STUDI PENGARUH PEMBANGUNAN CLOSURE DIKE WADUK WONOGIRI TERHADAP KONDISI BANJIR DI HILIR WADUK

ADANI AMALIA, Dr. Ir. Rachmad Jayadi, M.Eng.

2015 | Tesis | S2 Teknik Sipil

Waduk Wonogiri merupakan bangunan air serbaguna yang difungsikan untuk penyediaan air irigasi, PLTA dan pengendali banjir di wilayah Sungai Bengawan Solo hulu. Akibat peningkatan endapan sedimen terutama pada intake waduk telah terjadi penurunan efektifitas fungsi waduk untuk keperluan irigasi. Pemerintah melalui Balai Besar Wilayah Sungai (BBWS) Bengawan Solo melakukan kegiatan studi terkait penanganan sedimentasi waduk dan menghasilkan solusi yaitu dengan membangun closure dike berupa tanggul penyekat area genangan waduk untuk menampung sedimen dari DAS Keduang dan bangunan penggelontor sedimen berupa spillway baru. Perubahan karakteristik waduk tersebut diikuti dengan perubahan operasi waduk untuk tujuan pengendalian banjir. Oleh karena itu perlu dikaji dampak perubahan pola operasi waduk periode banjir terhadap kondisi hilir waduk dalam hal kerawanan banjir. Studi pengaruh pola operasi waduk pada wilayah hilir dilakukan dengan bantuan model matematis metode perhitungan hidrologi. Metode yang digunakan adalah channel routing di sepanjang Sungai Bengawan Solo dengan batas hulu Waduk Wonogiri dan hilir di Bendung Colo. Pada sepanjang sungai tersebut ada 12 lateral inflow yang menjadi masukan debit untuk proses perhitungan channel routing selain debit outflow waduk. Debit outflow waduk terdiri dari 6 (enam) tipe berdasarkan kondisi lama (kondisi sebelum ada closure dike) dan kondisi baru (kondisi setelah ada closure dike) dengan 3 (tiga) kala ulang (60 tahun, 500 tahun, dan 5500 tahun/PMF). Perhitungan dibantu dengan software HEC-HMS. Metode channel routing di ruas sungai hilir waduk menghasilkan debit banjir rancangan di Bendung Colo untuk 3 kala ulang pada kondisi lama dan baru. Peningkatan debit puncak kala ulang 60 tahun, 500 tahun, dan PMF masingmasing sebesar 35%, 13%, dan 67%. Walaupun debit puncak meningkat, volume debit Bendung Colo turun untuk kala ulang 60 dan 500 tahun masing-masing 45% dan 8%, untuk PMF volume debit meningkat 25%. Kontribusi lateral inflow terhadap debit banjir di Bendung Colo dinyatakan dalam perbandingan volume outflow lateral inflow terhadap volume outflow waduk. Pada kala ulang 60, 500, dan PMF pada kondisi baru kontribusi lateral inflow sebesar 39,2%, 29,9%, dan 17,3%. Persentase kontribusi lateral inflow semakin berpengaruh pada kala ulang kecil. Dalam hubungan debit dan elevasi muka air yang disajikan rating curve Bendung Colo, didapat elevasi maksimum muka air banjir pada debit puncak. Elevasi muka air maksimum bendung kondisi lama kala ulang 60 tahun dicapai jam ke-13 setinggi +109,72 m, kondisi baru pada jam ke-8 setinggi +110,21 m. Untuk kala ulang 500 tahun, kondisi lama dengan waktu puncak jam ke-6 tinggi elevasi muka airnya +110,61, kondisi baru waktu puncak pada jam ke-7 dengan tinggi elevasi +110,82 m. Pada kondisi PMF mencapai waktu puncak pada jam ke- 6 dan ke-52 pada elevasi +110,82 m untuk kondisi lama, sedangkan kondisi baru pada jam ke-8 dengan elevasi +111,51 m dan jam ke-15 elevasi +111,67 m. Dapat disimpulkan bahwa operasional Waduk Wonogiri periode banjir kondisi lama lebih aman dibandingkan kondisi baru, yaitu setelah dibangun closure dike. Kata kunci : Bendung Colo, channel routing, lateral inflow, HEC-HMS

Wonogiri Reservoir is a multipurpose reservoir in the upper part of Bengawan Solo. It is functionalized for irrigation water supply, hydropower and flood control at the upstream area of Bengawan Solo River Basin. The high sediment production at intake decreases the effectiveness of storage capacity for irrigation water supply purpose. The Government of the Republic of Indonesia through Balai Besar Wilayah Sungai (BBWS) Bengawan Solo has conducted a study of reducing sediment deposition at intake and constructed closure dike (dike that separate reservoir area) for sedimentation deposit of Keduang River and a new spillway for flushing sediment. The change of reservoir characteristic is followed by the change of reservoir operational for flood control. Therefore, it is necessary to study the change of reservoir operation system during flood period and learn the impact to downstream area of Wonogiri Reservoir. Study about the impact of reservoir operation rule at the downstream area has been done by using mathematic method for hydrology calculation. The calculation along Bengawan Solo River (with the upstream area is Wonogiri Reservoir and downstream area is Colo Weir) is using channel routing method. It has been 12 lateral inflows that contribute flow discharge for channel routing calculation besides reservoir outflow discharge. There are six types of reservoir outflow discharge, i.e.: existing condition (reservoir without closure dike) and new condition (reservoir with closure dike) with 3 types of return period (60 years, 500 years, and 5,500 years/PMF). The calculation is using HEC-HMS. Design floods at Colo Weir are calculated by using channel routing method at the downstream area of reservoir (for existing and new condition). The increase of 60 years, 500 years, and 5,500 years are 35%, 13%, and 67%. Despite the increase of peak discharge, the volume discharges are decreased. The percent decrease of volume discharges are 45% and 8% in 60 years and 500 years of return period. Volume discharge increase and the percent increase is 25% for PMF condition. The contribution of discharge from lateral inflow against the flood discharge at Colo Weir is shown by the ratio of volume discharge from lateral inflow and volume discharge from reservoir. The ratio of 60, 500, and 5,500 years return periods in new condition are 39.2%, 29.9%, and 17.3%. Maximum water elevation at Colo Weir can be determined from discharge-water level rating curve. The flood hydrograph of 60 years return period reaches its maximum water level at elevation of +109.72 m for existing condition and +110.21 m for new condition. The maximum elevation in new condition is reached within 5 hours earlier than the existing one. For 500 years return period, the flood hydrograph reaches its maximum at elevation of +110.61 m for existing condition (time to peak: 6th hours) and +110.82 m for new condition (time to peak: 7th hours). For PMF condition, the existing condition reaches the maximum water level at 6th hours and 52nd hours with the value of water level is +111.51 m, the new condition reaches maximum at 15th hours with the water level is +111.67 m. The conclusion is the operation rule of Wonogiri Reservoir during flood period in existing condition is more safety than the new one. Keywords: Colo Weir, channel routing, lateral inflow, HEC-HMS

Kata Kunci : Bendung Colo, channel routing, lateral inflow, HEC-HMS; Colo Weir, channel routing, lateral inflow, HEC-HMS