Evalution of potential for 2001 debris flow and debris flood in the vicinity of Nam Ko area, Amphoe Lom Sak Changwat Phetchabun, Central Thailand

Abstract

Thematic (GIS and remote sensing) data interpretation, field investigation, and laboratory analysis were carried out to investigate the parameters influencing the debris flow and debris flood (flow-flood) occurrence on 11ᵗʰ August 2001 (8/11) in Nam Ko area, Changwat Phetchabun, central Thailand. The purpose of study was to identify the potential source area, run-out zone, and depositional area, and to determine the evidences of the potential for hazards in Nam Ko Yai sub-catchment and its alluvial fan. The relationship between the sedimentary sequences and debris flow-flood occurrence in the alluvial fan was also defined. The relationship between debris flow-flood and relevant parameters was analyzed for debris flow-flood susceptibility assessment. In Nam Ko Yai sub-catchment, scar-scouring locations detected from remote sensing interpretation and field surveys were complied into a GIS database. Various maps were constructed from the flow-flood relevant parameters derived from the database. The parameters, univariant probability method, and calculation of debris flow-flood susceptibility were applied to analyze and produce the susceptibility map of debris flow-flood hazard in the sub-catchment. From the debris flow-flood event reconstruction and its potential, it was concluded that the disastrous event was not the work of the unusually heavy rainfall alone as previously concluded, but it was the work of combined parameters including the terrain characteristics with specific land cover, underlain-material geotechnical properties, and time-delay for accumulation of plant debris and sediments. Combination of parameters could lead to a debris flow-flood. The process could be worse with a natural temporary landslide dam formed and then the dam was destroyed under the weight of impounded water. After this disastrous event, it should take time to the next debris flow-flood to recur as accumulation of more plant debris and sediments in the sub-catchment would be needed.