Seite - 276 - in Book of Full Papers - Symposium Hydro Engineering

3. ANALYSIS AND RESULTS The study identified that the sediment rate per year is 20.25 ton/ha. The grain sediment analysis also showed that 50% of sediment which setlles in river bed is sand. The sources of the sediment were from the lake and from Asahan tributaries which are Bolon and Mandosi river. From the water quality analysis, chlorine has significant parameter for the Asahan water. Asahan River, of which a major part is used by the people around Toba Samosir Regency, is utilized as a place for washing, bathing, industry, and agricultural activity runoff. THE COUNTERMEASURES OF SEDIMENTATION Based on the Technical Plans for Rehabilitation of Forest and Land Asahan Barumun Water Catchment Area 2015–2029 by Ministry of Forestry, targets of erosion control in the study area are established to be at least equivalent to the target of erosion control in the Toba Asahan watershed. Table 1 Proposed Program for Sediment Countermeasure (data processing 2017) Another approach for sediment control management is dredging works. The analysis showed that dredging works have to be done regularly with estimate volume of 21.000 m3 in order to maintain sedimentation. 4. CONCLUSION The objective of this study is to identify the rate and sources of sedimentation at Upstream Asahan and the countermeasures as well in order to ensure the sustainability of Siruar Dam and Asahan hydroelectric power plant. From the analysis, the rate of sedimentation reached 20.25 ton/ha/year and 50% which settled in river bed was sand. Changes of land use has important role to speed up the sediment rate, therefore conservation of land, reforestation and civil conservation approach such as check dams, gully plugs have to be done. Another technical approach to maintain deposit sediment in the river is dredging works. The study recommends for regular dredging with estimate volume of 21.000 m3 sediment per year. 276