D. Regüés

Regolith behaviour and physical weathering of clayey mudrock as dependent on seasonal weather conditions in a badland area at Vallcebre, Eastern Pyrenees

Abstract Studies on badland areas in the Vallcebre basin (Pyrenees) suggest that erosion rates are controlled by the weathering rate of mudrocks. To obtain the temporal pattern of physical weathering and its control on the erosive processes, monitoring of regolith temperatures at different aspect, depth and lithology, and periodical determination of regolith moisture and bulk density were carried out. Changes in surface regolith have also been monitored by means of photographic techniques, using an especially designed tripod. Finally, the hydrological and erosive response of the regolith trough the seasons were studied using rainfall simulations. The results obtained support and confirm previous knowledge on the role of frost action in mudrock weathering and the role of subsequent regolith development and wetness status on infiltration and runoff generation. These seasonal changes of behaviour are so important that they raise serious questions about the validity of observations or experiments carried out in only one season. This work confirms that erosion processes are controlled by regolith formation during winter and regolith depletion by storm rainfall erosion during summer, spring and early autumn. It also raises important points about annual trends of infiltration and erodibility, that should be used as guidelines for modelling runoff and sediment production.

Within-storm soil surface dynamics and erosive effects of rainstorms

Abstract This study deals with the characteristics which make a rainstorm an event that can produce intense erosion and even trigger the formation of a badlands site. In order to keep the presentation closely linked to a real situation, a rainstorm which took place on an experimental farm equipped for soil erosion studies was selected. The effects of the erosive rainstorm, which fell on dry antecedent moisture conditions, are given in terms of total rill erosion and rill cross-section along the slope. Unfortunately, the data collected did not answer the basic question, i.e., what combination of factors makes a rainstorm critical? A set of rainfall simulation experiments was therefore carried out, in the field and in the laboratory, in order to evaluate the soil surface variations caused by the rainstorm. All the experiments were performed on dry antecedent soil moisture conditions. It was confirmed that the characteristics of the infiltration curve are modified considerably during such rain events. The saturated conductivity of the first thin top-layer is also modified and it can easily decrease by a factor of 10 due to drop impact forces. The runoff coefficient is also influenced by the raindrop impacting energy and it increases sharply with cumulate energy until a maximum value is reached. The surface micro-relief dynamics was also studied. It was very clearly shown that impacting drop kinetic energy is the rainfall characteristic which is linked to random roughness decay. Cumulative rainfall was not able to align all the data in a single trend. The effect of surface micro-relief decay on the rainstorm erosive power was examined using two equations, thus linking Mannings hydraulic roughness to random roughness. Using a simulated runoff over the field plots that were particularly eroded by the rainstorm, it was possible to observe that the runoff drag forces reached values of between 3 and 100 times the ones which would have been calculated if the random roughness had been constant during the same event. Many of the soil surface characteristics that are modified interact with one another and with erosion. Examining each of them in isolation cannot explain the drastic increase in erosivity of a rainstorm, as the latter is the result of the combined effects of all the surface modifications.

Geomorphic agents versus vegetation spreading as causes of badland occurrence in a Mediterranean subhumid mountainous area.

Abstract Previous evidence on the chief role of physical weathering on badland development in the Vallcebre area (South Eastern Pyrenees) led to the study of the amounts of energy available for the main geomorphic agents, and to analyze the results in the light of the patterns of vegetation cover and species distribution, in order to explain both the occurrence of these badland forms and their preference for north-facing (shady) hillslopes. Results showed that the weathering potential provided by freezing is about two orders of magnitude greater than rainfall kinetic energy, and that the former strongly depends on hillslope aspect. The study of vegetation demonstrated that vegetation is poorer in cover and species composition on shady hillslopes than on sunny ones, spontaneous revegetation of these surfaces being restrained by cold thermal conditions rather than by water availability. These results show that the occurrence of these badland areas depends on montane climatic attributes that control geomorphic and biological processes, and that are different from those that promote the occurrence of badlands in dry Mediterranean areas. The preference for northern aspects provides evidence for these differences.

