G. Desir

Erosion rates in badland areas recorded by collectors, erosion pins and profilometer techniques (Ebro Basin, NE-Spain)

Abstract In badland areas of the Ebro Basin, in a semiarid climate, two erosion plots (257 m2; 5° slope and 128 m2; 23° slope) on exposed Tertiary clays were monitored over two years (Nov. 1991–Nov. 1993). This material is characterized by high sodium absorption ratios which lead to high soil dispersivity. The dominant erosion processes in both plots are rilling and sheet erosion. Rainfall intensity was recorded at a weather station, connected to a data-logger, sediment production for single events was collected in tanks, and ground lowering was measured every six months by erosion pins and microtopographic profile gauge techniques. Significant runoff was produced only by rainfall events above 5 mm. Another threshold at 20 mm rain was noted. For rainfalls higher than 20 mm, the 23° slope plot shows a greater runoff response than the 5° one. Rainfall events exceeding this threshold showed a higher sediment production for the steeper slope. In the relationship between precipitation and sediment concentration, an envelope curve can be drawn indicating that any rainfall event of a given amount and intensity has a maximum sediment concentration which we speculate to be a function of the runoff sediment transport capacity. Runoff response and sediment yield in the studied plots are controlled by the rainfall and soil characteristics and their seasonal variations. In both plots, the erosion pins show that erosion rates in rill areas are 25–50% higher than in the interrill areas. Sediment yield recorded by collector devices was higher than the rates measured by erosion pins. The erosion rates based on rill cross-sections by profilometers were higher than the ones recorded by collectors.

Quantitative study of piping processes in badland areas of the Ebro Basin, NE Spain

Abstract In the Ebro Basin, piping is an ubiquitous erosion process in badland areas, occurring in two main situations: on slopes of exposed Tertiary clays and in recently incised silty-clay Holocene valley-fill sediments. The quantitative importance of the main factors controlling piping processes in those conditions was obtained by statistical analysis based on multiple and stepwise regressions. Piping erosion is strongly related to a high sodium absorption ratio (SAR) which favours clay dispersion. However, the statistical analysis indicates that the chemical characteristics and the physical-chemical behaviour are a necessary condition but are not sufficient on their own to develop piping processes. On the slopes, piping is controlled by the physical and chemical properties of the material such as the dispersion index, the granulometric fraction smaller than 4 μm, pH and crack density. In the valley-fill sediments, piping is mainly controlled by the sulphate concentration in the water extract from saturated paste, which is also associated with high SAR values, and the dispersion index. In both geomorphic situations, piping is always related to high crack density and high topographic gradient.

Yardangs in the semiarid central sector of the Ebro Depression (NE Spain)

Abstract This work reveals the existence of yardangs in the central sector of the semiarid Ebro Depression of Spain. Almost all the documented yardang fields are located in extreme arid environments. The yardangs are developed in horizontal tertiary gypsum and limestone. More than 100 closed depressions (solution dolines) have been formed in the soluble sediments of this area, some of them hosting saline lakes or playas. All the yardangs occur on the leeward margin of the larger playas and their mean orientation (N122E) coincides with the prevalent direction of the strong local wind called Cierzo. Two main types of yardang have been identified; 44 rock yardangs formed in the Miocene bedrock and six yardangs developed in unconsolidated lacustrine deposits. Nebkha dunes have been recognized and yardang-like morphologies in building rubble accumulations on the floor of a playa (

Are talus flatiron sequences in Spain climate-controlled landforms?

Summary. This study provides chronological evidence of the influence of climatic variability in the generation of late Quaternary talus flatiron sequences in Spain. The temporal clustering of the OSL and radiocarbon dates obtained from talus flatiron deposits indicates that warm/wet and cold/dry periods controlled the accumulation and incision processes in the slopes, respectively, that led to the development of talus flatirons. These results strongly suggest that talus flatiron sequences constitute valuable paleoclimatic records. Additional and more accurate geochronological data from Spain and other regions of the world would improve the potential of these poorly-known landforms in paleoenvironmental studies.