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排序方式: 共有186条查询结果,搜索用时 15 毫秒
181.
182.
Wilfried B. M. Ten Brinke Margriet M. Schoor Anne M. Sorber Henk J. A. Berendsen 《地球表面变化过程与地形》1998,23(9):809-824
Using aerial photographs and field measurements, sandy overbank deposits formed by the large-magnitude floods of 1993/94 and 1995 were quantified along two branches of the Dutch Rhine river system: the Waal (1993/94 and 1995) and the IJssel (1995). These deposits were laid down intermittently all along the length of these rivers on the top and landward slope of the natural levees, and covered about 4 per cent of the embanked floodplain on the Waal and about 1 per cent on the IJssel. The overbanks and transport mechanism is basically convective by nature. The spatial variability of overbank sedimentation points to the important role played by helicoidal currents in determining overbank deposition. The presence of embankments and training works appears to influence the sand transport to and morphological development of the floodplains along the Dutch Rhine river system. Overbank deposition volumes about equal present estimates of sand transport during a large-magnitude flood. It appears that studies on sand transport in the Dutch Rhine carried out so far underestimate sand transport during floods. © 1998 John Wiley & Sons, Ltd. 相似文献
183.
Ava D. Simms Colin Woodroffe Brian G. Jones Henk Heijnis Jennifer Harrison Rob A. Mann 《The Australian geographer》2008,39(4):445-465
Soil redistribution studies are important, especially in water supply catchments, because the rate at which denudation is occurring has implications for offsite water quality. However, the extent to which soil is redistributed within the landscape can be difficult to determine. This challenge can be overcome using fallout caesium-137 (137Cs). This paper describes the rates of soil loss and remobilisation in two sub-catchments within the Sydney Basin region, namely Kembla and Kentish Creeks, which drain to the Cordeaux reservoir. The total inventories of 137Cs in catchment soils were determined, a 137Cs-regression equation and a theoretical diffusion and migration model were used to established relationships between 137Cs inventories and the rates of soil loss. These relationships revealed relatively low occurrence of soil loss in Kentish Creek, but two slopes in the Kembla Creek sub-catchment had losses that appear to be moderate. However, there was no clear evidence to suggest whether slopes in upper and lower reaches of catchments had specific patterns of soil remobilisation. Qualitative categorisation of the slope elements using a 137Cs-sediment hillslope model can be a useful sentinel for land users and decision makers even if absolute rates of soil loss or gain are not certain. The findings suggest that sediments mobilised in the study sub-catchments are not likely to impact significantly on the water quality in the Cordeaux reservoir. 相似文献
184.
Karin M. H. Huijts Huib E. de Swart George P. Schramkowski Henk M. Schuttelaars 《Ocean Dynamics》2011,61(8):1067-1091
An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped
estuarine cross-sections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong
mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional
analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model.
The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter
space considered (average tidal velocity amplitude, 0.1–1 m s − 1 and maximum water depth, 10–60 m). Its skills are lower for along-channel residual flows if nonlinearities are moderate to
high (strong tides in deep estuaries) and for transverse flows and residual sediment concentrations if the Ekman number is
small (weak tides in deep estuaries). An important new aspect of the analytical model is the incorporation of tidal variations
in the across-channel density gradient, causing a double circulation pattern in the transverse flow during slack tides. The
gradient also leads to a new tidally rectified residual flow component via net advection of along-channel tidal momentum by
the density-induced transverse tidal flow. The component features landward currents in the channel and seaward currents over
the slopes and is particularly effective in deeper water. It acts jointly with components induced by horizontal density differences,
Coriolis-induced tidal rectification and Stokes discharge, resulting in different along-channel residual flow regimes. The
residual across-channel density gradient is crucial for the residual transverse circulation and for the residual sediment
concentration. The clockwise density-induced circulation traps sediment in the fresher water over the left slope (looking
up-estuary in the northern hemisphere). Model results are largely consistent with available field data of well-mixed estuaries. 相似文献
185.
186.
Andrea A. Cimatoribus Sybren S. Drijfhout Matthijs den Toom Henk A. Dijkstra 《Climate Dynamics》2012,39(9-10):2291-2306
The sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to changes in basin integrated net evaporation is highly dependent on the zonal salinity contrast at the southern border of the Atlantic. Biases in the freshwater budget strongly affect the stability of the AMOC in numerical models. The impact of these biases is investigated, by adding local anomaly patterns in the South Atlantic to the freshwater fluxes at the surface. These anomalies impact the freshwater and salt transport by the different components of the ocean circulation, in particular the basin-scale salt-advection feedback, completely changing the response of the AMOC to arbitrary perturbations. It is found that an appropriate dipole anomaly pattern at the southern border of the Atlantic Ocean can collapse the AMOC entirely even without a further hosing. The results suggest a new view on the stability of the AMOC, controlled by processes in the South Atlantic. 相似文献