Stationary Instrument Data


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. Gaps in the time series were filled with values of -9999 so they are easily identified. There was insufficient time to redesign the data acquisition system to eliminate these gaps. Gains and biases for converting the voltage time series to engineering units were included 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 180 degrees during the experiment, noted as a change in the sign of the gains, in order to perform a field calibration of gauge electronic offsets (biases). Other orientation changes that occurred and must be noted include: gauge CM31 rotated 20 degrees clockwise (visually estimated) at 1330 on 15 October, gauge CM10 rotated 2 degrees clockwise between the beginning of collection at 0834 and end of collection at 1455 on 11 October, and also re-aligned 18 degrees clockwise between 0624 and 1330 on 18 October, and a re-alignment of gauge CM20 rotated 28 degrees counter-clockwise between 0624 and 1330 on 18 October. In addition, a 10 degrees 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 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 moved were recorded and the time series headers.

The electronics package at position 60 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

A technique for determining when the current meters were exposed (airborne) was used to mark time series headers with a data quality parameter for exposed sensors. This technique is described in Appendix E of the full DELILAH report. Data headers for exposed gauges are flagged with the number of exposed points in the 4096-point record. A value of -2 in this field indicates this test was not done due to problems determining mean water levels for the corresponding pressure gauge, specifically pressure gauge PW10 and current meter CM10. Records with exposed sensors should be used with discretion and the understanding of how this estimate was computed. In particular, many current meters were not co-located with the pressure gauges, and that raw pressure variations were used as surface displacements, without hydrodynamic compensation of the pressure signal.

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 is described in Appendix E of the full DELILAH report. This parameter is the ratio of surface wave heights determine from a pressure gage to that from a current meter. Values greater than unity generally indicate biofouling of the current meter. Mean current velocities were computed for both X and Y channels of each gauge, as well as resultant cross-shore and longshore components, and recorded in the DELILAH statistics files. Data from each instrument in the primary cross-shore subarray, the trough array, and crest array are available.

Gauge 1 is a primary National Oceanic and Atmospheric Administration (NOAA) tide gauge located at the end of the FRF pier. Pressure gauge biases were adjusted so the mean water level for each record would match Gauge 1. This adjustment was necessary since these gauges exhibited drifting means. Depths at each gauge were estimated by hourly interpolation of daily CRAB surveys. The data was processed to handle data gaps with a linear interpolation through the gap.

Analysis has indicated the data set is of highest quality between 6 and 16 October. 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.

Digitized Timeseries File Description


To read the binary timeseries files see the program "READTS.FOR" as an example. Most UNIX computers require that you swap bytes. The byte order is ok for a VAX or PC.
Be sure that you check the diagnostic codes in the headers before you use the data! (see "Timeseries Header Description" below.)

FRF timeseries files are in 2 byte integer, binary form. The byte order is :

least significant byte(lsb) - most significant byte(msb).

Each record in the file consists of a 80 character data header and 4096 data points. Each data point is a 2 byte integer. To convert to engineering units :

EngUnit = (data/res - bias) * gain

Gain and bias are discussed in the "Header Description" below.

When the gain and bias are applied to DELILAH timeseries:

  • gauge 1116 is a barometer in meters
  • current meters are in meters/second
  • pressure gauges are in meters after you remove the barometer value

    These records are contiguous throughout the collection period but are broken into 4096 point records for reasons of historical consistency at the FRF. Data collections were typically contiguous for 8 hours. Inter-collection gaps occurred between these 8 hour periods while data tapes were being changed.

    Header Description

        VARIAB. COL #'S    FORMAT   DESCRIPTION
        ________________________________________________________
      
        IGID      1-5       I5      Gauge identification number
        ITYPE     6-7       I2      Gauge type
        IYR       8-11      I4      Year
        IMO      12-13      I2      Month
        IDA      14-15      I2      Day
        ITM      16-19      I4      Time (EST)
        ISMN     20-21      I2      Record in sequence (1)
        ISMX     22-23      I2      Total number of records in sequence(1)
        VAXNOV      24      I1      Computer data was collected on (2).
        IDTYPE      25      I1      Data type (0) 
        IFRQ     26-27      I2      Sample frequency * 10 (ie: 8Hz * 10 = 80)
        NDPTS    28-31      I4      Number of data points in record (4096)
        IGAIN    32-38      I7      Gauge calibration (GAIN * RES)
        IBIAS    39-44      I6      Gauge calibration (BIAS * RES)
        RES      45-48      I4      RESOLUTION
        IORIENT  49-52      I4      Gauge orientation (degrees clockwise of +X axis from true north)
        IGMSL    53-57      I5      Gauge elevation relative to NGVD, centimeters
        IDMSL    58-62      I5      Local bottom position relative to NGVD, centimeters
    
