Pumping steadiness and measurement repeatability
As mentioned previously, another system requirement for the recirculation
system was the capability of maintaining constant discharge rates for several
hours of continuous operation. This was needed to accommodate the time scales
associated with conducting sediment transport experiments. To help meet this
requirement, all of the components procured for the recirculation system were of
high quality design and construction. Nonetheless, maintaining constant
discharge rates for several hours of operation was still a concern. Therefore, tests
were conducted to verify the steadiness of flow through each of the 20 systems
and to evaluate the repeatability of the in-line flow sensors.
Tests were conducted by operating all of the pumps for 8 hr of continuous
operation to represent the case where an experiment was run continuously all day
long; an extreme case. The in-line flow sensors were sampled at 20 Hz and the
average flow rate was recorded every 30 sec in the 20 systems. Therefore, each
data point in the recorded time series represented the average value of the
previous 600 (20 Hz 30 sec) instantaneous values for each of the 20 systems.
For pump-and-piping System No. 10, the standard deviation was 0.40 ℓ/sec for
an average discharge rate of 70 ℓ/sec. Therefore, the average discharge rate
remained within +2 percent of the desired flow rate of 70 ℓ/sec during 8 hr of
continuous operation. The 20 pump-and-piping systems gave similar results.
Unfortunately, it was not possible to determine if this variation was caused by
actual oscillations in the pump discharge rate or by errors associated with
measuring the flow rate with the in-line flow sensors. Nonetheless, this
magnitude of variation is relatively small, and it was concluded that the
recirculation system met the design requirements for the facility.
Summary
This chapter documents the design process used to develop the longshore
current recirculation system for the LSTF. Prior to construction, a design wave
condition, of Hs = 0.3 m, Tp = 2.5 s, and θ = 20 deg, with an offshore water depth
of 0.9 m, was used to numerically estimate the magnitude and cross-shore
distribution of the longshore current that would be generated in the new facility.
It was concluded that 20 independent pump-and-piping systems would be
required to provide adequate cross-shore resolution to recirculate a broad range
of longshore current distributions within the facility.
The 20 independent pump-and-piping systems have a total discharge capacity
of 1,250 ℓ/sec. Using a combination of variable speed vertical turbine pumps
along with a low- and high-flow measurement mechanism in each piping system,
the recirculation system has the capability to accurately recirculate between 10
and 100 percent of the maximum design capacity at the 20 cross-shore locations.
This unique design allows a wide range of longshore current magnitudes and
cross-shore distributions to be accurately controlled and recirculated for a wide
range of incident wave conditions, water levels, and bathymetric conditions.
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Chapter 3
Longshore Current Recirculation System
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