WAVE CLIMATE
An understanding of the local wave climate was developed from long-term wave hind casts and
with measurements from a directional wave located near Sebastian Inlet during the 1994-95 period.
Wave Information System (WIS) hind cast data are available from the U.S. Army Engineer Research
and Development Center, Coastal and Hydraulics Laboratory. These were generated by basin-scale
modeling of water wave heights from historical wind fields, providing the results at a number of
coastal stations, roughly following the 25-m contour (Hubertz, et. al. 1996). Significant wave
height, dominant period, and dominant direction were extracted from the Level 2 station nearest to
Sebastian Inlet, A20017 (located approximately 20 km offshore, Fig. 1).
Wave power and direction for 22 years of WIS data (1976-1998) were divided into two groups in
a joint probability analysis for Spring/Summer, between April and September, and for Fall/Winter,
between October and March. Figure 3 displays the joint probability distributions for both
Spring/Summer and Winter/Fall. The Spring/Summer season is dominated by low energy (power)
events arriving from an azimuth of 85 to 100 deg. The Fall/Winter probabilities do not present such
a coherent picture. There is a small predominance of low energy directed straight onshore, from 50
to 70 deg and a secondary peak in the same range as the predominant Spring/Summer peak. The
much lower probabilities of the low power Fall/Winter events indicate a much higher occurrence of
higher power events relative to the Spring/Summer.
If events are considered having wave power greater than 2.5x104 W/m corresponding to wave
heights greater than 1.5-2 m and periods of 5-8 sec, peak frequencies occur in a directional bin
centered around 62 deg, which is nearly shore perpendicular. If all events during the Fall/Winter
above 2.5x104 W/m are integrated with respect to power, there is nearly the same total probability of
power arriving from southerly directions as from more northerly directions. When only extreme
events are considered having wave powers greater that 8x105 W/m (corresponding to 3-m, 9-sec
waves), the peak frequency of occurrence in the Fall/Winter is from the approximately 55 deg, an
oblique angle north of the shore normal direction. After these probabilities are integrated with
respect to power, the frequency of events generating waves from north of the shore normal direction
is about 2.5 times greater compared with similar events from the south. Figure 4 presents the joint
probability of events having a power of greater than 8x105 W/m. Such extreme events from
northerly directions are mainly due to powerful temperate cyclones, or northeasters, that sometimes
spin off of the U.S. East Coast in the fall, winter, and occasionally during the early spring. The
Spring/Summer probabilities are much lower and much more scattered with respect to direction,
mainly due to tropical cyclones, which generally follow much more diverse paths compared to
extratropical storms.
The joint probability analysis of wave power versus direction reveals that modal conditions are
generally low energy waves approaching the shoreline from a quadrant that actually provides a weak
south to north longshore forcing, which is opposite that indicated by the local inlet morphology,
especially in the spring and summer months. Thus, it is the relatively infrequent powerful events,
occurring primarily between the months of October and March and arriving from the northeast that
provide the necessary longshore forcing for the net north to south transport that is apparent in this
region of central Florida (Coastal Technology Corp. 1989; Hoeke 2001).
3
Zarillo et al.
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