Showing papers by "C.J. van Westen published in 2005"

Analysing the relation between rainfall characteristics and lahar activity at Mount Pinatubo, Philippines

Abstract: The eruption of Mount Pinatubo in June 1991 altered the conditions of the surrounding river catchments. Pyroclastic flows and tephra fall were deposited over extensive areas, stripping off the forest cover and burying drainage divides. These recent deposits are very loosely consolidated and generally consist of sand-sized particles, which commonly mobilize into lahars in response to rainfall of a certain magnitude. Several devastating lahar occurrences have buried settlements covering tens to several hundred square kilometres in a single event. Correlation of storm rainfall intensities and durations with lahar activity as recorded by acoustic flow monitors is used to investigate trends in the initiation conditions for lahar activity. This research confirms that the relationships of rainfall intensity and duration with lahar initiation threshold values are not linear but rather approximate a power relation. Different relations were found for lahar initiation in different years, from 1991 to 1997, as a result of the dynamic changes in hydrologic and geomorphic conditions of the affected catchments. Data from acoustic flow monitors are used to distinguish debris flow and hyperconcentrated flow activity from that of muddy water. Copyright © 2005 John Wiley & Sons, Ltd.

Accuracy assessment of InSAR derived input maps for landslide susceptibility analysis: a case study from the Swiss Alps

Abstract: In recent years SAR interferometry has become a widely used technique for measuring altitude and displacement of the surface of the earth. Both these capabilities are highly relevant for landslide susceptibility studies. Although there are many problems that make the use of SAR interferometry less suitable for landslide inventory mapping, it’s use in landslide monitoring and in the generation of input maps for landslide susceptibility assessment looks very promising. The present work attempts to evaluate the usefulness and limitations of this technique based on a case study in the Swiss Alps. Input maps were generated from ERS repeat pass data using SAR interferometry. A land cover map has been generated by image classification of multi-temporal SAR intensity images. An InSAR DEM was generated and a number of maps were derived from it, such as slope-, aspect, altitude- and slope form classes. These maps were used to generate landslide and rockfall susceptibility maps, which give fairly well acceptable results. However, a comparison of the InSAR DEM with the conventional Swisstopo DEM, indicated significant errors in the absolute height and slope angles derived from InSAR, especially along the ridges and in the valleys. These errors are caused by low coherence mostly due to layover and shadow effects. Visual comparison of stereo images created from hillshading maps and corresponding DEMs demonstrate that a considerable amount of topographic details have been lost in the InSAR-derived DEM. It is concluded that InSAR derived input maps are not ideal for landslide susceptibility assessment, but could be used if more accurate data is lacking.

Community-based Flood Risk Assessment using GIS for the Town of San Sebastian, Guatemala

Abstract: The municipality of San Sebastian, considered one of the most prone to natural hazards in Guatemala, was selected as one of the pilot areas of the UNESCO program on “Capacity Building for Natural Disaster Reduction”. The town is located within the Samala basin, near the active channel of the river, which is heavily affected by lahars related to the active Santiaguito-Santa Maria volcanic complex. The town is threatened both by lahars as well as by flooding. As is often the case in towns such as San Sebastian, very little data (e.g., cadastral, topographical, hydrological and meteorological) was available to perform a flood risk assessment based on traditional hydrological modeling. To perform a basic flood risk assessment, an alternative method for producing several sets of data was implemented by combining aerial photo interpretation, the use of data questionnaires in a community-based field data collection campaign, and subsequent analysis using Geographical Information Systems (GIS). A cadastral base map with an attribute database was generated as key inputs for flood hazard and vulnerability assessment. Information collected by means of a small-scale survey on a home-by-home basis, in cooperation with a local NGO, was used to reconstruct the last flood that affected the town during Hurricane Mitch in 1998. Based on this historic flood scenario vulnerability curves relating water height and speed with expected level of damage were constructed for three of the main elements at risk within the urban area: buildings, building contents, and road network. Vulnerability maps were generated using GIS and combined with cost information to derive estimated losses. The resulting cadastral database containing physical and social information at the parcel level as well as detailed information on flood hazard, vulnerability and expected losses, can be used by municipal authorities as a valuable tool in municipal planning, including a wider range of applications in addition to risk management.

Development of a system for landslide risk assessment for Cuba

Abstract: In many developing countries, such as Cuba, landslide-affected areas are not systematically mapped in a national inventory and no “regional to local” approach for landslide hazard and risk assessment has been carried out so far. Most of the conventional landslide studies in Cuba are qualitative in nature and focus on description of phenomena or on landslide susceptibility assessment. Moreover, most of the quantitative landslides risk assessment methodologies that have been developed elsewhere are very case-specific and require many types of data, on landslide occurrences and impacts, most of which are not available in Cuba. This research is aiming at the development of a methodology for spatial landslide risk assessment for the Civil Defence in Cuba, taking into account the conditions of Cuba regarding the required levels (scales) of analysis, the triggering factors and the data availability. It aims at implementing the methodology in four case study areas at national level (1:1,000,000 scale), provincial level (1:100,000 scale), municipal level (1:50,000 scale) and local level (1:25,000 scale). This hierarchical approach will allow comparing the suitability of risk assessment methods at each level for the specific conditions of Cuba. The research will focus on the generation of a national landslide inventory database, which should provide the magnitude and frequency information needed for risk assessment. The landslide inventory will be carried out by setting up a landslide-reporting system by staff from the 169 local Civil Defence branches in Cuba, and by multitemporal airphoto and satellite image interpretation, using visual stereoscopic interpretation methods.