Mapping Stability and Soil Saturation Indices in the Huehuetán River Basin, Chiapas, Using the SINMAP Model

Abstract

Landslides are among the most common natural events that cause mass destruction worldwide. For this reason, studying the causes, behavior, and mitigation of landslides is essential to reduce their adverse impacts. These occur most frequently in tropical environments because of the volume and intensity of rainfall that, combined with steep relief, causes damages to urban, livestock, agricultural, and industrial infrastructure, leading to changes in the environment and even losses of human life. Several studies have investigated the origin and effect of morphodynamic processes associated with landslides. However, given their complexity, there is still uncertainty regarding the prediction of their incidence. Although landslides result from various interactions, where relief morphology and the evolution of hillslopes play a primary role, a single factor, such as precipitation or an earthquake, is considered the external triggering factor. The Huehuetán River basin, located on the Chiapas coast, is highly susceptible to landslides caused by heavy rains. Therefore, this study aimed to identify areas susceptible to landslides due to soil saturation associated with precipitation. To this end, we calculated the Stability Index (SI) and the Soil Saturation Index (SSI) through the SINMAP (Stability INdex MAPping) model for use as support elements in urban planning and the prevention or mitigation of landslides through structural and non-structural measures. The Huehuetán River basin is located in southeast Chiapas, covering an area of 319.27 km2, and is part of the Sierra Madre of Chiapas. It is characterized by a rugged relief, mainly in the upper portion, with elevations ranging from 19 to 2690 meters above sea level and 83.15% of the total area with hillslopes steeper than 10%. The predominant soil is Lithosol (36.63%), followed by Andosol (28.71%), and its geology is dominated by igneous (more than 85%) and sedimentary rocks. The basin was delimited using the Soil and Water Assessment Tool (SWAT) model based on the Digital Elevation Model (DEM) with a 10-m spatial resolution and the drainage network from the Instituto Nacional de Estadística y Geografía (Mexico's National Institute of Statistics and Geography). We built a landslide inventory for the period 2005 to 2021. First, we performed field trips across the Huehuetán River basin, supported by GPS equipment, for the spatial location of landslide polygons. Landsat-7 and Sentinel-2A satellite images were used as support tools for locating and detecting landslides larger than 100 m2; smaller landslides were recorded based on field trips and located on Google Earth®. The SINMAP model was executed using the DEM, the mechanical and hydrogeological properties of the soils, and the landslide inventory. The land stability was classified by estimating the spatial distribution of SI and SSI across the Huehuetán River basin. We determined that 24.36% (77.76 km2) of the basin is an unstable area and 75.64% (241.51 km2) is stable. The highest hillslope instability values were observed in the upper parts due to the relief conditions, soil type, and vegetation cover. The performance of the SINMAP model was evaluated by comparing the spatial distribution of the SI with the landslide inventory obtained for the basin with the ROC curve method; it yielded an 85.60% accuracy that grades it as excellent. The SINMAP model allowed for determining the SI and SSI easily and accurately. Both indices can be used for urban planning and to reduce serious damage from landslides. The results of this study suggest that using the SINMAP model as a tool for mapping areas susceptible to landslides is a feasible option. It will support decision-makers involved in urban growth planning by identifying and zoning vulnerable areas and applying structural and non-structural control measures.