VI. COASTAL FEATURES 



A number of selected coastal features in the study area were noted 

 during analysis of the ERTS-1 images. Interpretation of these features 

 is important to coastal engineering because it provides vital clues to 

 the littoral budget and behavior of shorelines and inlets. This section 

 treats each feature separately with accompanying ERTS-1 photos and perti- 

 nent ground- truth data. 



1. Sediment Plumes . 



Because sediment plumes act as tracers, bodies of suspended sediment 

 shown in aerial and space photos have long been used by coastal engineers 

 in interpreting current structures and estuarine flushing patterns. With 

 ERTS-1 imagery, it is possible to observe sediment plumes of areal extent 

 measuring in thousands of square miles. These sediment bodies (plumes) 

 are seen in spectral bands 4 and 5. 



Figure 3 shows bands 5, 6, and 7 of the study area observed on 4 

 December 1972 (ERTS 1134-15211) (band 4 was not reproducible). Band 5 

 reveals sediment plumes at the mouths of Carolina Beach and Masonboro 

 Inlets. The visible part of the plume at Carolina Beach Inlet is almost 

 semicircular with its longest diameter against the shoreline, measuring 

 about 2.8 nautical miles. Maximum seaward extent of the plume is about 

 2.1 nautical miles. Masonboro Inlet has a smaller, more linear plume 

 extending seaward about 1.5 nautical miles and trending southeast. 



Tide data (Table 4) obtained from a station at Masonboro Inlet indi- 

 cate an ebbtide occurrence during the ERTS-1 observation (National Oceanic 

 and Atmospheric Administration, 1972b, 1973b). Tide level was 1.9 feet 

 above mean low water (MLW) (slack waters were 4.3 and 0.3 feet above MLW, 

 respectively). Daily weather data obtained from the National Weather 

 Service Office, Wilmington, North Carolina (National Oceanic and 

 Atmospheric Administration, 1972a, 1973a), for 4 December 1972 and the 

 preceding 3 days showed zero precipitation. Hence, the sediment plumes 

 do not reflect abnormal quantities of runoff due to heavy precipitation ; 

 they are more likely normal discharges associated with ebbtide. 



The sediment plume off Carolina Beach Inlet is displaced slightly 

 toward the south, indicating the presence of a southbound current. The 

 near-semicircular configuration suggests that this current was relatively 

 weak near the inlet. There is no ground-truth data available to substan- 

 tiate the existence of a predominant southward littoral drift during the 

 observation that may be a contributing factor to this movement. Wave 

 gage data (Table 5) obtained at Wrightsville Beach for 0100, 0700, 1300, 

 and 1900 hours (e.s.t.) on 4 December 1972 show lower significant wave 

 heights and longer wave periods than the average for the month of Decem- 

 ber 1972. Therefore, wave energy was lower than average. Wave observa- 

 tion data obtained by volunteer observers at Wrightsville Beach under the 

 Beach Evaluation Program (Galvin and DeWall, 1971) managed by CERC showed 



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