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101.
What controls submarine channel development and the morphology of deltas entering deep-water fjords?
Jenny A. Gales Peter J. Talling Matthieu J.B. Cartigny John Hughes Clarke Gwyn Lintern Cooper Stacey Michael A. Clare 《地球表面变化过程与地形》2019,44(2):535-551
River deltas and associated turbidity current systems produce some of the largest and most rapid sediment accumulations on our planet. These systems bury globally significant volumes of organic carbon and determine the runout distance of potentially hazardous sediment flows and the shape of their deposits. Here we seek to understand the main factors that determine the morphology of turbidity current systems linked to deltas in fjords, and why some locations have well developed submarine channels while others do not. Deltas and associated turbidity current systems are analysed initially in five fjord systems from British Columbia in Canada, and then more widely. This provides the basis for a general classification of delta and turbidity current system types, where rivers enter relatively deep (>200 m) water. Fjord-delta area is found to be strongly bimodal. Avalanching of coarse-grained bedload delivered by steep mountainous rivers produces small Gilbert-type fan deltas, whose steep gradient (11°–25°) approaches the sediment's angle of repose. Bigger fjord-head deltas are associated with much larger and finer-grained rivers. These deltas have much lower gradients (1.5°–10°) that decrease offshore in a near exponential fashion. The lengths of turbidity current channels are highly variable, even in settings fed by rivers with similar discharges. This may be due to resetting of channel systems by delta-top channel avulsions or major offshore landslides, as well as the amount and rate of sediment supplied to the delta front by rivers. © 2018 John Wiley & Sons, Ltd. 相似文献
102.
Kaptein Steven J. van de Wal Koen J. Kamp Leon P. J. Armenio Vincenzo Clercx Herman J. H. Duran-Matute Matias 《Ocean Dynamics》2020,70(1):41-56
Ocean Dynamics - One-dimensional models of exchange flows driven by horizontal density gradients are well known for performing poorly in situations with weak turbulent mixing. The main issue with... 相似文献
103.
In October 1965 and February 1966, 55 Young-pits were installed in tropical Northern Territory (NT) and temperate New South Wales (NSW). Pits were monitored in 1968, 1971 and 1974; also, for the NT only, in 1988. In each region, half of the pits are on weathered granite, and half on sandstone. Local relief is 30m or less, and slopes are up to 20°. Annual rainfall is evenly distributed in the NSW sites (800 mm a−1), but is confined to the five to six month wet season in NT (1200 mm a−1). Six pits suffered external disturbance and so were not analysed. Analysis of 160 rods in 49 undisturbed pits shows: (1) vectorial movement generally not downslope parallel to the ground surface, but dominated by a vertically downward component; (2) significant uphill and vertically upward components of movement for many rods; (3) a weak correlation between total movement and sine of slope; (4) rapid movement during 1965-68, and slow movement thereafter; (5) significantly higher creep rates on the NT granites than on all other sites, perhaps because mound-building termites are especially active there. We conclude that our data do not support soil creep models which assume that all movement is downslope and slope-parallel. Repeated long-term measurements are essential to distinguish long-term creep rates from the short-term effects of disturbance. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
104.
A new hominid skeleton from Sterkfontein Member 2 attaches to foot bones recovered from loose blocks during the 1980s and first described in 1995. Several flowstone horizons are present above and below the skeleton and have given clear palaeomagnetic signatures. Five changes in magnetic polarity have been identified; when constrained by the available biostratigraphy, this sequence can be placed confidently between 3.22 and 3.58 Ma. Interpolation of sedimentation rates over the small intervals between reversals allows this range to be reduced to 3.30–3.33 Ma. The skeleton is thus the oldest yet discovered and is considered to belong to a species of Australopithecus other than africanus. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
105.
This paper describes global (whole of Leeds) and local (super output area) analyses of the relationship between childhood obesity and many 'obesogenic environment' variables, such as deprivation, urbanisation, access to local amenities, and perceived local safety, as well as dietary and physical activity behaviours. The analyses identify the covariates with the strongest relationships with obesity, and highlight variation in these relationships across Leeds, thus identifying 'at-risk' populations. This paper seeks to demonstrate the importance of analysis at the micro-level in order to provide health planners with additional information with which to tailor interventions and health policies to prevent childhood obesity. 相似文献
106.
Karyn Morrissey Graham Clarke Dimitris Ballas Stephen Hynes Cathal O'Donoghue 《Area》2008,40(3):354-364
This paper analyses the effect accessibility has on General Practitioner (GP) utilisation rates at the sub-national level for Ireland. Specifically, the aim of this paper is to estimate whether there is an urban–rural differential in GP utilisation rates. We do this by simulating micro-level healthcare data. Using this synthetic data, simple logit models are employed to estimate the likelihood that individuals in different jurisdictions will attend a GP surgery. These individual logit estimates are then inputted into a spatial interaction model to highlight areas with low GP accessibility given their health status. The policy implications of these results are discussed in relation to both the healthcare literature and current Irish healthcare policy. 相似文献
107.
