LANDSLIDES AND DEBRIS FLOWS AT KHAO PHANOM BENJA, KRABI, SOUTHERN THAILAND

Abstract

A detailed investigation of the characteristics of the 2011 catastrophic landslides of Khao
Phanom Benja, Krabi, Thailand was conducted. The landslides on the slope faces of the high relief granitic
mountain led to devastating debris flows of large rock boulders that buried houses in the foothill area. The
unfavorable orientations of four major joint sets of the mountain created high kinematic instability of rock
wedge slides along the stream channel direction as well as rock wedge formation on the channel walls in the
eastern sides of the mountain. Weathering and joint opening in the areas of slopes and stream channels increase
the vulnerability of rock slides, in particular for a prolonged heavy rainfall. The heavy rainfall and stream
channel flow on slab block of rock slope erosion along stream bank caused down bank slope failure. The
increasing upward water pressure caused the increase in uplift force behind the rock block and rock wedge
failure. In addition, the hazard zone delineation of debris flows deposition was mapped out using a MATLAB
codenamed as FLOWS. Majority of slides on the eastern slopes of the mountain were granitic rock slide modes
while soil slide and surface erosion on the residual soil to completely weathered sedimentary rock were the
predominant modes of failure on the western and northern slopes in lower areas at foothills of the mountain.
As found in this investigation the factors influencing landslides and debris flows in 2011 at Khao Phanom
Benja, Krabi, southern Thailand were not only limited to the prolonged rainfall but also to slope gradient, rock
types and weathering degree, discontinuity characteristics.