muhammad; Fujita, Masaharu (2008): A Study on SustainableSediment Management in Merapi Volcanic Area. In: Sekiguchi, Hideo (Hg.): Proceedings 4thInternational Conference on Scour and Erosion (ICSE-4). November 5-7, 2008, Tokyo,Japan. Tokyo: The Japanese Geotechnical Society. S. 553-557.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.1  A STUDY ON SUSTAINABLE SEDIMENT MANAGEMENT IN MERAPI VOLCANIC AREA    Jazaul IKHSAN1, Muhammad SULAIMAN2 and Masaharu FUJITA3  1Member of JSCE, Graduate Student, Dept. of Civil and Earth Resc. Eng., Kyoto University  (Shimomisu, Yoko-oji, Fushimi, Kyoto 612-8235, Japan) E-mail: jazaul.ikhsan@at4.ecs.kyoto-u.ac.jp 2Dept. of Civil and Env. Eng., Gadjah Mada University  (Jl. Yacaranda No.1, Sekip Unit IV, Yogyakarta, Indonesia) E-mail: muhammad_sulaiman@yahoo.com 3Member of JSCE, Professor, Disaster Prevention Research Institute, Kyoto University  (Shimomisu, Yoko-oji, Fushimi, Kyoto 612-8235, Japan) E-mail: fujita@sabom.dpri.kyoto-u.ac.jp        Mt. Merapi is one of the most active volcanoes in the world and located at 30 km north-northeast from Yogyakarta, Indonesia. A large amount of sediment supply from Mt. Merapi area is serious threat to people, but works also as an important resources for people.  Thus, the sediment from the volcano has given some advantages and disadvantages.  Sustainable sediment management is urgently necessary to mitigate the sediment disasters and provide the people with benefits. It is considered that sand mining activity can be used as one of the tools to control the sediment discharge, and the regional development. In this paper we discussed the basic concept of such sustainable sediment management considering the sediment production, the topographical and hydrological condition, and the actual sand mining activity.     Key Words : Mt. Merapi, sediment, sustainable management, control sediment, sand mining    1.  INTRODUCTION   (1) Volcanic activities Mt. Merapi is one of the most active volcanoes in the world8). Mt. Merapi is located at approximately 30 km north of the city of Yogyakarta, Indonesia. Mt. Merapi has been giving various volcanic activities, such as eruptions, lava flows, pyroclastic flows, glowing clouds, volcanic ash falls and volcanic debris flows. Mt. Merapi has erupted once 5 years, or once 14 years for major eruptions during the last 200 years. In the last 50 years, Mt. Merapi erupted once 3 years, and major eruption occurred at an interval of 9 years. Mt. Merapi has still been producing actively a huge amount of sediment. The sediment has been causing many sediment disasters, and threatening local residents. Particularly, pyroclastic flows due to collapse of lava dome or lava tip result in disasters and a tremendous amount of volcanic loose deposits on the slope of Mt. Merapi. Pyroclastic flows have run down during the last 100 years on every slope of Mt. Merapi2), 8). However, they have occurred most on southwest slope during 37 years from 1961 to 1997. The occurences of pyroclastic flows in 1998 and 2001 were limited on the western slope. However, in the eruption on June 2006, the pyroclastic flows took place in the Gendol River and the Woro River5) . The total number of debris flows recorded from 1931 to 1996 was more than 500 times. The debris flows were recorded in almost all the rivers on the slopes of Mt. Merapi2).  (2) Sand mining activities The sediment can be important resources for people. The sediment deposit produced by the eruptions of Mt. Merapi has market value, and its quality attracts sand miners. The sand mining activities have given some advantages for rural/local people, local government and reduced sediment run off. Sand and gravel material in Mt. Merapi offer many benefits such as employment opportunity, and an increase in economical benefit to farmers. 