Characterization of experimental dynamos     

Nicholas L. Peffley  Alexei G. Goumilevski  A. B. Cawthrone  & Daniel P. Lathrop 《Geophysical Journal International》2000,142(1):52-58

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Chute channel dynamics in large,sand‐bed meandering rivers     

Michael Grenfell  Rolf Aalto  Andrew Nicholas 《地球表面变化过程与地形》2012,37(3):315-331

Meander bends of many large, sand‐bed meandering rivers are partitioned by chute channels that convey permanent flow, and co‐exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely‐spaced scrolls with intervening sloughs that are positively aligned with primary over‐bar flow. The rivers plot in order of increasing chute activity on an empirical meandering‐braided pattern continuum defined by potential specific stream power (ω_pv) and bedload calibre (D₅₀). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single‐thread meandering and braided. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Interaction between meander dynamics and floodplain heterogeneity in a large tropical sand‐bed river: the Rio Beni,Bolivian Amazon          下载免费PDF全文

Arved C. Schwendel  Andrew P. Nicholas  Rolf E. Aalto  Greg H. Sambrook Smith  Simon Buckley 《地球表面变化过程与地形》2015,40(15):2026-2040

The evolution of meandering river floodplains is predominantly controlled by the interplay between overbank sedimentation and channel migration. The resulting spatial heterogeneity in floodplain deposits leads to variability in bank erodibility, which in turn influences channel migration and planform development. Despite the potential significance of these feedbacks, few studies have quantified their impact upon channel evolution and floodplain construction in dynamic settings (e.g. locations characterized by rapid channel migration and high rates of overbank sedimentation). This study employs a combination of field observations, geographic information system (GIS) analysis of satellite imagery and numerical modelling to investigate these issues along a 375 km reach of the Rio Beni in the Bolivian Amazon. Results demonstrate that the occurrence of clay‐rich floodplain deposits promotes a significant reduction in channel migration rates and distinctive styles of channel evolution, including channel straightening and immobilization of bend apices leading to channel narrowing. Clay bodies act as stable locations limiting the propagation of planform disturbances in both upstream and downstream directions, and operate as ‘hinge’ points, around which the channel migrates. Spatial variations in the erodibility of clay‐rich floodplain material also promote large‐scale (10–50 km) differences in channel sinuosity and migration, although these variables are also likely to be influenced by channel gradient and tectonic effects that are difficult to quantify. Numerical model results suggest that spatial heterogeneity in bank erodibility, driven by variable bank composition, may force a substantial (c. 30%) reduction in average channel sinuosity, compared to situations in which bank strength is spatially homogeneous. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Repeat Sampling and Coliform Bacteria Detection Rates in New Jersey Domestic Wells          下载免费PDF全文

Thomas B. Atherholt  Leo R. Korn  Judith B. Louis  Nicholas A. Procopio 《Ground Water Monitoring & Remediation》2015,35(2):70-80

In compliance with the New Jersey Private Well Testing Act, 78,546 wells (93,787 samples, including samples from 13,290 wells that were analyzed more than once) were analyzed for total coliform (TC) bacteria by one or more of 39 laboratories over a 10‐year period. Samples containing TC bacteria were further analyzed for the presence of either fecal coliform or E. coli (FC/EC) bacteria. The large population of wells sampled multiple times permitted a systematic study of the effect of repeat sampling on coliform bacteria detection rates. The detection rate increased with the number of times wells were sampled. In bedrock, TC bacteria were detected in 21% of the population of wells analyzed only once, 33% in the population sampled twice, and 43% in the population sampled three times. It was estimated that TC bacteria would be detected in 90% of all wells if each well was analyzed 10 times. For FC/EC bacteria, it was estimated that 21 and 68 samples, respectively, would be required to reach the 50% and 90% population detection rates. In the Coastal Plain (CP), many more samples would be required to achieve the same estimated population detection rates. The population detection rate estimates were also dependent on the type of method used, the pH of the well water, and the geologic formation in which wells were located. A single sample was not sufficient to detect coliform bacteria when present in well water.  相似文献   

High-levels of microplastic pollution in a large,remote, mountain lake   总被引：5，自引：0，他引：5  

Christopher M. Free  Olaf P. Jensen  Sherri A. Mason  Marcus Eriksen  Nicholas J. Williamson  Bazartseren Boldgiv 《Marine pollution bulletin》2014

Despite the large and growing literature on microplastics in the ocean, little information exists on microplastics in freshwater systems. This study is the first to evaluate the abundance, distribution, and composition of pelagic microplastic pollution in a large, remote, mountain lake. We quantified pelagic microplastics and shoreline anthropogenic debris in Lake Hovsgol, Mongolia. With an average microplastic density of 20,264 particles km⁻², Lake Hovsgol is more heavily polluted with microplastics than the more developed Lakes Huron and Superior in the Laurentian Great Lakes. Fragments and films were the most abundant microplastic types; no plastic microbeads and few pellets were observed. Household plastics dominated the shoreline debris and were comprised largely of plastic bottles, fishing gear, and bags. Microplastic density decreased with distance from the southwestern shore, the most populated and accessible section of the park, and was distributed by the prevailing winds. These results demonstrate that without proper waste management, low-density populations can heavily pollute freshwater systems with consumer plastics.  相似文献   

