Thus, a gauge number of 2503 is a DELILAH instrument, located at the 50 position, the fifth offshore gauge on the primary cross shore subarray, and is the Paroscientific pressure gauge (type 3).

The Gauge Name is used only to describe the particular gauge. It is not stored with either the raw time series or the FRF's summary statistics file. The following conventions are used:

Current meters and pressure gauges used in DELILAH are listed in Tables E1 and E2, respectively. The tables include gauge name, gauge number, serial number plots, gain, bias, and coordinates relative to the FRF coordinate system. The signs of the gains may differ from those listed in Calibrations section. The signs have been adjusted to compensate for instruments being mounted downward. A negative gain flips the data of the channel so that they correspond to the conventions of:

DELILAH array data collection

Data were sampled at 8 Hz except for the Paroscientific sensors, which were sampled at 1 Hz. The primary cross-shore array had an unanticipated 4 to 8-sec gap in the data which occurred approximately every 20 min as a result of the microprocessors used. The 10 current meters in the three subarrays did not have these data gaps. These gaps in the time series were filled with values of -9999 so they are easily identified. There was no way to eliminate these gaps.


Gains and biases applied can be found in the data file headers of each time series. Several of the gauges (CM73 at 0939 on 20 October, CM71 at 1030 on 20 October, and CM50 at 1358 on 4 October) were rotated 180during the experiment, noted as a change in the sign of the gains, in order to conform to other Marsh McBirney current meter orientations. Other orientation changes that occurred and must be noted include: gauge CM31 rotated 20 clockwise (visually estimated) at 1330 on 15 October, gauge CM10 rotated 2 clockwise between the beginning of collection at 0834 and end of collection at 1455 on 11 October and also re-aligned 17.6 clockwise between 0624 and 1330 on 18 October, and a re-alignment of gauge CM20 rotated 28.1 counter-clockwise between 0624 and 1330 on 18 October. In addition, a 10 landward bend from vertical in Gauge CM73 was corrected at 0939 on 20 October. These rotations were noted in the time series headers.

Throughout the experiment, two current meters, CM10 and CM20, were in a zone that experienced large bathymetric change. This necessitated moving the gauges up and down in order to ensure that they remained submerged in the water but were sufficiently far above the sediment surface. Gauge CM10 was moved up on 7 and 8 October and down on 9,11, and 18 October. Gauge CM20 was moved up on 7 and 8 October. Measurements of how far up or down these gauges were moved were recorded and the time series headers were modified.

The electronics package at position 60 (Figure E1) was lost during the high waves of Hurricane Lili at approximately 0700 on October 11. The sensors at position 30 stopped functioning a few days later.

DELILAH array data analysis

Although several of the current meters were vertically adjusted to maintain submergence and to remain above the bottom, several of the shallower gauges would occasionally became exposed in the wave troughs, particulary during low tide. A technique for determining when the current meters were exposed is presented, and was used to mark time series headers with a data quality parameter for exposed sensors. Several methods for estimating gauge exposure could be envisioned, the one presented here was used since it was fairly simple to employ. This technique finds the lowest trough elevation in each pressure gauge record (Setra gauges), then determines if the position of the corresponding current meter is above the lowest trough. This was a reasonable approach, particularly for the primary cross-shore array, since the current meters and pressure sensors were only separated by 1.5 m. The 10 current meters in the sub-arrays did not have co-located pressure gauges, so comparisons were made with primary cross-shore array pressure gages at the same cross-shore coordinate. Longshore homogeneity of the wave field was assumed.

DELILAH array pressure gauges recorded offsets and daily variations which were corrected by comparison to the Paroscientific pressure gauge LA33 (gauge number 231) in the FRF's permanent 8-m array. Gaps sometimes occurred in the data from gauge 231. If the gaps were less than 30 minutes in length, interpolation was used to attain water levels for those times. On 20 October, 1990 gauge 231 stopped functioning, from that time forward, the tide gauge located at the end of the pier was used for water level comparisons (Gauge 1, Figure E3). The tide gauge was not used for correcting water levels through the entire experiment because the signal from this gauge is typically not as accurate as the signal from the Paroscientific gauge. It samples a single point every six minutes which results in a noisier signal. This method does not take into account any potential wave setup between the 8-m array and the nearshore DELILAH array. The changes in the water levels for the current meters resulting from this analysis are presented in Figure E4.

