FRF CROSS-SHORE
WAVE AND CURRENT ARRAY

Figure 1. FRF cross-shore wave and current array off Duck, North Carolina

Array Configuration To collect essential data required to better understand coastal wave transformation, and facilitate development of the next generation coastal numerical wave models, the US Army Corps of Engineers has deployed a cross-shore wave and current array in the energetic shelf environment off Duck, North Carolina. As depicted in Figure 1, the array consists of 4 bottom-mounted NORTEK Acoustic Wave and Current (AWAC) sensors at depths of 5, 6.5, 8.5 and 11.2 m and two Datawell Directional Waverider buoys at 17- and 26-m depths. An instrument tower at the end of the FRF pier (8-m water depth) includes a meteorological station with a marine anemometer (RM Young Model 09101) located at an elevation of 19.5m. The wave stations are in a direct line with a 3-m discus buoy (National Data Buoy Center station 44014) that includes a meteorological station and directional wave sensor at 48-m depth. The array spans 95-km and captures all phases of wave transformation from the outer continental shelf to within the surf zone. Furthermore a the nearshore portion of the array is within field of view of a 43-m high Argus video station providing high-resolution digital color images of surf zone behavior. A profile view of the nearshore (AWAC) portion of the array appears in Figure 2.

Figure 2. Cross-shore bathymetry showing AWAC sensor placement (relative to 0-m water elevation).

Cross-Shore Wave Evolution We use swell from Hurricane Bill (August 2009) to demonstrate cross-shore measurement fidelity and characterize swell transformation as it moves into shallow water. Sample cross-shore data from this event appears in Figure 3. The lower panel shows the evolution of significant wave height (Hs) at each station over the life of the event. The effect of wave shoaling can be observed early in the record (10:00 - 17:00 on 22 Aug) as the inner 5- and 6-m stations have the highest wave heights. Near the peak of the event, when we most certainly had wave breaking over the inner stations, the influence of wave dissipation processes are evident as wave heights evolve from nearly 4 m at the outer 26- and 17-m stations to less than 3 m at the inner station. The top left panel of Figure 4 depicts the energy-frequency spectra at each station across the array during the peak of the swell event (see vertical reference line on lower panel). Evidence of wave nonlinearity appears as harmonic sub-peaks down-shifted from the dominant peak. These harmonics are likely a result of triad resonant interactions in shallow water (Holthuijsen, 2007) and are characterized by waveforms that substantially deviate from sinusoidal shapes. The transformation of wave energy across the shelf during the peak of Hurricane Bill is represented by the top right panel of Figure 3. The fraction of incoming wave energy, represented by the ratio of total energy at each station to total energy at the outer (26-m) station, is presented as a function of station depth. These observations show three different wave transformation zones. Starting with the outer 26-m station and working shoreward, Zone A (Figure 3 top right panel) represents a net energy loss by bottom friction and refraction, Zone B represents energy gain through wave shoaling, and finally Zone C represents energy loss by depth-induced breaking. As linear wave theory dictates, the existence and extent of these zones are a strong function of the incoming wave properties. For example, when the shallow water breaker index H/d does not reach the breaking threshold of approximately 0.78, Zone C is absent and the waves continue to shoal all the way up to the 5-m station.

Figure 3. Hurricane Bill cross-shore array observations. Lower panel: Wave height time series at all 6 stations. Upper left panel: Wave energy frequency spectra at event peak. Upper right panel: Attenuation of wave energy across the array at event peak. Dominant processes controlling wave energy across the array are bottom friction and refraction (zone A), shoaling (zone B), and depth breaking (zone C).