Wave characteristics derived from video

Abstract

Collection of long-term data sets of wave characteristics and bathymetry with high resolution in time is a prerequisite to improve our understanding of nearshore coastal dynamics. Advanced video monitoring systems allow to sample these data automatically, on a long-term basis and without the need to deploy in situ instruments in a hostile environment. Recently, a technique was developed to derive wave frequency-direction spectra from time series of pixel intensities (Holman and Chickadel, 2004). A second technique (Stockdon and Holman, 2000) quantifies nearshore bathymetry from videoderived estimates of wave phase speed. Both techniques were successfully tested against field data collected at Duck (NC) and near San Diego (CA); their applicability for typical North Sea conditions as found along the Dutch coast is unknown yet. The central aim of this work is to apply and assess the applicability of the existing methods to estimate wave characteristics from video imagery with data from the Dutch coast. To achieve this central aim, the Holman and Chickadel (2004) model is used to determine directional wave spectra at Egmond aan Zee (NL) and the accuracy of the results is assessed. Furthermore a new analysis technique is developed to determine wave phase speed and corresponding bathymetry from video using cross correlation analysis, as opposed to the spectral technique developed by Stockdon and Holman (2000). The work described here is part of the ONR funded BeachWizard project (Cohen et al, 2006), which aims at the nowcasting of bathymetrical evolution through assimilation of remote sensing data and model computations of nearshore coastal processes. The analysis of wave direction, period and celerity from time stack images was performed with data collected at Egmond aan Zee along the Dutch coast. The field experiment was carried out from 20 September 2005 to 29 October 2005 in cooperation with the Utrecht University. During this field campaign data were simultaneously collected by video (half hourly time stack images) and in situ wave instruments in the intertidal zone. During the field campaign mostly mild wave conditions occurred and a complex, highly irregular beach morphology was noticed. The final dataset consists of 2.5 days of good quality time stacks at half hourly intervals and concurrent in situ measurements (water level, wave height, period and direction) at three locations, resulting in 43 time-series for ground-truthing. Wave data sampled from an offshore buoy at 21 m water depth are used for ground-truthing the deeper optical instruments (outside the intertidal area).