Laboratory measurements of wave height variations and currents along a steep-sided channel

Abstract

A small-scale physical model test was conducted in a three-dimensional shallow water wave basin to document wave height variation, velocity and current along a steep-sided channel. The purpose of the test was to provide a benchmark data set for numerical model predictions of wave transformation. A frequently applied model, REFDIF1, was used to compare with the measurements. Orbital velocities and mean currents were also measured to document any wave and bathymetry induced changes in particle velocity. The bathymetry simulated a dredged entrance channel for a harbor. Two cases were conducted using monochromatic waves with periods of 0.73 s and 1.22 s. The instantaneous free surface oscillations were measured at 405 positions using seven surface piercing wave gages. Particle velocities were measured at 49 positions with a side looking three-dimensional acoustic Doppler velocimeter. A spectral analysis was performed on each free surface time series to obtain H[m]₀, as well as first and second harmonics. The conclusions from the experiment were that the wave height data and REFDIF1 predictions exhibit good quantitative agreement approximately one wavelength after the channel onset, which indicates that the mild slope equation used by REFDIF1 accurately simulates wave transformation along a steep-sided channel. Interesting time series were generated from the steep-sided channel and large second harmonics were observed at the 1:16 slope and outside the channel. Particle velocity data showed the existence of a weak undertow, and the channel did significantly affect wave orbital velocities.