Simultaneous normal faulting and extensional flexuring during rifting: an example from the southernmost Upper Rhine Graben

The southern end of the Upper Rhine Graben (URG) is formed by a major continental transfer zone, which was localised by the reactivation of ENE-oriented basement faults of Late Palaeozoic origin. A combination of subcrop data (derived from exploration wells and reflection seismic lines) and palaeostress analysis provided new constraints on the timing and kinematics of interacting basement faults. Rifting in the southern URG began in the Upper Priabonian under regional WNW–ESE-directed extension, oriented roughly perpendicular to the graben axis. In the study area, this led to the formation of NNE-trending half-grabens. Simultaneously, ENE-trending basement faults, situated in the area of the future Rhine-Bresse Transfer Zone (RBTZ), were reactivated in a sinistrally transtensive mode. In the sedimentary cover the strike-slip component was accommodated by the development of en-échelon aligned extensional flexures. Flexuring and interference between the differently oriented basement faults imposed additional, but locally confined extension in the sedimentary cover, which deviated by as much as 90° from the regional WNW–ESE extension. The interference of regional and local stresses led to a regime approaching radial extension at the intersection between the URG and RBTZ.     

Fluid flow impact on slope failure from 3D seismic data: a case study in the Storegga Slide

Analysis of three‐dimensional (3D) seismic data from the headwall area of the Storegga Slide on the mid‐Norwegian margin provides new insights into buried mass movements and their failure mechanisms. These mass movements are located above the Ormen Lange dome, a Tertiary dome structure, which hosts a large gas reservoir. Slope instabilities occurred as early as the start of the Plio‐Pleistocene glacial–interglacial cycles. The 3D seismic data provide geophysical evidence for gas that leaks from the reservoir and migrates upward into the shallow geosphere. Sediments with increased gas content might have liquefied during mobilization of the sliding and show different flow mechanisms than sediments containing less gas. In areas where there is no evidence for gas, the sediments remained intact. This stability is inherited by overlying strata. The distribution of gas in the shallow subsurface (<600 m) may explain the shape of the lower Storegga headwall in the Ormen Lange area.     

Thresholds in small rivers? Hypotheses developed from fluvial morphological research in western Germany

The objective of this study was the formulation of fluvial morphological regularities for small rivers with a wide range of morphological and geological characteristics in North-Rhine-Westphalia (Germany) based on a statistical research methodology. Such empirical quantitative information on reference conditions is required for the restoration of small rivers in the former highly industrialised Ruhr-Area. Following the approach of some classic empirical works in fluvial morphology of the last century, several natural reference rivers in the entire research area have been observed in order to provide a statistical correlation between independent and dependent morphological variables. Regressions between valley-floor slope, bankfull discharge and stream power on the one hand and several variables describing the longitudinal profile and river planform on the other hand have shown some significant results. The regularities found are a quantitative contribution to the establishment of reference conditions as well as a useful tool for the restoration of small rivers, if the specific properties and values of the underlying random sampling are taken into account. In addition, the relation between stream power and sinuosity shows the likely existence of a threshold: Exceeding a stream power of 100 W/m, the sinuosity decreases after an increase for lower stream power values. Comparable thresholds were found for the relation between stream power and pool depth as well as stream power and step steepness. The thresholds could be explained by a change in the type of energy dissipation, due to different physio-geographical settings in highland rivers within forested v-shaped valleys. Here, large-woody debris seems to increase the channel roughness and possibly replaces the significance of coarse-grained bed material, pool depth and step steepness as contributors to energy dissipation.     

Evaluation of the streambed leakage concept in analytical models using data from three pumping tests

Two-dimensional analytical models of pumping induced drawdown and stream depletion account for the streambed properties and stream geometry. Using data from three pumping tests performed under various hydrologic conditions, the following hypotheses were tested: (1) a partially penetrating stream at a certain distance from the pumping well can be represented by the streambed leakage term in the analytical models; (2) in the streambed leakage term, the streambed conductance coefficient λ=(WK′)/m′ accounts for the average stream width W, the streambed thickness m′, and hydraulic conductivity K′. The first hypothesis was tested by comparing results from tests under flow and no-flow stream conditions. The second hypothesis was tested by comparing results from two tests under low and high streamflow conditions. Similar estimates of the hydraulic conductivity and storativity for all tests indicate the validity of the streambed leakage term. The drawdown data of the test under low- and high-flow conditions (varying W) do not follow the predictions of the analytical models, which results in inconsistent λ estimates. Thus, for different hydrologic conditions, λ cannot be scaled solely by the stream width W. One possible explanation for this result is streambed dynamics caused by the changes in the stream stage.

