Shipboard Acoustic Doppler Current Profile Survey
Dr. Jesse E. McNinch, Virginia Institute of Marine Science
Christopher W. Freeman, UNC-Chapel Hill Institute of Marine Science
In order to quantify the tidal prism and determine the relative contribution of the primary tidal constituents (M2, S2, O1, K1, N2, M4, M6) at the entrance of the Cape Fear River, two +13-hour Acoustic Doppler Current Profile (ADCP) transects were performed during spring tidal conditions (Figure 1). The instrument deployed for this study was a 600 kHz Workhorse Rio Grande Ô manufactured by RD Instruments of San Diego California. The instrument takes advantage of BroadBand ADCP technology reducing variance in the velocity estimates by as much as 100 times when compared with narrowband ADCP’s (RDI, 1996).
The Rio Grande is a vessel-mounted class of ADCP (Figure 2) in which the instrument tracks the bottom thereby allowing the user to collect current velocities throughout the water column while steaming the course of a transect. Transects were in the form of a square or rectangular track that was passable in one hour and covered important sections of the main channel and margin areas. The ability to complete the loop of an entire transect in one hour allowed for a velocity profile at the same place along each segment of the transect over the course of a full tidal cycle (Geyer, 1993). The +13-hour transects span the primary tidal constituent periods and thus provide an accurate measurement of currents forced by astronomical tides.
Each acoustic ping was tagged with a differential corrected position from a NorthStar 12-channel DGPS receiver. Data was acquired and merged in RD Instruments and the United States Geologic Survey (USGS) WinRiver 1.02 software. Output from WinRiver was processed using harmonic analysis algorithms created in Matlab 6 software. Threshold values for good velocity measurements were based on both WinRiver’s percent good and error velocity calculations. Flow velocities were averaged horizontally over 50 m lengths of each transect and integrated vertically to determine near-surface and near-bottom velocities. Individual profiles illustrate velocity vectors at several stages of the tidal cycle and Tran1.movand Tran2.mov displays a loop of near-bottom and near-surface velocity vectors in one-hour increments over the complete tidal cycle.
Open the links below to view the velocity profiles from Transect 1 and 2 at several stages of the tidal cycle. Near-bottom velocities are shown in blue and near-surface velocities are shown in red. A maximum velocity of 1.58 m/s was reached during peak ebb stage and 1.45 m/s during the peak flood stage.
Using the shipboard ADCP measurements from September 2000 and the LARC bathymetric survey of the river mouth cross-section completed in August 2000, flood and ebb tidal prisms were calculated (Figure 3). The results agree well with previous computations by Jarrett, 1977 and is consistent with the relationships outlined in the Shore Protection Manual. Velocity profiles and tidal prism calculations will be repeated annually for the duration of the Wilmington Harbor Project. The next ADCP survey is scheduled for October 2001.
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