Inference of tidal elevation in shallow water using a vessel-towed acoustic Doppler current profiler

Vessel-towed acoustic Doppler current profilers (ADCPs) have been widely us ed to measure velocity profiles. Since the instrument is usually mounted on a catamaran floating on the surface, previous studies have used the water surface as the reference level from which the vertical coordinate for the v elocity profile is defined. However, because of the tidal oscillation, the vertical coordinate thus defined is time-dependent in an Earth-coordinate s ystem, which introduces an error to the estimated harmonic constants for th e velocity. As a result, the total transport will also be in error. This is particularly a problem in shallow waters where the tidal elevation is rela tively large. Therefore tidal elevation needs to be resolved to make a corr ect harmonic analysis for the velocity. The present study is aimed at resol ving the tidal elevation change in shallow water using a vessel-towed ADCP. Semidiurnal and diurnal tidal elevations across the lower Chesapeake Bay h ave been determined using a vessel-towed ADCP. Data from four cruises rangi ng from 25 to 92 hours in 1996 and 1997 are used. Water depth averaged ever y 30 s by the ADCP is studied by harmonic and statistical analysis. By sele cting only the data within a narrow band (similar to 320 m) over the planne d transect, we are able to improve the reliability of the data. We then gri d the depth data along the 16 km transect into 200 equal segments and use h armonic analysis to resolve the semidiurnal and diurnal tidal variations wi thin each segment. We find that (1) the depth data from the ADCP contain bo th semidiurnal and diurnal signals that can be resolved, from which the sur face elevation can be inferred, (2) the major error appears to come from sp atial variation of the depth, (3) the semidiurnal and diurnal tidal variati ons of elevation inferred over Aat bottom topography account for almost 100 % of the total variability, while those measurements over large bottom slop es account for a much lower percentage of the total variability, (4) at lea st 70% of the variability of depth can be explained by semidiurnal and diur nal tides if the bottom slope is smaller than 0.006, and (5) the spatial va riation of both amplitude and phase of the elevation along the transect app ears to be small with a slightly lower tidal amplitude at the south of the Chesapeake Bay entrance, consistent with the Coriolis effect. The inferred elevations from the ADCP readings are consistent with sea level measurement s at a tide station 10 km inside the estuary.