Bed load transport on the shoreface by currents and waves     

Maarten G. Kleinhans  Bart T. Grasmeijer   《Coastal Engineering》2006,53(12):983-996

Tide-driven bed load transport is an important portion of the net annual sediment transport rate in many shoreface and shelf environments. However, bed load transport under waves cannot be measured in the field and bed load transport by currents without waves is barely measurable, even in spring tidal conditions. There is, consequently, a strong lack of field data and validated models. The present field site was on the shoreface and inner shelf at 2 to 8.5 km offshore the central Dutch coast (far outside the surfzone), where tidal currents flow parallel to the coast. Bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13–18 m with fine and medium sands. The near-bed flow was measured over nearly a year and used for integration to annual transport rates. An empirical bed load model was derived, which predicts bed load transports that are a factor of > 5 smaller than predicted by existing models. However, they agree with laboratory data of sand and gravel transport in currents near incipient motion. The damped transport rates may have been caused by cohesion of sediment or turbulence damping due to mud or biological activity. The annual bed load transport rate was calculated using a probability density function (pdf) derived from the near-bed current and orbital velocity data which represented the current and wave climate well when compared to 30 years of data from a nearby wave station. The effect of wave stirring was included in the transport calculations. The net bed load transport rate is a few m²/year. This is much less than predicted in an earlier model study, which is partly due to different bed load models but also due to the difference in velocity pdf. The annual transport rate is very sensitive to the probability of the largest current velocities.  相似文献   

Globules and star formation     

Bart J. Bok 《Earth, Moon, and Planets》1978,19(2):153-155

The principal properties of dark globules are reviewed. Optical and radio (CO) data suggest masses in the range 20 to 750M , radii 0.15 to 1.2 parsecs. There is no evidence for fragmentation. The dust density inside near-spherical globules varies inversely as the cube of the distance from the center. There is evidence to show that several globules are in a state of collapse.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.Much of the work was done while the author was a Visiting Scientist at Cerro Tololo Interamerican Observatory in Chile, supported by the National Science Foundation under Contract No. NSF-C866.  相似文献   

Faulting history at the eastern termination of the High Atlas Fault (Western High Atlas,Morocco)     

Sintubin  Manuel  Nefly  Mohammed  Rijpens  Jan  Van Zegbroek  Bart 《Geologie en Mijnbouw》1997,76(3):187-195

At its eastern termination, the High Atlas Fault in the Western High Atlas in Morocco, consists of a splay of three faults. In the interjacent fault blocks, Neo- and Paleoproterozoic basement, forming the northernmost extremity of the NW-African Craton, is cropping out. The Precambrian basement witnesses a long history of brittle deformation starting at the end of the Pan-African Orogeny. A subsequent episode of normal faulting can be related to the development of a Hercynian basin along the northern passive margin of the cratonic promontory. With regard to the main tectonic activity in the Western High Atlas, basically two models exist: one emphasising block tectonics reflecting Mesozoic rifting followed by Alpine uplift and inversion, the other emphasising Late Paleozoic dextral wrench tectonics. The analysis of the fault activity along the splay faults reveals a predominantly Alpine history, consisting of the Triassic development of the Atlas Rift along the axial zone of the orogen, followed by uplift and inversion. The Late Jurassic to Cenozoic fault activity took place in a sinistral transpressive regime and was partitioned over the three splay faults. Dextral strike-slip fault activity could not be demonstrated in the fault blocks nor along the splay faults. Therefore the faults were probably not involved in Late Paleozoic dextral wrench tectonics.  相似文献   

Exploring the hydrological effects of normal faults at the boundary of the Roer Valley Graben in Belgium using a catchment-scale groundwater flow model     

Casillas-Trasvina  Alberto  Rogiers  Bart  Beerten  Koen  Wouters  Laurent  Walraevens  Kristine 《Hydrogeology Journal》2022,30(1):133-149

Hydrogeology Journal - Faults may impact regional groundwater flow and transport, so it is important to include them during aquifer-system conceptualization and while constructing groundwater flow...  相似文献   

