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Pamela Naden 《地球表面变化过程与地形》1987,12(4):353-367
A simple queueing model which generates bed topography consistent with the mechanics of gravel motion is presented. The equations on which it is based are derived from the application of simple theoretical ideas and an analysis of published flume data (Meland and Norrman, 1966; Francis, 1973; Abbott and Francis, 1977). The simulation produces an extremely variable bed topography in which at least two scales of bedform may be identified. Features of 5–10 grain wavelengths, similar to pebble clusters, are superimposed on other forms whose wavelength is up to 30–40 grains and which resemble step-pools systems and antidunes. Sediment transport shows many of the characteristics found in the field and the model is, therefore, thought to provide a useful starting point from which to examine the interaction between flow, sediment transport, and bedforms in gravel-bed and cobble streams. 相似文献
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Jian Shen Taiping Wang Julie Herman Pam Mason Gretchen L. Arnold 《Estuaries and Coasts》2008,31(4):652-663
Two distinct hypoxic patterns were revealed from high-frequency dissolved oxygen (DO) data collected from North Branch of Onancock Creek, a shallow coastal estuary of the Chesapeake Bay, from July to October 2004. Diurnal hypoxia developed associated with large DO swings during fair weather and hypoxia/anoxia developed for prolonged 2–5-day periods following rainfall events. A simplified diagnostic DO-algae model was used to investigate DO dynamics in the creek. The model results show that the modeling approach enables important features of the DO dynamics in the creek to be captured and analyzed. Large anthropogenic inputs of nutrients to the creek stimulated macroalgae blooms in the embayment. High DO production resulted in supersaturated DO in daytime, whereas DO was depleted at night as the high respiration overwhelmed the DO supply, leading to hypoxia. Unlike deep-water environments, in this shallow-water system, biological processes dominate DO variations. High macroalgae biomass interacting with low light and high temperature trigger the development of prolonged hypoxic/anoxic postrainfall events. 相似文献
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I. D. L. Foster A. L. Collins P. S. Naden D. A. Sear J. I. Jones Y. Zhang 《Journal of Paleolimnology》2011,45(2):287-306
Establishment of water quality criteria to guide catchment sediment management is required by the European Union (EU) Water
Framework Directive. The topic, however, is hotly contested among scientists and policy makers. Existing legislation with
regard to fine sediment was set by the EU Freshwater Fish Directive. Its guideline, i.e. mean annual suspended sediment concentration,
is 25 mg l−1. Such a static target fails to capture the episodic nature of sediment transport. Furthermore, application of such global
standards is inappropriate for a pollutant that is strongly controlled by spatial variation in key catchment drivers. Paleolimnology
offers an approach for assessing background sediment pressures on watercourses, enabling determination of values for times
pre-dating agricultural intensification. We propose that Modern Background Sediment Delivery to Rivers (MBSDR) across England
and Wales can be determined using paleolimnology to quantify maximum feasible sediment reduction. No management programme
should aim to reduce sediment loss to values below those resulting from background, natural physiographic and/or hydrological
controls. Lacking generic tools to quantify process linkages between sediment pressures and biological impact, we propose
that MBSDR could be taken to represent ecological demand for sediment inputs into watercourses required to support healthy
aquatic habitats. In situations where generic tools exist for coupling sediment pressures and ecological impacts, assessment
of MBSDR could be used to correct the gap between current or future projected sediment loss and biological condition. Existing
paleolimnological data on sediment yields across England and Wales are presented to illustrate the approach and provide preliminary
national estimates of MBSDR. We briefly consider the basis for reconstructing sediment yields using a paleolimnological approach
and analyse temporal trends in published sediment yield, inferred for a range of landscape types. We also attempt to correlate
sediment accumulation rates (SARs) with sediment yields to extend the MBSDR data base. Preliminary maps were generated to
identify regions where further sediment yield data are needed to produce a more robust estimate of the spatial distribution
of MBSDR across England and Wales. 相似文献
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