An additional data quality analysis was performed on each current meter record to determine the effect of biofouling on signal attenuation. This procedure, referred to as the PUV-test, Z(s), used a ratio of surface wave H_mo computed from pressure gauges and current meters, to estimate a gain correction for the current meters. The method to calculate the PUV-test follows.

Pressure gauge data (in units of meters of sea water) were surface normalized by the pressure response function , where is gauge depth from (ensemble) mean sea level, is total water depth, and is radian wavenumber (related to radian frequency by the dispersion relation ) immediately after Fourier transformation, by dividing the complex Fourier coefficients at frequencies by . Current meter data were surface normalized in a similar fashion except that the Fourier coefficients were divided by . In linearized wave theory, the auto-spectrum from surface normalized pressure is equal to the sum of the auto-spectra from the two surface normalized velocity components and for and , respectively. A routine check on data quality for co-located pressure and current meter data , the -test, is to compute the function, Z²(s)=/[+], which was expected to be unity in the wind-wave pass band of frequencies in regions where linear theory applies.

The PUV-test analysis used the same gauge pairs as the exposed current meter analysis, current meters without colocated pressure gauges were matched with primary array pressure gauges at the same cross-shore coordinate. Again, longshore homogeneity was assumed. An average PUV-test is calculated for each current meter record over a select frequency range of the Z(s). This PUV-test is averaged over the half-power bandwidth in the pressure gauge energy spectra, for the spectral peak that lies in the wind-wave band (0.4 to 0.05 Hz). These PUV-test values are used as a multiplier to adjust the mean current amplitudes. Inherent in this treatment is the assumption of uniform fouling between both axes of each current meter. Corrected and uncorrected current velocities, and PUV-test values are recorded in the DELILAH statistics database. Plots of the PUV-test values are presented in Figures E5 and E6. These data are noisy because averages are taken over short time segments. PUV-test plots for current meters that are not colocated to pressure gauges are especially noisy. The noisiness increases with the distance between current meter and pressure gauge. Plots CM71, CM72, CM73 and CM74 demonstrate this spatial inhomogeneity. The consistent PUV-test value of less than one for gauge CM90 indicates the pressure gauge was either lower in the water than believed or the current meter was higher.

Statistical data from each instrument in the primary cross-shore subarray, the trough array, and crest array are plotted in Figures E7 through E12. The shoreward most gauge CM10 (2101 and 2102) was exposed more often than not consequently, unlike all other gauges in figures E7 and E8, it did not undergo the PUV-test. Data from 11 October through 15 October appears to be the only portion of the collection when this gauge is consistently submerged. Gauge 1 in these plots is a primary National Oceanic and Atmospheric Administration tide gauge located at the end of the FRF pier. These plots have been processed to handle data gaps, establish gauge depths relative to mean sea surface, and eliminate gauges that became exposed at low tide. Analysis has indicated the data set is of highest quality between 6 and 16 October. In the first few days of the experiment problems existed in the collection system and gauge elevations and orientation were being adjusted. After the 16 October biofouling of current meters had significantly attenuated the sensor response, this was especially true for the Scripps Open Frame gauges. For these Open Frame gauges, there appears to be three identifiable portions of the experiment where three separate gains can appropriately be applied to each Open Frame current meter. The PUV-test values remained fairly constant during the first portion of the experiment, from 1 October through 8 October, then increased from 9 October through 13 October, and stabilized again from 14 October through 19 October. When these PUV-test multipliers are applied to the data, differences in the current velocity as great as 0.24 m/sec (27.5 % change in velocity) can result. For these reasons DATA COLLECTED USING OPEN FRAME SENSORS SHOULD BE USED WITH CAUTION.

FRF Permanent Instrumentation

A group of instruments that is permanently installed at the FRF allows for the continuous collection of oceanographic and meteorologic data. During large cooperative experiments, such as DELILAH, these instruments not only provide researchers with information on conditions during the experiment, but also with an invaluable archive of data from which to compare and interpret trends. The permanent instrumentation includes the 8-m array; Baylor wave gauges; waverider buoy; and gauges for wind speed, wind direction, tide, atmospheric pressure, and temperature. Table E3 lists the FRF permanent instruments by name and number and includes gain, bias, coordinates relative to the FRF coordinate system, and gauge depth. Figure E3 is a schematic depicting the location of these instruments.