Fluvial geomorphology and hydrology in the dispersal and fate of pyrite mud particles released by the Aznalcóllar mine tailings spill

The Agrio-Guadiamar River reach affected by the Aznalcollar mine tailings spill can be divided into three main sectors taking account of geomorphic characteristics and human-induced changes. Along the first 15 km, the valley presents the characteristics of a middle fluvial reach of moderate sinuosity and gradient; the floodplain declines gently towards the main channel and is built up mainly by lateral accretion deposits, which are mined by open pits. The second sector is located between 15 and 30 km downstream of the mine, and is characterised by higher channel sinuosity and a lower gradient; the floodplain is built on overbank deposits, separated from the main channel by natural levees, showing some low sinuosity flood channels. Finally, the lowest reach, with a similar low gradient, can be described as the proximal area of the Donana marshlands, whose geomorphology has been modified by agricultural practices for the last 40 years. The spread of mining tailings deposited a mat of pyrite mud (fine milled heavy metal-rich ore) over the main channel and floodplain with a thickness of up to 50 cm in the first sector and then, progressively decreasing downstream. The pyrite mud was deposited over the natural sediments without significant mobilisation of the latter and without relevant geomorphic changes. The subsequent cleaning up of the pyrite mud with the help of heavy machinery resulted in the removal of most of the bushy and herbaceous vegetation as well as the reworking of bed sediments and the destruction of their natural armouring. Relatively small amounts of submerged pyrite mud that could not be removed from the main channel and that mixed with the shallow natural sediments during the cleaning works provides a long-term pollution element in the system. Pyrite mud remnants on floodplains are not being directly removed by fluvial activity in the mid-term because these sedimentation areas are susceptible to erosion only by the wandering of the main channel. However, these materials can be easily oxidised, therefore, plant uptake and underground water pollution should be monitored. On the other hand, the channel bed and banks are source areas of sediments and pyrite materials that can be easily eroded due to the destruction of the natural protection features (vegetation and armouring). Once eroded, the pyrite particles will be transported over longer distances than the coarser host natural alluvium, and deposited on floodplains or areas with low flow velocity.

Forests and Their Hydrological Effects in Mediterranean Mountains - The Case of the Central Spanish Pyrenees

Abstract This article considers the effects of forests on the hydrology of a Mediterranean mountain area. Variations of climate factors, discharge, interception, and water table depth in the San Salvador forested experimental catchment in the Central Spanish Pyrenees were studied and the results compared with those from two deforested catchments. The hydrological response of the San Salvador catchment had the following properties: 1) it had both smaller peak flows and smaller low flows than the deforested catchments; 2) most rainstorm events produced almost no discharge response; 3) the intensity of precipitation had no influence on the magnitude of peak flows; and 4) depth to the water table was the most important factor in the relationship between precipitation and discharge. These results confirm that forest conservation reduces floods and soil erosion, particularly on steep slopes.

Chapter 2 Catchment dynamics in a Mediterranean mountain environment: the Vallcebre research basins (southeastern Pyrenees) II: Temporal and spatial dynamics of erosion and stream sediment transport

Abstract Erosion processes and sediment dynamics were studied in a set of Mediterranean mountain catchments with very active badlands during a period of over 10 years. The relatively small badlands produced most of the sediments, whereas both the dense vegetation and the old soil conservation structures impeded significant erosion from the main area of the catchments. The dynamics of the badland surfaces are driven by intense frost weathering in winter, and the subsequent compaction and erosion of the regolith throughout the year. Infiltration rates measured by means of rainfall experiments were found to depend on regolith moisture and density and reached the lowest values in autumn, whereas regolith detachability by splash was higher in autumn and spring. Nevertheless, erosion rates at the plot scale (1430 m 2 ) were especially high between May and September due to the role of high intensity rainstorms. The main runoff events at the small catchment scale (1.3 km 2 ) occurred between November and January due to the large rainfall events of moderate intensity and the wet antecedent conditions; secondary events occurred in late spring and scattered flash floods occurred during summer. The main sediment transport events occurred in November and December, although less important events occurred in late spring and late summer. As little as 3% of the events produced 73% of sediment yield during 5 years. There was, therefore, a temporal shift of the sediment conveyance with increasing spatial scale: the regolith was produced during winter and, subsequently, eroded during summer, when intense rainstorms produced runoff only on the badland surfaces; nevertheless, most of this sediment was deposited in the channels because the catchment was dry and did not contribute to stream discharge. Subsequently, large flood events produced outside of the badlands transported the sediment deposited on the channels during the previous summer and eventually that from preceding years.

