Study on Integrated Sediment Management in an Active Title Volcanic Basin( Dissertation_全文 ) Author(s) IKHSAN, JAZAUL Citation Kyoto University (京都大学) Issue Date 2010-09-24 URL https://dx.doi.org/10.14989/doctor.k15649 Right Type Thesis or Dissertation Textversion author Kyoto University Study on Integrated Sediment Management in an Active Volcanic Basin By IKHSAN JAZAUL 2010 Abstract Mass and sediment movements in a river basin give us benefit as well as natural disasters. Mass movement such as pyroclastic flows, debris flows, and landslides causes often destructive sediment disasters to human being as a short term event. Also, sediment movement such as erosion, bed load, suspended load and wash load generates important problems on basin management such as reservoir sedimentation as a long-term event. The mass and sediment movements give negative impacts on a river basin and human being. This is an aspect of natural disaster in mass and sediment movements. On the other hand, sediment deposits of bed load, suspended load, debris flow, and so on are the potential sediment resources in a basin. Huge mass movement brings excess sediment, but it is also one of the important sediment resources. This is an aspect of natural resources in mass and sediment movements. In sediment management for a basin, it is essential to consider these two aspects together. In other words, sediment disaster management and sediment resources management should be well synchronized. For example, sabo works and channel works for sediment disaster mitigation influence a role of sediment as resources in a river basin. Inappropriate sediment resources management could result in devastation of the basin and produce many weak points for sediment disaster management. The sediment disaster management can function only under an appropriate sediment resources management. Safety, river environment, and utilization are commonly the target elements of sediment management. As a change in an element by sediment management may affect the other two elements, and the priority among three elements depends on stakeholders. Current conditions on these elements in a basin are changed toward the targets by sediment management and consequently the socio-economic conditions are expected to be improved. Hence, it is necessary to develop a method to evaluate the effect of sediment management on each element and an integrated evaluation method for socio-economic effect. In this study, taking Mount Merapi basin as an investigation field, these methods for an active volcanic basin were developed and some case studies were conducted. Mount Merapi located in Yogyakarta is one of the most active volcanoes in Indonesia. It has erupted regularly and the eruption has been more active in the last i 20 years. The eruptions have produced huge sediment and caused pyroclastic flows and debris flows, threatening people live and assets in the downstream area. Sediment disaster mitigation has been implemented and sabo facilities have been constructed to mitigate the sediment disasters. On the other hand, local people have used deposited sediment as construction material. People came to take the sediment as much as possible for supporting regional development and their additional incomes. Moreover, huge eruption has accelerated their sand mining activities. Once sand mining is much activated, it is difficult to reduce a sand mining business. Rather sand mining area has spread through the basin from the river. Uncontrolled sand mining has caused the bed degradation and basin wasting, and has given negative impact for safety and environment. This study focuses on developing a framework of integrated sediment management in Mount Merapi volcanic basin considering three elements, namely utilization, safety and environment. Then, some case studies of sediment management were conducted and the effectiveness of sediment management on socio-economic condition was also discussed considering the current situation. The objectives of this study are: (1) to figure out the socio-economical and environmental conditions in Mount Merapi volcanic basin as a background of sediment management for the basin, (2) to develop a concept of sustainable sand mining management as one of the main management tools in Mount Merapi basin, (3) to develop a framework of integrated sediment management, which considers both of sediment disasters and sediment resources and (4) to develop a method to evaluate the effect of sediment management from a socio-economic point of view. To achieve first objective of this study, a questionnaire survey for inhabitants and literature investigation were carried out. The result shows that sediment is an important resource to support inhabitants’ daily life through sand mining activity. The activity has a positive socio-economic impact on Mount Merapi basin by providing job opportunities and giving additional income for inhabitants as well as local government. Inhabitants and local government give a good awareness to sabo works. Sabo works have constructed for two purposes; first for sediment disaster mitigation and second for supporting regional development such as bridges and an irrigation water intakes. As a result, safety is secured and transportation access is more convenient. However, due to the excessive sediment resources use, the environmental condition in Mount Merapi basin tends to be worse due to riverbed degradation and instability of river ii infrastructures. Hence, it is necessary to develop a new concept of sediment management in Mount Merapi basin. Hence, a concept of sustainable sand mining management combined with consolidation works was discussed as one of the main management tools in Mount Merapi basin. In this management, the sediment produced at Mount Merapi is used as much as possible with preventing severe riverbed degradation and aggradation. This is implemented to overcome the sediment problems on both of disaster and resource. Steps to estimate the allowable sand mining volume were proposed as follows; (i) determining a designed bed slope, (ii) calculating sediment discharge to the sea, (iii) deciding the allowable sand mining volume based on the designed sediment supply rate, and (iv) determining the location of groundsills. The concept of sustainable sand mining management with consolidation works was applied to Progo River in Mount Merapi basin. Simulations of riverbed variation were carried out under a sediment supply rate equal to the averaged sediment production rate in the basin by means of one-dimensional bed deformation model. The simulation result under no sand mining condition shows rather large riverbed aggradation in the upstream reach. Therefore, sand mining could be one of the tools in sediment run off control. If the sand mining volume is 39 % of the annual average sediment production, the riverbed is in equilibrium condition without aggradation and degradation. Also the sand mining is an important role in the socio-economic condition. So, the simulation for sediment management with sand mining control and consolidation works was performed. The simulation result shows that severe bed degradation takes place in the upstream reach if the present sand mining activity is kept without groundsill. Consolidation works are necessary to prevent the bed degradation. Thus, the bed variation was simulated for the coupled management of sand mining control and consolidation works. The simulation result shows that a series of groundsill can protect the riverbed from degradation even if more than 39% of sediment production is taken. However, the groundsills cannot prevent the serious bed degradation if the current sand mining activity is kept. A huge eruption frequently takes place at Mount Merapi. At that time, severe bed aggradation occurs and all the groundsills installed by the above management could be buried with sediment. Sabo works should be combined with the proposed sediment management to prevent the excess sediment supply to the main river. A simulation was conducted considering sabo works in a tributary as well as sand iii mining control and consolidation works. The sediment discharge to the main river is estimated and it is found that the tributary installed some sabo dams can function as a buffer zone for the huge eruption. This sediment management composed of sand mining control, consolidation works and sabo works is very effective for Mount Merapi basin. Finally, an integrated evaluation method for sediment management was discussed from a socio-economic point on safety, environment, and sediment utilization and the case study of sediment management was evaluated by means of this method. To evaluate the effects of the sediment management, some parameters on safety, utilization, and environment have been introduced. From a utilization point of view, job opportunity, additional income of local people and tax income to local government were used to evaluate the effectiveness of the sediment management. The risk degree of river infrastructures was used to describe the effect of the sediment management on a safety aspect. To evaluate the effects of the sediment management on environment, the mean diameter of grain size distribution of riverbed surface was used. On the coordinate system designating these elements, the direction of change in basin condition by the sediment management can be predicted, so that the most preferable sediment management can be decided. The results indicate that the proposed sediment management tends to give the negative impacts