An additional numbering convention for FRF permanent instrumentation is used to refer to combinations of gauges. This is a four digit gauge number that associates a name with the results of a multi-sensor data analysis. Among these four-digit combination gauges are 3111, which is the pressure sensors in the 8-m array; 3519, which is the vector averaged currents of the PUV gauge in the 8-m array; and 3932, which is the vector averaged wind speed and direction. Statistics from these and other gauges show the basic climatology during DELILAH in Figure E13.

8-m Array

The 8-m array is composed of a linear longshore array of ten pressure sensors (Nos. 101-191), a cross-shore array of five pressure sensors (Nos. 211-251), and a combination pressure/current meter (PUV) (Nos. 511, 519, and 529) within the array. The fifteen pressure gauges are mounted approximately 0.5 m off the bottom in the vicinity of the 8-m isobath. The array employs pressure sensors, manufactured by Senso-Metrics Inc., with a range of 0-25 psir (pounds per square inch relative to 1 atm). Each sensor was statically calibrated prior to deployment and mounted on a 2-in.-diam pipe jetted into the ocean bottom. A complete analysis of the data results in a three-dimensional directional spectra as illustrated in Figure E14. 8-m array representative spectra and data for all days of the experiment are available for viewing and the downloading. Characteristic wave heights from spectral observation are most frequently given as H_mo, which is four times the standard deviation of sea-surface displacement. It can be determined from the volume under the frequency-direction spectrum by the equation:

It can also be found from the integrated frequency spectrum by:

which is its more conventional definition, or from the integrated direction spectrum (Equation 3) by:

Peak wave period (T_p) and peak direction (q_p), can be determined by integrating the data in both direction and frequency as shown by the graphs on the vertical panels in Figure E14. Note that as defined, the peak direction does not necessarily correspond to the waves which occurred at the peak period (i.e. peak energy). The PUV directional wave gauge consists of a Marsh-McBirney electromagnetic current meter and a Senso-Metrics Co., Inc. pressure sensor. It is mounted on a tripod located within the linear array. Wave direction is measured in degrees relative to true North and indicates the direction the waves are coming from. A detailed discussion of the data processing required to transform measured time series to estimates of frequency-direction spectra may be found in Long and Atmadja (1994)⁽²⁾.

Surface Wave Gauges

The Baylor gauges and the waverider buoy are two different types of gauges used to collect wave information. The Baylor gauges (nos. 625 and 645) are surface piercing inductance staff gauges mounted on the pier at the locations indicated in Table E3 and Figure E3. The waverider (no. 630) is an accelerometer buoy located four kilometer offshore. Data analysis is similar for the two gauge types. Data were collected in 34 min records, which consisted of 4,096 data values (representing the voltage output of the sensor) sampled at 2 Hz. After the voltages were converted to engineering units using the sensor calibration factors, the time series were edited to eliminate erroneous jumps and spikes.

Wind Gauge

Measurements of the wind speed (no. 933) and direction (no. 932) were made at the seaward end of the research pier using a Qualimetrics Corporation Skyvane Model 2101 anemometer. Wind directions are reported relative to true North with onshore winds having a direction of 71.8^o. Summary statistics of 34 minute mean values of these data are presented as the wind vector plot in Figure E11.

Tide Gauge

Water level data were collected by a National Oceanic and Atmospheric Administration (NOAA) tide gage located at the seaward end of the research pier (Gage 1). A 6 minute mean value from the tide gauge, computed once per hour, is plotted as water level from NGVD in Figures E7 through E13.

Barometer and Thermometer

Atmospheric pressure (gauge 616) and air temperature (gauge 624) were measured by Yellow Springs Instrument Co. sensors installed at the FRF building. Summary statistics of 34 minute mean values of atmospheric pressure is included in Figure E13.

1. Personal Communication, 29 April 1997, Dr. Edward Thornton, Dept of Oceanography, Naval Postgraduate School, Monterey CA

2. Long, Charles E., and Atmadja, Juliana. (1994). "Index and Bulk Parameters for Frequency-Direction Spectra Measured at CERC Field Research Facility, September 1990 to August 1991," Miscellaneous Paper CERC-94-5, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.