Comments and Corrections to: “The Emperor's Old Clothes: An Inconvenient Truth About Currently Accepted Vapor Intrusion Assessment Methods,” and “Emperor's Old Clothes Revisited,” Two Recent Editorials by Mark Kram   总被引：1，自引：0，他引：1       下载免费PDF全文

Todd McAlary  Tom McHugh  Bart Eklund  Chris Lutes  Eric Suuberg  Heidi Hayes  Kelly G. Pennell  David Folkes  Helen Dawson  Robert Truesdale  Lila Beckley  Chase Holton 《Ground Water Monitoring & Remediation》2016,36(3):84-87

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Biomarkers in upper Holocene Eastern North Sea and Wadden Sea sediments     

Bart E. van Dongen  W. Irene C. Rijpstra  Catharina J. M. Philippart  Jan W. de Leeuw  Jaap S. Sinninghe Damst 《Organic Geochemistry》2000,31(12)

Total extracts of sediment cores from five different stations in the North Sea and Wadden Sea were analysed for their biomarker composition. Only sediments of the Skagerrak contained significant amounts of marine biomarkers (mainly alkenones), other sites contained predominantly terrestrial biomarkers. Bioturbation in the Skagerrak is, however, far too high to determine sea surface temperature (SST) changes within short time intervals. These results indicate that biomarkers contained in these sediments are not useful to reconstruct climate fluctuations during the upper Holocene. High amounts of α-, β- and ω-hydroxy fatty acids as well as small amounts of α, β-dihydroxy fatty acids were released from the insoluble organic material of the sediments from the Wadden Sea station, indicating a significant input of the eelgrass Zostera marina. This was confirmed by microscopic observations. This is the first time the α,β-dihydroxy fatty acids have been found in a sediment core and they have proven to be potential biomarkers for these seagrass species.  相似文献   

Oblique aggradation: a novel explanation for sinuosity of low‐energy streams in peat‐filled valley systems          下载免费PDF全文

Jasper H.J. Candel  Bart Makaske  Joep E.A. Storms  Jakob Wallinga 《地球表面变化过程与地形》2017,42(15):2679-2696

Low‐energy streams in peatlands often have a high sinuosity. However, it is unknown how this sinuous planform formed, since lateral migration of the channel is hindered by relatively erosion‐resistant banks. We present a conceptual model of Holocene morphodynamic evolution of a stream in a peat‐filled valley, based on a palaeohydrological reconstruction. Coring, ground‐penetrating radar (GPR) data, and ¹⁴C and OSL dating were used for the reconstruction. We found that the stream planform is partly inherited from the Late‐Glacial topography, reflecting stream morphology prior to peat growth in the valley. Most importantly, we show that aggrading streams in a peat‐filled valley combine vertical aggradation with lateral displacement caused by attraction to the sandy valley sides, which are more erodible than the co‐evally aggrading valley‐fill. Owing to this oblique aggradation in combination with floodplain widening, the stream becomes stretched out as channel reaches may alternately aggrade along opposed valley sides, resulting in increased sinuosity over time. Hence, highly sinuous planforms can form in peat‐filled valleys without the traditional morphodynamics of alluvial bed lateral migration. Improved understanding of the evolution of streams provides inspiration for stream restoration. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Assessing Methane in Shallow Groundwater in Unconventional Oil and Gas Play Areas,Eastern Kentucky          下载免费PDF全文

Junfeng Zhu  Thomas M. Parris  Charles J. Taylor  Steven E. Webb  Bart Davidson  Richard Smath  Stephen D. Richardson  Lisa J. Molofsky  Jenna S. Kromann  Ann P. Smith 《Ground water》2018,56(3):413-424