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Résumé Les rabattements dans les nappes et cours d’eau générés par des modèles analytiques de pompages en deux dimensions dépendent des propriétés du lit du cours d’eau considéré et de sa géométrie. En utilisant les données de trois essais de pompages effectués en conditions hydrologiques contrastées, les hypothèses suivantes ont été testées: (1) un ruisseau à pénétration partielle situé à une certaine distance du puits de pompage peut être exprimé par le terme de drainance dans les modèles analytiques; (2) dans le terme de drainance du lit du cours d’eau, le coefficient de conductivité du lit λ=(WK′)/m′ représente la largeur moyenne du cours d’eau W, l’épaisseur du lit m′, et la perméabilité K′. La première hypothèse a été testée en comparant les résultats des tests en conditions d’assèchement et d’écoulement. Pour éprouver la seconde hypothèse, les résultats de deux tests en conditions d’étiage et de hautes eaux ont été comparés. Sur l’ensemble des tests, la concordance entre les perméabilités calculées a permis de valider le terme de drainance du cours d’eau. Les rabattements observés en conditions d’étiage et de hautes eaux (W variable) ne suivent pas les prédictions des modèles analytiques, fournissant au final des estimations discordantes de λ. Aussi, dans le cas de conditions hydrologiques différentes, la seule largeur du cours d’eau ne suffit pas à estimer λ. Une explication possible de ce résultat réside dans la dynamique du lit causée par les variations de niveau du cours d’eau.

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Resumen Modelos analíticos en dos dimensiones de descenso inducido por bombeo y de agotamiento de la corriente del río representan las propiedades del lecho del río y la geometría del mismo. Usando datos de tres ensayos de bombeo llevados a cabo bajo condiciones hidrológicas variadas, se probaron las siguientes hipótesis: (1) puede representarse un río parcialmente penetrante a una cierta distancia del pozo de bombeo por el término de goteo a través del lecho del río en los modelos analíticos; (2) en el término de goteo a través del lecho del río, el coeficiente de conductancia del lecho del río λ=(WK′)/m′ representa el ancho promedio del río W, la potencia del lecho del río m′, y la conductividad hidráulica K′. La primera hipótesis fue comprobada comparando los resultados de pruebas bajo condiciones de flujo y no flujo de corriente. La segunda hipótesis fue probada comparando los resultados de dos pruebas bajo condiciones de corriente altas y bajas. Estimaciones similares de la conductividad hidráulica y el almacenamiento para todas las pruebas apuntan hacia la validez del término de goteo a través del lecho del río. Los datos de descensos de la prueba bajo condiciones de flujo altos y bajos (variando W) no siguen las predicciones de los modelos analíticos, lo que resulta en estimaciones inconsistentes de λ. Así, para condiciones hidrológicas diferentes, λ no puede ser escalada solamente con el ancho de la corriente W. Una posible explicación para este resultado es la dinámica del lecho del río causada por los cambios en el nivel del río.

     

The groundwater–land-surface–atmosphere connection: Soil moisture effects on the atmospheric boundary layer in fully-coupled simulations

This study combines a variably-saturated groundwater flow model and a mesoscale atmospheric model to examine the effects of soil moisture heterogeneity on atmospheric boundary layer processes. This parallel, integrated model can simulate spatial variations in land-surface forcing driven by three-dimensional (3D) atmospheric and subsurface components. The development of atmospheric flow is studied in a series of idealized test cases with different initial soil moisture distributions generated by an offline spin-up procedure or interpolated from a coarse-resolution dataset. These test cases are performed with both the fully-coupled model (which includes 3D groundwater flow and surface water routing) and the uncoupled atmospheric model. The effects of the different soil moisture initializations and lateral subsurface and surface water flow are seen in the differences in atmospheric evolution over a 36-h period. The fully-coupled model maintains a realistic topographically-driven soil moisture distribution, while the uncoupled atmospheric model does not. Furthermore, the coupled model shows spatial and temporal correlations between surface and lower atmospheric variables and water table depth. These correlations are particularly strong during times when the land-surface temperatures trigger shifts in wind behavior, such as during early morning surface heating.     

The distribution of large mixotrophic ciliates (Stentor) in deep North Patagonian lakes (Chile): First results

This study showed first results on the geographical distribution of mixotrophic ciliates of the genus Stentor in the Chilean North Patagonian Lake District. Thirteen deep North Patagonian lakes were sampled during 2004 and 2005, and important ecological parameters as nutrients, light climate, chlorophyll a and all plankton groups were analysed. The data were evaluated using correlation, cluster and discriminant function analyses.In four of 13 lakes, Stentor was an important constituent of the plankton community and contributed significantly to the total zooplankton biomass. It coinhabited lakes with calanoid copepods (Boeckella, Tumeodiaptomus) and small cladocerans (Eubosmina, Ceriodaphnia). Stentor was negatively correlated with cyclopoid copepods and large cladocerans. Cyclopoid copepods were a very good predictor to discriminate between lakes with and without Stentor. It is suggested that cyclopoid copepods have top-down impact on Stentor.     

An inter-comparison of regional climate models for Europe: model performance in present-day climate

The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.