Spatially homogeneous offshore wave conditions.
Steady waves, currents, and winds.
Linear refraction and shoaling.
Depth-uniform current.
Negligible bottom friction.
STWAVE is a half-plane model, meaning that only waves propagating toward the coast are represented. Waves
reflected from the coast or waves generated by winds blowing offshore are neglected. Wave breaking in the surf
zone limits the maximum wave height based on the local water depth and wave steepness. STWAVE is a finite-
difference model and calculates wave spectra on a rectangular grid with square grid cells. The model outputs
include zero-moment wave height, peak wave period (Tp), and mean wave direction (αm) at all grid points and two-
dimensional spectra at selected grid points.
The STWAVE bathymetry grid was developed using high-resolution data available from the FRF. The grid extends
from the shoreline to WIS Station 218 in 25-m water depth. The grid is 163 cells across the shore and 260 cells
along the shore with a resolution of 100 m. The grid is orientated with x-axis perpendicular to the shoreline (waves
from 70 deg are parallel to the grid x-axis). Input wave spectra were generated using two methods: 1) WIS spectra
were truncated to align with the STWAVE half-plane grid. Model runs using the WIS spectra as input will be
referred to as the `spectral results'. 2) WIS-generated bulk wave parameters were used to generate spectra of the
TMA shape (Bouws et al. 1985) in frequency and a cosn directional distribution, with wave height, peak period, and
mean direction defined by WIS. The values of the spectral peakedness parameters and spreading exponent used
varied with peak period as estimated by Thompson et al. (1996). Model runs using the WIS parameters as input will
be referred to as the `parametric results'. Local wind speed and direction used in the simulations were taken from
WIS Station 218. Tide elevation measured at the 8-m depth at the FRF was input as a tidal adjustment. When
measured tide was not available, the predicted tide was substituted. Currents were neglected in the simulations.
3.
COMPARISONS TO FIELD MEASUREMENTS
Wave measurements from the nearshore 15-element array of bottom-mounted pressure gauges (Gauge 111), located
north of the FRF pier at 36.1872 deg N, 75.7429 W in a depth of approximately 8 m were used for model
comparisons. The gauge array provides high-resolution directional measurements. Wave spectra are based on
8192-sec time series of data collected at 2 Hz and analyzed using the Iterative Maximum Likelihood Method (Long
and Oltman-Shay 1991, Long and Atmadja 1994). Spectral information is available every 3 hrs starting a 1 hr
GMT. Note that the measurements lag 1 hr from the WIS standard output times.
Table 2 provides error statistics for the STWAVE simulations based on the 8-m array measurements. The bias and
RMS error are defined by Equations 1 and 2, respectively. Positive biases represent underestimates by the model.
The statistics cover the full years, less 20 3-hr measurement gaps in the 8-m array data for 1997 and 15 gaps in
1998. In general, wave height is represented with similar accuracy with the parametric and spectral methods. Wave
periods and mean wave directions show significant improvement using the spectral method. Figures 1-4 show
comparisons of the measured and modeled wave heights and mean directions for April 1997 and October1998.
One-month plots are selected because full-year plots are too compressed to show the details of the comparisons.
These two months were shown because they include events with the largest errors in the model results, which will
be discussed in more detail in later sections. A wave direction of 70 deg is normal to the shoreline.
Table 2. Wave model errors based on FRF 8-m array measurements
Wave Height
Peak Wave Period
Mean Direction
Year
Method
Bias (m)
RMS Error (m)
Bias (sec)
RMS Error (sec)
Bias (deg)
RMS Error (deg)
1997
Parametric
0.17
0.40
3.0
4.5
3.2
32.1
Spectral
0.14
0.37
0.6
3.8
1.8
23.3
1998
Parametric
0.18
0.44
3.7
4.9
3.3
35.7
Spectral
0.17
0.34
1.2
3.4
2.3
22.8
Previous Page
