interval. The number and location of transects can be programmed in advance,
repositioning of the bridge can be handled automatically, and movement from
one location to another is done rather quickly. Acquisition of data at five
transects in this manner takes approximately 1 hr.
Four FOBS arrays were available, which occasionally required repositioning.
To obtain sediment concentration data at a denser spatial resolution in the cross-
shore direction, sensor arrays had to be manually repositioned along the bridge,
and 10-min test segments had to be repeated. Also, since the current sensors
measure velocity at only one elevation in the water column, repositioning of the
sensors was required to make measurements for characterizing the vertical
current structure. Measurements at other elevations required manual adjustment
of the sensor elevation. Manual repositioning of sensors increased the amount of
time needed to make measurements. Approximately 2 to 2.5 hr were required to
obtain velocity measurements at eight or nine positions throughout the water
column, at one alongshore location.
Making measurements over this length of time raises questions concerning
measurement repeatability and steadiness of hydrodynamic conditions. In
Chapter 9, measurements made during fixed-bed concrete beach experiments,
which lasted for several hours, showed that conditions were steady during the
experiment duration. Figure 72 shows the mean longshore current measured at a
number of cross-shore locations at transect Y22 (near the center of the beach at
the alongshore coordinate of 22 m). Measurements were made at the beginning
(Y22a), middle (Y22b), and end (Y22c) of an approximately 2-hr-long wave run
designed to acquire a high alongshore density of cross-shore transects and to
examine steadiness of the mean longshore current for the spilling breaker case.
The curve labeled "LB's" in the legend refers to the lateral boundary conditions,
i.e. the pumped discharges converted to current speed. Results show that for the
movable-bed case, steadiness in mean longshore current is good. Gaps in the
measured data reflect sensor malfunctions.
The issue of measurement repeatability is related but different, in the sense
that steadiness refers to conditions within a single wave run or test segment,
whereas, repeatability refers to variations from test to test for the same beach,
wave, and current conditions. To examine repeatability, measurements were
made at the same transect and then compared for a number of wave runs in which
a near-equilibrium beach condition existed, and wave/current forcing was held
constant. Figures 73 and 74 show measured significant wave height for both the
spilling- and plunging-breaker wave cases, respectively. Measurements were
made near the center of the beach (at alongshore coordinates of 22 m for the
spilling case, and 24 m for the plunging case). The beach was at its near-
equilibrium profile shape in both instances. Repeatability in wave measurements
is good from run to run.
128
Chapter 10
Longshore Sediment Transport Experiments
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