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D. N. Mottershead 《地球表面变化过程与地形》1989,14(5):383-398
Rapid surface lowering of bedrock is taking place in the supratidal zone by salt spray weathering. A seven-year run of data demonstrates a mean rate of lowering of 0·625 mm a?1. Considerable variation exists in annual point lowering values within measurement sites, although between-site variation is not significant. Aggregate year to year variations in surface lowering are not significant. Spatial variation in individual point values may be compensated by temporal variation over an 11-year period. There is a marked summer maximum in surface lowering rate, and this is strongly correlated with monthly air temperature. Spatially and temporally episodic swelling of the rock surface is demonstrated. This does not correlate statistically with any available climatic variable and is deemed to be a real and largely stochastic phenomenon. It is interpreted as rock bursting at the granular scale due to haloclasty. The processes most likely to be responsible for the observed rapid denudation are crystallization and thermal expansion of halite, both of which are enhanced by high summer temperatures. 相似文献
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Martha M. Van Eerdt 《地球表面变化过程与地形》1985,10(2):95-106
Salt marsh cliff erosion in the Oosterschelde, due to basal scour and small-scale failure, was monitored during a two-year period using reference stakes. The composite marsh cliffs have a cantilever profile. Their stability is evaluated from beam failure analysis. A model is proposed, in which the cantilever weight is taken as the motive force; the major resistive force is the tensile strength. By substituting height, undermining width and soil mechanical properties of the cliff in the model, one can identify the cliffs that are likely to fail. Salt marsh cliffs, which combine a large tensile strength due to roots at the top of the profile with a large compressive stress at the cliff base due to the sandy texture of the subsoil, display the highest resistance to beam failure. The critical cliff dimensions, observed in the field, correspond with the values calculated from the proposed model of beam failure. 相似文献
235.
Long-term changes in salt marsh extent affected by channel deepening in a modified estuary 总被引:2,自引:0,他引:2
The aim of this study is to quantify the long-term (54 years) rates of marsh extension and retreat at two sites in the Westerschelde Estuary, SW Netherlands. Nine sets of aerial photographs were obtained for each of the two salt marsh sites, Zuidgors and Waarde, taken at various times between 1944 and 1998. The seaward edges of the marshes were digitised from rectified images after the photographs had been scanned and georegistered to the Dutch National Grid. Comparison of the extents of the marshes at these nine time points revealed that the areas of both marshes had fluctuated during these 54 years with periods of both extension and retreat of the seaward marsh edges. These periods of extension and retreat appeared to be approximately synchronised until the 1990s, with mean changes in position of marsh front ranging from −7.92 to 6.04 m a−1.The rate of landward retreat and seaward extension of the marsh front is shown to be related to an increase in the tidal prism brought about by dredging operations to maintain or increase the depth of the main navigable channel of the estuary. The consequent greater frequency with which the high tides reach the edge of the fringing marshes increases the risk of erosion. Strong westerly winds may also be a factor in increasing erosion. No relationship between the volume of shipping traffic using the estuary and marsh erosion was found. 相似文献
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Analyses of independent laboratory- and field-scale measurements from two sites on Sapelo Island, Georgia reveal heterogeneity in hydraulic parameters across the upland–estuary interface. Regardless of the method used (short-duration pumping tests, amplitude attenuation of tidal pumping data, sediment grain size distributions, and falling head permeameter tests), we obtain hydraulic conductivity of 10−4 m s−1 for the fine-grained, well-sorted, clean sands that make up the upland areas. Proximal to the upland–estuary boundary, the tidal pumping analyses and permeameter tests suggest that hydraulic conductivities decrease by more than two orders of magnitude, a result consistent with the presence of a clogging layer. Such a clogging layer may arise due to a variety of physical, chemical, or biological processes. The extent and orientation of the layers of reduced hydraulic conductivity near the upland–estuary boundary influence the nature of the aquifer's response to tidal forcing. Where the lower conductivity layer forms a relatively flat creek bank, tidal pumping produces a primarily mechanical response in the adjacent aquifer. Where the creek bank is nearly vertical, there is a more direct hydraulic connection between the tidal creek and the adjacent aquifer. The clogging layer likely contributes to the development of complicated flow pathways across the upland–estuary boundary. Effective flow paths calculated from tidal pumping data terminate within the marsh, beyond the boundary of the upland aquifer, suggesting a diffuse regime of groundwater discharge in the marsh. We postulate that, in many settings, submarsh flow may be as important as seepage faces for groundwater discharge into the marsh–estuary complex. 相似文献
238.
负压地层形成机制及其对油气藏的影响 总被引:10,自引:1,他引:10
具异常压力场特征的盆地在我国比较普遍,但更多的是研究超压体系,对于异常低压体系的成因与机制在国内还没有很好的解释。盆地内的广泛沉积造成的巨厚沉积地层,并在随后的地层抬升过程中造成的严重剥蚀,导致剥蚀反弹现象,从而形成负压区。由于地层的温度或其他因素的影响,也会造成局部的负压现象。根据百色盆地的负压现象,从其成因着手,分析其形成过程有其对该地区油气藏的影响。负压造成流体的局部回流,形成低压封隔窗,促使油气二次运移,从而改造油气藏的分布,形成一些与负压有关的油气藏。 相似文献
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