Manly Hydraulics Laboratory

NSW Public Works

Manly Hydraulics Laboratory

Offshore Ocean Tide Data Collection

Introduction

The data collection in this program started in December 1992 at Tweed Heads and is ongoing. The Offshore Tidal Data Collection Program provides offshore water levels which are a measure of the ocean tide minimising any effects of nearshore processes. The program is funded by the Floodplain Management Program which was administered by the then NSW Public Works until April 1995 when it was transferred to the Specialist Coastal and Flood Services Section of the Department of Land and Water Conservation. The program provides data that forms the offshore boundary conditions for flood studies, as follows:

ocean tidal data, with minimal effects of nearshore influences, (such as river flows, wave setup and attenuation/amplification due to shallow inshore waters)
a measure of ocean tide anomalies (recorded tides that exceed or are less than the predicted astronomical tides.)

The actual ocean tide levels are necessary to provide a consistent approach to flood studies by defining the tailwater conditions. Tidal anomalies which produce water levels higher than the predicted tides are of vital importance to flood and coastal studies.

Instrumentation

MHL uses Anderaa water level recorders for this project, which are high precision recording instruments for determining water level in the open seas. The total pressure is measured by an ultra precise quartz pressure sensor. The total pressure can then be corrected for water density and atmospheric pressure to determine the water level. The operation depth is limited by the range of the pressure transducer.

The water level recorder is specially designed to measure ocean water levels. Placed on seabeds, the instrument records pressure, temperature and conductivity at regular intervals. The data is stored to reusable solid state data storage unit (DSU). Five channels of 10 bits each are recorded in sequence. The channels are reference (channel 1), temperature (channel 2), pressure (most significant part, channel 3), pressure (least significant part, channel 4), conductivity (channel 5).

Instrument Calibration

The calibration of the instruments prior to and following deployments is carried out at MHL. The calibration coefficients of the Aanderaa's are checked using the MHL dead weight tester (DWT) which applies a known pressure to the sensor. The instrument is checked over a range of pressures in accordance with the MHL QA procedures. If the deviation of the instrument reading and the DWT is outside the accepted range then a new set of coefficients is calculated.

Instrument Retrieval and Deployment

The instrument deployment is a specialised diving operation. This activity is carried out by MHL divers using local boat and diving operators as support. The retrieval and deployment routines have been developed over the period of the program to the present standard. This involves a single trip to the site to retrieve the existing instrument and deploy the new instrument. Global positioning system (GPS) and tracklines are used to locate the site prior to diving. Deployment check sheets are used to record the details of the retrieval and deployments

Data Reduction

Generally temperature and conductivity data are available for the WLR7 deployments and density could be derived from these records. However since this data represents only one value of density at the bottom of the water column and since there is no such data for the WLR5 deployments a single value for water density has been adopted for correcting the raw data. The single standard adopted is 1023.74 kgm³ representing a salinity of 35 psu and temperature of 25 Deg. C. This methodology was considered appropriate for the data reduction and for the use of the data in determining short-term events and storm surges as a consistent water density is assumed during events.

Barometric pressure from the closest barograph station is used to correct the raw data file for atmospheric pressure. Generally MHL barograph stations were used for these corrections. Bureau of Meteorology data from Coffs Harbour was used to correct the data until around July 1987 when MHL barometric data became available. There are many places in the early datasets where processing was not possible due to gaps in the BOM barometric data.