    
    

    Description of gauge by "ITYPE"

    GAUGE TYPE  DESCRIPTION                 GAUGE NAME      UNITS
    _______________________________________________________________________
         3      DEPTH FROM GAGE TO SURFACE   PRESSURE        METERS CH2O
         4      CURRENT SPEED & DIRECTION    CURRENT MTR     METERS/SECOND
         6      WIND SPEED                   ANEMOMETER      METERS/SECOND
         7      WIND DIRECTION               ANEMOMETER      DEGREES CLOCKWISE FROM TRUE NORTH
         9      ATMS PRESSURE                BAROMETER       METERS CH2O
    Meters CH2O = meters of sea water assuming a water density of 1.023 g/cm**3 and gravitational acceleration of 9.79789 m/s**2.

    Statistics File Description

       COLUMN #     FORMAT     DESCRIPTION
      ______________________________________
          1            I4       YEAR
          2            I3       MONTH
          3            I3       DAY
          4            I5       HOUR MINUTE
          5            I6       GAUGE IDENTIFICATION NUMBER
          6            I6       KEY TO DATA FILE RECORD NUMBER
          7-10        4F8.3     DATA (ACCORDING TO GAUGE TYPE)
         11-12        2F6.2     DATA (ACCORDING TO GAUGE TYPE)
          13           I5       NUMBER OF EXPOSED POINTS 
          14           I3       NUMBER OF GAP POINTS
    
    DELILAH array gauge numbering is described in Appendix E of the full report. Each gauge identification number can be referenced to a particular gauge type. The data columns 7-12 are described below according to the type of gauge.

    Pressure gauges

       COLUMN #    DESCRIPTION                         UNITS
      _______________________________________________________
         7         MEAN PRESSURE                      METERS CH2O
         8         SIGNIFICANT WAVE HEIGHT            METERS
         9         WAVE PERIOD                        SECONDS
        10         WAVE DIRECTION                     DEGREES FROM TRUE NORTH
        11         TIDE                               METERS
        12         DEPTH (BOTTOM TO MEAN WATER)       METERS
    

    Current meters

       COLUMN #    DESCRIPTION                         UNITS
      ______________________________________________________________
         7         X or Y CHANNEL MEAN                METERS/SECOND
         8         CROSS- or LONG-SHORE MEAN          METERS/SECOND
         9         VARIANCE                          (METERS/SECOND)**2
        10         PUV TEST                       
        11         GAUGE TO MEAN WATER LEVEL          METERS 
        12         DEPTH (BOTTOM TO MEAN WATER)       METERS

    Resultant current meters

       COLUMN #    DESCRIPTION                         UNITS
      ______________________________________________________________
         7         CURRENT SPEED                      METERS/SECOND
         8         CURRENT DIRECTION                  DEGREES CLOCKWISE FROM TRUE NORTH
         9         COMBINED VARIANCE                 (METERS/SECOND)**2
        10         PUV TEST                     
        11         GAUGE TO MEAN WATER LEVEL          METERS 
        12         DEPTH (BOTTOM TO MEAN WATER)       METERS
    The resultant current meter is given a gauge identification of the cross-shore channel plus 10000 (e.g., 12201 for gauge with cross-shore channel 2201).

    The number of data points a gauge is exposed is indicated in column 13. If the pressure gauge data failed the criteria for the PUV test, this column contains a -1. Because the pressure gauge PW10 had a severe drift in the mean pressure the PUV-test was not be applied; this is indicated by a -2 in column 13.

    A -6 in the last data column indicates gaps of greater than 99 data points.

    A variance of less than .006 is indicative of poor instrument response. Data falling into this category was not plotted.

    Statistics of the data are labeled according to date and time of the beginning of sampling.