PERENCANAAN GROUNDSILL SECTION 2 PADA SUNGAI SERAYU OPAK, DAERAH ISTIMEWA YOGYAKARTA

Serayu Opak River which is located around Rogem, Tamanmartani, Kec. Kalasan, Sleman Regency, Yogyakarta Special Region, as a river that was affected by cold lava floods due to the eruption of Mount Merapi in 2010, left deposits of material that caused erosion and degradation of the river channel which would endanger the foundation of the Yogyakarta-Solo National Bridge. To prevent this, it is necessary to build a river bed control weir (Groundsill) to control the river bed elevation and protect the bridge. Groundsill Section II on the Serayu Opak River has a catchment area of 55,066 km2 with a trace length of 4.5 km2. Rainfall analysis was calculated using the Pearson log type III method with a 25-year return period of 192,338 m3/s. Based on the calculation results, the dimensions of the groundsill are 35 meters wide, 3 meters high, the lighthouse is 3 meters wide and the edge walls are 7 meters high with a surface width of 1.5 meters. The sediment storage capacity for 25 years is 41,409 m3. Based on the dimensions of the groundsill building, the stability analysis results under normal water level and flood water conditions were declared safe against overturning, sliding, and the bearing capacity of the soil.