MITIGASI BENCANA ALIRAN LAHAR DENGAN CARA NORMALISASI SUNGAI DI GUNUNG MERAPI, JAWA TENGAH

A. Djumarma Wirakusumah1, Apud Djadjulie1, Dewi S. Sayudi2
1STEM “Akamigas”, Jl. Gajah Mada No. 38, Cepu
2Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi
E-mail: ade.wirakusumah@gmail.com

ABSTRAK

Sejumlah 140 juta meter kubik endapan awan panas (berupa material lepas) merupakan produk letusan Merapi 2010 (salah satu letusan terbesar Merapi) terkumpul pada sungai-sungai di lereng Merapi lereng barat laut, selatan, dan tenggara. 30% dari padanya terdapat di sepanjang Sungai Gendol di lereng selatan, dan sebagian lagi antara lain terdapat di Sungai Putih (lereng barat), dan Sungai Woro (lereng tenggara). Tumpukan endapan tersebut berpotensi banjir lahar di masa yang akan datang apabila terjadi curah hujan cukup besar dan cepat. Pemetaan dan penghitungan endapan bahan lahar pada tahun 2012 memperlihatkan bahwa volume bahan lahar di sepanjang Sungai Putih terhitung 5,3 juta m³ dengan volume daya tampung lahar 9,8 juta m³, volume bahan lahar di sepanjang Sungai Gendol terhitung 7,4 juta m³ dengan volume daya tampung 11,5 juta m³, dan volume bahan lahar di sepanjang Sungai Woro terhitung 6,6 juta m³ dengan daya tampung lahar 13,2 juta m³. Di banyak tempat endapan tersebut tertumpuk sehingga tidak terlihat mana sungai dan mana bantaran sungai. Oleh karena itu melalui manjemen darurat dan dengan tujuan mitigasi bencana lahar di daerah Gunung Merapi, perlu dilakukan upaya kebijakan normalisasi sungai. Mengingat berbagai kendala terkait peraturan tentang penambangan di daerah Gunung Merapi, maka kebijakan diberlakukan selama kegiatan mitigasi berlangsung.

Kata kunci: lahar, mitigasi, normalisasi sungai.

ABSTRACT

About 140 million cubic meters of loose materials of pyroclastic flow deposit was produced by the 2010 Merapi eruption, one of the biggest Merapi eruptions. The deposit was distributed mostly at north-west, south, and south-east flanks of Merapi. Thirty percents of them were deposited along Gendol River (South flank), and smaller part of them were deposited along Putih River (Nort-west flank), and Woro River (South-east flank) respectively. When a heavy rain falls at this area, the huge of deposits would be producing lahar flow which was dangerous especially for people who live in downstream areas. Mapping and calculation of the deposits of Lahar candidate at those rivers in early December 2012 showed that the deposit of lahar candidate along the Putih River was 5.3 millions m³ with the empty volume of 9.8 millions m³ for accommodating lahar deposits, the deposit of lahar candidate along the Gendol river was 7.4 millions m³ with the empty volume of 11.5 millions m³ for accommodating lahar deposits, and the deposit of lahar candidate along the Woro River was 6.6 millions m³ with the empty volume of 13.2 millions m³ for accommodating lahar deposits. In many places it could not be distinguished which the middle of the river and the river bank were. By using emergency management and the mitigation purposes of lahar dangerous at Marapi area, a policy of river normalization is important to be done . As a lot of problem related with regulation about mining applied in Merapi area, therefore, the policy is valid only during the activity of lahar dangerous mitigation.

Key words: lahar, mitigation, river normalization.