Metadata Report for BODC Series Reference Number 1815540

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Quality Report for Wave statistics data collected by a Waverider buoy at Site A in the Bristol Channel

Data were screened by BODC using in house visualisation software were any obvious outliers and spikes were looked at in closer detail. There were a few spikes in the GCMXVA01 channel but it was not deemed that these spikes required flagging. Flags were also applied to the start of the series dated 21 JUN 2010 - 13 AUG 2010 as the waverider was undergoing testing resulting in lower GCMXVA01 and GAVGVA01 values than the rest of the data.

During the mooring, the mooring line parted from the ballast on three occasions. When this occurred and the Waverider continued to collect wave statistics data, these data have been flagged suspect. The flagged periods are 23/04/2010 22:36 - 18/05/2010 09:36 and 10/08/2010 08:50 - 13/08/2010 09:50.

Series: 08 MAR 2010 - 18 MAY 2010

The first series of the dataset starts on 08 MAR 2010 which differs from the metadata reports that accompanied the data, as the reports specify the deployment of the waverider as occuring on 09 MAR 2010. When looking at all parameters together it is difficult to establish when the waverider was deployed and so these data have been left unflagged. The time channel of this first series should be used with caution.

Data Access Policy

Public domain data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

Narrative Documents

Datawell Directional Waverider Buoy

General

The Datawell Datawell Directional Waverider is a 90cm diameter spherical buoy which measures wave height, period and direction. The directional measurement is based on a translational principle so that horizontal motions rather than wave slopes are measured. Consequently the measurement is independent of buoy roll motions and a relatively small spherical buoy may be used. A single point vertical mooring ensures adequate symmetrical horizontal buoy response to resolve small motions at low frequencies. Two 15m lengths of rubber cord are normally included in the mooring. Data are transmitted to a shore based station and/or an Argos satellite.

Sensors

• Heave-pitch-roll sensor Hippy40
• Three axis fluxgate compass
• Two fixed "x" and "y" accelerometers
• Temperature sensor

Principles of Measurement

The Directional Waverider measures vertical accelerations by means of an accelerometer placed on a gravity stabilised platform. This platform is formed by a disk which is suspended in the fluid within a plastic sphere placed in the bottom of the buoy. Two vertical coils are wound around the plastic sphere and one small horizontal coil is placed on the platform. The pitch and roll angles are defined by the amount of coupling between the fixed coils and the coil on the platform. This coupling is measured. The result of the measurement gives the sine of the angles between the coil axes (x and y axes) and the horizontal plane (=platform plane). The fluxgate compass measures the components of the Earth's magnetic field in the direction of the x, y and z axes. All of these measurements define the buoy's position with respect to fixed (north, south and vertical) coordinates.

A transformation matrix of the relation between vector components in buoy coordinates and fixed coordinates is formed. The acceleration of the buoy is measured in the x and y directions (two fixed accelerometers) and in the vertical direction (accelerometer mounted on a stabilised platform).

With the help of the transformation matrix and these measured accelerations, the accelerations in the north-south and east-west directions can be calculated. All the analogue outouts of the sensors are filtered using a low-pass filter with a cut-off frequency of 1.5Hz. At this frequency, the buoy's motions are already strongly attenuated due to the small wavelengths compared to the buoy's dimensions (geometric attenuation).

The filter outputs are sampled at a rate of 3.84Hz. As transformation calculations are performed at this rate by the on-board processor, the three accelerations are available at the same rate. By means of further digital processing, the acceleration data are filtered (low-pass 10th order filter with a cut-off at 0.6Hz) and double integrated to give three translation signals in the frequency range 0.033Hz - 0.6Hz (3dB) (0.05Hz - 0.55Hz 0.31dB). These translation signals are transmitted over the radio link in real time.

A complete spectrum is calculated every half hour. Parameters are calculated at each of 64 frequency points (0.025Hz - 0.100HZ at 0.005Hz intervals and 0.110Hz to 0.580Hz at 0.010Hz intervals). The parameters calculated at each frequecny point are: mean direction, angle of spread, second harmonics of angular energy distribution check factor and relative power density.

Reference

Datawell bv, Zomerlustraat 4, 2012 LM Haarlem, the Netherlands (1995). Operation and Service Manual for "Directional Waverider" from serial no. 30109 and including 30076, 30101, 30106, 30108.

