[an error occurred while processing this directive]

DELILAH Current Meter Calibrations
[an error occurred while processing this directive]

Two types of electromagnetic current meters were used in the DELILAH array, Marsh-McBirney 551's (Figure 4 in the main text) and Scripps Institution of Oceanography "open frame" sensors (Figure D1). All of the current meters were calibrated in a constant flow tank by the Naval Oceanographic Office in Bay St. Louis, MS. Each instrument was calibrated at five speeds ranging from 0.0 to 1.91 m/s and four probe orientations (± X, ±Y).

All instruments deployed during the experiment were also postcalibrated. Because of the biological growth that developed on the open frame current meters, they were post-calibrated with and without the growth. Although the growth had decayed somewhat by the time the instruments were recalibrated, the results give some indication of their performance under fouled conditions. The calibration results, including pre- and postcalibration gains and bias values, and computed changes between the calibrations, are listed in Tables D1 and D2.

Pre- and postcalibrations were used to adjust the collected time series data. In general, the precalibration was used unless the postcalibration was significantly different. Then, either the postcalibration was used or the pre- and postcalibrations were averaged. The decision to use only the postcalibration was based primarily on a comparison with nearby gauges during a time of high flow over linear bathymetry. In most cases, the current meter offsets were determined using the calibration curve for each channel of a given gauge and applied as a bias in volts. After plotting the time series in engineering units, however, further offsets of the velocities from zero were obvious for open frame channels 2711, 2731, and 2741 in the crest subarray, and for Marsh-McBirney channels 2701 and 2901 in the primary cross-shore subarray. These channels all measured cross-shore currents. The offsets appeared to be systematic throughout the duration of each time series, suggesting that a constant value applied to these data would be a satisfactory adjustment. To correct for these offsets, current velocities taken from these channels during a calm period (6-10 October) were correlated against data from channels which did not appear to require further offsets. The y-intercept of the regression curve for each channel was the offset applied in meters per second to the current velocities. Correlation coefficient (r2) values for these regressions ranged from 0.77 m/sec to 0.90 m/s. The greatest offset of -0.231 m/s was applied to channel 2741. New biases were computed from the changed offset using the following equation:

biasnew = biasorig. + (Doffset / gainorig.).(1)

The open frame electromagnetic current meters posed a particular problem because of unanticipated biological fouling. Normally, the offset can be field determined by comparing data collected at orientations of 0 deg and 180 deg. However, the high wave activity during the last 2 weeks of the experiment prevented this check from being accomplished. An analysis performed on the current meter data from the crest and trough subarrays indicates an increase in the gains of the open frame sensors from the beginning to end of the experiment. This analysis is described in detail in the "Data Processing" portion of the Stationary Instrument Data section.

A comparison calibration plot for the for sensor S385-X, where the post-calibration differed from the pre-calibration, is shown below.

All other sensor calibration plots can also be viewed from this table.

S1013X.GIF S1013Y.GIF S1081X.GIF S1081Y.GIF S1082X.GIF S1082Y.GIF
S1084X.GIF S1084Y.GIF S385Y.GIF S476X.GIF S476Y.GIF S762X.GIF

[an error occurred while processing this directive]