INCIDENT BOUNDARY CONDITIONS FOR WAVE TRANSFORMATION
Jane McKee Smith and Mark B. Gravens
U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory,
Vicksburg, Mississippi, USA
1.
INTRODUCTION
The nearshore wave transformation model STWAVE is used to transform hindcast wave time histories to the shore
to estimate longshore sediment transport rates. These transport rates are used to evaluate engineering design of
beach fills and coastal shore protection structures. The boundary conditions used to force STWAVE are typically
derived from the Wave Information Studies (WIS) wind-wave hindcast database. In the past, only bulk wave
parameters (height, period, and direction) were available from the database, and these parameters were used to
generate parametric spectra to drive the transformation model. Thus, the detailed spectral information from the
hindcast was lost in the transition to the nearshore. The updated WIS hindcast is archiving wave spectra, as well as
parameters, for intermediate-depth sites along the coast. The purpose of this paper is to evaluate the feasibility of
driving a nearshore model with hindcast spectra versus parameters for a two-year period at Duck, North Carolina.
Comparisons are made with nearshore gauge measurements. The relative impacts of the incident boundary input on
sediment transport estimates for the two-year period are evaluated.
2.
WAVE MODELING
The spectral wave model STWAVE is used to transformation hindcast waves from a depth of 25 m to 8 m. The
input hindcast information from WIS and the STWAVE model are discussed in the following sections.
2.1 Wave Information Studies
The role of WIS is to generate wave climatologies for U.S. coastlines to provide wave information for coastal
engineering studies. For many project sites, WIS hindcasts provide the only wave information available for design.
Even when field measurements are available, they typically lack the temporal coverage necessary to represent the
local wave climate. WIS hindcasts have covered the period 1956-1995. WIS uses the numerical wave generation
model WISWAVE (Resio 1981, Hubertz 1992) together with input wind fields and bathymetry to simulate wave
generation and propagation. WISWAVE is a second-generation discrete directional spectral wave model. The
model includes source and sink terms of atmospheric input, parameterized nonlinear interactions, and dissipation.
WIS recently completed an updated hindcast of the U.S. Atlantic and Gulf of Mexico coastal wave climate (Tracy
2002) for 1990-1999. The updated hindcast employs improved wind fields, higher spatial resolution (nested from 1
deg to 1/4 deg to 1/12 deg), and upgrades to WISWAVE wind input and nonlinear source terms. The update also
included a comprehensive verification effort. Nearshore wave information is also being saved at higher resolution
(1/12th deg along the coast in depths of 15-30 m for the Atlantic). In addition to the wave parameters (height,
period, and direction) saved in previous hindcasts, the full spectra are being archived at the nearshore save locations
as well. Wave spectra are available every 3 hrs and parameters are available every 1 hr, starting at 0 hr Greenwich
Mean Time (GMT) each day.
The Atlantic WIS station closest to the Field Research Facility (FRF) at Duck, North Carolina, is Station 218 located
at 36.25 deg N and 75.58 deg W in a depth of 25 m. The years 1997 and 1998 was chosen for analysis because the
hindcast for these years was complete at the time of this study and a nearly complete nearshore wave measurement
record at the FRF was available. WIS data extracted from the archive for Station 218 includes the wave parameters
zero-moment wave height (Hmo), peak spectral period (Tp), and mean wave direction (θm) and the wave spectra. The
spectra include 20 frequency bins (ranging from 0.03 to 0.30 Hz) and 16 direction bins (22.5 deg resolution).
Parametric wave energy in frequencies higher than 0.30 Hz are included in the wave heights, but not in the spectra.
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