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A Bispectral Analysis of
Nearshore Flow Meter Data

Devon Danielson

The movement of beach sediments, leading to erosion and accretion, has been shown by many researchers to be fundamentally associated with nearshore wave field properties such as wave skewness. With recent advancements in velocity sensors, large collections of high quality velocity data are being gathered and examined throughout the world. The data used herein, is a result of one such collaborative effort of experimenters in SandyDuck ’97. Faculty and students from Dalhousie University , the University of Manitoba and Memorial University of Newfoundland formed the Canadian part of this collaboration. Over two months of semi-continuous data were collected from five different locations in the nearshore. Two of these locations were equipped with Acoustic Doppler Velocimeter (ADV) sensors in addition to traditional Electromagnetic Current Meters (EMCM) and pressure sensors.

Using the ADV and EMCM from one of the two frames, a large dataset of concurrent and relatively collocated velocity records has been compiled. These data facilitate a detailed comparison of the instrumentation that addresses concerns over the accuracy of the EMCM’s. The results indicate very good agreement between these records. Deviations between the measurements from the two instruments are discussed and can likely be explained simply by local variations, especially vertical variations in the wave field.

Using the ADV records from over a two month period, contributions to velocity skewness are examined. The majority of the velocity skewness at this location is shown to arise from only three of a possible ten terms resulting from an expansion of the cross-shore velocity into three parts; the mean flow, the short wave part and the long wave part. The three dominant terms involve; i) the skewness from the short wave part, ii) the correlation between incident short waves and the mean flow, and iii) the correlation between the wave envelope and the long wave motion.

Also using this ADV data, parameterization techniques that attempt to relate skewness to nearshore parameters are evaluated. The main parameter used for these parameterization is the Ursell number. The formulations suggested by these parameter techniques are shown to inadequately describe the skewness. Modifications to these formulations are presented that suggest that there general forms have potential.

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