Leakage below dam sites in limestone terrains by enhanced karstification: a modeling approach

Unnaturally high hydraulic gradients below dam sites enhance dissolutional widening of fractures in limestone. A model is presented which suggests that under unfavorable conditions, leakage rates could become unbearably high during the lifetime of the dam. At the beginning when water is impounded, leakage rates are low and increase slowly. A positive feedback loop, however, causes a sudden dramatic increase of leakage within a few years. Deep grouting becomes necessary to prevent such failures. Inefficiencies in grouting may leave some open fractures in the grouting curtain. These fractures widen faster than pathways below the grouting curtain, and reduce the effect of the grouted region located below. Therefore, open fractures act in a similar way as reduction of the grouting depth.     

Evaluating Subsurface Parameterization to Simulate Hyporheic Exchange: The Steinlach River Test Site

Hyporheic exchange is the interaction of river water and groundwater, and is difficult to predict. One of the largest contributions to predictive uncertainty for hyporheic exchange has been attributed to the representation of heterogeneous subsurface properties. Our study evaluates the trade-offs between intrinsic (irreducible) and epistemic (reducible) model errors when choosing between homogeneous and highly complex subsurface parameter structures. We modeled the Steinlach River Test Site in Southwest Germany using a fully coupled surface water-groundwater model to simulate hyporheic exchange and to assess the predictive errors and uncertainties of transit time distributions. A highly parameterized model was built, treated as a “virtual reality” and used as a reference. We found that if the parameter structure is too simple, it will be limited by intrinsic model errors. By increasing subsurface complexity through the addition of zones or heterogeneity, we can begin to exchange intrinsic for epistemic errors. Thus, the appropriate level of detail to represent the subsurface depends on the acceptable range of intrinsic structural errors for the given modeling objectives and the available site data. We found that a zonated model is capable of reproducing the transit time distributions of a more detailed model, but only if the geological structures are known. An interpolated heterogeneous parameter field (cf. pilot points) showed the best trade-offs between the two errors, indicating fitness for practical applications. Parameter fields generated by multiple-point geostatistics (MPS) produce transit time distributions with the largest uncertainties, however, these are reducible by additional hydrogeological data, particularly flux measurements.     

Das Zirkulationstheorem von V. Bjerknes und seine Anwendung in der Ozeanographie

Zusammenfassung Aus dem Zirkulationstheorem von V. Bjerknes wird eine Formel (Seite17, Formel (6)) für die praktische Berechnung von Meeresströmungen abgeleitet, welche über jene von B. Helland-Hansen und J.W. Sandström hinausgeht. Der neuen Formel lä\t sich der Gültigkeitsbereich der alten entnehmen, wenn man die Grö\enordnung der horizontalen Scherung sowie der Divergenzen kennt.

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V. Bjerknes' circulation theorem and its application to oceanographic research

Summary From V. Bjerknes' circulation theorem a new formula (see page 17, (6)) is derived allowing practical computations of ocean currents to be made. The new formula surpasses that one given by B. Helland-Hansen and J. W. Sandström. The range of validity of the old formula can be deduced from the new one if the order of magnitude of horizontal shearing as well as that of the divergences are known.

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Le théorème de circulation d'après V. Bjerknes et son application à des recherches océanographiques

Résumé On déduit du théorème de circulation d'après V. Bjerknes une nouvelle formule (voir page 17, (6)) permettant le calcul pratique des courants océaniques. Cette nouvelle formule est supérieure à celle présentée par B. Helland-Hansen et J. W. Sandström. La portée de validité de la formule antérieure se laisse dériver de la nouvelle formule lorsqu'on connaÎt l'ordre de magnitude du cisaillement horizontal et celui des divergences.

     

On the relation between residual currents and the wind field in the German Bight

Summary The long-period part (T>25 h) of currents measured at close spacing in the German Bight in June 1968 is investigated. The north component of the bottom current is shown to be highly correlated with the east component of the wind in this area, the wind-generated changes of the mass-field giving rise to a quasi-geostrophic current near the bottom.This current responds without perceptible delay to the variations of wind fields with periods considerably longer than those of the seiches of the North Sea. The accelerations linked to such long periods are so small as to be negligable and the residual currents can be regarded as quasi-stationary.

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Über die Beziehung zwischen Reststrom und Windfeld in der Deutschen Bucht

Zusammenfassung Es werden engabständige Strommessungen in der Deutschen Bucht aus dem Juni 1968 in ihrem langperiodischen Teil (T>25 h) untersucht. Dabei ergibt sich, daß die Nordkomponente des Bodenstromes (v) stark korreliert mit der Ostkomponente des Windes (U) in diesem Gebiet. Die winderzeugten Veränderungen des Massenfeldes verursachen eine quasi-geostrophische Strömung am Boden.Diese Strömung reagiert ohne erkennbare Verzögerung auf solche Veränderungen des Windfeldes, die erheblich größere Perioden haben als die Eigenschwingungen der Nordsee. Die damit verbundenen Beschleunigungen sind sehr klein und so zu vernachlässigen; daher kann man die Restströmung auch als quasi-stationär ansehen.

