Gully evolution and geomorphic adjustments of badlands to reforestation

International audienceBadlands and gullied areas are among those geomorphic environments with the highest erosion rates worldwide. Nevertheless, records of their evolution and their relations with anthropogenic land transformation are scarcer. Here we combine historical data with aerial photographs and tree-ring records to reconstruct the evolution of a badland in a Mediterranean environment of Central Spain. Historical sources suggest an anthropogenic origin of this badland landscape, caused by intense quarrying activities during the 18th century. Aerial photographs allowed detection of dramatic geomorphic changes and the evolution of an emerging vegetation cover since the 1960s, due to widespread reforestation. Finally, tree-ring analyses of exposed roots allowed quantification of recent channel incision of the main gully, and sheet erosion processes. Our results suggest that reforestation practices have influenced the initiation of an episode of incision in the main channel in the 1980s, through the hypothesized creation of disequilibrium in water-sediment balance following decoupling of hillslopes from channel processes. These findings imply an asymmetry in the geomorphic response of badlands to erosion such that in the early evolution stages, vegetation removal results in gullying, but that reforestation alone does not necessarily stabilize the landforms and may even promote renewed incision

Avulsions and the spatio-temporal evolution of debris-flow fans

Debris flows are water-laden masses of sediment that move rapidly through channel networks and over alluvial fans, where they can devastate people and property. Episodic shifts in the position of a debris-flow channel, termed avulsions, are critical for debris-flow fan evolution and for understanding flow hazards because avulsions distribute debris-flow deposits through space and time. However, both the mechanisms of flow avulsion and their effects on the long-term evolution of debris-flow fans are poorly understood. Here, we document and analyze the spatial and temporal patterns of debris-flow activity obtained by repeat topographic analyses, dendrogeomorphic and lichenometric reconstructions, and cosmogenic radionuclide dating on 16 fans from Japan, USA, Switzerland, France, and Kyrgyzstan. Where possible, we analyze the observed spatio-temporal patterns of debris-flow activity in conjunction with high-resolution topographic data to identify the main controls on avulsion. We identify two main processes that control avulsions on debris-flow fans, operating over distinct time scales. First, during individual flows or flow surges, deposition of sediment plugs locally blocks channels and forces subsequent flows to avulse into alternative flow paths. Plug deposition is a stochastic process but appears to depend in part on the sequence of flow volumes, the geometry of the channel, and the composition of the flows. Second, over time scales of tens of events, the average locus of debris-flow deposition gradually shifts toward the topographically lower parts of a fan, highlighting the importance of topographic compensation for fan evolution. Our documented debris-flow avulsions often, but not always, follow a pattern of channel plugging, backstepping of deposition toward the fan apex over one or more flows, avulsion and establishment of a new active channel. Large flows can have contrasting impacts, depending on whether or not they follow smaller flows that have deposited channel plugs. These results suggest that avulsions and spatio-temporal patterns of debris-flow fan formation strongly depend on both the magnitude-frequency distribution and the sequence of the flows feeding a fan. While individual avulsions are generally abrupt and difficult to predict, the presence of debris-flow plugs and patterns of backstepping may be useful as indicators of impending avulsions. Over longer time scales, the compensational tendency of flows to avulse into topographic depressions on the fan may also be used to identify sectors of the fan that are at risk of future inundation

Debris-flow risk analysis in a managed torrent based on a stochastic life-cycle performance

International audienc

Can tree tilting be used for paleoflood discharge estimations?

