| dc.description.abstract | Lahar has been applied as a general term for rapidly flowing, high-concentration, poorly sorted sediment-laden
mixtures of rock debris and water (other than normal streamflow) from a volcano. Lahars are one of the most destructive
phenomena associated with composite volcanoes, which are dominant in Java Island. Resulting deposits of lahar are poorly
sorted, massive, made up of clasts (chiefly of volcanic composition), that generally include a mud-poor matrix. The aim of
this research is threefold: to discuss the initiation of lahars occurrences, their dynamics, to assess the hazard and to analyse
the deposition. Lahars are either a direct result of eruptive activity or not temporally related to eruptions. Syn-eruptive
lahars may result from the transformation on pyroclastic flows or debris avalanches which transform to aqueous flows (e.g.
at Papandayan in November 2002); They may be also generated through lake outburst or breaching (e.g. at Kelut in
1909 or 1966), and through removal of pyroclastic debris by subsequent heavy rainstorms. Post-eruptive lahar occurs
during several years after an eruption. At Merapi, lahars are commonly rain-triggered by rainfalls having an average
intensity of about 40 mm in 2 hours. Most occur during the rainy season from November to April. Non-eruptive lahars
are flows generated without eruptive activity, particularly in the case of a debris avalanche or a lake outburst (e.g., Kelut).
A lahar may include one or more discrete flow processes and encompass a variety of rheological flow types and flow
transformations. As such, lahars encompass a continuum between debris flows and hyperconcentrated flows, as observed at
Merapi, Kelut and Semeru volcanoes. Debris flows, with water contents ranging from 10 to no more than about 25%
weight, are non-newtonian fluids that move as fairly coherent masses in what is thought to be predominantly laminar
fashion. However, the relative importance of laminar versus turbulent regime is still debatable. Hyperconcentrated streamflows
contain 25- to about 40%-weight-water; these flows possess some yield stress, but they are characteristically turbulent.
Hazard-zone maps for lahar were produced for most of the the Javanese volcanoes, but these maps are on too small-scale
to meet modern zoning requirements. More recently, a few large-scale maps (1/10,000 and 1/2,000-scale) and risk
assessments have been completed for a few critical river systems at Merapi. | en_US |
