Near-bed horizontal (cross-shore) and vertical velocity measurements were acquired in a laboratory wave flume over a 1:8 sloping sand beach of finite depth. Data were acquired using a three-component acoustic Doppler velocimeter to measure the velocity field close to, but at a fixed distance from the bed. The near-bed velocity field is examined as close as 1.5 cm above a trough and crest of a ripple under three different types of wave forcing (Stokes waves, Stokes groups, and irregular waves). Although both horizontal and vertical velocity measurements were made, attention is focused primarily on the vertical velocity. The results clearly indicate that the measured near-bed vertical velocity (which was outside the wave-bottom boundary layer) is distinctly nonzero and not well predicted by linear theory. Spectral and bispectral analysis techniques indicate that the vertical velocity responds differently depending on the location over a ripple, and that ripple-induced effects on the velocity field are present as high as 4–8 cm above the bed (for vortex ripples with wavelengths on the order of 8 cm and amplitudes on the order of 2 cm). At greater heights above the bed, the observed wave-induced motion is adequately predicted by the linear theory. 相似文献
The area studied is a north–south oriented, V-shaped, clayey lagoonal depression bordered by Pleistocene sands on the west
and Recent dune sands on the east. The freshwater aquifer in the area is the main source of potable water for the urban centres
of Gongoni and Timboni and the main Mombasa Salt Works processing plant. The aim of the study was to look at the impact of
sand harvesting, and possible saline contamination of the aquifer by activities at the nearby salt harvesting plant and by
seawater intrusion. The major factor abetting pollution of the freshwater aquifer is the sand harvesting activity, which exposes
the aquifer to the atmosphere. Extension of the salt works closer to the aquifer field, combined with high abstraction rates,
may also lead to saline water intrusion and contamination of the aquifer.
Received: 6 April 1999 · Accepted: 23 May 2000 相似文献
The long-term effects of sand extraction on macrozoobenthic communities were investigated in an offshore area in the Northern Adriatic Sea characterised by relict sands formed during the last Adriatic post-glacial transgression. Surveys were carried out before, during and 1, 6, 12, 18, 24 and 30 months after extraction at three impacted and seven reference stations. The operations did not influence the physical characteristics of the sediment, but they caused almost complete defaunation at dredged sites. Univariate and multivariate analyses highlighted that the macrozoobenthic community responses to the dredging operations were (1) a rapid initial recolonisation phase by the dominant taxa present before dredging, which took place 6-12 months after sand extraction; (2) a slower recovery phase, that ended 30 months after the operations, when the composition and structure of the communities were similar in the dredged and reference areas. This pattern of recolonisation-recovery fits well with the commonly encountered scenario where the substratum merely remains unchanged after marine aggregate extraction. 相似文献
Quaternary desert loess and sandstone-loessite relationships in the geological record raise questions regarding causes and mechanisms of silt formation and accretion. In the northern Sinai-Negev desert carbonate terrain, only sand abrasion in active erg could have produced the large quantities of quartzo-feldspathic silts constituting the late Quaternary northwestern Negev loess. In the continuum of source (medium to fine sand of dunes) to sink (silts in loess) the very fine sand is unaccounted for in the record. This weakens the sand abrasion model of silt formation as a global process. Here, we demonstrate that, as predicted by experiments, abrasion by advancing dunes generated large quantities of very fine sand (60-110 μm) deposited within the dune field and in close proximity downwind. This very fine sand was generated 13-11 ka, possibly synchronous with the Younger Dryas under gusty sand/dust storms in the southeastern Mediterranean and specifically in the northern Sinai-Negev erg. These very fine sands were washed down slope and filled small basins blocked by the advancing dunes; outside these sampling basins it is difficult to identify these sands as a distinct product. We conclude that ergs are mega-grinders of sand into very fine sand and silt under windy Quaternary and ancient aeolian desert environments. 相似文献
Conflicts between different interest groups for use of natural resources is one area where state geological surveys can provide
assistance. A state geological survey working within the scientific constraints of specific issues can remain objective in
its presentations and maintain the faith of both the conflicting interest groups and the public. One cannot vary from the
objective view or you will quickly be criticized. Criticism can still occur from one side of a natural resource issue as your
data might counter their views. However, the final decisions are almost always made in some legislators, or regulators, area
of responsibility. The responsibility of the state geological survey is to provide the important data that will assist in
making correct decisions. Should one party in the conflict become extreme in their demands, a potential compromise that is
beneficial to both sides can be lost. In Kansas, the classical natural resource problem of resource/recreation in a populated
area is presented as a case study. The state geological survey presented data on sand resources in the Kansas River and its
valley in northeast Kansas. That information was important to both recreation and dredging interests where the political problem
is a conflict of sand use as a construction material resource versus use of the alluvial river as an important recreation
area, especially for canoeing. However, when a reasonable compromise was near completion in the Kansas Legislature one side,
in a bold move to develop an advantage, ruined that potential for compromise.
Received: 12 May 1998 · Accepted: 6 July 1998 相似文献
Data are reported on the shrinkage and desiccation cracking exhibited by bentonite-enhanced sand mixtures (BES) upon air-drying. Mixtures containing 10 and 20% bentonite by dry weight, compacted at moisture contents ranging from 8 to 32%, were investigated. Hydraulic conductivity data for BES specimens saturated and tested immediately after compaction, and for similar specimens that had no visible damage after air-drying, are also presented.
All the mixtures exhibited volumetric shrinkage upon air-drying with the amount of shrinkage increasing with increasing moisture content during compaction. At any initial moisture content mixtures containing 20% bentonite shrink more than those containing 10% bentonite, but the shrinkage is insensitive to the compactive effort. Compacted beds of BES containing 10 and 20% bentonite exhibit no visible desiccation cracking as the top surface is dried when compacted at 15 and 14% moisture content, respectively, and only minor cracking when compacted at initial moisture contents of 20 and 15%, respectively. For the range of mixtures tested, it appears that cracking only occurs when BES undergoes more than about 4% volumetric shrinkage when air-dried. The saturated hydraulic conductivity of intact BES specimens is unaffected by a drying episode prior to testing. 相似文献