Abstract
Soil creep is common along the hillslopes of the tropical Andes of Colombia, where very heterogeneous soils develop on old debris flow deposits and are subjected to abundant rainfall with a bimodal annual regime. In particular, the western hillside of the city of Medellín, Colombia, is comprised of a series of debris and earth flow deposits in which landslides and soil creep are common. To explore linkages between soil creep and hydrology, we selected an experimental site in the western hillslope of the Medellín valley to assess the behavior of water within the soil mass, its relationship with rainfall, and its connection with soil displacement. In experimental plots, we systematically measured runoff, percolation, water table levels, and volumetric water content, for a period of almost 2 years; we also conducted several alti-planimetric positioning surveys to estimate relative displacements of the soil surface. Moisture content of the soil remained above field capacity for most of the year (~68% of the time) and active and quasi-permanent lateral subsurface flow occurred within the upper 80 cm of the profile. The shallow flow likely facilitates the downslope movement. Additionally, our results suggest that displacement magnitudes are largest during the wet season of September–October–November, when a highly humid soil experiences changes in water content, so it is during this time that the effects of expansion / contraction of the soil particles (associated to wetting / drying cycles) contribute the most to the movement. This observational study represents a contribution to the understanding of soil creep in tropical hillslopes, where it responds to the wetting / drying cycles, with the particularities of a rainy weather (>1500 mm/year), warm temperatures (~22 °C on average), and a bimodal precipitation seasonality.
Highlights
The Tropical Andes of northern South America are highly susceptible to mass movements given the climate, topography, and susceptibility to soil erosion
We focus on seasonal soil creep, which is highly controlled by the slope angle, curvature of the topography, soil depth where hydroclimate exerts a major control, soil texture and density, and fluctuations of moisture within the soil
Water table fluctuations are closely related with precipitation
Summary
The Tropical Andes of northern South America are highly susceptible to mass movements given the climate, topography, and susceptibility to soil erosion. Within the framework of this research, it is noted that mass movements such as landslides, debris flows, and subsidence, among mass movements, can be triggered by soil creep and its response is accelerated by rainfall [13,14,15,16] For this reason, the analysis of the different processes in the climate-soil system depends on the temporal and spatial scales addressed. This study describes a more elaborate hydrological data set at the same site and aims to characterize the motion itself and its relationship with the soil hydrology using qualitative observations and quantitative relative positioning measurements These observations will allow us to infer some aspects of how the well-known soil creep mechanisms [18] are linked to the hydrological characteristics of this mountainous tropical area and to the associated soil response
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