Moving boulders in flash floods and estimating flow conditions using boulders in ancient deposits          下载免费PDF全文

Jan Alexander  Mark J. Cooker 《Sedimentology》2016,63(6):1582-1595

Boulders moving in flash floods cause considerable damage and casualties. More and bigger boulders move in flash floods than predicted from published theory. The interpretation of flow conditions from the size of large particles within flash flood deposits has, until now, generally assumed that the velocity (or discharge) is unchanging in time (i.e. flow is steady), or changes instantaneously between periods of constant conditions. Standard practice is to apply theories developed for steady flow conditions to flash floods, which are however inherently very unsteady flows. This is likely to lead to overestimates of peak flow velocity (or discharge). Flash floods are characterised by extremely rapid variations in flow that generate significant transient forces in addition to the mean‐flow drag. These transient forces, generated by rapid velocity changes, are generally ignored in published theories, but they are briefly so large that they could initiate the motion of boulders. This paper develops a theory for the initiation of boulder movement due to the additional impulsive force generated by unsteady flow, and discusses the implications.  相似文献   

Links between circulation indices and precipitation in the Mediterranean in an ensemble of regional climate models     

Romana Beranová  Jan Kyselý 《Theoretical and Applied Climatology》2016,123(3-4):693-701

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Convective and stratiform precipitation characteristics in an ensemble of regional climate model simulations   总被引：1，自引：0，他引：1  

Jan Kyselý  Zuzana Rulfová  Aleš Farda  Martin Hanel 《Climate Dynamics》2016,46(1-2):227-243

We apply a recently proposed algorithm for disaggregating observed precipitation data into predominantly convective and stratiform, and evaluate biases in characteristics of parameterized convective (subgrid) and stratiform (large-scale) precipitation in an ensemble of 11 RCM simulations for recent climate in Central Europe. All RCMs have a resolution of 25 km and are driven by the ERA-40 reanalysis. We focus on mean annual cycle, proportion of convective precipitation, dependence on altitude, and extremes. The results show that characteristics of total precipitation are often better simulated than are those of convective and stratiform precipitation evaluated separately. While annual cycles of convective and stratiform precipitation are reproduced reasonably well in most RCMs, some of them consistently and substantially overestimate or underestimate the proportion of convective precipitation throughout the year. Intensity of convective precipitation is underestimated in all RCMs. Dependence on altitude is also simulated better for stratiform and total precipitation than for convective precipitation, for which several RCMs produce unrealistic slopes. Extremes are underestimated for convective precipitation while they tend to be slightly overestimated for stratiform precipitation, thus resulting in a relatively good reproduction of extremes in total precipitation amounts. The results suggest that the examined ensemble of RCMs suffers from substantial deficiencies in reproducing precipitation processes and support previous findings that climate models’ errors in precipitation characteristics are mainly related to deficiencies in the representation of convection.  相似文献   

High-frequency Earth rotation variations deduced from altimetry-based ocean tides     

Matthias Madzak  Michael Schindelegger  Johannes Böhm  Wolfgang Bosch  Jan Hagedoorn 《Journal of Geodesy》2016,90(11):1237-1253

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Disaster risk profiling in southern Africa: inventories,impacts and implications     

Jan de Waal  Coleen Vogel 《Natural Hazards》2016,84(3):1921-1942

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Identification of Greigite in the Pliocene Sediments of Qinghai Lake and Its Geological Significance     

FU Chaofeng  Jan BLOEMENDAL  QIANG Xiaoke  Mimi J. HILL  AN Zhisheng 《《地质学报》英文版》2016,90(6):2261-2262

正Objective The Qinghai Lake in North China is the largest interior plateau lake in Central Asia,and is sensitive to climate change and the environmental effects of Tibetan Plateau uplift.An almost continuous 626 m long sediment core was drilled in an in-filled part of the southern lake basin in the Scientific Drilling at Qinghai Lake.Previous  相似文献   

Ocean currents determine functional connectivity in an Antarctic deep‐sea shrimp          下载免费PDF全文

Johannes Dambach  Michael J. Raupach  Florian Leese  Julia Schwarzer  Jan O. Engler 《Marine Ecology》2016,37(6):1336-1344