553D-8 Fourth International Conference on Scour and Erosion 2008 2                            Total number of mining workers in Mt. Merapi area amounts to about 21,000 man/day. The local government of Magelang Regency obtained benefit from the sand mining activities and the regency income increased from Rp. 236,000,000 (in fiscal 1997) into Rp. 2,218,000,000 (from fiscal 1998)4). Klaten regency imposed a mining tax revenue of Rp 92,00,00 (in fiscal 1999) and Sleman regency imposed a mining tax revenue of Rp 500,000,000 (in fiscal 2000). It means that exploitation of sand and gravel material provides rural areas with considerable opportunities for economic development. However, uncontrolled sand mining has caused serious problems in the watershed such as unstableness of sediment control facility, bridge and irrigation intake by digging nearby, channel and riverbank instability due to riverbed degradation, and destruction of aquatic and riparian habitat due to natural and artificial armoring. As long as the sand mining is controlled, it can be one of measures for sediment control plan to give an extra empty in the sediment reservoirs and contribute to the rural economy. From this point, sustainable sediment management assisted by sand mining is urgently necessary to mitigate the above issues. In this paper, the basic concept of such sustainable sediment management assisted by sand mining is discussed.                             2. SEDIMENT BALANCE  The current situation of sediment balance in Mt. Merapi area is influenced by sediment production, sediment mining and sediment discharge to sea. Figure 1 shows the current situation of sediment balance in Mt. Merapi area.  (1) Sediment production The lava production data from 1890 to 1992 have been compiled by Siswowidjoyo et al., (1995)6). The production volume of individual eruptive events is varied widely from less than 106 m3 to more than 20 x 106  m3, but the cumulative volume is proportionally increased and the annual average lava production rate is approximately estimated at around 1.2 x 106  m3/year.  In Mt. Merapi area, sediment production from the non-volcanic basin can not be neglected. The sediment production from non-volcanic basin is estimated at 20% of the sediment production from volcanic active basin2), therefore, the annual average sediment production is equal to 0.24  x 106 m3/year. Thus, the annual average sediment production rate from Merapi volcano (volcanic active basin) and non-volcanic basin, Qspm , is 1.44 x 106 m3/year.   0 Km 15  N Indonesia Sea Progo River Opak River Yogyakarta City Mt. Merapi  Lower-middle area Sediment discharge to sea 1.46x106m3/year  (calculated) Average lava production  1.2x106m3/year Sand mining  5~6x106 m3/year  (2000) Sand mining  1.07x106m3/year  (1999) Slope = 0.0015 (2001) Sediment from Non volcanic basin 0.24x106 m3/year Upper area Mt. Merapi  Mean grain size = 1 mm  Mean grain size = 14 mm  Slope = 0.0107 (2001) Fig.1 Sediment balance in Mt. Merapi area 554 3                Fig.2 Cumulative volume of the lava productions in Mt. Merapi and the sediment production in non volcanic basin.  Figure 2 shows the sediment production from volcanic active basin and non-volcanic basin in Mt. Merapi.  (2) Sand mining volume The sand mining volume in the foothills (upper area) of Mt. Merapi in 2000 was estimated at 5-6 x 106 m3/year1). The sand mining persists not only in the foothills of Mt. Merapi but also in the lower reach of river channel, especially in the Progo River. In the Progo River, the sand mining activities are concentrated in the lower reach area. The mining rate in the Lower Progo is estimated at about 2,933 m3/day or 1.07 x 106 m3/year3).  (3) The future condition According to DGWR report, the hydrological and topographical conditions in the lower Progo River are as follows. The annual average discharge is 83.1 m3/s. The mean diameter of bed material is 1 mm, the average river width is 200 m, and the average bed slope is 0.0015. Under this condition, the total sediment discharge in the lower Progo River, Qs, is estimated at 1.46 x 106 m3/year using Ashida and Michiue’s bed load transport formula and Ashida and Michiues’s suspended load formula. This result shows annual average sediment discharge is almost equal to annual average sediment production rate. Therefore, the sediment discharge to sea balances with the sediment production rate. If