The expanding use of horizontal drilling and hydraulic fracturing technology to produce oil and gas from tight rock formations has increased public concern about potential impacts on the environment, especially on shallow drinking water aquifers. In eastern Kentucky, horizontal drilling and hydraulic fracturing have been used to develop the Berea Sandstone and the Rogersville Shale. To assess baseline groundwater chemistry and evaluate methane detected in groundwater overlying the Berea and Rogersville plays, we sampled 51 water wells and analyzed the samples for concentrations of major cations and anions, metals, dissolved methane, and other light hydrocarbon gases. In addition, the stable carbon and hydrogen isotopic composition of methane (δ¹³C‐CH₄ and δ²H‐CH₄) was analyzed for samples with methane concentration exceeding 1 mg/L. Our study indicates that methane is a relatively common constituent in shallow groundwater in eastern Kentucky, where methane was detected in 78% of the sampled wells (40 of 51 wells) with 51% of wells (26 of 51 wells) exhibiting methane concentrations above 1 mg/L. The δ¹³C‐CH₄ and δ²H‐CH₄ ranged from ?84.0‰ to ?58.3‰ and from ?246.5‰ to ?146.0‰, respectively. Isotopic analysis indicated that dissolved methane was primarily microbial in origin formed through CO₂ reduction pathway. Results from this study provide a first assessment of methane in the shallow aquifers in the Berea and Rogersville play areas and can be used as a reference to evaluate potential impacts of future horizontal drilling and hydraulic fracturing activities on groundwater quality in the region.  相似文献   

Evaluation of aDcp processing options for secondary flow identification at river junctions     

Gelare Moradi  Bart Vermeulen  Colin D. Rennie  Romain Cardot  Stuart N. Lane 《地球表面变化过程与地形》2019,44(14):2903-2921

Secondary circulation in river confluences results in a spatial and temporal variation of fluid motion and a relatively high level of morphodynamic change. Acoustic Doppler current profiler (aDcp) vessel-mounted flow measurements are now commonly used to quantify such circulation in shallow water fluvial environments. It is well established that such quantification using vessel-mounted aDcps requires repeated survey of the same cross-section. However, less attention has been given to how to process these data. Most aDcp data processing techniques make the assumption of homogeneity between the measured radial components of velocity. As acoustic beams diverge with distance from the aDcp probe, the volume of the flow that must be assumed to be homogeneous between the beams increases. In the presence of secondary circulation cells, and where there are strong rates of shear in the flow, the homogeneity assumption may not apply, especially deeper in the water column and close to the bed. To reduce dependence on this assumption, we apply a newly-established method to aDcp data obtained for two medium-sized (~60–80 m wide) gravel-bed river confluences and compare the results with those from more conventional data processing approaches. The comparison confirms that in the presence of strong shear our method produces different results to more conventional approaches. In the absence of a third set of fully independent data, we cannot demonstrate conclusively which method is best, but our method involves less averaging and so in the presence of strong shear is likely to be more reliable. We conclude that it is wise to apply both our method and more conventional methods to identify where data analysis might be impacted upon by strong shear and where inferences of secondary circulation may need to be made more cautiously. © 2019 John Wiley & Sons, Ltd.  相似文献   

Tectonics of the Virgin Islands Basin,north eastern Caribbean     

Sara Raussen  Holger Lykke‐Andersen  Antoon Kuijpers 《地学学报》2013,25(3):252-257

The tectonics of the Virgin Islands Basin are controlled by the plate boundary between the Puerto Rico‐Virgin Islands Microplate and the stable part of the Caribbean Plate. Several contradicting theories about the formation and development of this basin have previously been proposed. As part of the Danish Galathea 3 expedition, extensive marine geological investigations of the basin were carried out in March 2007 including sediment coring and acquisition of multi‐beam and two‐dimensional seismic data. This paper represents a summary of the key observations from the multi‐beam and the seismic data set. The interpretation of these observations leads to the proposition of a tectonic model for the Virgin Islands Basin. The model consists of N–S to NW–SE directed extension combined with E‐W trending sinistral strike‐slip and the new structural evidence from the Virgin Islands Basin is entirely consistent with the most recently published GPS data.  相似文献