Near-Bottom Water Motion under Ocean Waves

Robert A. Grace
1977 Coastal Engineering 1976   unpublished
A two-year ocean experiment involving wave-induced forces on a test pipe mounted on the sea floor [Grace and Nicinski (1976) ] involved the measurement of various quantities other than the pipe forces per se. A pair of these involved surface wave characteristics and wave-induced water motion at the level of the pipe centerline but off to one end of the pipe. These wave-kinematics data have been combined, and the results of this work make up this paper in which the emphasis is on the
more » ... c approach to data interpretation. Presented are comparisons of the velocity and acceleration data with the predictions of Airy and stream function theories plus discussion of the dispersion of the field data. The primary intent of the paper is to suggest to designers of bottom-laid structures, such as pipes, how values of the peak velocity and maximum acceleration of the water motion associated with a non-breaking design wave of specified characteristics can be chosen. TEST SITE AND WAVE CONDITIONS A site was chosen on a moderately level area of coral rock bottom 1400 feet from the reclaimed shoreline near Kewalo Basin, the fishing and tour boat harbor for Honolulu, where the water depth was 37 feet. Peak-to-trough tidal variations in Hawaiian waters are in the l-to-2-foot range, so that the depth can be considered constant for all practical purposes. We installed at this test site various structures. The major one related to the topic of this paper consisted of a heavy base composed of steel I beams and a wave mast bolted to it. This mast consisted of two parts; the lower one remained vertical and occupied approximately half the water column, whereas the upper one tilted down when not deployed and vertically upwards when a buoyancy chamber mounted permanently on it was blown. Both 3-inch and 2-inchdiameter steel pipe were used in the mast. The graduated upper part of the mast in part resembled the mast of a sailboat. A line over a pulley enabled us to pull the top of a 15-foot-long, wire-wound electrical wave staff to the summit; an acme thread bracket well down the tilting mast portion permitted us to then tighten the staff parallel to the mast. A cable ran from an oscillator at the top of the staff over the water to the project boat where suitable power supply and recording instrumentation were available. The boat, incidentally, was a 31-foot-long catamaran with 12-foot beam, an excellent work platform for our purposes. A ducted-impeller velocity sensor was attached, by means of U bolts, to a small pipe cantilevered out from the end of the base for the 16-inch test pipe.
doi:10.1061/9780872620834.139 fatcat:uhik5lrmgbd53lqvwt5ul63thq