the bed material is not removed by sand mining, degradation does not occur. However, actually total sand mining in the foot hill area and the lower Progo River are 6.07~7.07 x 106 m3/year. Thus, the riverbed degradation has occurred in the lower Progo River and caused unstableness of existing river structures such as sediment control dam, bridge foundation and irrigation intake. In April 2000, one of the bridges, Srandakan Bridge located in lower Progo River collapsed. If no sediment is supplied to the lower reach of Progo River because of active sand mining in the upper reach, the annual average degradation depth is estimated at 1.10 m/year. According the field survey, this estimation agrees with the actual bed degradation. If sand mining activities in the upper reach is not suppressed, it means sediment does not supplied into the lower reach for a long term. Under this condition, the slope decreased from 0.0015 until the static equilibrium state of sediment transport is reached. The static equilibrium estimated at 0.000156.   3. SUSTAINABLE SEDIMENT MANAGEMENT  (1) Present condition Sediment deposit in Mt. Merapi area has threatened people; however, its quality attracts sand miners. Sand in Mt. Merapi area has a good quality and is useful as construction material. However, uncontrolled sand mining has caused serious problems. Excessive sand mining at a particular site reduces sediment discharge and causes the river bed degradation in the downstream.   (2) Sand mining management The sand mining activities are prospering around Mt. Merapi. It is could be realized that sustainable sediment management consideres sustainable sand mining management. Sand mining management is one of alternative to control sediment discharge in Mt. Merapi area. The view point in sustainable sand mining is how to determine the allowable sand mining volume in the upper area around Mt. Merapi.  Determining the allowable sand mining volume, the following steps are necessary to do. First, the designed bed slope in the lower reach, ibd, is decided. In consequence of first step, it is necessary to estimate how many groundsills must be installed. If the designed bed slope is less than the original bed slopes, the number of groundsills becomes larger. Next step, sediment discharge to sea, Qs1, is calculated for the designed bed slope. Finally, the allowable sand mining volume, Qsa, can be calculated upon the design sediment supply rate, Qspd, and the sediment discharge to sea as follows.    Qsa = Qspd – Qs1          (1)  1900 1950 199250100YEARCUMULATIVE VOLUME (x106 m3 )1500CUMULATIVE VOLUME (x106 m3 )Annual average lava production 1.2x106 m3/year Annual sediment production  in non volcanic basin  0.24x106 m3/year 555 4 00.20.40.60.811.21.41.60 0.0005 0.001 0.0015 0.002i bdQ sa (x 106  m3 /year)Average sediment supply rate 00.511.522.533.50 0.0005 0.001 0.0015 0.002i bdQ s2 (x 106  m3 /year)Average sediment supply rate Assumed that Qspd is equal to Qspm (1.44 x 106 m3/year), Qsa is expressed by Qspm – Qs1. For instance, if the designed bed slope is 0.0015, the sediment discharge to sea, Qs1, is 1.46 x 106 m3/year. Thus, under this condition, the allowable sand mining volume is around zero. In the other case, if the designed bed slope is 0.0010, the sediment discharge to sea is 0.78 x 106 m3/year, and therefore the allowable sand mining volume is estimated at 0.66 x 106 m3/year. Relation between ibd and the allowable sand mining volume, Qsa, is shown in Figure 3. In the Mt. Merapi area, the maximum allowable sand mining volume is limited to 1.44 x 106 m3/year that is the sediment resource annually provided from Mt. Merapi volcanic and non volcanic area.    4. OTHER PROBLEM  The lava production volume of individual eruptive events is varied widely from less than 106  m3 to more than 20 x 106 m3. Therefore, the sediment supply rate, Qsupply, from the Mt. Merapi also changes very much. Thus, it is very important to determine the allowable sediment supply to the lower Progo River, Qs2, for each designed bed slope. Qs2 is defined as sediment supply rate that causes the designed bed slope