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