Deep Structure of the Swiss Alps,Results of NRP 20. Edited by O. A. Pfiffner,P. Lehner,P Heitzmann,St. Müller and A. Steck,Birkhauser Verlag,Basel 1997, 380pp.

Studia Geophysica et Geodaetica -     

C.—W. M. O.—O. M. M.

Abstract

The intrusion of seawater in a tidal river is treated as a diffusion problem, characterized by a coefficient of longitudinal diffusivity.

In order to analyse the longitudinal diffusivity, a mathematical model is set up, consisting of two bodies of water, either one besides the other or one on top of the other. The two bodies are assumed to move relatively to each other, as a secondary effect of the tidal flow. It is moreover assumed that there is turbulent exchange of salt between the bodies.

It is demonstrated that the diffusion of salt into the river is greatest for an optimum value of the coefficient of exchange between the two bodies.

Exchange weaker or stronger than this optimum both diminish the salt intrusion.

The theory is applied to the Rotterdam Waterway, for which estimates of the exchange are made. Estimation of the reduction of the turbulence by stratification and hence of the vertical exchange, shows that the observed strong intrusion is explainable.

Intensified vertical mixing, for instance as provoked by compressed air, need not always result in less intrusion, and hence should be considered carefully.     

Subglacial drumlins and englacial fractures at the surge-type glacier,Múlajökull,Iceland

The interaction between drumlins and overriding glacier ice is not well studied, largely due to the difficulty of identifying and accessing suitable active subglacial environments. The surge-type glacier Múlajökull, in central Iceland, overlies a known field of actively forming drumlins and therefore provides a rare opportunity to investigate the englacial structures that have developed in association with ice flow over the subglacial drumlins. In this study detailed ground-penetrating radar surveys are combined with field observations to identify clear sets of up-glacier and down-glacier dipping fractures at Múlajökull's margin. These are interpreted as conjugate shear planes or P- and R-type Reidel shears that developed and filled with saturated sediment derived from the glacier bed, during a previous surge. The fracture sets exhibit focused spatial distributions that are influenced by the subglacial topography. In particular, down-glacier dipping fractures are strongly focused over drumlin stoss slopes. These fractures, although well developed at depth, were mostly unable to transmit basal water and sediment up to the glacier surface during the surge cycle. In contrast, up-glacier dipping fractures formed over drumlin lee sides and in more gently sloping swales, and more frequently connected to the glacier surface, providing a pathway for the evacuation of basal water and water-saturated sediment. The study suggests that the subglacial drumlins under Múlajökull's margin have influenced the nature and distribution of englacial fractures, which could potentially contribute to spatial variations in basal water pressure during a surge. BGS © UKRI 2018     

Comment on Hearty,P.J. and O'Leary,M.J., 2008. Carbonate eolianites,quartz sands,and Quaternary sea-level cycles,Western Australia: A chronostratigraphic approach. Quaternary Geochronology 3: 26–55

A proposal that the yellow sands which commonly overlie Pleistocene eolianites along the west coast of Western Australia may have their origin from major extensions of inland deserts during times of severe aridity, is refuted on several grounds. The eolianites typically are deeply degraded remnants of the original coastal dunes, which contained a high content of non-calcareous detritus. Hence plentiful residual sand will accumulate during weathering of a dune. Studies on heavy mineral content of the yellow sands have shown that there is an inland ageing trend consistent with local derivation from successive dune belts, and have also pointed to local sources for yellow color in the sand, without need to postulate a distant source. Localized denudations have however occurred, during which sand is blown inland to leave tracts of bared limestone. No remnants of the alleged desert extensions exist in the hinterland of the coastal belt.     

FFECTS OF FLOW INTENSITY, OBSTACLE ALIGNMENT AND CROSS-SECTION GEOMETRY ON SCOUR AT BRIDGE ABUTMENTS"?CARDOSO, A.H., SANTOS, J.S. and ROCA, M.

1 FLOW INTENSITY, PF Local scour processes differ between clear-water (sub-threshold) and live-bed (post-threshold) conditions of sediment transport on the channel bed ahead of the bridge abutment. As described by Melville and Coleman (2000), the stage of sediment transport can be represented by the mean velocity ratio U/Uc, termed the flow intensity. For U/Uc 1, clear-water conditions pertain, while live-bed scour occurs for U/Uc > 1. The variation of local scour depth at bridge …     

Comment on “Complete Eulerian-mean tracer equation for coarse resolution OGCMs” by M. S. Dubovikov and V. M. Canuto

Dubovikov and Canuto (Dubovikov, M.S. and Canuto, V.M., Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. Astrophys. Fluid Dyn., 2006, 100, 197–214), after averaging the tracer conservation equation in density coordinates and transforming to height coordinates, concluded that present ocean models are missing important terms in their mean tracer equations. Here we point out some errors made by Dubovikov and Canuto (2006 Dubovikov, MS and Canuto, VM. 2006. Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. & Astrophys. Fluid Dynam., 100: 197–214. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) in their isopycnal averaging procedure. We draw on the temporal-residual-mean (TRM) theory and show that when the isopycnal averaging and coordinate transformation are performed correctly, the tracer equations of present ocean circulation models are recovered. We therefore conclude that present ocean circulation models are not neglecting the leading order terms identified by Dubovikov and Canuto (2006 Dubovikov, MS and Canuto, VM. 2006. Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. & Astrophys. Fluid Dynam., 100: 197–214. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]).     

Aussergewöhnliche Rauschstörung im Anschluss an einen Mögel-Dellinger-Effekt

Zusammenfassung Während der grossen Sonnen-Eruption vom 29.3.60 wurde bei Registrierungen der kosmischen Radio-Strahlung zuerst eine Abnahme beobachtet, der hohe Intensität nachfolgte, die mehrere Stunden anhielt. Die Abnahme entspricht der Mögel-Dellinger-Absorption; es ergibt sich, dass der Radio-Strahlungs-Ausbruch viel länger dauerte als der UV- oder Röntgen-Strahlungs-Ausbruch, der den Mögel Dellinger-Effekt verursachte.

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Summary Recording cosmic radio noise during the large solar eruption of march 29, 1960 a decrease of intensity has been observed at first which was followed by high intensity lasting several hours. The decrease being due to SID absorption it appears that the noise-burst lasted much longer than the uv- or x-ray eruption causing the SID.

     

Comment on Sousa, M. L. and Costa, A. C., ��Ground motion scenarios consistent with probabilistic seismic hazard disaggregation analysis. Application to Mainland Portugal��

Disaggregation of the seismic hazard has become a popular technique to convey information on the main sources contributing to the hazard at a particular site. Recently published work adopts geographic disaggregation analysis of seismic hazard as a tool to identify dominant rupture scenarios for municipalities in Mainland Portugal. The authors conclude that the seismic hazard in South and Central Portugal is dominated by the seismicity that takes place offshore, around 70km WSW of Cape S. Vicente, both for the return periods of 475 years and 975 years. Whilst recognizing the merits of the approach taken and the utility of the tools developed, we take issue with this last conclusion. We consider that the proposed disaggregation returns a picture of the biases in the hazard analysis, more than any real feature of the distribution of relevant seismogenic sources.