Model‐based multi‐metric control of uniaxial shake tables          下载免费PDF全文

Brian M. Phillips  Nicholas E. Wierschem  B. F. Spencer Jr. 《地震工程与结构动力学》2014,43(5):681-699

Shake tables provide a direct means by which to evaluate structural performance under earthquake excitation. Because the entire structure is mounted on the base plate and subjected to the ground motion in real time, dynamic effects and rate‐dependent behavior can be accurately represented. Shake table control is not straightforward as the desired signal is an acceleration record, while most actuators operate in displacement feedback for stability. At the same time, the payload is typically large relative to the capacity of the actuator, leading to pronounced control‐structure interaction. Through this interaction, the dynamics of the specimen influence the dynamics of the shake table, which can be problematic when specimens change behavior because of damage or other nonlinearities. Moreover, shake tables are themselves inherently nonlinear, making it difficult to accurately recreate a desired acceleration record over a broad range of amplitudes and frequencies. A model‐based multi‐metric shake table control strategy is proposed to improve tracking of the desired acceleration of a uniaxial shake table, remaining robust to nonlinearities including changes in specimen condition. The proposed strategy is verified for the shake table testing of both linear and nonlinear structures. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Invasive crayfish as drivers of fine sediment dynamics in rivers: field and laboratory evidence          下载免费PDF全文

Gemma L Harvey  Alexander James Henshaw  Tom P Moorhouse  Nicholas J Clifford  Helen Holah  Jonathan Grey  David W Macdonald 《地球表面变化过程与地形》2014,39(2):259-271

Despite increasing recognition of the potential of aquatic biota to act as ‘geomorphic agents’, key knowledge gaps exist in relation to biotic drivers of fine sediment dynamics at microscales and particularly the role of invasive species. This study explores the impacts of invasive signal crayfish on suspended sediment dynamics at the patch scale through laboratory and field study. Three hypotheses are presented and tested: (1) that signal crayfish generate pulses of fine sediment mobilisation through burrowing and movement that are detectable in the flow field; (2) that such pulses may be more frequent during nocturnal periods when signal crayfish are known to be most active; and (3) that cumulatively the pulses would be sufficient to drive an overall increase in turbidity. Laboratory mesocosm experiments were used to explore crayfish impacts on suspended sediment concentrations for two treatments: clay banks and clay bed substrate. For the field study, high frequency near‐bed and mid‐flow turbidity time series from a lowland river with known high densities of signal crayfish were examined. Laboratory data demonstrate the direct influence of signal crayfish on mobilisation of pulses of fine sediment through burrowing into banks and fine bed material, with evidence of enhanced activity levels around the mid‐point of the nocturnal period. Similar patterns of pulsed fine sediment mobilisation identified under field conditions follow a clear nocturnal trend and appear capable of driving an increase in ambient turbidity levels. The findings indicate that signal crayfish have the potential to influence suspended sediment yields, with implications for morphological change, physical habitat quality and the transfer of nutrients and contaminants. This is particularly important given the spread of signal crayfish across Europe and their presence in extremely high densities in many catchments. Further process‐based studies are required to develop a full understanding of impacts across a range of river styles. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Numerical and experimental study of the performance of a single‐sided vibro‐impact track nonlinear energy sink          下载免费PDF全文

Jingjing Wang  Nicholas Wierschem  Billie F. Spencer Jr.  Xilin Lu 《地震工程与结构动力学》2016,45(4):635-652

This paper proposes a single‐sided vibro‐impact track nonlinear energy sink (SSVI track NES) as an effective way to mitigate the effects of impulsive and seismic excitation on building structures. The SSVI track NES is a passive energy dissipation device, which consists of a mass moving along a track, the shape of which provides a nonlinear restoring force to the mass. Previous studies have analyzed the track NES, which considers the track shape to be smooth and symmetric. By introducing a discontinuity into the shape of the track (e.g., through impact), energy in the primary structure can be scattered to higher frequency responses where it can be dissipated at a faster rate. First, the SSVI track NES is analytically investigated and numerically optimized base on a two degree‐of‐freedom primary structure. The results of numerical simulations show that the SSVI track NES can be more efficient than both the track NES and tuned mass damper in reducing the response of the primary structure. Based on the analytical studies, the SSVI track NES is experimentally realized and investigated when subjected to both impulse‐like and seismic excitations, confirming the numerical predictions and validating the analytical model of the device. Finally, the robustness of the SSVI track NES is investigated numerically. The results of this investigation indicate that the SSVI track NES remains effective over a broad range of input excitation energy levels, as well as during significant changes in the stiffness of the primary structure. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

A note on John Playfair and the statistics of directional data     

Nicholas J. Cox 《Mathematical Geology》1990,22(2):211-212

John Playfair recommended the resultant vector method of averaging directions as early as 1802.  相似文献   

Spatially explicit estimates of erosion-risk indices and variable riparian buffer widths in watersheds   总被引：3，自引：0，他引：3  

Robert?C.?WissmarEmail author  W.?Nicholas?Beer  Raymond?K.?TimmII 《Aquatic Sciences - Research Across Boundaries》2004,66(4):446-455

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