Measuring short‐period wind waves in a tidally forced environment with a subsurface pressure gauge

Abstract

The ability of a current meter equipped with a standard pressure sensor to estimate the wave spectra of underdeveloped, fetch‐limited waves (peak period ~2 s) in a tidally forced, estuarine environment was investigated. Instruments of this type were placed at four heights throughout the 2.5 m water column, and data were recorded synchronously with a co‐located capacitance wave gauge. Linear wave theory was found to adequately describe the wave field in the lower frequency portion of the spectra (<0.6 Hz). The capacitance wave gauge data revealed that the equilibrium range of the spectrum could be modeled as ƒ−4. Large variability in the noise floor of the identical pressure gauges was found. The optimal cutoff frequency (beyond which the ƒ−4 model was applied) was defined as the location where the pressure spectra was 12 times the noise floor but was at least 1.1 times the peak frequency. Using these criteria, the pressure sensors measured root‐mean‐squared wave heights and mean wave periods with an uncertainty of 0.035 m and 0.13 s, respectively (95% confidence level). Applying a point measurement of the current in the transformation of the pressure spectra was found to be as reliable as applying a vertically averaged current. Using the criteria found to best estimate the wave spectra, an expression that determines the ability of a pressure sensor to measure the wave field of interest was developed. This expression enables the user to specify the wind velocity and peak wave frequency at a site and determine the maximum depth below the water surface at which the pressure gauge can be placed.