ABSTRACT: Aeolian sediment transport conditions on a beach:II. Thresholds, intermittency, and high frequency variability.

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This paper presents results from field measurements of wind flow and the intensity of sediment transport during a storm on October 11, 2004 at Greenwich Dunes, Prince Edward Island. Continuous measurements of saltation intensity on the beach were made using 5 saltation probes (safires) deployed along a shore - transect. The saltation probes were collocated with R.M. Young cup anemometers deployed at a height of 0.6 m. Additional wind speed measurements were made with a vertical array of 5 anemometers mounted on a mast located on the mid-beach and at the top of the mast at a height of 4 m. Wind speed and direction were measured with a 2-D Windsonic anemometer. The anemometers and safires were connected by cable to a notebook computer data-logging system. Instruments were sampled at a frequency of 1 Hz over an 8-hour period from 9:00-17:00. Surface moisture content and temperature on the beach were measured at a spacing of 2.5 m intervals across three profiles at 1-2 hourly intervals using a Delta-T moistureprobe. The fetch length to each instrument location was determined from the wind angle and the distance to the saturated zone. Early in the day when wind speed varied across the threshold of initiation, sediment transport was intermittent at all locations. As the storm strengthened, sediment transport was essentially continuous (but variable) on the upper beach where sand entrainment was due primarily to impact rather than fluid forces. At stations closer to the waterline where interstitial moisture content was significant, sediment transport remained intermittent throughout the day. This zone of intermittent transport migrated landward with rising water levels due to storm surge. Visual comparison of instantaneous transport intensity and wind speeds showed an obvious relationship but quantitative correlations between the two records are relatively weak, though significant. Correlations improve with transport intensity lagged by 1 s, reaffirming the observations of others that saltation responds almost instantaneously to fluctuations in wind speed. While transport intensity is generally a function of Ub or U*b, the value of b is shown to vary considerably (ranging from <2 to >5) as a function of fetch distance and depending on the time interval used for analysis.

Bibliography of Canadian Geomorphology