Procesos geomorfológicos activos en cárcavas del borde del piedemonte norte de la Sierra de Guadarrama (Provincia de Segovia, España)

Resumo

Two areas located at the edge of the North piedmont of the Guadarrama Mountains (province of Segovia) show a conspicuous development of gullies. A series of valley side gullies, or ravines, cut on arkosic sand sediments of Miocene age at the Río Chico Valley (southwest of the Segovia province); and slope gullies, cut on silica sand, shale and gravel sediments of Upper Cretaceous age, on the slopes of a set of mesas and cuestas at the río Cega piedmont (Pedraza region, south centre of the Segovia province). This paper characterizes the geomorphic activity of both types of gullies. In addition, the quantification of their current activity is initiated. The objectives are: to know which processes are eroding and mobilizing the sediments within the gullies, and to evaluate their rates, intensity, frequency and connectivity. A detailed field survey shows that weathering processes (sandstone sheeting, and the formation of popcorn structures on shale layers), and splash, sheet and rill erosion, along with sand and mud falls, slides and flows move sediment from the gully slopes and internal divides to the dry washes. These materials are temporary stored in ephemeral stream channels (filled with sandy bed material), evacuated by stream-bed scour and fill processes, and deposited in alluvial cones. In order to get a first approximation of the amount of erosion and sediment movement, a set of field methods have been applied to these areas. These methods include: erosion pins, measurement of pedestals, system of rods and washers, pit traps (box and gabion check-dam types), and topographic surveys after extensive sedimentation on alluvial cones, along with the installation of pluviographs. The results show how on the arkosic valley side gullies, the precipitation events of low intensity and high frequency tend to fill the sandy stream-beds, whereas the snowmelt and the high intensity and low frequency precipitation events scour the sandy stream-beds. The sedimentation occurs here at the apex part of the alluvial cones, which rest directly on the nearby floodplain of the Chico River. On the silica sand slope gullies, the first gathered data point at very high rates of erosion and sedimentation (44.1 tons/ha for a single event), which is interpreted due to the fact that runoff and water erosion occurs here almost instantaneously after precipitation. Here, the precipitation events of low intensity and high frequency produce sedimentation at the apex part of the alluvial cones, whereas the high intensity and low frequency precipitation events sediment in the form of telescopic lobes, on existing alluvial cones, which rest on pediments.