Basic HTML Version
ACT Protocols for Wave Measurement Systems
July 2012
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Oceanography in La Jolla, CA. Additional ferris wheels could be manufactured as geographic
coverage necessitates.
When deploying the Datawell, no additional superstructure or sub-structure mounts must be
attached. The accuracy of the wave measurements is dependent upon the frequency response of
the buoy. The mooring system consists of a 30 meter bungee and terminations supplied by
Datawell, polypropylene line for the appropriate depth (2*depth, or 1.5*depth in deep water with
little current) and large anchor chain links for the 1200-pound anchor. Datawell approved
mooring line must be used. Deployment protocol as specified by Datawell is explicitly described
in the manual. The Mark III deployment is very straightforward, and depending on sea
conditions and distance offshore, can be performed from small vessels. Additional information
can be found at: http://cdip.ucsd.edu/?nav=documents&sub=index&pub=public&map_
stati=1,2,3&xitem=gauge#buoys. It is suggested that the buoy be capable of both iridium and
HF. As mentioned, HF is necessary in case the buoy breaks loose. The hand held GPS tracking
unit is essential in recovery (which communicates via HF). Iridium communication has proven
to be very reliable and solid.
10.7.2 FRF Pressure Array
The FRF long linear pressure array is perhaps the only location for which real and accepted “sea
truth” exist for incident directional wave spectra. The fundamental work on this nearshore
directional wave sensing system was performed by
Long and Oltman-Shay (1991). The array
consists of 15 bottom-mounted pressure sensors centered on the 8-m contour about 900 m
offshore. Under most conditions this location is beyond the active surf zone. Records of 136 min
duration are collected at 3 hourly intervals at a sampling rate of 2 Hz. Time series are windowed
and Fourier transformed to compute cross-spectra between all unique pairs of gages. Cross-
spectra of pressure are surface corrected to sea-surface displacement cross-spectra using linear
wave theory, and then ensemble and band averaged for a final resolution bandwidth of 0.01 Hz
(approx. 160 DOF). Directional spectral estimates are made using the Iterative Maximum
Likelihood Estimator on both the full 15-gage array (360 degree processing) and on just the 9
gages of the longshore linear array (180 degree processing). The latter calculation is necessary
because refractive blurring occurs in the 360-degree analysis (Long, 1994). This high-resolution
directional array produces estimates of incident directional wave spectra to approximately 2
degrees under nominal east coast wave conditions (Long & Oltman-Shay, 1991). Hence, it
represents an ideal shallow water test and evaluation location.
11.0 Data Management
The purpose of the CDIP wave instrument intercomparison tool (CDIPtool) is to allow wave data
collectors and instrument developers to easily compare wave spectral data from different
sources, and view the differences between spectral data sets in a standardized form. It compares
wave spectra data sets that have been calculated by the user or by using the processing software
provided by an instrument manufacturer. The sampling rates are sensor/manufacturer issues and
not related to the CDIPtool.