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LETTER REPORT: BATHYMETRY OF THE CAPE FEAR RIVER EBB
TIDAL DELTA, 2001-2002
Dr. Jesse E. McNinch
Bathymetry was collected using an interferometric swath sonar system integrated with a motion sensor that removed vessel motion in real-time. Dual-channel RTK GPS provided horizontal and vertical control. The GPS base receiver and radio were placed 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, NAVD88). Soundings were corrected for water level fluctuations forced by astronomical tides and wind-driven tides using the vertical RTK-GPS measurements. Coastal Oceanographics Hypack survey program was utilized for navigating survey track lines and logging 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). Speed of sound variations were addressed using repeated conductivity-temperature-density casts during the survey and processing algorithms across known bathymetry. 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 NAVD88, 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 surveys were also imported within the x-y-z spreadsheet and gridded with the soundings collected by the interferometric system. A final measure of quality control was undertaken by comparing the soundings collected by the LARC with the interferometric soundings. In particular, the seaward end of the LARC profiles were compared to the interferometric soundings measured along the 10-m isobath. A Matlab script was written to compare all of the soundings found within 3 m (horizontal) and to determine a mean depth difference. A standard deviation of these comparisons was calculated as well. Of the nearly 54,000 soundings collected by the LARC, approximately 600 fell within 3 m of an interferometric depth. The average depth difference was 4.2 cm with a standard deviation of 9.4 cm. This exceptional comparison, given the multitude of possible errors associated with inner-shelf bathymetry and the use of different depth measuring methodologies, suggest the two data sets can be safely combined and that collection and processing errors were minimal. Contour and 3-D surface plots were generated from Surfer grids. Final x, y, z data were exported from surfer grid files in ascii format.
Processed soundings from the LARC survey, which include speed of sound and tide corrections, are provided as an x-y-z ascii spreadsheet (Raw LARC Data). Final processed and gridded soundings from the interferometric and LARC surveys are provided as an x-y-z ascii spreadsheet (Cape Fear 20m Grid) at a spatial interval of 20 m. Gross patterns of seafloor morphology across the ebb tidal delta is shown in Figure 1. Three linear shoals compose much of the ebb tidal delta. Two shoals are present on the west side of the shipping channel, and a well-developed flood margin channel is apparent flanking Oak Island. Interestingly, there is no evidence of a flood margin channel present on the Baldhead Island side of the ebb tidal delta. This suggests that flooding currents are accommodated through the west-side flood margin channel and the main shipping channel (see Shipboard 2002 ADCP Current Results). The linear nature of the two shoals on the west side of the ebb tidal delta also suggests that mean currents are focused through the low areas or swales and are probably not flowing as a broad, evenly distributed flow across the ebb tidal delta. The presence of the new channel is also apparent in Figure 1. The sill that appears in the bathymetry between the new channel and the point of connection with the original shipping channel is not likely present any longer. The dredge was located at this position (presumable completing the last part of the dredged connection) during the two month period in which much of these soundings were collected (December 2001- January 2002).
A comparison of depths around the mouth of the Cape Fear Inlet between soundings collected in 2000 and 2001 is shown in Figure 2. The most obvious differences (shown as changes in excess of 50 cm) include: 1) deepening of the main shipping channel on the order of 1-2 m; 2) possible accretion of the southwest point of Baldhead Island; and 3) isolated areas of accretion and erosion on the western flank of the ebb tidal delta. Deepening of the main shipping channel around the inlet mouth is possibly a result of the recent dredging of the river channel between Wilmington and the mouth. An increase in basin volume would likely result in a comparable increase in flow speed creating adequate space through the inlet mouth via deepening and/or widening of the channel. The apparent accretion of the southwest corner of Baldhead Island is probably a result of beach renourishment that was placed on Baldhead Island in 2001 and suggests either a direct placement of renourish sediment at that location or a west-directed sediment transport in that immediate region. Lastly, the pockets of erosion and accretion seen on the western portion of the ebb tidal delta indicate the shoals and lows across the delta are temporally and spatially dynamic. It is unclear if, or to what degree, these changes may result from dredging activities. The dynamic nature of ebb tidal deltas, particularly sediment transport through natural bypassing via shoal migration, is well documented in the literature. Although channel deepening, which extended from Wilmington to near the inlet mouth, likely increased basin volume of the Cape Fear River and presumably increased the volume of water that must ingress/egress the inlet mouth, a clearly defined deepening of the flood margin channel is not found.
The seaward portion of the ebb tidal delta is shown in Figure 3. As expected, a comparison of the depth changes between 2000 and 2001 reflects the dredging of the new channel across the southeast region of the delta. The overall morphology of the delta, especially the flanking shoals, appears to have changed little (< 50 cm vertical). Also, a shift in sediment transport patterns that may be reflected in the morphology or migration of large-scale bedforms is not apparent. The areas of most change are the seaward-end of Jay Bird Shoals (west shoal flanking the main channel) where accretion appears to have occurred and the far seaward region of the ebb tidal delta where there may be slight deepening. Subsequent surveys in 2002-2003 may provide additional insight as to possible changes in sediment transport pathways (as defined by large-scale bedforms) and may also help establish whether the changes seen between 2000 and 2001 are sustained patterns or simply a reflection of ephemeral hydrographic conditions just prior to the surveys.