Temporal variability in hydrological response within a small catchment with badland areas, central Pyrenees

Abstract The lower Araguás catchment, central Pyrenees, is characterized by extensive badlands (25% of the total catchment), whereas the upper catchment is covered by dense plantation forest. The catchment (45 ha) has been monitored since October 2005 with the aim of studying its hydrological response. The 44 floods recorded over this period were analysed to identify the factors that control the rainfall—runoff relationship. The first relevant feature of the catchment was its responsiveness. The catchment reacted to all rainfall events, but the irregular nature of the hydrological response was the most characteristic feature of the response. No single variable could explain the response of the Araguás catchment. It was found that stormflow coefficients mainly depend on the combination of rainfall volume and antecedent baseflow. A significant correlation was observed between maximum rainfall intensity and peak flow values. The shapes of the different hydrographs are very similar, regardless of the peak flow magnitude; they show a short time lag, relatively narrow peak flow, and steep recession limb. This indicates a large contribution by overland flow, resulting mainly from the generation of infiltration excess runoff in badland areas.

Hydrological and Erosion Processes in the Research Catchments of Vallcebre (Pyrenees)

The research catchments of Vallcebre were selected in 1988 for studying the hydrological and geomorphological consequences of Mediterranean middle mountain land abandonment and badland activity. The results obtained to date show that the hydrological response of old farmed areas is dominated by saturation mechanisms and yield very low sediment loads, because of the dense vegetation cover and the good condition of the old conservation structures (terraces and ditches). On the other hand, badland surfaces suffer very high erosion rates (about 9 mm per year), with a very dynamic behaviour regulated by physical weathering processes during winter and Hortonian overland flow and erosion when intense rainstorms occur. Catchments with significant badland area show a clear non-linear hydrological and sediment response. Characteristic unit hydrographs show the superposition of two peaks, the first one comes from degraded areas and is controlled by rainfall intensity, whereas the second and broader one comes from saturated areas and is controlled by the antecedent condition of the catchment. Suspended sediment concentrations measured during events occurring in dry periods are as high as 100 g l-1, but they fall by one order of magnitude for events during wet periods, because of the dilution with clear water contributed by the saturated stable areas.

Sediment yield and transport in badlands located in the Inner Depression (Araguás Basin, Central Pyrenees)

This study shows the analyses of suspended sediment transport from badland areas developed in the Inner Depression (Central Pyrenees). The importance of these morphologies is related to its frequency, and to the high impact that they produce on the drainage net, because of their intense geomorphological dynamics. The occurrence and development of these morphologies is associated to two main factors: the geological characteristics of the substratum constituted by marls, and the strong climatic seasonality of the submediterranean climate. The area has been studied since 2004, through the selection of a small basin, which was monitored with sensors and plots for observation of weathering and erosion processes (between January 2004 and December 2006). This instrumentation was completed with a gauging station (October 2005) on the main drainage (Rebullesa ravine) that facilitates the record of hydrological response and suspended sediment transport. This paper shows an analysis, based on 79 recorded events between October 2005 and April 2007. The results, that correspond to a short period of time, have allowed pointing out some general characteristics of hydro-sedimentological responses in these morphologies: very high hydrological response, even during the driest period; great capacity of sediment yield and exportation, with punctual concentrations than can reach more than 1000 g·l-1, and the strong sensibility against climatic seasonality. Likewise, it was observed that precipitation intensity is well correlated with sediment concentration, especially during the driest periods, which demonstrates that the main runoff generation process is associated to infiltration excess mechanisms (Hortonian flow). Palabras clave: carcavas, respuesta hidrologica, concentracion de sedimento, transporte en suspension.

Rainfall, run-off, and sediment transport dynamics in a humid mountain badland area: Long-term results from a small catchment

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