Download weathering, erosion and transport processes in

Badland areas as the main sediment source areas in mountain basins: weathering,
erosion and transport processes in marls (Central Spanish Pyrenees)
Badlands are usually defined as intensely dissected natural landscape where
vegetation is sparse or absent and which are useless for agriculture (Bryan and Yair,
1982).
Badlands develop within a wide range of climatic environments, particularly in
semiarid areas, and to a lesser extent in humid and subhumid regions. In such areas, the
formation of badlands is favoured by lithology, topography, human influence and a
climate that is characterized by strong seasonal contrast in temperature and rainfall
distribution. Badlands usually consist of bedrock covered by a layer of disturbed rock
known as regolith which can be extremely susceptible to erosion. Rapid and deep
weathering, together with intense soil erosion, explains the fact that erosion rates in
badlands areas are much higher than those in surrounding areas in Central Spanish
Pyrenees. It’s obvious, that badlands development in these areas affects directly to
environmental and socio-economical aspects: water quality and water availability,
reservoir sedimentation or reduction of its useful life, imbalance in the rivers dynamics,
damage of communication routes and urban infrastructures, and soil and biomass loss.
In the Central Spanish Pyrenees, Eocene marls are the most important erodible
rock substratum, particularly within the Inner Depression where the present study area,
the Araguás catchment is located. This small research catchment (0.45 km2) has been
the site of different geomorphological studies since 2004. Badlands are mainly
developed within the lower part of the Araguás Catchment, where the marls are
massive.
The aim of this work is to analyse the geomorphological dynamics of a
subhumid badland areas developed in the Inner Depression, through the study of the
temporal and spatial patterns of weathering, erosion and sediment transport processes.
The Araguás catchment was first monitored in 2004 to study physical and
chemical weathering processes. Two representative slopes (north- and south-facing)
were selected to study regolith development and weathering processes. The physical
condition of regolith can be determined from physical indicators: bulk density and
surface mechanical resistance. Also regolith moisture (crust, 0-5 and 5-10 cm) and
regolith temperature (1, 10, 20 and 30 cm) were recorded to studying its relationship
with respect to the variations of physical conditions. In order to determine the effect of
regolith dynamics on sediment detachment and infiltration processes, rainfall
simulations were conducted on small plots using a pressure nozzle. Finally, at the outlet
of the Araguás catchment a gauging station has been installed to measure precipitation,
discharge and suspended sediment load.
This work has documented the complexity of geomorphological dynamics in
subhumid Mediterranean badlands areas.
Badlands dynamics is complex and depends on extrinsic factors, such as rainfall
distribution, and intrinsic factors, such as regolith characteristics. In areas subjected to
freezing-thawing cycles, the weathering role of geliviation may be much more
important than wetting-drying cycles. In addition, the regolith development and
dynamics in north-facing slopes is more active than that on south-facing slopes.
Weathering processes show clear temporal and spatial variability that is stronger in
winter, when high values of regolith humidity, lower temperatures and frequent freezethaw cycles occur. Physical weathering processes, during the winter, act to expand the
depth of regolith and favour the development of popcorn morphology.
The weathering processes that take place during the cold and humid season are
strong enough to supply large quantities of fine material to the channel. However, it was
observed that there is a lag between weathering, erosion, and transport processes, with
the latter being directly dependent on the occurrence of rainstorms and floods. Erosion
processes depends on seasonal differences of regolith development that produces
important variations of detachment and infiltration processes. The highest marls
fragility occurs between the end of winter and half spring, associated with popcorn
morphology. During summer and autumn, the greatest resistance occurs related to
development of crust surface.
In the Araguás catchment, the seasonal trend of the hydrological and
sedimentological dynamics was linked to seasonal dynamics in weathering and erosion
processes. The hydrological response tends to be simple, being mainly dependent on the
rainfall intensity and rainfall volume. Floods were generally flashy, suggesting the
importance of bare areas and hortonian flows, in terms of runoff generation and
sediment sources.
The sediment response in the Araguás catchment is complex and heterogeneous.
Badlands areas always yield significant volumes of sediment, but most of the annual
sediment yield was a result of just a few precipitation events. The availability of
sediment in such environments is so high that even short, low-intensity rainfall and
flood events record high suspended sediment concentrations. The consequence of this
sediment supply was a total loss of 57,000 Mg Km-2 yr-1, considering badland areas the
main sediment source. Such behaviour contrasts with that in more highly vegetated
areas of neighbouring catchment in Central Pyrenees, where sediment load was about 3
orders of magnitude lower than the registered in the Araguás catchment.
Global climatic change may alter the relationships between processes and
modify the erosion rates. Changes in climatic variables are predicted: a mean increase in
annual temperature, a wintertime warming of 2.5 ºC and a reduction of snowfalls and
snow accumulation. As a result, physical weathering processes, in these subhumid
Mediterranean badlands areas, will be alleviating. So, what would happen was a
decrease in sediment supply capable of be exported, and a qualitative change in regolith
dynamics, with more persistent crust conditions. Consequently, hydrological responses
would be characterized by fast and intense runoff and sediment dynamics would be less
intense, because there will be less sediment availability.
To conclude, severe soil erosion is a frequent problem, especially in mountain
areas. This erosion, which is mainly due to superficial runoff, causes problems to
society due to soil loss upslope and torrential floods downslope. Future studies could be
focused on avoid these problems. To achieve this objective, sustainable control of soil
erosion and sediment is necessary. However, installing vegetation cover on severely
eroded mountain areas remains difficult due to the strong hydrological and erosive
forces.
Therefore, the Araguás catchment could be considered as representative of
highly degraded environments affected by a dense network of badlands and further
investigation in this issue is clearly warranted.