to return to the original bed slope (ib = 0.0015). The maximum allowable sediment supply to the lower area is 2.9 x 106 m3/year for the designed bed slope of 0.000156 and this is the maximum allowable supply rate. Relation between ibd and Qs2 is shown in Figure 4. If Qsupply is less than or equal to Qs2, a series of groundsill is never buried with sediment. But if Qsupply is much bigger than Qs2, this condition will cause severe agradation. For instance, if a huge eruption occurs with the sediment production rate of 20 x 106 m3/year like 1930, it is predicted that the bed slope changes from the designed bed slope to 0.0086. This condition is quite danger for the lower reach. If the river bed increased rapidly, it caused some river structures such as irrigation intakes and bridges functioned ineffectively7. In order to reduce the predicted sediment disasters, the excess sediment supply should be controlled by the structural method such as sand pockets. For sustainable sand mining management, it is important to release the sediment deposit from the sand pocket at a rate of Qsa. Considering the actual situation of the volcanic activities in Mt. Merapi, a buffer zone such as a sand pocket is strongly required in the deposition area of pyroclastic flows/debris flows for sustainable sediment management.               Fig.3 Relation between the allowable sand mining volume, Qsa, and the designed bed slope, ibd               Fig.4 Relation between the allowable sediment supply, Qs2, and the designed bed slope, ibd   5. CONCLUSION  Considering the actual situation of sediment production, bed variation, sand mining activities, we have shown a concept of sand mining management. In this management, firstly the designed bed slope is determined and the difference between the sediment supply and the sediment discharge for the designed slope is used as the resource for sand mining. The relation between the allowable sand mining volume and the designed bed slope was shown. However, a huge eruption that sometimes takes place would cause the severe bed agradation. Thus, we mentioned the necessity of sabo works such as sand pocket that could be a buffer zone for sediment run off. The sediment management system composed of sand mining management and sabo works is strongly required. 556 5 REFERENCES  1) DGWR, Ministry of Settlement and Regional Infrastructure, Republic of Indonesia: Review master plan study on Mt. Merapi, Main Report, 2001a. 2) DGWR, Ministry of Settlement and Regional Infrastructure, Republic of Indonesia: Review master plan study on Mt. Merapi, Supporting Report [B] Volcanic Disaster Mitigation Plan, 2001b. 3) Indra Karya : Survey of sediment balance and management in Progo river, Final Report, 1999. (in Indonesian)  4) Karnawati, D., Pramumijoyo, S., and Hendrayana, H.: Geology of Yogyakarta, Java: the dynamic volcanic arc city, IAEG paper number 363, Geological Society of London, 2006. 5) Mananoma, T., Rahmat, A., and Legono, D. : Prediction of sediment storage capacity of Gendol river after eruption of Mt. Merapi in 2006 Proc. Annual Meeting of Indonesia Association of Hydraulic Engineers XXIII, Manado, Indonesia ,2006. (in Indonesian) 6) Siswowidjoyo, S., Suryo, I., and Yokoyama, I.: Magma eruption rates of Merapi volcano, Central Java, Indonesia, during one century (1890-1992), Bulletin Volcanology, Vol.57, pp.111-116, 1995. 7) Sumaryono, A., Churiyah, and Artha, I.G.M: Geomorphological changes of Kali Progo caused by lahar flow from Mt. Merapi, Proceedings of Workshop on Disasters Caused by Floods and Geomorphological Changes and Their Mitigations, pp. 198-202, Yogyakarta, Indonesia, 1996. 8) Voight, B., Constantine, E.K., Siswowidjoyo, S., and Torley, R.: Historical eruptions of Merapi volcano, Central Java, Indonesia, 1768-1998, Journal of Volcanology and Geothermal Research, vol. 100, pp. 69-138, 2000. Mt. 557

A Study on Sustainable Sediment Management in Merapi Volcanic Area

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A Study on Sustainable Sediment Management in Merapi Volcanic Area

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