The coherency among larval stages of marine taxa, ocean currents and population connectivity is still subject to discussion. A common view is that organisms with pelagic larval stages have higher dispersal abilities and therefore show a relatively homogeneous population genetic structure. Contrary to this, local genetic differentiation is assumed for many benthic direct developers. Specific larval or adult migratory behavior and hydrographic effects may significantly influence distribution patterns, rather than passive drifting abilities alone. The Southern Ocean is an ideal environment to test for the effects of ocean currents on population connectivity as it is characterized by several well‐defined and strong isolating current systems. In this study we studied the genetic structure of the decapod deep‐sea shrimp Nematocarcinus lanceopes, which has planktotrophic larval stages. We analysed 194 individuals from different sample localities around the Antarctic continent using nine microsatellite markers. Consistent with a previous study based on mitochondrial DNA markers, primarily weak genetic patterns among N. lanceopes populations around the continent were found. Using ocean resistance modeling approaches we were able to show that subtle genetic differences among populations are more likely explained by ocean currents rather than by geographic distance for the Atlantic Sector of the Southern Ocean.  相似文献   

Seasonal variations in the nitrogen isotopic composition of settling particles at station K2 in the western subarctic North Pacific     

Yoshihisa?MinoEmail author  Chiho?Sukigara  Makio?C.?Honda  Hajime?Kawakami  Kazuhiko?Matsumoto  Masahide?Wakita  Minoru?Kitamura  Tetsuichi?Fujiki  Kosei?Sasaoka  Osamu?Abe  Jan?Kaiser  Toshiro?Saino 《Journal of Oceanography》2016,72(6):819-836

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific Western Subarctic Gyre (WSG) revealed seasonal changes in δ ¹⁵N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting sediment traps (DST; 100–200 m) and moored sediment traps (MST; 200 and 500 m). All particles showed higher δ ¹⁵N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ ¹⁵N(SUS) of 0.4–3.1 ‰ in the euphotic zone (EZ). The δ ¹⁵N(SUS) signature was reflected by δ ¹⁵N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ ¹⁵N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ ¹⁵N(DST) variations of 2.4–7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ ¹⁵N(DST) vs. PP regression to δ ¹⁵N(MST) of 1.9–8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. This new approach to estimate productivity can be a powerful tool for further understanding of the biological pump in the WSG, even though its validity needs to be examined carefully.  相似文献   

Mapping the hydraulic connection between a coalbed and adjacent aquifer: example of the coal-seam gas resource area,north Galilee Basin,Australia     

Zhenjiao Jiang  Gregoire Mariethoz  Christoph Schrank  Malcolm Cox  Wendy Timms 《Hydrogeology Journal》2016,24(8):2143-2155

Coal-seam gas production requires groundwater extraction from coal-bearing formations to reduce the hydraulic pressure and improve gas recovery. In layered sedimentary basins, the coalbeds are often separated from freshwater aquifers by low-permeability aquitards. However, hydraulic connection between the coalbed and aquifers is possible due to the heterogeneity in the aquitard such as the existence of conductive faults or sandy channel deposits. For coal-seam gas extraction operations, it is desirable to identify areas in a basin where the probability of hydraulic connection between the coalbed and aquifers is low in order to avoid unnecessary loss of groundwater from aquifers and gas production problems. A connection indicator, the groundwater age indictor (GAI), is proposed, to quantify the degree of hydraulic connection. The spatial distribution of GAI can indicate the optimum positions for gas/water extraction in the coalbed. Depressurizing the coalbed at locations with a low GAI would result in little or no interaction with the aquifer when compared to the other positions. The concept of GAI is validated on synthetic cases and is then applied to the north Galilee Basin, Australia, to assess the degree of hydraulic connection between the Aramac Coal Measure and the water-bearing formations in the Great Artesian Basin, which are separated by an aquitard, the Betts Creek Beds. It is found that the GAI is higher in the western part of the basin, indicating a higher risk to depressurization of the coalbed in this region due to the strong hydraulic connection between the coalbed and the overlying aquifer.  相似文献   

Improved regional groundwater flow modeling using drainage features: a case study of the central northern karst aquifer system of Puerto Rico (USA)     

Reza Ghasemizadeh  Xue Yu  Christoph Butscher  Ingrid Y. Padilla  Akram Alshawabkeh 《Hydrogeology Journal》2016,24(6):1463-1478

In northern Puerto Rico (USA), subsurface conduit networks with unknown characteristics, and surface features such as springs, rivers, lagoons and wetlands, drain the coastal karst aquifers. In this study, drain lines connecting sinkholes and springs are used to improve the developed regional model by simulating the drainage effects of conduit networks. Implemented in an equivalent porous media (EPM) approach, the model with drains is able to roughly reproduce the spring discharge hydrographs in response to rainfall. Hydraulic conductivities are found to be scale dependent and significantly increase with higher test radius, indicating scale dependency of the EPM approach. Similar to other karst regions in the world, hydraulic gradients are steeper where the transmissivity is lower approaching the coastline. This study enhances current understanding of the complex flow patterns in karst aquifers and suggests that using a drainage feature improves modeling results where available data on conduit characteristics are minimal.  相似文献