Sedimentation rate in gajah mungkur reservoir central java, Indonesia

Gajah Mungkur reservoir is a multipurpose reservoir with broadness of 8,800 ha . Its functions mainly for irrigation, hydroelectric power, source of drinking water, tourism, aquaculture and fisheries. Presently the sustainability of reservoir function is distressed by emerging of sedimentation. The sedimentation may be reduced water volme, create siltation, decrease carrying capacity, create rust and corrosion on the turbine engine. The aims of the research is to observe the sedimentation rate at Gajah Mungkur Reservoir as a base line study on the reservoir management model. The research has been counducted at Gajah Mungkur Reservoir, Central of Jawa during the period of February to Desember 2011. In order to estimate bathymetry of this reservoir the acoustic survey methods has been applied whilest the sediment traps is applied to estimate the sedimentation rate. The results of this research showed that maximum depth was 16 m, average depth was 6.07 m, average water volume was 428,912,270 m3. Compared to the situation when the reservoir was built in 1981/1982 the average depth was 9 m, average water volume was 630.000.000 m3. It is mean that after 28 years, there is a reduction of water volume about 201.088.000 m3 and reduction of water depth 2.93 m. The sedimentation rate was higher during the rainy season i.e. 2.76 m3/ha/day, whilest during the dry season i.e. 0.96 m3/ha/day. The average sedimentation rate in one year was 6 cm / year, 593.71 m3/ha/year, 822.53 tons/ha/ year.