Longshore Subarray Directional Wave Measurements

The following discussion describes one approach to processing some of the data collected during DELILAH. It is not the only acceptable processing method, but it is described here in detail to demonstrate one way of handling the data complexities. A preliminary analysis of the wind-wave directional spectra from the two longshore subarrays during DELILAH was performed by Dr. Charles Long. The arrays were not ideal for wind-wave analysis, consequently simplifying assumptions as well as some modifications to the data were applied. Some of the data processing techniques described here may also be useful for other than wind-wave analysis.

Basic assumptions

The following assumptions were made:

Gauge selection

The two longshore subarrays are used in the analysis. The crest subarray, seaward of the nearshore bar crest, consisted of the pressure gauge 2704, and five two-axis (x/y) current meters: Marsh-McBirney 2701/2702, and the Scripps Open Frame current meters 2711/2712, 2721/2722, 2731/2732, and 2741/2742. The set comprises an 11-element array with each current meter representing two elements.

The trough subarray was generally on the trough side of the nearshore bar crest. It used pressure gauge 2304, and five Marsh-McBirney two-axis current meters 2301/2302, 2311/2312, 2321/2322, 2331/2332, and 2341/2342. This set also constitutes an 11-element array.

Data selection

The intent was to compare nearshore results with spectra from the FRF 8-m array. 8-m array frequency-direction spectra were computed from records of 2 hr 16 min duration (16,384 data points at 2 Hz, or 8,192 sec) for each gauge, with collection times at 3-hr intervals, starting daily at 0100, 0400, 0700, 1000, 1300, 1600, 1900, and 2200 Eastern Standard Time. To compare data of the same duration and collection start times required finding 16 contiguous records from the crest and trough subarrays that began at about (± 4 minutes) the same times as the FRF collections. Because of the logistics involved with changing tapes, not all FRF collections had a matching crest or trough collection, so some cases are missing from the analyzed set.

Data gaps

To deal with the gaps, and make maximum use of available data, processing occurred as follows:

The pressure gauge from each array (2704 for crest, or 2304 for trough) was processed first. The 8-Hz data were subsampled at 2 Hz by taking every fourth point, starting with the first point in the first of the 16 contiguous records to be processed, and storing the points in a single long array of 16,384 points. The long array was then broken into 31 half-lapped ensembles of 512- sec duration (1,024 points), and each ensemble was searched for data gaps.

If no gaps were detected, an ensemble record was accepted as it was. If a gap occurred in the second half of any ensemble record except the first, the bounding indices of the ensemble record were shifted to earlier times so that the last point in the ensemble record was the last good data point before the gap. If a gap occurred in the second half of the first ensemble record, that record was not used in analysis. The ensemble bounding indices from the long array were stored for later use with current meter data.

If a gap occurred in the first half of any ensemble record except for the final one, the bounding indices of the ensemble record were shifted to later times so that the first point in the ensemble record was the first good data point after the gap. If a gap occurred in the first half of the last ensemble record, that record was not used in analysis. Again, the ensemble bounding indices from the long array were stored for later use with current meter data.

This procedure resulted in occasional redundancy of the ensemble records, and occasional loss of one ensemble record (or half an ensemble's worth of data points), so that there were only 30 ensembles instead of 31. However, this method retained most of the useful data and none of the gapped data.

After the pressure gauge data had been analyzed for gaps, the current meter data were processed. Current meter data were also decimated to 2 Hz from

8 Hz and written into long arrays. To ensure no offsets in time, the ensemble record bounding indices from the pressure gauge were used to define the current meter ensemble records. Consequently, a given ensemble spanned the same period of time from all gauges in an array, so that relative phasing was preserved for all ensembles. In this way, a working database of ensemble records for each collection was created.

Modifications to pressure data

To establish realistic gauge depths relative to the (collection) mean sea surface, the barometric (gauge 616) mean value (in meters of water) was subtracted from the pressure gauge mean value (in meters of water). When the (positive) gauge depth relative to NGVD was added to the barometrically corrected gauge mean, the record should have reflected the tide plus any setup or setdown from wind or radiation stresses. However, this was not what was observed. Mean pressures from both gauges 2704 and 2304 seemed low. High tide didn't get very high above NGVD, and low tide was much lower.