Wind conditions 



43. Wind direction and mean scalar speed in the study area are given 

 in Figures 16 and 17. Mean annual velocities increase slightly to the south. 

 A velocity of 16 km/hour, 10 m above the ground, is required to initiate sand 

 movement. Speeds of 25 km/hour are required to sustain transport (Bagnold 

 1941). Winds at or above these speeds are predominantly onshore from the 

 northeast and occur most frequently during the winter months. The effects of 

 northwest winds, which are potentially important, may be lessened because of 

 local sheltering due to forests on the west side of the barriers (Hennigar 

 1979). 



Waves 



44. Changes in shoreline configuration result from a combination of 

 (a) wave action which mobilizes sediment and (b) wave-, wind-, and tide- 

 induced currents which transport the mobilized sediment. 



45. Wave data are available from gages situated at Virginia Beach, 

 Virginia, and Nags Head, North Carolina (Thompson 1977). The Virginia Beach 

 gage, located at a depth of 5.5 to 6 m of water msl on the north side and near 

 the seaward end of the 15th Street fishing pier, was a step resistance, staff 

 relay gage in noncontinuous operation between 1962 and 1971. At Nags Head a 

 step resistance, staff relay gage was in operation, with some short periods of 

 inoperation, between 1963 and 1972. In 1972 a continuous wire staff gage was 

 installed at a depth of 5 m of water msl on the north side and 50 m from the 

 end of Jeannettes Fishing Pier. A third gaging site, recently operational, 



is the CERC Field Research Facility, just north of Duck, North Carolina. Wave 

 data have been available from that site since 1979. 



46. The Wave Information Study, Phase III (Jensen 1983), provides hind- 

 cast wave data for 20-year time periods for the study area. Using those hind- 

 cast results, Figure 18 shows the annual cumulative significant wave height 

 distribution for waves which approach from all directions at station 81 at a 

 10-m water depth off Kitty Hawk, North Carolina. The mean and maximum sig- 

 nificant wave heights are, respectively, 0.89 m and 4.70 m. Figure 19 is a 

 wave rose diagram for the same location off Kitty Hawk showing the significant 

 wave height and direction of wave propagation for the combined 20-year hind- 

 cast data. 



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