BOTTOM PRESSURES DUE TO LONG WAVES: LABORATORY AND FIELD MEASUREMENTS

Frederic Raichlen, Jerald D. Ramsden, James R. Walker

Abstract


A convenient and attractive way of determining the nearshore wave environment is through the use of bottom (or near bottom) pressure measurements. However, it has been realized for some time that care must be taken in interpreting such pressure measurements and relating them to the local waves amplitudes in the region where the waves become highly nonlinear and shoal near the shore, e.g., see Grace (1978), Guza and Thornton (1980), Bishop and Donelan (1987), Lee and Wang (1984), Bodge and Dean (1984), and Nielsen (1989 ) among others. This study is directed to questions raised in connection with the interpretation of the water surface time-history from bottom pressure measurements for highly nonlinear long waves. In particular the waves which were primarily of interest were large solitary and cnoidal waves, i.e., non-periodic and periodic long waves. The motivation for this work was the requirement to document the water surface-time histories of near breaking waves, nearly solitary in shape, with heights up to 2 m in a large fresh water wave basin. Wave staffs, either capacitance or resistance types, were not readily available for these measurements. In place of this technique, video recordings were made of the water surface at the location of surveyor's staff gages, but data reduction of these video recordings, except for a few cases, was estimated to be excessively time consuming. Hence, pressure transducers mounted near the bottom were installed at the wave staff locations to obtain simultaneous measurements which could be related to the variation of the water surface elevation with time.

Keywords


bottom pressure; long waves; laboratory measurements; field measurements

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