Wind Wave Frequency- Direction Spectral Measurements


Investigator

Charles E. Long

Funding Source

Directionality of Waves in Shallow Water Work Unit,

US Army Corps of Engineers, Coastal Research and Development Program

Objective

Fundamental to many nearshore processes is the frequency and directional (f-d) distribution of wind wave (0.04 - 0.32 Hz, 360 deg) energy. The primary intent of this research is to continue the ongoing collection of wind wave f-d spectra to establish a long-term wave energy climatology from measurements in 8 m water depth at the FRF.

Approach

The specific results of this study will be wind wave f-d spectra derived from an array of 15 bottom-mounted pressure sensors centered on the 8-m contour about 900 m offshore. Records of 136 min duration will be collected at 3 hourly intervals at a sampling rate of 2 Hz. Time series will be windowed and Fourier transformed to compute cross-spectra between all unique pairs of gages. Cross-spectra of pressure will be surface corrected to sea-surface displacement cross-spectra using linear wave theory, and then ensemble and band averaged for a final resolution bandwidth of 0.01 Hz (approx. 160 DOF). Directional spectral estimates will be made using the Iterative Maximum Likelihood Estimator on both the full 15-gage array (360 degree processing) and on just the 9 gages of the longshore linear array (180 degree processing). The latter calculation is necessary because refractive blurring occurs in the 360-degree analysis.

Results will be plotted and posted for immediate viewing, and spectra will be archived for electronic retrieval by all investigators. Raw time series will be archived in the routine FRF data base for use in other types of array processing (e.g., infragravity wave studies or alternative wind wave directional estimation algorithms).

For a list of addresses of the investigators involved in Duck 94 click here.