Sistem mitigasi bencana sekunder Gunungapi Gamalama di Sungai Tubo (Tugurara) Kota Ternate

SALEH, Muslim, Prof. Dr. Ir. Bambang Agus Kironoto

2009 | Tesis | S2 Teknik Sipil

Gunung Gamalama yang mempunyai ketinggian 1715 meter di atas permukaan laut terletak di Pulau Ternate sebelah barat Halmahera, dalam aktifitasnya sering menimbulkan letusan, yang menyebabkan terbentuknya endapan material vulkanik dan piroklastik hasil letusan sebelumnya yang menumpuk disekitar puncak gunung. Pada musim hujan, endapan material tersebut dapat menyebabkan terjadinya bencana sekunder berupa banjir lahar, sehingga untuk mengantisipasi bencana sedimen berupa aliran lahar hingga saat ini telah dibangun bangunan sabo sebanyak 6 (enam) buah. Bangunan paling hulu adalah chek dam 1 dan 2 yang kemudian diikuti oleh bangunan groundsill. Usaha penambangan Galian C yang dilakukan secara terus menerus oleh masyarakat disekitar Sungai Tubo dapat menimbulkan kerusakan lingkungan dan menurunnya fungsi bangunan sabo. Penelitian ini dilaksanakan dengan mengkaji imbangan sedimen. Analisis volume sedimen yang masuk sebagai produksi sedimen (Vs) didasarkan rumus empiris Takahashi dan Mizuyama, sedangkan volume sedimen yang melimpas (VE) berdasarkan rumus empiris Shimoda. Analisis imbangan sedimen diperhitungkan berdasarkan volume sedimen yang masuk dengan curah hujan harian rencana (R24) kala ulang 25 tahun. Hasil penelitian menunjukan bahwa sistim sabo yang diterapkan di Sungai Tubo belum cukup efektif dalam mengendalikan aliran sedimen/debris karena produksi sedimen yang masuk sebesar 42.267,03 m3 dengan curah hujan harian rencana (R24) kala ulang 25 tahun, sedangkan kapasitas bangunan untuk kedua chek dam sebesar 15.308,15 m3. Tingkat keberhasilan sistem pengendalian sedimen di Sungai Tubo sangat dipengaruhi oleh beberapa aspek yaitu aspek sistem pengelolaan imbangan sedimen dan aspek penambangan pasir. Imbangan sedimen dapat tercapai apabila pengelolaan sedimen dilakukan secara terkendali dengan melakukan pengawasan penambangan pasir melalui penerapan perundangan dan peraturan daerah secara baik. Upaya mitigasi bencana aliran sedimen belum cukup optimal disebabkan penyelenggaraan pendidikan dan pelatihan belum dilaksanakan secara berkala dan berkelanjutan, serta belum memiliki dokumen RAD-PRB sebagai dasar dalam penanggulangan bencana.

Gamalama Mountain with the height of 1,715 meters above sea level is located in the western Halmahera of Ternate Island which often erupts and causing the formation of volcanic material sediments and piroclastic from the previous eruptions deposited around the peak of the mountain. During the rainy season, those material sediments could bring secondary disaster in the form of debris flood and therefore six sabo constructions had been built to prevent sediment disaster created by debris flow. The most upstream constructions are check dam 1 and 2 that are supported by groundsill constructions. On the other hand, continuing traditional sand mining extraction by surrounding communities along Tubo River may cause environmental degradation and decreased function of the sabo constructions. The research was carried out to study the sediment balance. Analysis of volume of the sediment inflow as sediment production (Vs) was approached with Takahashi and Mizuyama empirical formula meanwhile the sediment outflow (VE) was approached with Zhimoda empirical formula. Analysis of sediment balance was accounted by volume of sediment inflow mixed with daily designed rainfall (R24) for 25 years of return period. The research showed that sabo system applied in Tubo River is inadequate to control debris flow since the inflow of sediment product is 42,267.03 m3 for daily designed rainfall (R24) for 25 years of return period comparing to the total storage capacity of both check dam constructions is 15,308.15 m3. Performance level of Tubo River sediment control system was influenced by several factors such as management system of sediment balance and sand mining extraction. Sediment balance would be achieved if sediment management is controlled through sand mining inspections by practicing the enforcement of both constitution and the local regulation. Disaster mitigation efforts on debris flow was insufficient due to lack of periodical and continuous educational and training courses offered, and the absence of Local Action Plan on Disaster Risk Reduction as an essential reference in disaster mitigation.

Kata Kunci : Mitigasi,Dam Sabo,Penambangan,Imbangan sedimen,Mitigation,Sabo Dam,Mining,Sediment Balance.