Kajian Hidrologi-Hidrolika Banjir di Sungai Ancar, Kota Mataram Akibat Hujan Tanggal 6 Juli 2025

Abstract

Abstract: The flood event that occurred in Mataram City on July 6, 2025, highlights the need to evaluate the relationship between rainfall characteristics, discharge generation, and the capacity of the Ancar River. This study aims to investigate the hydrological and hydraulic characteristics of the flood and to evaluate the capacity of the Ancar River cross-sections to convey the flood discharge during the event and under several return periods. The hydrological analysis used rainfall data from three stations, namely Sesaot, Bertais, and Gunungsari, covering the period from 2010 to 2024. Areal rainfall was calculated using the Thiessen Polygon method, data consistency was assessed using the Rescaled Adjusted Partial Sums (RAPS) method, rainfall distributions were analyzed using the Log-Pearson Type III distribution, rainfall intensity was calculated using the Mononobe method, and effective rainfall was transformed into a hydrograph using the Nakayasu Synthetic Unit Hydrograph (SUH) method. Hydraulic analysis was conducted using HEC-RAS 6.6 through an Unsteady Flow simulation with river geometry derived from a Digital Elevation Model (DEM). The results indicate that the areal rainfall on July 6, 2025, was 152.65 mm, which was closest to the 25-year return-period design rainfall of 155.84 mm. The peak discharge generated by the Nakayasu SUH reached 57.93 m³/s. The simulation results indicate that the existing capacity of the Ancar River was insufficient to accommodate this discharge. The percentage of safe cross-sections decreased from 34.13% for the 2-year return period to 0% for the 25-year return period, while the proportion of unsafe cross-sections increased from 65.87% to 100%. These findings indicate a high vulnerability of the river's conveyance capacity and highlight the need for capacity enhancement and flood mitigation measures. The identified contributing factors include extreme rainfall, changes in riverbed elevation, differences in bank elevations, relatively small cross-sectional dimensions, non-uniform Manning's roughness coefficients, and sediment deposition at several locations. The findings can serve as a basis for technical evaluation and flood preparedness along the Ancar River.

Abstrak: Banjir pada 6 Juli 2025 di Kota Mataram menunjukkan perlunya evaluasi hubungan antara karakteristik hujan, pembentukan debit, dan kapasitas Sungai Ancar. Penelitian ini bertujuan mengkaji karakteristik hidrologi-hidrolika banjir serta mengevaluasi kemampuan penampang Sungai Ancar terhadap debit banjir pada kejadian tersebut dan beberapa kala ulang. Analisis hidrologi menggunakan data curah hujan tiga stasiun, yaitu Sesaot, Bertais, dan Gunungsari, periode 2010–2024. Curah hujan wilayah dihitung dengan Polygon Thiessen, konsistensi data diuji menggunakan Rescaled Adjusted Partial Sums (RAPS), distribusi hujan dianalisis dengan Log Pearson Tipe III, intensitas hujan dihitung dengan Mononobe, kemudian hujan efektif ditransformasikan menjadi hidrograf menggunakan Hidrograf Satuan Sintetik (HSS) Nakayasu. Analisis hidrolika dilakukan dengan HEC-RAS 6.6 menggunakan simulasi Unsteady Flow dan geometri sungai berbasis Digital Elevation Model (DEM). Hasil menunjukkan curah hujan wilayah 6 Juli 2025 sebesar 152,65 mm dan paling mendekati hujan rancangan kala ulang 25 tahun sebesar 155,84 mm. Debit puncak HSS Nakayasu mencapai 57,93 m³/det. Simulasi menunjukkan kapasitas eksisting Sungai Ancar belum mampu menampung debit tersebut. Persentase penampang aman menurun dari 34,13% pada kala ulang 2 tahun menjadi 0% pada kala ulang 25 tahun, sedangkan kondisi tidak aman meningkat dari 65,87% menjadi 100%. Temuan ini menunjukkan kerentanan kapasitas sungai yang tinggi dan perlunya peningkatan kapasitas serta mitigasi banjir. Faktor penyebab yang teridentifikasi meliputi hujan ekstrem, perubahan elevasi dasar sungai, perbedaan elevasi tebing, penampang relatif kecil, ketidakseragaman kekasaran Manning, dan sedimentasi pada beberapa titik. Hasil kajian dapat menjadi dasar evaluasi teknis dan kesiapsiagaan banjir di sepanjang Sungai Ancar.