KAJIAN PENGARUH PEMBANGUNAN SEDIMENT BYPASS TUNNEL DI BENDUNGAN SUTAMI TERHADAP RUAS HILIR SUTAMI HINGGA LODOYO

Chairul Adam, Ir. Neil Andika, S.T., M.Sc., Ph.D. ; Dr. Ir. Istiarto, M.Eng., IPU.

2026 | Tesis | S2 Teknik Sipil

Bendungan Sutami mengalami penurunan kapasitas tampungan efektif yang signifikan akibat sedimentasi, dari 350 juta m³ (1973) menjadi 172,87 juta m³ (2022). Sebagai solusi, direncanakan pembangunan Sediment Bypass Tunnel (SBT), yaitu terowongan sepanjang 10,95 km dengan kapasitas 120 m³/s yang dirancang untuk mengalihkan sedimen dari hulu Waduk Sutami langsung ke hilir Bendungan Sutami secara gravitasi. Penelitian ini bertujuan mengkaji dampak pembangunan SBT terhadap morfologi dan kualitas air di hilir Bendungan Sutami hingga Bendungan Lodoyo, serta merumuskan pola operasi optimal.

Penelitian menggunakan pemodelan hidrolik 1D dengan HEC–RAS untuk menyimulasikan transpor sedimen periode 2013–2023. Tiga skenario dianalisis, yaitu kondisi eksisting tanpa SBT, operasi SBT musim penghujan (November–Maret), dan operasi SBT pada debit banjir kala ulang 1 tahun (Q? = 194,8 m³/s). Data yang digunakan meliputi debit harian AWLR Sutami, gradasi sedimen dari Jembatan Ngembul dan Jembatan Glondong, serta topografi dari LiDAR. Angkutan sedimen dimodelkan dengan persamaan Meyer–Peter Müller, Toffaleti, dan Engelund–Hansen serta divalidasi dengan observasi elevasi dasar sungai tahun 2022. Hasil terbaik yang mendekati nilai observasi diperoleh menggunakan persamaan Meyer-Peter Müller dengan NSE 0,93, R2 0,98, dan RMSE 3,69 m.

Hasil simulasi menunjukkan bahwa pembangunan SBT memberikan dampak positif berupa pemulihan kesinambungan sedimen dari hulu ke hilir, dengan peningkatan transpor sedimen sebesar 20% pada skenario 2 (91.256.541 ton/tahun) dan 4% pada skenario 3 (78.270.784 ton/tahun) terhadap kondisi eksisting (75.246.127 ton/tahun). Di sisi lain, ketiga skenario menunjukkan dominasi agradasi dengan rata-rata perubahan elevasi dasar sungai 1,5 m (0,13 m/tahun), disertai peningkatan konsentrasi sedimen tersuspensi (SSC) rata-rata menjadi 4,36 mg/L (+99%) pada skenario 2 dan 8,27 mg/L (+289%) pada skenario 3 terhadap kondisi eksisting 2,18 mg/L, serta pelebaran penampang ekstrim pada skenario 2 hingga 460,72 m di STA 8+200 yang berpotensi mengancam kapasitas tampung Waduk Wlingi. Berdasarkan analisis komparatif, skenario 3 dengan pola operasi berbasis debit banjir kala ulang 1 tahun (Q? = 194,8 m³/s, rata-rata 5 hari/tahun) ditetapkan sebagai pola operasi optimal karena menghasilkan lebar sungai rata-rata paling mendekati kondisi eksisting (60,67 m vs. 60,96 m), tidak menimbulkan rebound effect pasca penghentian operasi, dan memberikan stabilitas pengendalian kualitas air terbaik.

Sutami Dam has experienced a significant reduction in effective storage capacity due to sedimentation, declining from 350 million m³ (1973) to 172,87 million m³ (2022). As a solution, the construction of a Sediment Bypass Tunnel (SBT) has been proposed a 10,95 km tunnel with a discharge capacity of 120 m³/s, designed to divert sediment from the upstream reach of Sutami Reservoir directly to the downstream side of Sutami Dam by gravity flow. This study aims to investigate the impact of SBT construction on the morphology and water quality of the river reach downstream of Sutami Dam to Lodoyo Dam, as well as to formulate an optimal operation scheme. 

The study employs 1D hydraulic modeling using HEC–RAS to simulate sediment transport over the period 2013–2023. Three scenarios were analyzed  the existing condition without SBT, SBT operation during the wet season (November–March), and SBT operation at a 1–year return period flood discharge (Q? = 194.8 m³/s). Data used include daily discharge records from the Sutami AWLR station, sediment gradation data from Ngembul Bridge and Glondong Bridge, and topographic data derived from LiDAR surveys. Sediment transport was modeled using the Meyer–Peter Müller, Toffaleti, and Engelund–Hansen equations, and validated against observed riverbed elevation data from 2022. The best results, closest to observed values, were obtained using the Meyer–Peter Müller equation, with NSE = 0.93, R² = 0.98, and RMSE = 3.69 m.

The simulation results indicate that SBT construction yields a positive impact through the restoration of sediment continuity from upstream to downstream, with sediment transport increasing by 20% under Scenario 2 (91,256,541 tons/year) and 4% under Scenario 3 (78,270,784 tons/year) relative to the existing condition (75,246,127 tons/year). Conversely, all three scenarios exhibit dominant aggradation processes, with an average riverbed elevation change of 1.5 m (0.13 m/year), accompanied by increases in mean suspended sediment concentration (SSC) to 4.36 mg/L (+99%) under Scenario 2 and 8.27 mg/L (+289%) under Scenario 3 compared to the existing condition of 2.18 mg/L, as well as extreme channel widening under Scenario 2 reaching 460.72 m at STA 8+200, which potentially threatens the storage capacity of Wlingi Reservoir. Based on comparative analysis, Scenario 3 — operating under a 1–year return period flood discharge (Q? = 194.8 m³/s, averaging 5 days/year) was determined to be the optimal operation scheme, as it produces an average channel width most closely approximating the existing condition (60.67 m vs. 60.96 m), generates no rebound effect following cessation of operations, and delivers the best water quality control stability with SSC consistently 22–37?low the existing condition after SBT deactivation.

Kata Kunci : Sediment Bypass Tunnel, Bendungan Sutami, transpor sedimen, HEC–RAS, morfologi sungai.