Pengaruh data hilang pada perkiraan debit rancangan dalam skema pengendalian banjir

ANGGRAINI I., Novi, Prof. Dr. Ir. Sri Harto Br, Dip.H

2008 | Tesis | S2 Teknik Sipil

Usaha yang dilakukan untuk mencegah terjadinya kerugian akibat banjir adalah dengan bangunan pengendali banjir, dimana dalam perancangannya diperlukan perkiraan debit rancangan. Debit rancangan ditetapkan melalui prosedur analisis hidrologi. Permasalahan yang banyak terjadi dalam analisis hidrologi adalah kelengkapan data. Sering terjadi data hujan tidak terekam atau catatan sementara hilang atau rusak sehingga data hujan pada hari tertentu tidak diketahui. Apabila kesalahan tersebut sering terjadi maka akan sangat merugikan. Penelitian ini menggunakan lima DAS yaitu DAS Bogowonto, DAS Keduang, DAS Opak Hulu, DAS Progo Hulu dan DAS Winongo. Prosedur yang dilakukan adalah dengan penerapan skenario penghilangan data hujan harian, yang kemudian diselesaikan dengan 4 metode yaitu normal ratio method, rata-rata aritmatika sederhana, reciprocal method dan pengabaian stasiun hujan data hilang dalam perhitungan hujan harian rata-rata DAS. Melalui prosedur analisis hidrologi akan diperoleh debit banjir rancangan terhitung yang kemudian dibandingkan dengan debit banjir rancangan hasil analisis frekuensi data debit terukur. Hasil analisis menunjukkan bahwa reciprocal method memberikan perbedaan kesalahan relatif debit rancangan untuk berbagai skenario penghilangan data kecil bagi DAS Bogowonto, DAS Keduang, DAS Opak Hulu dan DAS Winongo dan normal ratio method bagi DAS Progo Hulu. Pengabaian stasiun hujan dalam perhitungan hujan harian rata-rata DAS untuk DAS Bogowonto, DAS Keduang, DAS Opak Hulu, DAS progo Hulu dan DAS Winongo menunjukkan perbedaan kesalahan relatif debit rancangan untuk berbagai skenario penghilangan data yaitu berturut-turut 8,26%, 37,52%, 12,19%, 53,17%, 31,735% (kala ulang 20 tahun) dan 9,60%, 49,61%, 15,30%, 76,721%, 41,360% (kala ulang 50 tahun).

Design flood, which becomes a main consideration in designing flood control structures, is estimated by applying hydrologic analysis procedure. The common problem in executing the procedure is the data deficiency in computing catchment average daily rainfall. It becomes familiar that rainfall data are not well-recorded; either by the cause of malfunctioned gauges or improper process of archiving; which result in the unavailability of data at certain times. This research was conducted to identify the influence of data unavailability in determining design flood. Five catchment areas were analyzed in this research: Bogowonto, Keduang, Upper Opak, Upper Progo and Winongo. The procedure was executed by applying scenarios of daily rainfall data elimination. These scenarios were later solved by applying 4 methods: normal ratio, simple arithmetic mean, reciprocal and the omission of stations which experienced data deficiency. Computed design flood discharge as the output of analysis was then compared to design flood discharge which was generated by applying frequency analysis towards measured data. The result showed that small relative error differences were yielded by the application of varying scenarios of data elimination. In Bogowonto, Keduang, Upper Opak and Winongo catchment areas, the small relative error differences were generated by reciprocal method, while in Upper Progo by normal ratio method. The omission of station method yielded in varying deviation values in Bogowonto, Keduang, Upper Opak, Upper Progo and Winongo catchment areas, sequentially 8.26%, 37.52%, 12.19%, 53.17%, 31.735% for 20-year-return period, and 9.60%, 49.61%, 15.30%, 76.721%, 41.36% for 50-year-return period.

Kata Kunci : Data hilang,Debit rancangan,Banjir, Unavailability of data, design discharge, flood