Banjir Bengawan Solo sering terjadi akhir-akhir ini dan menimbulkan banyak kerugian. Salah satu upaya pengendalian banjir tersebut, di Wilayah Sungai Bengawan Solo Hulu, melalui pengoperasian Waduk Wonogiri. Perubahan tataguna lahan dan karakteristik hujan di daerah tangkapan air Waduk Wonogiri memungkinkan terjadinya perubahan hidrograf banjir inflow rancangannya sehingga diperlukan pula kajian lanjutan terhadap pola operasi pintu spillway Waduk Wonogiri guna meningkatkan kinerjanya sebagai pengendali banjir. Analisis utama yang dilakukan dalam pengendalian banjir ini ada 2 (dua) yaitu analisis hidrograf banjir inflow Waduk Wonogiri berdasarkan kondisi tata guna lahan dan analisis operasional pintu spillway pada periode banjir. Perhitungan hidrogfar banjir inflow menggunakan model semi distribusi hujanaliran. Simulasi hidrograf banjir rancangannya dilakukan untuk kala ulang 2, 60, 500 dan 5500 (PMF) tahun dengan input adalah parameter DAS hasil kalibrasi dan hujan rancangan jam-jaman untuk kondisi saat ini. Penentuan pola operasi Waduk Wonogiri yang update dan optimum pada periode banjir dilakukan melalui penelusuran banjir dengan beberapa skenario. Batasan yang dipakai adalah elevasi muka air di waduk saat awal penelusuran: +135.3 m (controlled water level) dan elevasi muka air maksimum di waduk: +139.1 m (extra flood water level) sebagai elevasi kritis pada periode banjir. Hasil penelitian ini menunjukkan telah terjadi perubahan respon DAS pada daerah tangkapan air Waduk Wonogiri. Hidrograf banjir inflow rancangannya cenderung meningkat, debit puncak maupun volumenya, dibandingkan desain awal. Untuk menghindari risiko overtopping pada bendungan, pola operasi pintu spillway Waduk Wonogiri yang update dan optimum periode banjir PMF adalah pola operasi: debit outflow maksimum melalui bukaan pintu sebagian adalah 400 m 3 /s; selang waktu operasi pintu spillway: 1 jam dan berubah menjadi 15 menit (dimulai saat elev. muka air di waduk mencapai +136.4 m); full opening gate dilakukan saat elev. muka air di waduk mencapai +136.6 m (dimana ketentuan sebelumnya adalah +138.2 m) dan pengaturan debit outflow merupakan kelipatan 50 m 3 /s. Waduk Wonogiri masih cukup aman untuk banjir T: 2, 60 dan 500 tahun. Real time flood forecasting sangat diperlukan agar kinerja operasi waduk sebagai pengendali banjir dapat ditingkatkan.

Bengawan Solo often produces flooding and creates sufficient damage and loss. One of flood control efforts in the upstream Bengawan Solo River Basin is conducted through the operation of Wonogiri Reservoir during flood season. Changes in land use and characteristics of high resolution rainfall in the catchment area of Wonogiri Reservoir allow changes of inflow hydrograph. Therefore, a study to evaluate the operational pattern of Wonogiri Reservoir and to improve the performance of flood control is urgently required. Two important analyses are carried out to clarify the flood control issue, i.e. inflow flood hydrograph based on the current land use and the operating rule of spillway gates for flood period. The current inflow flood hydrographs are obtained by using a semi distributed rainfall-runoff model. For this purpose the 2, 60, 500 and 5500 (PMF) year return period of flood hydrographs are simulated with input the calibrated watershed parameters and hourly design rainfall distribution of the current condition. The hydrologic simulation results are then used to simulate several scenarios of reservoir flood routings for obtaining the optimum spillway gates operation. The optimum flood control is determined by considering the controlled water level (+ 135.3 m MSL) and the extra flood water level (+ 139.1 m MSL) as the critical boundaries of water level during flood period. The results of the study indicate that the watershed response to the peak discharge changes considerably. The current land use produces flood inflow hydrographs that higher than the design flood hydrographs, both peak discharge and total runoff volume. For the spillway gates operation, it is found that the operating rule for the PMF has to be modified. In order to avoid high risk of dam over-topping, full opening gate is operated when the water level in the reservoir reaches +136.6 m MSL instead of +138.2 m MSL. Besides, the spillway gates operation time should be monitored every 15 minutes when the water level reaches +136.4 m MSL and outflow discharge arrangement is a multiple of 50 m 3 /s. Generally, it is concluded that Wonogiri Reservoir is still able to control the extreme floods with special consideration for the PMF. Real time flood forecasting facility is necessary to improve the performance of reservoir operation for flood control purpose.

Kata Kunci : Waduk Wonogiri, pengendalian banjir, hidrograf banjir saat ini, operasi spillway