Current Recirculation
This section discusses the method used to calculate the required discharge
rates of the pumps to recirculate the desired cross-shore distribution of the mean
longshore current. The procedure used to estimate this cross-shore distribution
will be discussed in detail in Chapter 9, and is based on the longshore current
data measured at several transects along the beach during the previous segment
of the experiment.
Calculating pump discharge rates
The method used to calculate the required discharge rates for the 20 vertical
turbine pumps is illustrated in Table 3. These data used in the calculations were
taken from a longshore sediment transport experiment.
Column 1 lists the pump number (or flow channel number) starting at Pump
No. 1 (most shoreward) and ending with Pump No. 20 (farthest offshore).
Column 2 lists the cross-shore position of the center line of each flow channel,
with respect to the coordinate system used in the facility. A plan view of the
layout of the pumps in the LSTF was shown previously in Figure 1, and an
oblique view, looking in the offshore direction, was shown in Figure 16.
Water depth is shown in Column 3, measured from the still-water level to the
elevation of the initial sand beach profile, which was mechanically graded using
a four-wheel drive tractor prior to the experiment. This initial beach profile was
shown previously in Figure 4. Depths listed in this column were measured along
the beach profile at the center line of each flow channel.
Column 4 lists the average beach profile depth in front of each flow channel,
based on the beach survey conducted immediately following the completion of
the previous test segment. These values were measured relative to the still-water
level. Each depth listed is the average value of 150 data points, obtained by
measuring the elevation of the sand bed every 0.005 m along the beach profile
transects. In addition, the values listed in column 4 are longshore averaged
values based on six standard beach profile transects obtained using the beach
profile indicator; namely Y = 18, 22, 26, 30, 34 and 38 m.
Column 5 is the ratio of Column 4 divided by Column 3 and indicates the
longshore averaged change in the most recently measured beach profile depth
relative to the initial beach profile depth in front of each flow channel. For
example, in this experiment the water depth in front of Flow Channel No. 1 had
increased by approximately 50 percent relative to the initial beach profile depth.
This indicates that the beach profile in that region (i.e., in front of Flow Channel
No. 1) had eroded relative to the elevation of the initial beach profile in that
region. In a similar manner, the water depth in front of Flow Channel No. 16 had
decreased (accretion) by approximately 10 percent. This was caused by the
formation of the offshore bar in that region of the beach profile.
68
Chapter 7
Wave Generation and Current Recirculation
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