Kajian karakteristik hidrograf banjir inflow Waduk Wonogiri terhadap kinerja pengendalian banjir :: Studi kasus banjir Bengawan Solo Desember 2007

NUGROHO, Alexander Armin, Dr. Ir. Rachmad Jayadi, M.Eng

2009 | Tesis | S2 Magister Pengelolaan Bencana Alam

Waduk Wonogiri dibangun dengan fungsi utama sebagai pengendali banjir terutama di daerah rawan banjir sepanjang Sungai Bengawan Solo. Untuk mengetahui kinerja Waduk Wonogiri dalam pengendalian banjir Bengawan Solo pada akhir Desember 2007 dilakukan kajian karakteristik hidrograf banjir inflow Waduk Wonogiri dengan mempertimbangkan kontribusi inflow dari seluruh Sub DAS di daerah tangkapan air pada waduk. Analisis perhitungan hidrograf banjir inflow Waduk Wonogiri dilakukan dengan kalibrasi Sub DAS Wuryantoro dan Keduang. Hasil kalibrasi ini dijadikan acuan untuk mensimulasikan hidrograf banjir inflow pada masingmasing Sub DAS daerah tangkapan air Waduk Wonogiri. Penelusuran banjir waduk dilakukan dengan asumsi bahwa debit inflow yang masuk ke Waduk Wonogiri dibiarkan sampai ketinggian muka air di waduk 135,3 m (batas bawah flood control) dan 138,3 m (batas atas flood control) kemudian pintu spillway dibuka penuh semua tanpa pengaturan bukaan pintu. Hasil penelitian menunjukkan bahwa nilai debit inflow maksimum yang masuk ke Waduk Wonogiri pada kejadian banjir akhir Desember 2007 berkisar antara 3.331 – 4.993 m3/s, yang terjadi pada tanggal 26 Desember 2007 antara pukul 04:00 – 06:00. Kontribusi hidrograf banjir paling dominan yang masuk ke Waduk Wonogiri berasal dari Sub DAS Keduang. Penelusuran banjir di waduk disimulasikan bahwa pintu spillway ditutup sampai ketinggian muka air di waduk mencapai minimal 135,3 m dan 138,3 m kemudian pintu spillway dibuka penuh semuanya. Elevasi muka air waduk mencapai 135,48 m pada tanggal 26 Desember 2007 jam 6:00 dan outflow yang dihasilkan saat pintu dibuka mencapai 550 m3/s kemudian meningkat sampai 642 m3/s pada jam 14:00 dan terus berangsur-angsur turun. Sedangkan simulasi untuk mendapatkan elevasi muka air waduk mencapai 138,3 m tidak terpenuhi karena sampai dengan tanggal 27 Desember 2007 pukul 23:00 elevasi muka air di waduk baru mencapai 163,44 m. Fungsi pengendalian banjir Waduk Wonogiri masih dapat dijalankan dengan baik dan mampu meredam banjir sebesar 85 %.

The Wonogiri reservoir was built with a primary function as flood control, especially in areas prone to flooding along the Bengawan Solo River. To find out the performance of the Wonogiri reservoir in flood control of Bengawan Solo, a study was conducted on flood hydrograph characteristics of the reservoir inflow by considering the contribution inflow from all sub-watersheds in the reservoir catchment area, at the end of December 2007. Calculation analysis flood hydrograph of Wonogiri Reservoir inflow is done with the calibration of Wuryantoro and Keduang sub-watersheds. Results of the calibration were then used reference to simulate flood hydrograph inflow in each sub-watershed catchment areas. Tracing the reservoir flood is done with the assumption that the inflow of the reservoir was left to face up a height of water in the reservoir 135.3 m (the lower flood control limit) and 138.3 m (the upper flood control limit) and then the spillway gates full-opening. Results of this research indicate that the maximum discharge inflow into the reservoir on the event of Wonogiri flood at the end of December 2007 ranged from 3,331 to 4,993 m3/s, which occurred on December 26, 2007, between 04:00 - 06:00 am. The most dominant flood hydrograph contribution into the reservoir was derived from Keduang sub-watershed. The flood in the reservoir was simulated as that the spillway gates were closed until water level of reservoir reached the minimum height of 135.3 m and 138.3 m and only until then the spillway gates fullopening. The reservoir water level reached 135.48 m on December 26, 2007 at 6:00 am and outflow was generated when the gates opened to reach 550 m3/s and then increased up to 642 m3/s at 14:00 after then it gradually decreases. The water level simulation was unable to reach 138.3 m because up to December 27, 2007 at 23:00 the water level reservoir reaches only 136.44 m. The Wonogiri reservoir flood control function still can run well and is able to reduce the peak flood of 85%.

Kata Kunci : hidrograf banjir inflow, Waduk Wonogiri, pengendalian banjir, flood hydrograph inflow, Wonogiri Reservoir, flood control