e. Should have a design life of 15 to 20 years given the long-term
requirements of the facility.
Numerical Simulation of Longshore Current
The first part of this section provides a brief description and configuration of
the numerical model used to estimate, prior to construction, the magnitude and
cross-shore distribution of the longshore current that would be generated in the
LSTF. The next section validates the results obtained from the numerical model
using dye measurements in the LSTF. The final section shows the influence of
wave height, period, and direction on the estimated magnitude and cross-shore
distribution of the longshore current that would need to be recirculated.
Description of the numerical model
Kraus and Larson's (1991) numerical model NMLONG (Numerical Model
of the LONGshore current) was used to estimate the wave-driven longshore
current in the LSTF, prior to design and construction. NMLONG is a PC-based
model that calculates cross-shore distribution pattern of wave transformation,
mean water surface elevation, and longshore current. The major assumptions in
NMLONG are longshore homogeneity and linear wave theory.
Using the data from Visser (1982), Kraus and Larson (1991) illustrated that
NMLONG can be calibrated to reproduce the LSC measured in the laboratory
with reasonable success. In particular, the magnitude and cross-shore location of
the peak of the LSC distribution was reproduced with reasonable accuracy. This
suggests that NMLONG can be used to provide a reasonable estimate of the
magnitude and cross-shore distribution of LSC that can be generated in the
LSTF. However, in the Visser cases, NMLONG predicted that the magnitude of
the offshore tail of the LSC distribution was higher than measured by Visser,
even after the numerical model had been calibrated.
NMLONG requires the following input parameters: offshore wave height,
period, and direction, specification of regular or random waves, offshore water
depth, and beach profile elevation relative to mean water level. Random waves
are characterized in NMLONG using the root-mean-square wave height, Hrms.
Values of Hrms were converted to significant wave height, Hs assuming Hs =
1.414 Hrms. Nonlinear bottom friction with a friction coefficient equal to 0.01
was used in the LSTF simulations. For one wave condition, the sensitivity of
results to this value was evaluated by reducing and increasing the coefficient to
0.005 and 0.02, respectively. All other empirical parameters were set to the
default values: incipient breaking-wave-height-to-water-depth ratio equal to 0.8,
stable wave-height-to-water-depth ratio equal to 0.4, energy flux dissipation rate
equal to 0.15, and lateral mixing coefficient equal to 0.3.
NMLONG was used to calculate the depth-averaged LSC velocities at each
cell spaced 1.0 m across the beach profile. These depth-averaged velocities were
multiplied by the corresponding mean water depth and integrated across the
entire profile to estimate the total longshore volume flux for each wave
Longshore Current Recirculation System