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Sur la relation entre les courants résiduels et le champ du vent en baie Allemande

Résumé La partie à longue période (T>25 h) de courants measurés en des points rapprochés en baie Allemande en juin 1968 est étudiée. La composante Nord du courant de fond se révèle largement corrélée avec la composante Est du vent dans cette zone. Les modifications du champ de masse dues au vent donnent naissance à un courant quasi-géostrophique près du fond.Ce courant répond sans délai perceptible aux variations du champ du vent avec des périodes considérablement plus longues que celles des seiches de la mer du Nord. Les accélérations liées à de telles longues périodes sont si petites qu'elles peuvent être négligées et les courants peuvent être considérés comme quasi-stationnaires.

     

Buchbesprechungen

Ohne Zusammenfassung     

The use of mycorrhiza for eco‐engineering measures in steep alpine environments: effects on soil aggregate formation and fine‐root development

Steep erosion‐prone and vegetation‐free slopes are widespread in alpine areas and are often discussed since they have a high socio‐economic damage potential. We present an eco‐engineering approach to test whether a mycorrhizal inoculum improves the establishment of hedge brush layers and in turn soil structural stability on a steep, coarse‐grained vegetation‐free slope in the eastern Swiss Alps. We established (i) mycorrhizal and (ii) non‐mycorrhizal treated eco‐engineered research plots on a field experimental scale, covering a total area of approximately 1000 m² on an east‐northeast (ENE) exposed slope, where many environmental parameters can be regarded as homogeneous. After a full vegetation period, we quantified soil aggregate stability, the formation of water stable aggregates and the fine‐root development. Our results illustrate that the establishment of brush layers without mycorrhizal inoculum increased aggregate stability significantly. Against our expectation and glasshouse experiments, the addition of mycorrhizal inoculum did not have a statistically significant effect after one vegetation period although it tended to increase aggregate stability. Analogously, root length density (RLD) tended to be higher at the non‐mycorrhizal treated site. Aggregate stability was significantly correlated with RLD. Studies on a bigger field experimental scale are inevitable, complement glasshouse studies and lead to a better understanding for a successful application of sustainable eco‐engineering measures in alpine environments. Based on our results and considering the fact that the response time in natural ecosystems may be slower than in laboratory approaches, we conclude that long‐term field studies are necessary to validate results gained through laboratory experiments. Copyright © 2014 John Wiley & Sons, Ltd.     

Hotspot distribution, gravity, mantle tomography: evidence for plumes

Thermal convection is the motor of Earth dynamics and therefore is the link between plate motions, hotspots, seismic velocity variations in the mantle, and anomalies of the gravity field. Small scale mantle anomalies, such as plumes, do, however, generally escape detection by tomographic methods. It is attempted to approach the problem of detection in a somewhat statistical manner. Correlations are sought between spherical harmonic expansions of the fields under study: the hotspot distribution, mantle velocity variations, gravity, heat flow. Using spherical harmonic representations of global fields implies integration and averaging over the whole globe. Thus, although relationships may remain masked in the space domain by a multitude of effects, tendencies may become visible in the spectra or in appropriate averages.The main results are the following: There is a significant long wavelength (n=2,3) negative correlation between the hotspot density and the P-wave velocity variation in the lower mantle. Positive hotspot density of degree 2 to 9 generally correlates with low seismic velocity in all depths of the upper mantle and with positive gravity. This fits well with plume-type convection. These results are also confirmed regionally for a number of individual mid-ocean ridges and hotspots. The hotspot density and the free air anomalies are distinctly positive above regions of low velocity extending to great depth. The effect is not distinct at ridges with shallow velocity anomalies. In a general way, we suggest that the antipodal upwellings (Pacific, Africa) are divided by downwelling currents around the shrinking Pacific. Plate boundaries can easily move away from their past connections with the deeper mantle. Small scale plume currents seem to be depicted in the hotspot expansion. © 1999 Elsevier Science Ltd. All rights reserved.     

Normal Point Algorithm For Reduction Of Two Colour Slr Observations

Two colour laser ranging to artificial satellites is an attractivetechnique, which is capable to provide refraction corrected ranges without the need of an atmospheric model by measuring the dispersive delay of laser pulses of different wavelength. Although the required accuracy of the detection scheme is stringent, the technique has matured so far, that routine two colour observationsbecame feasible.The present paper describes a normal point procedure reducing two colour laser range observations with respect to the dispersive delay,exploiting the knowledge of satellite response signatures in conjunction with detector characteristics and the appropriate center of mass correction models.Moreover the dispersion model of the atmosphere is briefly reviewed, paying attention to the wavelength domains provided by modern twocolour ranging lasers, e.g., the Ti:SAP laser.Preliminary data is presented and compared to both, normal point data reduced with a standard procedure and zenith path equivalent meteorological parameters.     

Cladocerans and chironomids as indicators of lake level changes in north temperate lakes

Water level fluctuations affect the size of the pelagic zone relative to the size of littoral habitats, and thus may influence the relative abundance of remains from planktonic and littoral cladocerans in sediment. The application of this planktonic/littoral ratio for the reconstruction of past water level changes is discussed using examples of: (1) surficial profundal sediments from lakes of different water depths; (2) Holocene variation in a profundal sediment core; (3) horizontal variation in surficial sediments within a lake; and (4) long term variation in an inshore sediment core. The latter seemed to be the most promising application of this ratio. Maximum effects of water depth changes on the lake fauna are expected in the littoral zone. It is, however, difficult to read this effect directly from subfossil cladoceran and chironomid assemblages from inshore sediments as shown by a sediment profile from a site exposed to a long term decrease of water depth.