Design wave condition
To determine the design capacity of a LSC recirculation system for the
LSTF, a maximum design wave condition was selected. After observing the
location of the break point for a number of different wave height and period
combinations in the LSTF (and comparing these results with the numerical
simulations), it was decided that Hs = 0.3 m and Tp = 2.5 sec is probably the most
energetic wave condition that will be generated during the moveable-bed LST
experiments. As mentioned previously, the maximum angle-of-wave incidence
(measured at the wave generators) to be tested in the LSTF is 20 deg. Therefore,
the "design wave condition" used to determine the maximum required pumping
capacity of the LSC recirculation system was characterized by Hs = 0.30 m,
Tp = 2.5 sec, and θ = 20 deg with an offshore water depth of 0.9 m. This is the
LSC distribution shown previously in Figure 11 (middle curve) and Figures 12
and 13 (upper curve). Results obtained using NMLONG to predict the magnitude
and cross-shore distribution of LSC for this "design wave condition" indicated a
total longshore flux of 0.76 m3/sec within the surf zone.
Determining a factor of safety
A factor of safety was incorporated into the design to compensate for
inaccuracies in calculating the magnitude and cross-shore distribution of LSC
that will be generated in the LSTF. Our concerns included: (a) having inadequate
data to calibrate the bottom friction coefficient and the lateral mixing coefficient
in the numerical model; and (b) changes in the bottom roughness between the
initial hydrodynamic tests on a fixed-bed concrete beach and multiple grain sizes
used for the moveable-bed beach for the LST experiments. The surface of the
concrete beach was broom-finished to simulate the roughness of a course grained
sand. However, because bed-forms would develop in the LST experiments, it
was assumed that the moveable-bed beach would have a higher friction
coefficient for a given wave condition. Because of these uncertainties, the
magnitude of the LSC at each cross-shore location was increased by 10 percent
(i.e., a Factor of Safety = 1.1) for the purpose of determining the required
pumping capacity of the LSC recirculation system.
Allowance for maximum depth of erosion
At this point in the design process, predictions of the LSC magnitude and
cross-shore distribution to be generated in the LSTF were based on a 1:30 plane
sloping fixed-bed beach having parallel contours. However, for future moveable-
bed experiments, the beach profile would adjust in response to the incident wave
conditions, creating a shore-parallel bar and trough feature near the location of
initial wave breaking. Assuming longshore uniformity in the beach bathymetry,
the capacity of the LSC recirculation system had to be increased wherever the
beach profile would erode, because of the increased longshore flux at that cross-
shore location.
To investigate this problem, 2-D flume tests were conducted to estimate the
maximum depth of erosion during moveable-bed LST experiments in the LSTF.
22
Chapter 3
Longshore Current Recirculation System
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