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LETTER REPORT: MEAN CURRENT FLOW ACROSS THE MOUTH OF THE CAPE FEAR RIVER TIDAL INLET AND THE NEW CHANNEL REGION, 2004

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

Methodology
Acquisition System:
·        600 kHz 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 and original shipping channel using a ship-mounted ADCP. The location of the two survey transects are shown on Figures 01 and 21, respectively. The vessel steamed continuously around each transect for over 13 hours, making a complete loop every hour or less, from 15:30 11 January to 05:00 12 January 2004 around the new channel region and from 10:00 to 23:30 13 January 2004 around the inlet. This technique provides a measure of current magnitude and direction at every location 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 12-hour snapshot of currents. Each transect was run within several days of the predicted spring high tide. Wind conditions prior to the surveys were relatively light and did not likely play a significant role in the measured flows. Although only a long-term time series of currents and water level around the inlet could precisely determine the relative percentage of influence the various tidal constituents and meteorological forces (wind, discharge) may play, we believe 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. The goals motivating the design of the transect locations and the ADCP measurements are to 1) provide critical verification and calibration for numerical simulations of mean currents, 2) measure ebb/flood exchange and calculate a tidal prism, and 3) qualitatively assess changes or similarities in flow patterns around the ebb tidal delta through time.

Measurements were collected using RDI’s WinRiver acquisition software with 1-m vertical bins in bottom tracking mode. No spatial (horizontal) or temporal ensembling was conducted during acquisition, other than the vertical 1-m binning. All original raw *.000 RDI files, one for each transect, will be provided in digital format. 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 (average values from 50 m horizontal intervals), and 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

Tidal Inlet Region
Results from each transect loop are provided in both raw, binary form (RDI format) from WinRiver and processed in ASCII text with the magnitude and direction of near-bottom and near-surface flow every 50 m along the inlet mouth transect wherein the magnitude and direction of near-surface are shown as red vectors and near-bottom are shown as blue vectors. A velocity scale is shown as a 1 m/s scale arrow on the lower left of each plot. The ADCP surveys reveal mean currents that are clearly influenced by localized bathymetry – where currents generally follow the contours through the channels (Figure 2). High magnitude flows during ebb, for example, funnel through the two deepest channels – the flood margin channel along Oak Island and the main ebb channel (exemplified in Figure 3). There does, however, appear to be a region near Jay Bird Shoals where water flows into the main channel at a fairly high angle relative to the local bathymetry and likely generates substantial horizontal sheer in this region (Figure 2). This same pattern was found in the same area in previous surveys. Current magnitudes consistently exceed 1 m/s (3.3 ft/s) during peak flows and are highest in the near-surface – this difference in near-surface and near-bottom magnitudes is most pronounced during the ebb (see Figure 3 and table in Figure 5). Vertical stratification is most apparent during periods of tidal change and the flood is led by near-bottom flows along the flood margin channel flanking Oak Island (see example in Figure 18).

A comparison of flows measured along the same transect in prior surveys reveal remarkably similar magnitudes and directions. The highest magnitude mean flows measured during each of the surveys – October 2000, April 2002, March 2003, and January 2004 – are shown in Figure 5 (table) and also indicate comparable velocities of roughly 1 m/s (3.3 ft/s) during the four surveys. The tidal prism was calculated from measured flows along a transect that extends across the entire inlet mouth, including the flood margin channel along Oak Island and the main ebb channel (see Figure 1 for transect location). The bar graph in Figure 6 shows the tidal prism calculated along the cross-section from velocities measured each survey year. Consistent with the velocity comparisons, tidal prism volumes are comparable between the four surveys. Similarly, the volume of water passing the survey transect for each hour spanning the survey is shown in Figure 7 and shows very comparable volumes to prior years.

Subtle differences revealed in direct comparisons of volume between the surveys likely result from the dynamic nature of other forces that 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).  Nevertheless, numerical simulations of flow that are verified and calibrated by these ADCP surveys should provide insight as to the possible effects of channel dredging to the tidal prism and regions not covered by the ADCP transects.

Figures 8-20 show each survey transect during the 13-hour survey. These will be placed in a movie format and embedded within a powerpoint after the draft report has been reviewed.

New channel region
Results from each transect loop are provided in both raw, binary form (RDI format) from WinRiver and processed in ASCII text with the magnitude and direction of near-bottom and near-surface flow every 50 m along the new channel transect (Figure 21). As seen in the other vector plots, examples of flow around the new channel is presented as near-surface (red vectors) and near-bottom (blue vector) flows plotted every 50 m along the transects with a 1 m/s scale arrow shown on the lower left of each plot.  Flow magnitudes are less than the inlet region but velocities of around 1 m/s (3.3 ft/s) were measured in the vicinity of the channels at the height of the ebb and decreased rapidly outside the channels, particularly along the western side of the ebb tidal delta (Figure 22).  This spatial difference in current magnitude is most pronounced during the ebb (see examples in Figures 25-26) and more uniform across the ebb tidal delta during the flood (Figures 28-30).  Flow patterns appear quite similar to those measured in 2002-2003 and currents measured across this region in 2000, prior to channel dredging, are also quite similar except for the region directly over the new channel (exemplified in Figures 35-37).  Strong flows are apparent in the new channel region during the early stages of the ebb (Figures 35-37) but flow does continue in the old shipping channel during the late stages of the ebb (Figure 24-26).  Also apparent in the later stages of the ebb is the vertical stratification wherein the bottom flow is directed shoreward (flooding) along the old channel while the near-surface flows remain seaward through the old channel (Figures 25-26).

The similarities in velocities through time, especially along the ebb shoal flanking the channels, do not indicate substantial changes in the mean current regime and appear consistent with the minimal change seen in the bathymetry of the ebb tidal delta.  Figures 25-37 show each survey transect from the new channel region during the 13-hour survey. These will be placed in a movie format and embedded within a powerpoint after the draft report has been reviewed.