Table 1. LSTF instrumentation and the sampling scheme for this study

Fig. 1. The LSTF during the plunging case, showing the flow channels (bottom) and the instrument bridge (top) carrying the current meters and wave gages (vertical rods)

Wave and beach conditions

wang_ebersole_smith_johnson
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P. Wang et al. / Coastal Engineering 46 (2002) 175211

Table 1

LSTF instrumentation and the sampling scheme for this study

Parameter to

Instrument type

Sampling

Number of

Vertical

be measured

rate

duration

cross-shore locations

profile

10a

Wave

capacitance gage

20 Hz

10 min

N/A

Current

Acoustic Doppler Velocimeter (ADV)

20 Hz

10 min

Yes

Sediment concentration

Fiber Optical Backscatter (FOBS)

16 Hz

10 min

Yes

Water depth

bottom-tracking profiler

every 5 mm cross-shore

continuous

3660

N/A

The 10 locations were 1.1 m (ADV1), 2.7 m (ADV2), 4.1 m (ADV3), 5.7 m (ADV4), 7.1 m (ADV5), 8.5 m (ADV6), 10.1 m (ADV7),

11.6 m (ADV8), 13.1 m (ADV9), 15.6 m (ADV10) seaward from the still-water shoreline.

column. The FOBS were sampled at 16 Hz and were

measured at twenty 0.75-m-wide downdrift bottom

operated through a separate computer independent of

traps, providing data on the cross-shore distribution of

the wave and current sampling system. Due to differ-

longshore sediment transport. The free surface posi-

ent lengths of startup time and limitations in the

tion was measured using capacitance wave gages

present data acquisition system, the sampling of wave

sampled at a frequency of 20 Hz. Acoustic Doppler

and current data was approximately 3 s behind that of

Velocimeters (ADVs) were used to measure current

sediment concentration data.

(Kraus et al., 1994). The wave and current sensors

The wave, current, and sediment concentration

were colocated at 10 cross-shore locations and

sensors were mounted on a steel bridge spanning the

synchronized in time (see Table 1 for cross-shore

basin in the cross-shore direction (Fig. 1). This instru-

sensor locations). Vertical profiles of velocity were

ment bridge can be moved precisely alongshore, with

measured by positioning the ADVs at different ele-

an accuracy of F 2 mm. By stationing the bridge at

vations in the water column at the same alongshore

different locations, cross-shore transects of wave,

location during a series of wave runs (Hamilton and

current, and concentration measurements can be con-

Ebersole, 2001). Therefore, there were time lags of

ducted at various longshore locations. The bridge also

approximately 15 min (10 min sampling and 5 min for

provides a platform for conducting dye experiments

positioning sensors) between velocity measurements

and beach-profile surveying. Beach profiles were

at different elevations. Given the constant input wave

surveyed using an automated bottom-tracking profiler

conditions and negligible beach changes during the

that travels along the bridge. For the present experi-

series of wave runs, time lags should not have

ments, the alongshore interval between adjacent beach

significant influences on data relevancy. Hamilton

profiles was 1.0 m (0.5 m near the updrift and down-

and Ebersole (2001) and Hamilton et al. (2001)

drift boundaries) and the profiler was programmed to

discussed the steadiness in hydrodynamic conditions

sample every 0.5 cm in the cross-shore direction.

and measurement repeatability and their implications

Experimental procedures similar to those described

on making measurements of the vertical structure of

in Wang et al. (2002) were followed in this study.

velocity.

Spectral analyses of water level, current, and sedi-

Profiles of sediment concentration were measured

ment concentration data were based on the Welch

using four arrays of Fiber Optical Backscatter Sensors

method (Welch, 1967). The 10-min wave and current

(FOBS). Each sensor of the array has a vertical

records, which were sampled at 20 Hz, were seg-

resolution of 0.5 cm (Beach et al., 1992) and was

mented into 1.71-min segments (2048 data points)

calibrated using the sand from the test beach. Eleva-

with 50% overlap. For the 16-Hz sediment-concen-

tions of the sensors were controlled by referring them

tration record, the 2048-point segment represented

to the bottom one, which was deployed directly on the

2.13 min. A confidence interval of 95% was used.

bed. Each array contains 19 sensors positioned at 0, 1,

A low cutoff at twice the generated peak wave period,

2, 3, 4, 5, 6, 7, 9, 11, 13, 15, 17, 21, 27, 33, 39, 45,

i.e., 3 s for the spilling case and 6 s for the plunging

and 51 cm from the bed. The near-bottom sensors

case, was applied during the calculation of significant

have a smaller spacing of 1 cm as compared to the

wave height (Hmo). This low cutoff had considerable

spacing of 4 to 6 cm for sensors higher in the water

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