1,000. As shown at the bottom of the table, a total longshore flow rate of 303.7
ℓ/sec will be actively recirculated during the next segment of the experiment.
It should be noted that Columns 3 and 5 are not required to calculate the
pump discharge rates. However, they are always included in these calculations,
because they provide a simple means of quantifying the change in beach profile
depth in front of each flow channel (Column 5) relative to the initial beach
profile depth (Column 3). This is an important parameter to quantify and
monitor. For example, even though the target longshore current distribution
(Column 9) may not be adjusted for the next segment of the experiment, the
pump discharge rates may need to be adjusted slightly as the beach profile
continues to make minor adjustments during later segments of the experiment.
Setting the speed of the pumps
There are several steps involved in setting the pumps to achieve the required
discharge rates from the 20 vertical turbine pumps. First, the operator must
determine whether to use the low-flow or the high-flow mechanism for each of
the pump-and-piping systems discussed previously in the "Calibration and
Testing" section of Chapter 3.
Second, an estimate of the output frequency for each of the variable speed
motor controllers is determined by using the calibration curves for each system.
For example, the calibration curve for Pump-and-Piping System No. 10 was
shown in Figure 19. As mentioned in Chapter 3, these calibration curves
generally have an accuracy of within +2 to 3 percent.
After the output frequency has been selected and the pumps have been turned
on, the flow rate through each pump-and-piping system is measured using the in-
line flow sensors. Experience has shown that to obtain accurate and repeatable
flow rates, the flow sensors must be measured for at least 500 sec at a frequency
of 20 Hz. If the measured flow rate is less than the required flow rate, the output
frequency of the motor controller is manually increased on the human interface
module, shown previously in Figure 17. In contrast, if the measured flow rate is
greater than the required flow rate, the output frequency is manually decreased
on the human interface module. This process is repeated until all of the
measured flow rates are within 1 percent of the desired flow rates for all 20
pumps. After the low-flow or high-flow mechanisms have been selected for each
pump, the process of converging in on the correct frequency for the motor
controllers requires approximately 30 min.
During this 30-min period, the pumps force a longshore current along the
moveable-bed beach. However, waves are not generated during this time.
Fortunately, for most conditions the sand is stable under longshore current
forcing only; i.e., the longshore current forcing alone does not usually generate
sufficient near-bed velocity to exceed the critical shear stress required to initiate
sediment transport for the sand particle size used in the LSTF.
70
Chapter 7
Wave Generation and Current Recirculation
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