Estimation of drag coefficient in James River Estuary using tidal velocity data from a vessel-towed ADCP

• Published in: Journal of Geophysical Research. C. Oceans 2004-03, Vol.109 (C3), p.C03034.1-n/a
• Main Authors: Li, Chunyan, Valle-Levinson, Arnoldo, Atkinson, Larry P., Wong, Kuo Chuin, Lwiza, Kamazima M. M.
• Format: Article
• Language: English
• Subjects: analytic model; Brackish; drag coefficient; Earth, ocean, space; Exact sciences and technology; Freshwater; Marine; tides
• ISSN: 0148-0227; 2169-9275; 2156-2202; 2169-9291
• DOI: 10.1029/2003JC001991

A phase‐matching method is introduced to calculate the bottom drag coefficient in tidal channels with significant lateral variation of depth. The method is based on the fact that the bottom friction in a tidal channel causes tidal velocity to have a phase difference across the channel. The calculation involves a few steps. First, the observed horizontal velocity components are analyzed to obtain the amplitude and phase of the velocity at the major tidal frequency. The phase of the longitudinal velocity is then fitted to a relationship derived from the linearized momentum balance. The drag coefficient is then calculated. This method is applicable to narrow (approximately a few kilometers wide) tidal channels without strong stratification and where the cross‐channel variation of surface elevation is negligible compared to tidal amplitude. This analytic approach is easy to implement when appropriate observational data are available. It allows a spatial variation of the drag coefficient, and the resolution is only limited by that of the observations. The method is validated by identical twin experiments and applied to tidal velocity data, obtained in the James River Estuary, using an acoustic Doppler current profiler during spring tides and neap tides in October–November 1996. The obtained bottom drag coefficient ranged from 1.2 × 10−3 to 6.9 × 10−3 at different positions along two cross‐channel transects each 4 km long and 2 to 14 m deep. The maximum drag coefficient is found in the shallowest water near the banks of the estuary, while the minimum values occur between 9 and 12 m in the center of the channel. The friction of the lateral boundary may have contributed to the apparent increase of the bottom friction on the banks. The transverse mean values of the drag coefficient ranges between 2.2 and 2.3 × 10−3 for the spring and neap tides, respectively.