Resumen

QUESADA ROMAN, Adolfo  y  ZAMORANO OROZCO, José Juan. Hillslope processes and floods zoning from a morphometric analysis in the Upper General Basin, Costa Rica. Invest. Geog [online]. 2019, n.99, e59843.  Epub 25-Sep-2019. ISSN 2448-7279.  https://doi.org/10.14350/rig.59843.

In tropical countries as Costa Rica, mass movements and floods are triggered by intense precipitation events, weathering, rock control, seismicity, morphology, and slopes angle. The recurrence and magnitude of these processes provoke economic losses and casualties in the communities of the Upper General River Basin in the center-south of Costa Rica. A morphometric analysis is performed to calculate and analyze seven variables (altimetry, slopes, dissection density, dissection profundity, relief energy, potential erosion, and total erosion). Within this analysis, five morphological regions were determined: mountain zone (Cordillera de Talamanca), floodplain, accumulative ramp (alluvial fans), minor mountain ranges (Fila Brunqueña), and mountain foothills covered by proluvial sediments. The mountain zone is intensely modeled with the presence of extensive weathering crusts on inclined and fractured slopes, which favors fluvial-gravitational processes. The minor mountain ranges serve as limit to the alluvial plain of the Pacuar and General River, that due to its sedimentary lithology, steep slopes, the rain regime and changes of land use, favor the slope instability of this region. The mountain foothills have a plateau morphology with alluvial covers on the base of the slopes. The accumulative proluvial ramp is the transition between the mountain zone and the floodplain, its morphology is defined by the presence of alluvial fans superimposed and continuous as a glacis. Finally, the floodplain is defined by the seasonal and extraordinary flooding areas.

The slopes map is a first approximation of the gravitational dynamics. It helps to identify landforms associated with the fluvial erosive-accumulative action. The dissection density map showed zones of greater or lesser erosion, where the channels concentration are linked to rainy zones (>2500 mm annually), the existence of a poorly consolidated and altered substratum (weathering crusts), the presence of disjunctive structures, large areas without significant lithological changes, and a heterogeneous morphology. In addition, the dissection profundity makes evident the influence of the disjunctive structures in the erosive and accumulative relief dynamics. In this context it is necessary to consider that the valleys vertical growth causes slopes instability and favor gravitational processes.

The relief energy integrates fluvial action (dissection density and dissection profundity) and slopes in its analysis. These three elements are closely linked to mass wasting. It also considers lithology, the influence of weathering, the faults activity, earthquakes and precipitation. On the other hand, potential erosion locates indirectly areas with denudation susceptibility, linked to factors such as specific lithological characteristics, topographic contrasts, morphological variety, hydroclimatic conditions, vegetation cover, and even land use changes. In the case of total erosion, areas with greater erodibility and with older morphologies were distinguished through the terrain roughness, and the role played by variables such as lithology age, disjunctive structures, the past climate (glaciations), the current rainfall patterns, and slopes are inferred.

To produce the flood and hillslope processes susceptibility maps, each morphometric variables result were grouped into three ranges (high, medium and low), which allowed a cross analysis. The territories where up to two morphometric indicators coincided, were considered as potential areas; the areas where three indexes were overcome, were classified as frequent occurrence areas, and the spaces classified as maximum occurrence were those where four or five variables were present. These criteria were applied for hillslope processes as well as for floods.

These maps are spatially compared with the disaster database DesInventar where 85.15% of the reports coincide with the zones identified as prone to flooding and 76.54% with susceptible areas to hillslope processes. This method can be easily replicated since only the contours lines and rivers are used, the measurements are simple and can be made from a Geographical Information System (GIS) in its entirety. This condition speeds up the digitization times of fluvial channels, the processing of information, the systematization of the procedure, and the practicality to make different measurements as well as to take precise data of contour lines and fluvial data.

The procedure can be implemented for watersheds territorial management and political-administrative units at local and regional scales. Geomorphometry is a practical tool for mitigating disaster risk by determining floods and hillslope processes susceptible areas. The morphometric method proves to be a low-cost method that could be replicated in other tropical regions and developing countries as a baseline input for decision making in disaster risk management and land use planning.

Palabras llave : geomorphological hazards; geomorphometry; disaster risk reduction; Pérez Zeledón; Central America.

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