Depth Compensation for Pressure Transducer Measurements of Boat Wakes

ELLIS, J. T; SHERMAN, D. J. and BAUER, B. O., 2006. Depth compensation for pressure transducer measurements of boat wakes. Journal of Coastal Research, SI 39 (Proceedings of the 8th International Coastal Symposium), 488 492. Itajai, SC, Brazil, ISSN0749-0208. Boat-generated waves are exceedingly difficult to parameterize because every wave in a wake has a slightly different period and size. In this study, several different sets of boat waves were measured using a vertical array of three pressure transducers (mounted at 0.44 m (PT1), 1.44 m (PT 2), and 2.44 m (PT 3) below the mean water surface in 3.0 m water depth) and a capacitance-type wave gage ("wave wire"). Linear theory was used to correct the pressure signals for depth attenuation. The results were compared to surface fluctuations measured by the wave wire, which were presumed to be accurate. Wave period was estimated by calculating the period of the largest boat generated wave (the "maximum" wave) and by obtaining average periods for the largest pair, trio, quartet (and so on) of consecutive waves in a given wake. Average instrument depth-to-wavelength (d/L) ratios were 0.15 (PT 1), 0.22 (PT 2), and 0.27 (PT 3), indicating that the use of linear wave theory should be applicable. Regression analysis indicated that the average r values decrease with increasing instrument depth: 0.92 atPT 1,0.59 atPT2, and 0.40 at PT 3. The slope of the regression equation is closest to unity when the shortest wave period is used for depth compensation. Average wave height underestimations for the corrected pressure transducer records are 15% at PT 1, 48% at PT 2, and 53% at PT 3. If only the shortest wave periods are considered, the maximum wave height underestimations are 6%, 31%, and 41 % for PTs 1 -3, respectively. ADDITIONAL INDEX WORDS: Linear wave theory, study design.