Investigaciones Geográficas (Mar 2017)
Estimating volume of deposits associated with landslides on volcanic landscapes in the SW flank of the volcano Pico de Orizaba, Puebla-Veracruz
Abstract
Landslides that occur along river systems are very common and have the potential to cause harm to human, to its infrastructure or affect their socio-economic activity. This dynamic is magnified in territories where morphological contrasts are very marked; as in the border between the mountains and subhorizontal land. This is especially true for volcanic terrains where volcanic activity can trigger voluminous landslides along stream systems by sector and flank collapse and where high seasonal rainfall on terrains covered by poorly consolidated materials produces small but hazardous landslides and debris flows that occur continually along stream systems during the volcanic repose periods. Those type of landslides can deliver volumes of hundreds and millions cubic meters that create a potentially hazardous situation for people and property down the valleys. The study of landslides in volcanic terrains through a Geographic Information System (GIS) and under a geomorphological criterion, have allowed to develop a comprehensive methodology linked to the development of multi-temporal inventory, with susceptibility and volume estimation of displaced material. The aim of this research is to develop a method (protocol) for landslide susceptibility and landslide volume assessment of potentially unstable volcanic landscapes in order to be helpful in mitigating landslide damages to human settlements. Pico de Orizaba volcano is the highest volcano in Mexico. The volcano has been affected by large flank collapse landslides throughout its geological history. These events have partially destroyed the cone as it happened in Bezymianny volcano and St. Elena volcano. In this volcano, the risk associated with landslide and debris flows, is increased by the growing of human settlements along the hillslopes and by the subsistence agriculture, and deforestation. This situation is favored by a volcanic calm that has lasted 147 years, approximate. These conditions create a dangerous situation for more than 360 000 people living on the southern flank of the volcano Pico de Orizaba, where landslides along the hillslopes and the river system threaten towns like Cordova, Orizaba, Rio Blanco, Nogales and Ciudad Mendoza. Today the most common and dangerous landslides are associated with unconsolidated volcanic deposits and heavy seasonal rains. In this paper, the cause, distribution, and link between landslides and the volcanic landscape relief susceptibility are analyzed. Similarly, the volume of displaced material is estimated in order to characterize the landslide instability in volcanic terrains. The Río El Estado watershed on the southwestern flank of Pico de Orizaba volcano is selected to describe and analyze susceptible areas of gravitational processes. The study area allows to show a systematic methodology for landslide mapping and volume calculation in areas with scarce information. The methodology encompasses three main stages of analysis. In the first stage, background information is collected to provide context and establish a generalized characterization of landslide processes, landsforms and volumes within the study area. Background information includes the following maps: topographic, geologic, land use, climate, slope, slope curvature, contributing area, flow direction, saturation, reclassified hypsomety, reclassified slope, and morphography. By retrieval and on-off switching of the background information in the GIS, a base map is created to assist in the digitizing of landslides. The base map and the theoretical aspects of the geomorphological mapping help to develop a conceptual base of support for mapping landslides. Landslides are digitized directly into a geographic information system (GIS), and in parallel, a spatial geodatabase of landslides attributes (eg. size, volume, activity, landslide type, etc.) is constructed. Previous landslide mapping in the study area is verified and new landslides are added to the landslide inventory. Once the locations of landslides are mapped and evaluated, similar potential landslide areas are grouped into individual landforms. During the second stage, the volcanic relief is analyzed under a geomorphological criterion based on aerial photographs, fieldwork, and detailed mapping and the results are adjusted to the landslide hazard zonation protocol from Washington State, Department of Natural Resources, USA. For each volcanic landform a semi-quantitative susceptibility rating is obtained from values that correspond to the total area of landslides and the number of landslides within each landform; and they are normalized for the total period of time spanned between sets of aerial orthophotographs (fourteen years period). These normalized variables are referred to as the landslide area rate and the landslide frequency rate, respectively. Finally, for the third stage, the volume of material delivered by landslides to the main river is estimated by using an empirical area-volume relationship. The results allow us to understand the long-term evolution of the fluvial system in southwestern flank of Pico de Orizaba volcano. They also show a useful mapping methodology for developing inventories, assessing susceptibility, and estimated volumes associated with landslides in volcanic terrains.
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