Measurement and time-frequency study of nearshore wind and wave processes

Paul C. Liu and Nathan Hawley

Collaborator: David J. Schwab (Advisory)

Summary:

The project is aimed at making long-term wind and waves time series measurements to collect and monitor currently lacking wind and wave data in the nearshore area and concurrently making time-frequency analysis on the measured data to advance new developments of coastal wave processes in the unexplored time and frequency domain.

For 1998 we propose to install underwater pressure measurement using pressure sensors (e.g. Paroscientific ) deployed during the late autumn through winter and early spring period to assess the currently unknown and unexplored nearshore wave activities during winter and winter-spring transition periods. It is hoped that the winter-spring transition period measurement can also coincide with and supplementing some of the measurements conducted in the EEGLE program.

Project Rationale:

Surface gravity waves represent a primary driving force for dynamical processes in the oceans and lakes. They induce temporal and spatial intermittent energetic mixing events at the underwater boundary layer which affect redistribution of sediment, heat, and biogeochemical substances such as nutrients and pollutants. The role of surface waves and their impact on other processes, while of fundamental importance, have not received very much attention, mainly because there have been few observations available and the predominant practice of frequency domain analysis is basically unable to resolve the temporal features of an episodic process. The connection between surface wave periodicity and that of subsurface orbital velocities, which is crucial for assessing sediment re-suspension processes, is still very much inadequately understood at the present and unable to provide accurate estimates for fruitful re-suspension studies. Additionally, the modern study of wind and wave processes over the last fifty years has basically evolved from the conjecture that the random stochastic nature of waves can be considered as a composite sum of a complete spectrum of simple harmonic waves with different frequency and energy. All the available models for wind wave prediction were developed within the framework of the existence of a frequency wave spectrum. The validity of the frequency wave spectrum, however, becomes ambiguous when data stationarity and wave grouping process are contemplated. To ameliorate the currently established but conceptually inadequate state of the art, the application of time-frequency analysis to the wind waves in the nearshore area as proposed in this project would be an important first step toward reconciling the current conceptual difficulties in the conventional wind wave analysis and modeling and providing tangible wave process information for multidisciplinary studies. So instead of perpetuating established approaches that has not been fruitful, this proposal represents an investment on a promising endeavor that could potentially leading to significant enhancement of current state of the art in coastal dynamics.

Project Linkages:

The new measurements proposed in this project is in conjunction with Nathan Hawley's Sediment Resuspension and Transport Project and closely coordinated with the EEGLE program's HF Radar Observations Project and Physical Oceanography Observations Project in which in-situ wave measurements were not included.