Evaluación de procesos de erosión en pistas forestales no pavimentadas(Sistema Ibérico noroccidental, La Rioja)

Résumé

Mountain areas have become recently places of great attraction, increasing the use of theirs resources and visits. This would not have been possible without the construction of a dense network of forest road in the world that has caused a great variety of environmental impacts, a lot of them negative, specially geomorphological impacts. Road building in mountain areas has a notable geomorphological impact. Several factors like: the breaking of the hillslope profile, the lack of vegetation protection and the alteration of hillslope hydrogeomorphological functioning, suggest an increase of erosion processes (both variety and intensity) and in sediment yield, particulary in those hill-roads that were built with low budgets, and carry heavy traffic and do not include even minimal conservation techniques. In the mountains of La Rioja, hill-road building has been increasing in recent years to connect villages, to facilitate livestock access to pastures, to manage afforestation, to build windparks... In this paper the author studies the geomorphological functioning of forest roads in the northwest Iberian System (La Rioja, Spain). The variety and significance of erosion processes have been analysed on the road-cut (T1), on sidecast fill (T2) and on the roadbed. A different behaviour has been observed in the three sectors. On T1 the activity of slides was important; on T2 the severe sheet wash erosion was very active; and incisions operated on the road bed. Geoecological and topographical controls explain the variety and activity of erosion processes. Different models of geomorphological evolution of road-cuts are here explained and erosion rates of processes vinculated to overland flow and its seasonal evolution have been estimated. After introducing this matter and explaining the methods used and the characteristics of the study area, the importance and distribution of the forest road network in the Iberian System in La Rioja, has been evaluated specially in the selected forest road including their morfometry, vegetation covert, erosion processes on the bed road, cut and shid road, environmental and topographical controls. The author describes, quantifies and classifies, with several techniques the erosion processes and analyzes in depth the superficial runoff and masse movements to set up behave models. The application of multivariate methods (cluster and discriminant analysis) has allowed to distinguish three models of geomorphological evolution on road-cuts that are related to different types of slope runoff regimes. These models explain the different geomorphological evolution of a forest road system (cutslope, roadbed and sidecast fill) and the hidrogeomorphological functioning (overland flow and mass movements) on hill-roads in mountain slopes. A mobile rainfall simulator was used to compare the information of the runoff coefficients, of the overland flow and of the transported material. Correlation coefficients and regression analysis showed that the gradient, the plant cover density and the gravel cover had statistically significant effects on runoff and erosion on the three parts of forest road system. Simple methods were applicated to classify road fills to establish typologies and to evaluate their evolution. An approximation method calculated the volume of material mobilized in mass movements; environmental controls were evaluated of several erosion processes; an analysis method of the susceptibility to landslides was tested and designed for this kind of lineal infrastructures. The results of the multivariate analysis were applied to obtain three landslide susceptibility maps using GIS in the forest road selected. Once validated in another section of the forest road studied a good result was obtained. Such methods could be applied in forest roads and hill-roads in order to hazard reduction and management. To finish, a list of conclusions of this study about erosion processes in forest road is showed that allows to give several recommendations that are parts of a good practices collection about the design, constructions and management of forest roads for technicians and managers. A specific planning of this forest road network is needed, as well as preventive measures and regulation of construction, utilization and restoration.