IDENTIFYING THE THREAT OF DEBRIS FLOW TO MAJOR ARTERIAL ROADS USING LANDSAT ETM+ IMAGERY AND GIS MODELING – AN EXAMPLE FROM CATANDUANES ISLAND, REPUBLIC OF THE PHILIPPINES

摘要

Debris flows are destructive events, caused when loose geological materials are saturated withrnprecipitation to the point when they become fluid enough to flow down a gradient. The power andrndestructive forces of such events is well documented, causing damage to buildings, roads and bridges,rnas well as leading to the loss of human life. One of the main problems faced by engineers trying tornidentify where a debris flow will occur, is that although the effects are often seen locally, the possiblyrnmultiple sources of the debris flow may be located many kilometres away, often in steep and dangerousrnterrain. This makes the task of manually identifying all the possible sources of a debris flow during fieldrnexpeditions both costly and time-consuming. Due to the spatial nature of the problem, it was consideredrnthat GIS and Remote Sensing could hold a timely and cost-effective solution to identifying the potentialrnsources of a debris flow. This study tests the abilities of these geo-spatial technologies to identify the riskrnposed by debris flow to a planned dual carriageway circumferential road that will be constructed alongrnthe route of an existing single-track road around Catanduanes Island in the Republic of the Philippines.rnBy combining one-time multi-spectral data and a digital elevation model, it was possible to identify areasrnof bare earth on slopes between 5°-15° and 15° - 25° inclination. These areas were deemed as beingrnpotential sources of geological material available for debris flow. By calculating the percentage area andrntotal area of these potential debris sources for each watershed draining out either immediately onto orrnabove the planned new road and modelling the gradient of the land surrounding the route, it was possiblernto identify those watersheds that undergo a high level of slope failure activity and an indication of thernamount of geological material available that may contribute towards a debris flow event. In this way, thernwatersheds were classified according to those that may be considered as having a high propensity torndebris flow and those that posed little or no risk. At the spatial resolution that this study was performedrn(Landsat 7 ETM+ 30m resolution), the results provide information to slope engineers and surveyors ofrnwhere to concentrate their field survey and the extent of any slope protection measures that may bernnecessary.