Analisis operasi Waduk Wonogiri untuk pengendalian banjir wilayah Sungai Bengawan Solo Hulu :: Studi kasus banjir akhir Desember 2007

SAVITRI, Dian, Dr. Ir. Istiarto, M.Eng

2009 | Tesis | S2 Teknik Sipil

Pada akhir Desember 2007 terjadi banjir di wilayah sungai Bengawan Solo, yang meyebabkan puluhan ribu orang menderita dan ribuan rumah tergenang. Upaya pengendalian banjir di wilayah Sungai Bengawan Solo Hulu adalah dengan pengoperasian Waduk Wonogiri. Salah satu fungsi Waduk Wonogiri adalah pengendali banjir, efektif sampai kota Surakarta. Selama kejadian banjir tersebut, terjadi operasi pengendalian banjir di Waduk Wonogiri dengan adanya operasi pembukaan pintu spillway. Tetapi hal tersebut oleh masyarakat justru dianggap sebagai penyebab banjir. Untuk mengetahui kontribusi outflow Waduk Wonogiri dan operasi waduk selama kejadian banjir akhir Desember 2007 perlu dilakukan rekonstruksi dan analisa kejadian pelepasan air dari Waduk Wonogiri tersebut serta pengaruhnya terhadap kondisi aliran sungai Bengawan Solo Hulu di hilir waduk. Rekonstruksi operasi waduk dilakukan dengan reservoir routing untuk mengetahui perubahan elevasi air waduk. Simulasi reservoir routing dilakukan untuk 3 kondisi yaitu outflow aktual, outflow jika sesuai manual OP dan tanpa outflow. Selanjutnya dilakukan perhitungan pengaruh outflow dari waduk terhadap sungai Bengawan Solo dengan titik kontrol di Bendung Colo. Perhitungan dilakukan dengan software HEC-HMS 3.3. Terdapat 3 kondisi simulasi pengaruh outflow yaitu, outflow jika sesuai OP, tidak ada outflow, dan outflow full opening. Hasil reservoir routing (aktual outflow) menunjukkan bahwa Waduk Wonogiri telah meredam puncak banjir dari 4.228 m3/s menjadi 200 m3/s. Reservoir routing (sesuai OP) Waduk Wonogiri telah meredam puncak banjir dari 4.228 m3/s menjadi 400 m3/s. Reservoir routing dengan kondisi pintu ditutup menunjukkan bahwa elevasi extra flood water level (+139,10 m) dicapai pada tanggal 9 Januari 2008 pukul 01:00. Dari hasil perhitungan dengan HEC-HMS diketahui bahwa Kali Walikan yang memberikan kontribusi debit pada puncak Banjir di Bendung Colo tanggal 26 Desember 2007 pukul 06:00, sedangkan Waduk Wonogiri mulai memberikan kontribusi pukul 14:00. Dari hasil simulasi berbagai kondisi outlow dari Waduk Wonogiri diketahui bahwa outflow aktual memberikan pengaruh paling kecil terhadap debit di hilir daripada outflow sesuai manual OP dan outflow full opening. Hal tersebut menunjukkan bahwa operasi waduk aktual memberikan kinerja yang lebih baik di hilir waduk

By the end of December 2007, flood event on Bengawan Solo river area, caused thousands of people suffered and thousands of house were flooded. Flood control effort in Bengawan Solo Hulu river area is by operating Wonogiri Reservoir. One of Wonogiri Reservoir function is flood control which effective down to Surakarta city. During flood event, flood control operation happened in Wonogiri Reservoir by opening spillway gates operation. But, this condition was reputed by society as a cause of flood. To identify outflow contribution from Wonogiri Reservoir and reservoir operation during flood event in last December 2007, it needs to do reconstruction and event analysis of water release from Wonogiri Reservoir’s effect to flow condition in Bengawan Solo Hulu river at reservoir downstream. Reconstruction of reservoir operation was carried out by using reservoir routing, in order to know reservoir water level changing. Reservoir routing was done for 3 conditions, namely actual outflow, outflow if suitable with OM manual, and without outflow. Furthermore impact calculation reservoir outflow was done for Bengawan Solo River in Colo weir control point. Calculation was done by using HEC-HMS 3.3 software. There are 3 simulation conditions, namely outflow if suitable with OM manual, no outflow, and full opening outflow. Reservoir routing (actual outflow) result represents Wonogiri Reservoir have dumped peak flow from 4,228 m3/s to 200 m3/s. Reservoir routing (suitable with OM manual), Wonogiri Reservoir have dumped peak flow from 4,228 m3/s to 400 m3/s. Reservoir routing in closed spillway gates condition represents extra flood water level (+139.10 m) elevation was reached in January 9th 2008 01:00 AM. From HEC-HMS calculation was known Kali Walikan gave peak flow contribution discharge in Colo Weir at December 26th 2007 06:00 AM, whereas Wonogiri Reservoir started to give contribution at 02:00 PM. Result from various simulation of Wonogiri Reservoir outflow, was known that actual reservoir operation gave less impact to downstream discharge than OM manual and full opening outflow. It case show that actual reservoir operation gave better performance than others to reservoir downstream.

Kata Kunci : Waduk Wonogiri,Pengendalian banjir,Outflow, Wonogiri Reservoir, flood control, outflow