Hydrogeological drives in mass transfers of Mount Telagalele and surrounding area, Banjarnegara Regency, Central Java, Indonesia

Abstract

Mount Telagalele is an ancient volcano, one of the eccentric cones of Mount Rogo Jembangan. It is situated in the Karangkobar District, Banjarnegara Regency, Central Java, Indonesia. Mount Telagalele and surrounding area is often subjected to either large or small-scale of mass movements, generally involving a thick volcanic rock and clayey formations. Geologically the area is constructed of plastic, clastic, clayey sedimentary rocks, Miocene aged, namely Merawu Formation and an elastic, brittle, massive volcanic rocks, andesitic to basaltic composition, Pleistocene aged, called Jembangan Formation. Hydrogeologically the study area is flown by Kali Urang (river Urang), which streams to the south towards the Merawu River. The volcanic rocks of Mount Telagalele which has begun to be weathered in general is able to hold and pass meteoric water so that it functioning as an aquifer. There are some springs on the contact between the volcanic rocks and the base rock. The local communities usually use springs and dug wells to get fresh water. However, when groundwater in this volcanic aquifer touches the base rock and making it oversaturated, various problems become to emerge. The clayey formation become to be slippery and muddy, causing the overlying volcanic rocks to slip and transfer. Parts of volcano’s body and rock blocks in the study area are able to move slowly but surely over a giant slip plane in the form of orographic fields, southward. Locally, this movement is manifested as mass slides, glides, and creeps, occurring any time. One of the last famous landslides involving apart of Mount Telagalele body happened in December 2014, assassinating 108 people. The movement can be classified into combination of rotational and translational, which then develops to be a flow, therefore it belongs to a complex type, indicating that the mass transfer is triggered by hydrogeology.