Aeolian sediment transport on a beach: Surface moisture, wind fetch, and mean transport

被引:189
作者
Bauer, B. O. [1 ]
Davidson-Arnott, R. G. D. [2 ]
Hesp, P. A. [3 ]
Namikas, S. L. [3 ]
Ollerhead, J. [4 ]
Walker, I. J. [5 ]
机构
[1] Univ British Columbia Okanagan, Kelowna, BC V1V 1V7, Canada
[2] Univ Guelph, Dept Geog, Guelph, ON N1G 2W1, Canada
[3] Louisiana State Univ, Dept Geog & Anthropol, Baton Rouge, LA 70803 USA
[4] Mt Allison Univ, Dept Geog, Sackville, NB E4L 1A7, Canada
[5] Univ Victoria, Dept Geog, Victoria, BC V8W 3P5, Canada
基金
加拿大自然科学与工程研究理事会;
关键词
Aeolian; Fetch effect; Wind angle; Tidal inundation; Boundary-layer adjustment; SAND TRANSPORT; BLOWN SAND; MASS FLUX; TUNNEL; ENTRAINMENT; WATER; EQUILIBRATION; THRESHOLDS; DYNAMICS; WILDWOOD;
D O I
10.1016/j.geomorph.2008.02.016
中图分类号
P9 [自然地理学];
学科分类号
0705 ; 070501 ;
摘要
Temporal and spatial changes in wind speed, wind direction, and moisture content are ubiquitous across sandy coastal beaches. Often these factors interact in unknown ways to create complexity that confounds our ability to model sediment transport at any point across the beach as well as our capacity to predict sediment delivery into the adjacent foredunes. This study was designed to measure wind flow and sediment transport over a beach and foredune at Greenwich Dunes, Prince Edward Island National Park, with the express purpose of addressing these complex interactions. Detailed measurements are reported for one stormy day, October 11, 2004, during which meteorological conditions were highly variable. Wind speed ranged from 4 ms(-1) to over 20 ms(-1), wind direction was highly oblique varying between 60 degrees and 85 degrees from shore perpendicular, and moisture content of the sand surface ranged from a minimum of about 3% (by mass) to complete saturation depending on precipitation, tidal excursion, and storm surge that progressively inundated the beach. The data indicate that short-term variations (i.e., minutes to hours) in sediment transport across this beach arise predominantly because of short-term changes in wind speed, as is expected, but also because of variations in wind direction, precipitation intensity, and tide level. Even slight increases in wind speed are capable of driving more intense saltation events, but this relationship is mediated by other factors on this characteristically narrow beach. As the angle of wind approach becomes more oblique, the fetch distance increases and allows greater opportunity for the saltation system to evolve toward an equilibrium transport state before reaching the foredunes. Whether the theoretically-predicted maximum rate of transport is ever achieved depends on the character of the sand surface (e.g., grain size, slope, roughness, vegetation, moisture content) and on various attributes of the wind field (e.g., average wind speed, unsteadiness, approach angle, flow compression, boundary layer development). Moisture content is widely acknowledged as an important factor in controlling release of sediment from the beach surface. All other things being equal, the rate of sediment transport over a wet surface is lesser than over a dry surface. On this beach, the moisture effect has two important influences: (a) in a temporal sense, the rate of sediment transport typically decreases in association with rainfall and increases when surface drying takes place; and (b) in a spatio-temporal sense, shoreline excursions associated with nearshore processes (such as wave run-up, storm surge, and tidal excursions) have the effect of constraining the fetch geometry of the beach -i.e., narrowing the width of the beach. Because saturated sand surfaces, such as found in the swash zone, will only reluctantly yield sediments to aeolian entrainment, the available beach surface across which aeolian transport can occur becomes narrower as the sea progressively inundates the beach. Under these constrained conditions, the transport system begins to shut down unless wind angle becomes highly oblique (thereby increasing fetch distance). In this study, maximum sediment transport was usually measured on the mid-beach rather than the upper beach (i.e., closer to the foredunes). This unusual finding is likely because of internal boundary layer development across the beach, which yields a decrease in near-surface wind speed (and hence, transport capacity) in the landward direction. Although widely recognized in the fluid mechanics literature, this decrease in near-surface shear stress as a by-product of a developing boundary layer in the downwind direction has not been adequately investigated in the context of coastal aeolian geomorphology. (C) 2008 Elsevier B.V. All rights reserved.
引用
收藏
页码:106 / 116
页数:11
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