Richard M. Warwick K. Robert Clarke Paul J. Somerfield 《Marine pollution bulletin》2010,60(4):554-559
The results of manipulating the input data to AMBI are explored using various transformations of numerical species abundance (NAMBI), biomass (BAMBI) and production (PAMBI) from a variety of stations on the NE Atlantic shelf at which the pollution/disturbance status is known. There is a close agreement between the proportions of species in the five AMBI ecological groups and a phylum level meta-analysis axis of increasing environmental impact. All AMBI measures provide a better monotonic relationship with the impact axis than do traditional species diversity measures, which show higher diversity at intermediate levels of disturbance. A marginally better relationship with the impact axis of the meta-analysis is achieved by a moderate (square root) transformation of the data. ‘Production’ data (an appropriate combination of abundance and biomass information) provide only a marginal improvement on abundance data, but are ecologically and functionally much more relevant. Severe transformation of the data, culminating in presence/absence, degrades the relationship with the impact axis, but if only simple species lists are available then these may still be useable in making an environmental assessment. 相似文献
108.
Qingfeng Guan Keith C. Clarke 《International journal of geographical information science》2013,27(5):695-722
A general-purpose parallel raster processing programming library (pRPL) was developed and applied to speed up a commonly used cellular automaton model with known tractability limitations. The library is suitable for use by geographic information scientists with basic programming skills, but who lack knowledge and experience of parallel computing and programming. pRPL is a general-purpose programming library that provides generic support for raster processing, including local-scope, neighborhood-scope, regional-scope, and global-scope algorithms as long as they are parallelizable. The library also supports multilayer algorithms. Besides the standard data domain decomposition methods, pRPL provides a spatially adaptive quad-tree-based decomposition to produce more evenly distributed workloads among processors. Data parallelism and task parallelism are supported, with both static and dynamic load-balancing. By grouping processors, pRPL also supports data–task hybrid parallelism, i.e., data parallelism within a processor group and task parallelism among processor groups. pSLEUTH, a parallel version of a well-known cellular automata model for simulating urban land-use change (SLEUTH), was developed to demonstrate full utilization of the advanced features of pRPL. Experiments with real-world data sets were conducted and the performance of pSLEUTH measured. We conclude not only that pRPL greatly reduces the development complexity of implementing a parallel raster-processing algorithm, it also greatly reduces the computing time of computationally intensive raster-processing algorithms, as demonstrated with pSLEUTH. 相似文献
109.
Jeff Onsted Keith C. Clarke 《International journal of geographical information science》2013,27(5):881-898
This research sought to understand the role that differentially assessed lands (lands in the United States given tax breaks in return for their guarantee to remain in agriculture) play in influencing urban growth. Our method was to calibrate the SLEUTH urban growth model under two different conditions. The first used an excluded layer that ignored such lands, effectively rendering them available for development. The second treated those lands as totally excluded from development. Our hypothesis was that excluding those lands would yield better metrics of fit with past data. Our results validate our hypothesis since two different metrics that evaluate goodness of fit both yielded higher values when differentially assessed lands are treated as excluded. This suggests that, at least in our study area, differential assessment, which protects farm and ranch lands for tenuous periods of time, has indeed allowed farmland to resist urban development. Including differentially assessed lands also yielded very different calibrated coefficients of growth as the model tried to account for the same growth patterns over two very different excluded areas. Excluded layer design can greatly affect model behavior. Since differentially assessed lands are quite common through the United States and are often ignored in urban growth modeling, the findings of this research can assist other urban growth modelers in designing excluded layers that result in more accurate model calibration and thus forecasting. 相似文献
110.
Joseph D. Conroy Leon Boegman Hongyan Zhang William J. Edwards David A. Culver 《Aquatic Sciences - Research Across Boundaries》2011,73(2):289-304
Calculated hypolimnetic oxygen depletion (HOD) rates depend not only on environmental factors but also logistical ones. In
particular, lack of understanding of the effects of weather in addition to how sampling effort determines calculated HOD rates
complicates ecological understanding and environmental management of lake ecosystems. To better determine the roles of weather
and sampling effort, we combined (1) weekly measurements of temperature and dissolved oxygen (DO) concentrations from seven
stations in the Sandusky subbasin of Lake Erie’s central basin during 2005, (2) contemporaneous measures of storm activity
and tributary discharge, and (3) a two-dimensional coupled hydrodynamic, chemical, and biological model of Lake Erie to investigate
(1) how increased storm activity and tributary discharge affected short- (daily) and long-term (seasonal) dynamics of hypolimnetic
hypoxia, and (2) how spatial (number of sites sampled) and temporal (sampling frequency) sampling effort affected calculated
HOD rates. Our model closely replicated field-observed DO dynamics. When comparing baseline modeled dynamics to those in a
second simulation with twice the number of days with high winds, however, we found that with more storm activity (1) periods
of entrainment became more frequent, (2) the hypolimnion was warmer, (3) thermal stratification occurred 1 month later, whereas
autumnal turnover occurred at least 1 week earlier shortening the duration of stratification by 1–2 months, and (4) HOD rates
increased 12%. Further, spatial and temporal sampling intensity also affected calculated HOD rates. Consequently, adequately
quantifying actual HOD rates requires sufficient sampling effort and the particular role of weather should be assessed with
rigorous field and simulation studies, especially if HOD rates are used to indicate management success. 相似文献