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LETTER REPORT: MEAN CURRENT FLOW ACROSS THE MOUTH OF THE CAPE FEAR RIVER TIDAL INLET AND THE NEW CHANNEL REGION, 2002

Dr. Jesse E. McNinch
Department of Physical Sciences
Virginia Institute of Marine Science
College of William and Mary

Methodology

Acquisition System:

  • 600 kHz Broadband RDI Acoustic Doppler Current Profiler
  • Differential GPS (Northstar)

    Mean currents were measured across the mouth of the Cape Fear River tidal inlet and the seaward portion of the ebb tidal delta around the new shipping channel using a ship-mounted ADCP. The location of the two survey transects are shown on Figure 1. The vessel steamed continuously around each transect for over 13 hours, making a complete loop every hour or less. This technique provides a measure of current magnitude and direction at discreet locations along the transect every hour and spans the periods of the primary tidal constituents (M2, S2). Other variables that typically force currents in tidal inlets, such as wind-driven flows and river discharge, are also incorporated within the 13-hour snapshot of currents. Each transect was run within several days of the predicted spring high tide and wind conditions prior to the surveys were light and did not likely play a significant role in the measured flows. A long-term time series of currents and water level around the inlet would best determine the relative influence the various tidal constituents and meteorological forces (wind, discharge) may play. We believe, nonetheless, that the transect measurements reflect near maximum magnitudes for astronomical flows and the spatial patterns seen across the transects fairly characterize recurring flow directions under similar conditions. Furthermore, the spatial coverage provided by the ADCP transects is well suited to provide calibration/verification of the modeled flows currently being simulated by USACE personnel.

    Measurements were collected using RDIís WinRiver acquisition software with 1-m vertical bins. No spatial (horizontal) or temporal ensembling was conducted during acquisition, other than the vertical 1-m binning. A Matlab script was written to convert the RDI proprietary raw binary file format to ascii text. Data were processed using Matlab and reduced to horizontal ensembles every 50 m along each transect. Near-surface and near-bottom averages were determined from an integration of the 1-m bins throughout the water column (upper half of the water column and lower half of the water column, respectively).

    Results

    Figures 2-17 show the magnitude and direction of flow for the near-surface (red vectors) and near-bottom (blue vector) every 50 m along the inlet mouth transect. Several observations are readily apparent. First, as surmised from the bathymetry of the ebb tidal delta (see bathymetry report, 2001-2002), current speed and direction are influenced by the shoals on the western flank of the delta particularly during flooding periods (transects 00-02; 11-15). Water appears to be funneled through the flood margin channel flanking Oak Island and around the two linear shoals before joining the main channel. The transect does not extend far enough seaward to measure possible flood margin channel flow that may exist in front of Baldhead Island. The large shoal that extends from Baldhead Island seaward, however, suggests that a significant tidal flow in the form of a flood margin channel does not occur on the east side of the inlet. Second, in contrast to the dispersed flow across the ebb tidal delta seen during flooding periods, much of the ebb flow appears to be concentrated in the main ebb channel. This funneling of ebb flow through the ebb channel is consistent with other observation in the tidal inlet literature. Lastly, current magnitudes consistently exceed 1 m/s (2 kts) and are highest in the near-surface. Vertical stratification is most apparent during near slack periods (transect 11).

    Cross-sectional flow across the tidal inlet is shown for each transect in Figures 18-33. Across-channel variations in flow magnitude and direction is apparent in many transects (e.g. transects 01-03) as well as vertical stratification. The northern leg of the transects extend across the entire inlet mouth, including the flood margin channel along Oak Island and the main ebb channel. Figures 34 and 35 show the measured tidal prism during the April 13, 2002 survey. These measurements indicate the flow was ebb dominated during this time, totaling roughly 4x108 cms during the ebb and 3x108 cms during the flood. The tidal prism measured in 2000 before the dredging also showed an ebb dominance but at a considerably smaller value of just under 2x108 cms. The absolute value of tidal prism volumes between 2000 and 2002, however, cannot be directly compared because the initial survey in 2000 only spanned the main shipping channel and did not extend across the entire inlet mouth. Further limitations to direct comparisons of volume are the dynamic nature of other forces which influence flow such as wind-forcing and river discharge as well as differences in astronomical tides at different times of the year and across a tidal epoch (i.e. spring tides are not necessarily equal through time). Future surveys will reoccupy the transect location established in 2002 and thus be more compatible for direct comparisons of flow patterns around the inlet mouth, particularly the ebb channel and flood margin channel. Although only a long-term time series of flow and water level measurements across the region prior to and after channel dredging would allow definitive observational evidence of changes in the tidal prism due to dredging, the spatial coverage offered by the ADCP transects will provide insight into possible changes in flow patterns across the ebb tidal delta in the future. Most importantly, these observations will serve as verification of numerical model simulations, which can, in turn, best address changes due to channel deepening from dredging.

    Flow direction and magnitude around the new channel and portions of the original shipping channel are shown in Figures 36-48. As seen in the other vector plots, near-surface (red vectors) and near-bottom (blue vector) flow is plotted every 50 m along the transect. Flow magnitudes are much reduced in this region as compared to the inlet mouth region. Velocities reach 1 m/s only in the vicinity of the old shipping channel at the height of the ebb (Figure 42, transect 06). Flow patterns also appear quite similar to those measured in 2000, prior to excavation of the new channel, except for small areas directly over the new channel. Interestingly, highest flow magnitudes appear to remain in the old shipping channel and only during transect 08 is there any evidence that the flow field is influenced by the new channel where near-bottom flow appears to lead in the flooding direction.