Analisis Debit Banjir Rencana di DAS Cimanceuri

Abstract

Debit banjir rencana penting untuk diketahui dalam upaya pengendalian banjir di suatu Daerah Aliran Sungai. Cara terbaik untuk menduga debit banjir rencana adalah menggunakan analisis frekuensi atau metode empiris berdasarkan debit historis, namun data debit historis jarang tersedia di Indonesia. Penelitian ini bertujuan menganalisis perbedaan model pendugaan debit banjir rencana menggunakan Metode Rasional dan Metode Hidrograf Satuan Sintetik Nakayasu di DAS Cimanceuri berdasarkan curah hujan CHIRPS dan curah hujan observasi. Hasil penelitian menunjukan bahwa Metode Rasional menghasilkan pendugaan debit banjir rencana lebih besar dibandingkan dengan Metode HSS Nakayasu baik menggunakan CHIRPS maupun curah hujan observasi. Data CHIRPS sendiri lebih kecil dibandingkan data hujan observasi. Hujan CHIRPS dengan kala ulang 2-100 tahun 61,4 mm/hr-118,9 mm/hr menghasilkan debit banjir rencana berdasarkan Metode Rasional 109,2 m3/det hingga 231,6 m3/det, sedangkan menggunakan Metode HSS Nakayasu 45,1 m3/det hingga 104,1 m3/det . Curah hujan observasi dengan kala ulang 2-100 tahun (94,4 mm/hr-241,8 mm/hr) menghasilkan debit banjir rencana dengan berdasarkan Metode Rasional (168,7 m3/det-470.6 m3/det) dan (66,5 m3/det hingga 211,7 m3/det) berdasarkan Metode HSS Nakayasu.

Design flood discharge is crucial for designing hydraulic structures to control flooding in watersheds. The most accurate way to estimate design flood discharges is through frequency analysis or empirical methods based on log-term historical discharge data. However, such long-term data are rarely available, especially in Indonesia. This study aims to compare the estimates of design flood discharge derived from the Rational Method and the Nakayasu Synthetic Unit Hydrograph method in the Cimanceuri watershed, using daily CHIRPS rainfall and observation rainfall. The results show that the Rational Method tends to produce higher estimate of design flood discharge than the Nakayasu Synthetic Unit Hydrograph Method, whatever using both observed rainfall or CHIRPS. The CHIRPS rainfall data itself records lower values than the observed rainfall data. CHIRPS rainfall with a recurrence period of 2-100 years 61.4 mm/day-118.9 mm/day yields a design flood discharge 109.2 m3/sec to 231.6 m3/sec using Rational Method, while the Nakayasu HSS Method gives 45.1 m3/sec-104.1 m3/sec. for observed rainfall with a recurrence period of 2-100 years (94.4 mm/day-241.8 mm/day) resulted in flood discharge ranges from (168.7 m3/sec-470.6 m3/sec) using Rational Method and from 66.5 m3/sec-211.7 m3/sec) based on the Nakayasu HSS Method.