BODC Processing of a Waverider Mooring in the Bristol Channel

Data Processing

Data were downloaded from the Crown Estate's Marine Data Exchange as part of the EMODnet Data Ingestion project. The data were in an Excel spreadsheet and were accompanied by a number of metadata reports in pdf format which were archived following BODC procedures. The spreadsheet contained wave statistics and wave spectral data recorded by a Datawell MkIII directional Waverider buoy in the Bristol Channel. The file was transferred to BODC internal format using standard BODC processing procedures.

The variables provided in the file were mapped to BODC parameter codes as follows:

Wave Statistics Parameters

Wave Spectral Parameters

Screening

Post transfer analysis and crosschecks were applied according to BODC procedures. This involved the screening of data using BODC's in house visualisation software where any suspect data were flagged but not removed.

Originator's Processing of Moorings in the Bristol Channel

Data Acquisition and Analysis

A number of moorings were deployed in the Bristol Channel between March 2010 and April 2011 by GEMS Survey Ltd who were contracted to collect metocean data for the environmental impact assessment of a proposed offshore wind farm. Moorings were deployed at three sites (A, B and D) with a Waverider buoy deployed on the mooring at site A and Acoustic Wave and Current (AWAC) profilers deployed at sites B and D along with water quality sondes.

During the AWAC moorings, the instruments were recovered and returned to shore for servicing, including the downloading of data, before being redeployed. However, after the first deployment of AWAC D on 10 MAR 2010, the frame containing the AWAC could not be located due to a fault with the floatation module. This was also the case when trying to recover AWAC B for the second time when recovery attempts of the AWAC on 12 JUN 2010 were unsuccesful. In order to continue collecting data, a second mooring was deployed at sites B and D consisting of a TRIAXYS directional wave buoy fitted with Nortek Aquadopp current profiler. A further mooring was also deployed at site B containing two YSI 6000 sondes.

The Waverider buoy was also recovered for servicing but during the first service visit, it was not possible to locate the Waverider. The buoy was found three weeks later during a cruise for a different project after which it was transported back to the laboratory where the data were downloaded and the Waverider serviced. A replacement Waverider buoy was later deployed at site A. Basic data, including a GPS position, were transmitted to the GEMS' office every 30 minutes but during subsequent service visits the full dataset was downloaded. If the buoy was noted to be drifting on the current, the buoy was recovered, serviced, data downloaded and redeployed. When the buoy was drifting, this was found to be due to the bungee cord being cut.

Data Quality

The lost AWAC B frame was located and recovered using a Remotely Operated Vehicle (ROV) but during this process the AWAC was inverted and so no valid data were collected. In addition to this loss of data, the water quality sonde data were not deemed to pass GEMS QC and so were rejected. This was also the case for the water quality sonde data at site D as the turbidity sensor failed. Again, the data were rejected.

A member of the public found the floatation module that failed on the AWAC D mooring and returned it to GEMS who were able to download the data logged by the unit. During QC it was found that the battery had started to deplete and so any data collected after this point were rejected.

There were also issues with the TRIAXYS B mooring as during QC it was found that the memory card was corrupted and there were no logged data available. Real-time data were then instead used for analysis. During the QC of the real-time data, values that were greater than 20 s in the peak wave period channel (BODC parameter code GTPKZZ01) were removed as the values were a result of the post processing software.

Project Information

No Project Information held for the Series

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

Fixed Station Information

Moorings in the Bristol Channel: Fixed Station, Site A

GEMS Surveys Limited were contracted to complete metocean data collection in the Bristol Channel by deploying a number of moorings as part of the Atlantic Array wind farm development project.

A Waverider mooring was deployed at site A which consisted of a Datawell Waverider Mk.III surface buoy with LED flash light, 30m bungee cord, 94m rope, 10kg float and chain ballast. During the first planned service visit, it was not possible to locate the waverider buoy but it was found three weeks later during a cruise for a separate project. It was found that the bungee had been cut and so a second waverider mooring was deployed at Site A. The second mooring consisted of a Datawell Waverider Mk.III surface buoy with LED flash light and Iridium satellite antenna, 30m bungee cord, 100m rope, 9kg float, chain ballast and sinker weight. Basic data were transmitted from the second waverider mooring to GEMS Survey's office every 30 minutes with the full dataset downloaded during servicing.

Related Fixed Station activities are detailed in Appendix 1

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

Appendix 1: Waverider_A

Related series for this Fixed Station are presented in the table below. Further information can be found by following the appropriate links.

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.