Risdiana Cholifatul Afifah, Farida Yudaningrum

Pemetaan Genangan banjir di Wilayah Hilir Bendungan Randugunting dengan Model HEC-RAS 2 (Dua) Dimensi

Introduction

Pemetaan genangan banjir di wilayah hilir bendungan randugunting dengan model hec-ras 2 (dua) dimensi. Pemetaan genangan banjir hilir Bendungan Randugunting Blora dengan HEC-RAS 2D mengklarifikasi kontribusi bendungan terhadap banjir Sungai Banyuasin Rembang. Ditemukan Kali Padas penyebab utama.

Abstract

Bendungan Randugunting di Kabupaten Blora dibangun sebagai infrastruktur pengendali banjir, penyedia air baku, dan irigasi. Namun, pada beberapa kejadian banjir di hilir Sungai Banyuasin, wilayah Rembang, muncul persepsi bahwa operasional Bendungan Randugunting turut berkontribusi terhadap peningkatan debit banjir di daerah hilir. Penelitian ini diharapkan dapat memberikan klarifikasi ilmiah terhadap dugaan pengaruh Bendungan Randugunting terhadap banjir di hilir Sungai Banyuasin Rembang. Penelitian ini bertujuan untuk mengevaluasi kontribusi pelepasan air bendungan terhadap debit puncak banjir di Sungai Banyuasin. Metode yang digunakan meliputi analisis hidrologi dengan pemodelan menggunakan perangkat lunak Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS), dan analisis hidrolika dengan bantuan model Hydrologic Engineering Center–River Analysis System (HEC-RAS) 2D untuk memodelkan genangan banjir. Hasil analisis pelepasan air Bendungan Randugunting, dapat disimpulkan bahwa waduk dapat menahan banjir di hulu, meredam sehingga rilis banjir cukup kecil dg kemampuan reduksi ±70% pada simulasi kala ulang 50th (outflow 63,4 m³/dt). Namun Kali Padas dengan luas Catchment area 2 (dua) kali luas DTA (Daerah Tangkapan Air) bendungan, menghasilkan pelepasan air sebesar 146 m3/dt. Melalui justifikasi teknis, banjir di daerah hilir waduk dipengaruhi oleh pelepasan air Kali Padas. Kondisi Kali Padas yang tanpa bangunan penahan banjir, diprediksi sebagai sumber limpasan terbesar penyebab banjir di pohlandak dan krikilan.

Review

The study, "Pemetaan Genangan banjir di Wilayah Hilir Bendungan Randugunting dengan Model HEC-RAS 2 (Dua) Dimensi," tackles a pertinent issue in water resource management: the perceived impact of infrastructure on downstream flood events. The research aims to scientifically evaluate the contribution of the Randugunting Dam to peak flood discharge in the Sungai Banyuasin, directly addressing public concerns that the dam's operation exacerbates flooding in the Rembang region. This objective is highly relevant, seeking to provide clear, evidence-based insights to resolve local perceptions and guide effective flood control strategies. Methodologically, the paper adopts a robust approach, leveraging industry-standard software for hydrological and hydraulic analyses. The use of Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS) for hydrological modeling and Hydrologic Engineering Center–River Analysis System (HEC-RAS) 2D for flood inundation mapping indicates a strong technical foundation. The findings significantly clarify the situation: the Randugunting Dam is shown to effectively retain upstream floods, reducing outflow by approximately 70% during a 50-year return period event (outflow 63.4 m³/s). Critically, the study identifies Kali Padas, with a catchment area twice that of the dam, as the primary source of downstream flooding, releasing a much larger volume of 146 m³/s. This technically justifies that the flooding in areas like Pohlandak and Krikilan is predominantly influenced by uncontrolled releases from Kali Padas, which lacks comparable flood retention structures. This research offers a valuable scientific debunking of a prevalent public misconception, providing a data-driven understanding of complex flood dynamics. The application of established and widely accepted models enhances the credibility and reliability of its conclusions. The implications are substantial, offering critical information for local authorities and water management bodies to re-focus their flood mitigation efforts. By clearly identifying Kali Padas as the major contributor to downstream floods, the study provides a strong basis for developing targeted flood control measures for this specific river, ultimately leading to more effective protection for the affected communities.

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