Integrated flood management for transboundary river basin : a case study in Bengawan solo river basin - Indonesia

Abstract

Bengawan Solo River flows from Upper Bengawan Solo Sub-catchment located in Wonogiri-Central Java Province to Java Ocean through some regions in Central Java and East Java Provinces, Indonesia. The length of the river is about 600 km, and the total area of Bengawan Solo River Basin is about 19,778 km2 . The average annual rainfall in the upper catchment of the river basin is approximately 1935 mm. In the end of December 2007, some regions in Central Java and East Java Provinces (Indonesia) located in Bengawan Solo River Basin were flooded due to high rainfall intensity which caused the overflow of River Bengawan Solo and release of excessive water from Wonogiri Dam in the upstream catchment. Some regions in the affected areas were inundated for several weeks. This flood phenomenon was the biggest flood in the river basin during the last 40 years. The flood continues to occur annually since then. This research aims to improve the understanding of the flood generating mechanisms and the nature of flooding in Indonesia, particularly in the Bengawan Solo River Basin, and to use this understanding to inform robust methods of flood risk management taking into account present and future climate as well as possible land use management strategies. The quality of the rainfall data in Bengawan Solo river basin is unsatisfactory with the average of cross-correlation 0.31. However, this research used the data because it is the only in situ data available in the river basin by selecting the most complete data from the available gauging stations. The Pettit test was applied in this research to detect the abrupt change of extreme rainfall in selected gauging stations. Moreover, the Mann-Kendall test was applied to investigate the trend of extreme rainfall. The annual maximum daily rainfall and monthly maximum daily rainfall during rainy season (October – March) were used in Pettit and MannKendall test. The SHETRAN simulation was implemented in this research to investigate the impact of land use change to peak discharge using three different land use scenarios. Besides using in situ data, the SHETRAN simulation also used 3-hourly TRMM rainfall data. According to the results of the SHETRAN simulation, the peak discharge rises considerably with expanding urbanization; consequently, planned control over land use change and development is required to prevent future issues. Improved hydrometry data availability is also necessary for more accurate flood modelling. The big flood event in December 2007 could be triggered by high precipitation (which was recorded as the highest maximum precipitation). The flood continues to occur recently, although the maximum rainfall is not as high as the one occurred in December 2007. Considering the result from SHETRAN simulation and the fact that the number of buildings is increasing in Bengawan Solo River Basin, it can be understood that flooding will continue to occur although there is no upward trend of extreme rainfall. Since flooding is likely to occur, attempts must be done to minimize the risk and reduce the severe impact of flooding. Learnt from this research with the limitation of reliable data, it is suggested that the agencies and authorities responsible in flood management in Bengawan Solo River Basin should improve the gauging networks and data base related to flood management, including hydrological and hydraulic data. Furthermore, the attempts have been done by Mekong River Commission and approach introduced by Cap-Net might be adopted by some modification appropriate to Bengawan Solo River Basin condition. Providing early good flood forecasting and warning system, conducting capacity building for all related stakeholders, enhance coordination and cooperation among related agencies and authorities, and development flood measure both structural and non-structural could be implemented to reduce the flood risk and minimize severe impact of flooding in Bengawan Solo River Basin.