requirements of the facility.

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.

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 =

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

condition.

17

Chapter 3

Longshore Current Recirculation System

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