DELILAH Instrumented Sled
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The instrumented sled was equipped with a 0- to 25- psir (psi relative to one
atmosphere) Senso-Metrics pressure gauge, a resistance wire wave gauge, and
five current meters arranged vertically. The current meters were mounted at
elevations 0.35, 0.60, 1.00, 1.35, and 1.75 m above the bed on a diagonally
sloping beam (Figure 7 of the Introduction section). The beam was parallel to
the shoreline, so the meters were aligned at approximately the same cross-shore
coordinate. The meters were spread over an alongshore distance of
approximately 3.5 m. The sled was deployed so that the lower end of the beam
was in the up-drift direction of the longshore current to reduce flow
interference, such that the current meters were up-drift of the beam. A
common timing pulse was used for all current meters to reduce interference
between instruments for this close proximity deployment. They were aligned
to measure longshore and cross-shore components of the current when the sled
was towed out normal to the shore. The sled was also instrumented with a
digital compass for orientation measurement, and a Met One anemometer. The
data were transmitted via radio modem to a personal computer located in a van
on the FRF pier.
The sled was pulled offshore with the CRAB (Figure 8 of the Introduction section) and then incrementally retrieved, stopping at several cross-shore locations, with the FRF's forklift. The sled was towed offshore past the breaker zone to a depth of approximately 3 m, typically to a cross-shore coordinate of 245 m. The runs were along a line just north of the primary cross-shore array and the retrieval stops were made adjacent to an instrument in the array, with three to eight stops for each cross-shore transect. Locations of the sled and the start time for each run are given in Table F1. The position and orientation of the sled were recorded using an electronic total station which sighted two prisms located on the sled mast.
At each sled position, data were collected for 34 min (except for run #SL1910A which was only 8 minutes long due to low battery). The collection period of 34 min was selected to provide a balance between the time needed for stable statistics on each run and to maintain a short enough transect time to assume stationarity of the incident waves.
The DELILAH array gauges provide background data on the horizontal structure of the hydrodynamics and on the stationarity of the waves and currents. Sled measurements were made during the final six days of the DELILAH experiment, from 16 to 21 October. Incident waves during these days provided a variety of conditions with wave heights of 0.5 to 1.5 m, peak spectral periods of 5 to 15 sec, wind speeds of 5 to 15 m/s, and wave directions both north and south of shore-normal. The maximum time-averaged current velocities exceeded 0.5 m/sec during measurements with the sled.
Quality control checks during the experiment indicated that the middle current meter (elevation 1.0 m) had a large offset, relative to the pre-experiment calibration. Calibrations were performed in the field during three runs (SL1808A, SL1909A, and SL1916A) by rotating the current meters and comparing the measured data to a similar run without meter rotation.
In shallow depths one or more current meters were often out of the water. An estimate of when this occurred was done with the same procedure as described for the DELILAH array current meters in the DELILAH array data analysis section in Appendix E. A minimum water elevation for each wave trough was computed from the pressure gauge record, then the elevation of each current meter was checked to determine if it exceeded the lowest wave trough. A data quality flag was set in each time series header for current meters that were believed to be exposed. An additional quality check of the current meter gains was then done with the PUV-test procedure as described in Appendix E.
