Structural attributes are fundamental biophysical parameters of forest, useful for ecological and environmental monitoring and planning. Canopy height is an important input for the estimation of several biophysical parameters as aboveground biomass and carbon stock, and can be related with forest degradation, deforestation, and emission reduction. Thus, an accurate canopy height estimation is a crucial issue in climate change studies and REDD+ initiatives. VHR imagery from unmanned aircraft systems has been studied as a low cost mean for canopy height estimation at local scales, but the accuracy in the estimation is a factor that determines its utility. We evaluated the ability of VHR imagery from unmanned aircraft systems to derive structural attributes, specifically tree-crown area and height, in a natural tropical forest fragment located in the foothills of the Andes Mountains, in the humid tropical forests of the region known as Biogeographic Chocó, South America. The region is one of the most biodiverse areas of the world and has a high level of endemism, but it is also at higher risk of natural-resource loss. We used a structure from motion approach to derive canopy height models of the forest fragment, and we applied mean-shift algorithms to identify single tree crowns. The accuracy assessment was performed using reference data derived from field campaigns and visually interpretation of VHR imagery. The estimated root-mean-square error of the population of vertical errors for the canopy height model was 3.6 m. The total accuracy for delineating tree crowns was 73.9%. We found that using VHR imagery, specific trees and canopy gaps can be identified and easily monitored, which is an important step in conservation programs. We also discuss the usefulness of these findings in the context of fragmented forests and the tradeoffs between the price of a LIDAR system and the accuracy of this approach.

Este artículo presenta una estimación del riesgo en edificaciones por deslizamientos causados por lluvias y sismos en la ciudad de Medellín, empleando una plataforma de Sistemas de Información Geográfica (SIG). Para esto se implementó un modelo que considera las propiedades topográficas, geológicas, geotécnicas e hidrológicas de la zona en estudio. El modelo de cálculo usado para la valoración de la amenaza por deslizamiento, está fundamentado en el Método seudoestático de NewmarN para el cálculo de estabilidad de laderas y usa un enfoque probabilista el cual se basa en el método de primer orden y segundo momento —FOSM— para estimar la varia- ción de los parámetros geomecánicos del suelo. El abordaje para la valoración de la vulnerabilidad de las construcciones en el área de estudio se realizó mediante índi- ces de fragilidad estructural que consideran la definición de niveles de daño a través de árboles de decisión haciendo uso de la información catastral de la ciudad. El modelo calcula la probabilidad de que ocurra un deslizamiento dado que se presente un sismo que produzca una aceleración (Ah), considerando la incertidumbre de los parámetros geotécnicos y las condiciones de saturación del suelo, y multiplica tales valores con los índices de debilidad estructural para estimar los índices de daño de las construcciones, las cuales se asocian con el riesgo al considerar la variable eco- nómica correspondiente al costo de reposición de tales construcciones.

Landslides constitute one of the natural phenomena that cause the most economic losses and deaths worldwide. After failure occurs, landslides can trigger mudflows. Understanding how mud is transported is very important in infrastructure projects that coincide with hillside areas due to the high risk of this phenomena occurring due to the high slopes, which can imply great risks and produce disasters, generating considerable costs. In this work, the evaluation of a mudflow is presented, from the execution of a scale experiment in the laboratory and its validation from numerical models, considering the material behaviour as a Newtonian fluid and as a non-Newtonian fluid. The physical model was developed using a 3m x 0.5m x 0.7m rectangular channel with dimensions, with slope control. A mud mixture composed of a silty material with 60% of moisture was tested producing a mudflow. Experimental tests were carried out with slopes of 5% and 10%. The numerical models were implemented in ANSYS FLUENT software. At first stage, the numerical model was calibrated with the results of the physical model with a slope of 5% and it was validated with the results of the model for the slope of 10%. Results of the numerical models were compared with the experimental results, and they have shown that these have a great capacity to reproduce what is observed experimentally. In addition, when the material was considerate as a Newtonian fluid, a similar behaviour was found respect to a mudflow as a non-Newtonian fluid, not finding considerable differences in the final deposition length of the flow. The simulation of mudflow, especially multiphase, using CFD is usually a complex process since the boundary conditions and physical and rheological properties of the soil must be correctly defined, considering the contribution of the solid fraction in the behaviour of the numerical model. Nevertheless, despite all the simplifications that a modelling of this type entails, the results are promising to improve the understanding of the phenomenon studied, and its application in risk assessment methodologies for mass movements and their derived effects.

This paper presents a methodology for the probabilistic estimation of the obstruction of water streams generated by shallow mass movements triggered by rainfall. The study focuses on the Ovejas River, a tributary stream of the Medellín River, in the jurisdiction of the municipality of San Vicente in the department of Antioquia (Colombia). The occurrence of a mass movements was evaluated by deterministic and probabilistic methods based on the automation of processes of Geographic Information Systems (GIS) and spatial modeling. The spatial distribution of the mass movement hazard was estimated in terms of Factor of Safety (FoS) values by the deterministic method with physical basis SLIDE (Slope - Infiltration - Distributed Equilibrium), which allows the hazard zonation by calculating a FoS for rainfall-induced mass movements with different return periods. The rainfall regimes of the study area are estimated by means of a simple scaling Log Normal Model. On the other hand, the Probability of Failure (PF) analysis was performed under Rosenblueth’s punctual estimates method (PEM), which allows incorporating the uncertainty of the soil parameters. Subsequently, the resulting zones with high hazard that could detach and reach the Ovejas River channel are identified as sources for runout modeling by means of the Flow R model, thus estimating the extent of mass movement in probabilistic terms. In all the analyzed scenarios, the sliding material from the critical stability zones has a high probability of spreading to the riverbed of the main river. This analysis makes possible to identify those areas of the riverbed that should be analyzed with more detail and require possible intervention for the protection of the riverbed.