A Comparison Of Measured And Predicted Ocean Wave Kinematics

ABSTRACT Measured surface profiles and wave induced horizontal particle velocities and accelerations are compared with predictions based on linear and Stokes 5th order wave theories and on both the symmetric and irregular forms of Stream function wave theory. Stream function and Stokes wave theories yielded identical results for the wave conditions studied. Except for the highest waves, there is little difference in the correlation of measured kinematics and predictions based on linear and Stokes theories. Overall, the irregular Stream function theory provides the best fit to the data. INTRODUCTION A number of wave theories are available for predicting water particle kinematics (velocities and accelerations). These kinematics are used with appropriate drag and mass coefficients to compute the wave loads on a structure. In addition, most experimental investigations to determine drag and mass coefficients have relied on wave theories for the evaluation of the kinematics. The resulting coefficients can be dependent upon the choice of wave theory(3). For whatever purpose, the selection of a wave theory is determined by water depth and wave height and period. Often the choice is limited to those readily available to the individual user. Unfortunately, there is only limited information available concerning the accuracy of the kinematics predicted by any wave theory. Dean(4) has compared the analytical validity of several wave theories. Lambrakos and Brannon(10) did a similar comparison with the extended velocity potential theory. However, as Dean mentioned, good analytical validity does not necessarily imply good correlation with natural phenomena. Le Mehaute et al(11) conducted a wave tank study of fairly high waves in the shallow and intermediate depth range. Their data included horizontal water particle velocity distributions beneath the crest for eight cases, the distribution of vertical velocity for one case, and one measured profile. They concluded that none of the commonly used wave theories were in exceptional agreement with the data. Dean and Le Mehaute(2,6) later found that Stream function theory provided reasonable agreement with the data. Iwagaki and Sakai(9) conducted a tank study of surface profiles and horizontal particle velocities and Tsuchiya and Yamaguchi(14) studied both horizontal and vertical velocities. Chaplin(l) presented the results of a wave tank study which was primarily concerned with particle motion at the surface. Forristall(7) mentioned many of the problems encountered in both measuring and predicting storm currents and wave kinematics. He noted a considerable scatter between predicted and measured velocities with a bias towards over prediction. The study presented herein, actually only a portion of an extensive measurement and analysis program(12), was undertaken to determine how well some of the commonly used wave theories predict the kinematics produced by storm waves. MEASUREMENT PROGRAM The test structure was the CAGC Eugene Island 266F platform. It is located in the Gulf of Mexico approximately 90 miles southwest of Morgan City, Louisiana. Mean water depth is 177 feet. Two electromagnetic current meters and a Baylor 9737 wave staff were mounted in the southeast corner of the structure. Signal conditioning and recording systems were placed in an instrument house on the cellar deck.