Two series of numerical simulations were conducted to represent two
different experimental configurations planned for the LSTF. The first series was
conducted with an offshore water depth equal to 0.6 m and represented the
configuration that would be used in the LSTF during the fixed-bed hydrodynamic
testing phase. The second series was conducted with a water depth equal to
0.9 m and represented the configuration to be used for the moveable-bed
sediment transport experiments. In the later configuration, it was assumed that a
0.3-m-thick uniform layer of sand would be placed on top of the existing
concrete beach. Increasing the water level from 0.6 to 0.9 m translates the
shoreline directly upward (i.e., there is no cross-shore translation of the nearshore
zone). However, the resulting magnitude and cross-shore distribution of the LSC
was slightly different for the two different configurations because of the slight
difference in wave transformation caused by the steeper toe of the beach in the
case with the moveable bed.
Numerical simulations in both test series covered the following range of
hydrodynamic conditions: significant wave height, Hs = 0.1 to 0.4 m, peak wave
period, Tp = 1.0 to 2.5 sec, and angle of wave incidence, θ = 5 to 20 deg relative
to shore normal. For these test conditions, the total longshore volume flux
ranged from 0.04 m3/sec for Hs = 0.1 m, Tp = 2.5 sec, and θ = 20 deg to 1.3
m3/sec for Hs = 0.4 m, Tp = 2.5 sec, and θ = 20 deg.
The sensitivity of the bottom friction coefficient was evaluated for a
relatively energetic wave condition; namely Hs = 0.3 m, Tp = 2.5 sec, and θ = 20
deg. The bottom friction coefficient was varied over a range of two times the
default value of 0.01 (i.e., from 0.005 to 0.02). Results obtained using the lower
friction coefficient (0.005) resulted in a total longshore volume flux of 1.1 and
1.2 m3/sec for the 0.6 and 0.9 m water-depth cases, respectively. The higher
for both water-depth cases. Therefore, the results obtained using NMLONG to
predict the total longshore volume flux in the LSTF are sensitive to the value of
the bottom friction coefficient.
Validation using dye measurements
Preliminary measurements of the magnitude of the longshore current
generated in the LSTF were obtained by injecting dye into the surf zone while
generating monochromatic waves. These measurements then were used to assess
the validity of the numerical simulations. However, the following facility
components had not been installed in the LSTF at the time these measurements
were required:
a. Lateral wave guides were not available to direct the incident waves from
the wave generators to the surf zone.
b. None of the longshore current recirculation system components (i.e.,
pumps and flow channels) were available to minimize the laboratory
effects caused by the lateral boundaries of the facility.
18
Chapter 3
Longshore Current Recirculation System
Previous Page
