Analysis of Band-Passed Directional Wave Data

Abstract

The last 10 years have seen the development of a new class of acoustic Doppler systems that can measure the wave directional spectrum in addition to their classical current profiler capability. Among these systems is the Nortek AWAC (Acoustic Waves and Current), which uses three acoustic beams angled at 25 degrees from vertical for wave direction estimation and profiling current velocities. A fourth beam pointed towards the sea surface is dedicated to measuring surface displacement. This particular system has been the subject of extensive studies worldwide. As a result, we have been able to quantify the AWAC's performance characteristics compared to conventional wave buoys and other forms of wave measurements. Historically, wave data have typically been represented in the form of a series of parameters that characterize the spectral and directional nature of the sea state (e.g. significant wave height, peak period, mean period, peak direction and mean direction). In many applications, such as structural response models or in studies of coastal sediment or pollutant transport in areas with variable bottom topography, this single-parameter representation is not sufficient. Instead, a more comprehensive parameterization is required where the wave data are separated into frequency bands and the wave energy and direction is provided as a function of each band. In turn, this puts more stringent requirements on the accuracy of the wave sensing system itself, which not only has to work in a bulk sense, but also has to be able to provide a true description of the sea state even if the energy content in a given frequency band is quite low. During a recent study near the Diablo Canyon, California, a Datawell Waverider buoy was located only 20 m away from an AWAC deployed in 25 m of water. In this paper, we will describe the results of the comparison between the two instruments and discuss the implications for the possibility of using the AWAC small scale wave array to characterize long waves.