Paleoflood hydrology typically deals with the reconstruction of floods in ungauged and poorly gauged basins by combining different sources of indirect evidence. Botanical indicators have been used repeatedly in the past, mostly through the study of scars in trees or germination dates of plants on newly created surfaces. In this paper we test the hypothesis that the inclination of trees – as induced by floods – can provide information on flood magnitude, and that this source of information can therefore be used for flood reconstructions. We used a mechanical root-plate rotational stiffness model in three gauged river reaches in Central Spain to test our hypothesis and combine approaches typically applied in dendrogeomorphic, dendrometric, mechanical structure, and hydraulic research. Results show a correlation between modeled and observed deformation at the stem base of trees induced by floods (coefficient of correlation 0.58 for all observations). However they also point to a clear underestimation of peak discharge reconstructions. We used different efficiency criteria to test the reliability of results and differences between river reaches. In addition, we carried out a sensitivity analysis and discussed sources of uncertainties which may reach up to 112%, mainly due to difficulties to determine the rotational stiffness of the root plate system a posteriori. The approach presented here is promising, but more research is clearly required to improve the quality of peak discharge estimations based on stem tilting. 2014 Elsevier B.V. All rights reserved.This study has been funded by the research project CGL2010-19274 (projects MAS Dendro-Avenidas) of the Spanish Ministry of Economy and Competitiveness. The authors would like to thank Mario Hernandez and Juan Gabriel Perez for their kind help during fieldwork.Eguíbar Galán, MÁ. (2014). Can tree tilting be used for paleoflood discharge estimations?. Journal of Hydrology. 1-10. https://doi.org/10.1016/j.jhydrol.2014.10.026S11

Dendrochronology Course In Valsaín Forest, Segovia, Spain

This report describes an international summer course, “Tree Rings, Climate, Natural Resources, and Human Interaction”, held in Valsaín, Spain, in summer of 2012. The course, with 14 participants from three countries (Spain, Algeria, and Russia), included basic training in dendrochronology skills as well as applied projects in dendroclimatology, dendroecology and dendrogeomorphology.This item is part of the Tree-Ring Research (formerly Tree-Ring Bulletin) archive. For more information about this peer-reviewed scholarly journal, please email the Editor of Tree-Ring Research at [email protected]

Challenges in paleoflood hydrology applied to risk analysis in mountainous watersheds A review

In many regions of the world flood events in mountain basins are one of the greatest risks to the local population, due to the pressure placed on land use by social and economic development. Conventional hydrologic hydraulic methodological approaches are not usually feasible in mountainous basins because they are not gauged at all or, in the best-case scenario, are poorly gauged. In this context, palaeohydrological research offers a valuable alternative to the above approaches. However, many palaeohydrological data sources and associated methods have been proposed and initially used in large basins with extensive floodplains. As a result, when they are used in mountainous areas they must be adapted to include different techniques, since the problems to be addressed are different and less data is usually available. In this paper, we review classic data sources and different analytical methods and discuss their advantages and shortcomings with particular attention to mountain basins. For this purpose, examples are provided where improvements in the palaeohydrologic methods are proposed by incorporating uncertainties, describing sources of error or putting forward hypotheses for hydraulic calculation to make palaeoflood hydrology more objective and useful in risk assessment. © 2014 Elsevier B.V. All rights reserved.Eguibar Galán, MÁ. (2014). Challenges in paleoflood hydrology applied to risk analysis in mountainous watersheds A review. Journal of Hydrology. 1-19. doi:10.1016 / j.jhydrol.2014.12.004S11

Dendrogeomorphology in badlands: Methods, case studies and prospects

Soil and vegetation are interacting factors controlling erosion. Soil degradation processes may affect the normal tree and shrub development and inversely, vegetation can modulate the velocity and intensity of soil development or denudation. A dendrogeomorphological approach can be used to study these interactions, allowing to obtain a date and estimate mean or specific erosion rates. This is especially useful in an unrecorded badlands and gullied environments,where the scarce vegetationmay be the only proxy available to quantify the different geomorphic processes which have occurred. This paper provides a fundamental review of the dendrogeomorphological methodology applied to erosion measurement in badlands. Focusing on the response of the vegetation to the geomorphic processes, this paper: (a) describes themethodology developed to estimate erosion rates with exposed roots; (b) shows newadvances through case studies; and finally, (c) discusses future lines of research to reduce methodological uncertainties and for making dendrogeomorphology more widely applicable