Page - 286 - in Book of Full Papers - Symposium Hydro Engineering

5.2 FLUSHING AND DREDGING BALANCE AGAINST RIVER BED DEGRADATION As described above that in sedimentation upper reach and degradation of river bed are also problem in water resources management. The degradation of river bed along Brantas River is between 2 s / d 8 m. To reduce of river bed degradation some efforts that have been done are:  Controlling illegal sand mining  Conducting public awareness on the impact of illegal sand mining  Implementation of flushing or dredging with riverine disposal to channel below. Over the past five years, the average sediment that flows to downstream is 350,865 m3/year. While the amount of sediment that can be supply form dredging with riverine disposal to channel below is 250,475 m3/year. If the amount of illegal sand mining along the Brantas River in lower reach of Wlingi dam is 2,470,600 m3/year, and there is no dredging with riverine disposal to channel below, flushing should be done 7 times a year. This is very difficult because the implementation of flushing is depend on the available inflow. Implementation of dredging 2 million m3/year with riverine disposal to channel below in Wlingi Reservoir requires a huge cost. The possible efforts have be done through sustainable public persuasive and constructive actions through controlling the illegal sand mining. 5. CONCLUSION 1. Implementation of flushing is very effective to reduce sediment in the reservoir which has river bed degradation in lower reach. Financially sedimentation flushing can save the cost of Rp. 35.85 billion compared to dredging with spoil bank disposal, while dredging with riverine disposal to channel below can save about Rp. 14.96 billion compared to the dredging with spoil bank disposal. However flushing implementation can not be implemented every year because it must consider the reservoir inflow. 2. If the inflow is not possible to carry out flushing, dredging with riverine disposal to channel below can be done. 3. Although not significant, the implementation of flushing and dredging with riverine disposal to channel below can reduce the river bed degradation occurring in lower reach. To balance the illegal sand mining in downstream it is necessary to flush 7 times a year and it is difficult to achieve. Other efforts to maintain the river bed are the control of illegal sand mining, and public campaign. 286