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Acquisition System:
·        234 kHz Interferometric Swath Sonar
·        TSS DMS05 Motion Sensor (0.05 degree measuring accuracy)
·        Trimble Dual-channel RTK GPS

Bathymetry was collected using an interferometric swath sonar system integrated with a motion sensor that removed vessel motion in real-time (Figure 1) in January 2003. Dual-channel RTK GPS provided horizontal and vertical control (Figure 2). Horizontal and vertical control during the 2003 survey was obtained from the GPS base and radio station operated by the USACE-SAW at the Oak Island Coast Guard Station (OICG). Geodetic coordinates for the OICG benchmark are 33°53'34.23090"N, 78°02'05.69403"W, with a height of 14.159m. The base radio is a TrimMark IIE broadcasting at a frequency of 148.725 MHz. The GPS base receiver and radio utilized during the 2000-2002 surveys were set-up on a benchmark located on Oak Island at Fort Caswell (B/M Fort Caswell 700711.15N, 16291.68E, m, state plane NAD83; elevation 6.76 m, NGVD29). Soundings were corrected for water level fluctuations forced by astronomical tides and wind-driven tides using the vertical RTK-GPS measurements. The Coastal Oceanographics Hypack survey program was used to navigate survey track lines and log vertical tide files.

Post-processing using proprietary Submetrix Interferometric swath sonar software, RTS2000, corrected for errors associated with speed of sound variations and low-frequency vessel motion (portion not removed by motion sensor). Changes in speed of sound, generated by density stratification in the water column, were measured periodically during the survey with CTD casts. Processing algorithms within RTS2000 also provide a continual assessment of speed of sound and vessel motion artifacts and allow removal of these artifacts during processing. Processed swath sonar files were then imported to Grid2000, a proprietary Submetrix program, for data gridding. A nearest neighbor, weighted gridding algorithm determined depths at irregularly-spaced, 2 m grid nodes from swath soundings. Grid soundings were exported in ascii format as x, y, z (m, state plane and NGVD29, respectively).

These highly anisotropic soundings were then imported to Golden Software Surfer 8.0 and despiked using a standard deviation threshold followed by gridding into a regularly-spaced, rectilinear grid using a kriging algorithm weighted for anisotropic data. Bathymetric data from the USACE LARC cross-shore surveys (Figure 3), conducted from Dec. 2002-February 2003, were also imported within the x-y-z spreadsheet and gridded at 20-25 m grid nodes with the soundings collected by the interferometric system. The combined survey track lines from the LARC and interferometric system are shown in Figure 4. A final measure of quality control was undertaken by comparing the soundings collected by the LARC with interferometric soundings. In particular, the seaward end of the LARC profiles were compared to the interferometric soundings measured along the 10-m isobath (2002/2003 survey). A Matlab script was written to compare all of the soundings found within 2 m (horizontal) and a mean depth difference as well as a standard deviation of these comparisons was calculated. Of the nearly 54,000 soundings collected by the LARC, approximately 890 fell within 2 m of an interferometric depth. This comparison yielded a very satisfactory result wherein the average depth difference was 4.9 cm with a standard deviation of 18.6 cm. Contour and 3-D surface plots were generated from Surfer grids of the combined LARC and interferometric data and exported as *.tiff image files (Appendix 1). Finally, x, y, z data in ascii format were exported from surfer grid files at a 20-25 m grid node spacing. All image files and ascii data files are posted on the USACE FRF website (www.frf.usace.army.mil/capefear).

Final processed and gridded soundings from the combined interferometric and LARC surveys (referred to as the 2002/2003 survey) are provided as an x-y-z ascii spreadsheet (filename: cfr etd03 20m grid xyz.dat) at a spatial interval of 25 m in North Carolina state plane, m, and NGVD29, m. Although these sounding were collected over a time span of several months and should not be used as an instantaneous measure of bathymetry or for navigation purposes, gross patterns of seafloor morphology can be seen (Figure 5). The ebb tidal delta surrounding the mouth of the Cape Fear River is shown in Figures 5 from the 2002/2003 survey. As apparent in previous years, three linear shoals compose much of the ebb tidal delta (Figure 6). Two shoals are present on the west side of the shipping channel and a well-developed flood margin channel is apparent flanking Oak Island. An annual comparison of this area is shown in Figure 7 and a change contour map illustrating the difference between the 1999/2000 to 2002/2003, where a negative value or red shows a deepening from 1999/2000 to 2002/2003 surveys as shown in Figure 8. These comparisons show a deepening of the flood margin channel on the Oak Island side and along the main shipping channel. The deepening seen in the main shipping channel resulted, in part, from dredging. Also, the shoreline of Baldhead Island adjacent to the inlet and Baldhead shoals appears to have been quite dynamic during this period displaying accretion in some areas and erosion in others (Figure 8). Lastly, the gross morphology of the ebb tidal delta appears to be largely static which suggests there have not been substantial changes in the sediment transport pathways around the ebb tidal delta since the 1999/2000 surveys.

An inset of the bathymetry surrounding the new channel at the distal end of the ebb tidal delta is shown in Figure 9; and a comparison of this region between 1999/2000 and 2002/2003 is provided in Figure 10 with a net change map presented in Figure 11. Construction of the new channel is apparent from the bathymetric charts with the 1999/2000 surveys reflecting a pre-dredge chart while the 2001/2002 were conducted at the very end of construction just prior to the cut-in with the old shipping channel. The bathymetry seen in the 2002/2003 survey, therefore, shows the morphology of this region after roughly 10 months of the new channel being opened. Surprisingly, little change appears to have occurred on the shoal between the two channels. We expect this shoal to be the most sensitive to changes in mean currents through the channels because it is located between the channels where the magnitude of mean currents around the distal end of the ebb tidal delta are the highest (see associated ADCP reports). The subsequent survey in January 2004 should provide further insight as to the evolution of the ebb tidal delta and whether or not it will likely remain static or respond more gradually to flows around both channels. In a similar manner, the larger shoals surrounding the new channel show minimal change over the three survey years and, like the minimal change near the inlet, suggest little change in sediment transport pathways along the offshore end of the ebb tidal delta.

The bathymetry near the western portion of Baldhead Island is shown on Figures 12 and 13. Although annual surveys show only endpoint differences or similarities and do not reflect the more temporally dynamic changes that may occur between surveys, these charts indicate minimal evolution of the shoals flanking the inlet in the Baldhead Island region. The shoreline, as defined by the 0-m isobath (NGVD29), indicates accretion along small portions of West Beach and the eastern portion of South Beach, while erosion is apparent along much of West Beach and a region of South Beach adjacent to Baldhead shoals. A similar inset of shorelines and attached shoals along Oak Island for the past three survey years is shown in Figure 14. Like the Baldhead Island side, there appears to be shoreline erosion along the beach flanking the inlet and a small area where the shoal is attached. The shoreline appears to have accreted across much of the south-facing beach and there has been minimal change in the nearshore bathymetry and attached shoals adjacent to Oak Island.