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Waterlevel, Current and Wave measements
 


Tide gauge at Timehri.                        Tide gauge at Adventure.
 

  Waterlevel measurements
In order to measure the water levels accurately during the Bathymetric Survey, tide gauges have been installed throughout the area that is being surveyed. Water levels were recorded at 6 locations i.e. Stabroek Stelling, Timehri, Haslington, Parika, Bartica, Adventure and Lima along the coast to enable reduction of the soundings to the Georgetown Datum reference level.

Water levels were measured using MiniTROLLpressure gauges. This system was selected for its reliability and ease of use. The system consists of an underwater pressure sensor measuring the water pressure and thus the water column above the sensor. A protective plastic bag filled with fresh water is fitted around the sensor, in order to protect the instrument from the corrosive environment. Subsequently, the instrument is placed at the bottom of a perforated PVC pipe, that is mounted on a jetty or other structure that is placed in water with sufficient depth. Pressure measurements are corrected for the atmospheric pressure by default, as all sensors are vented. Regular inspections of the tide gauge are carried out and the tide data stored in the internal memory is downloaded to a laptop computer, converted from pressure readings to a water level above the sensor, followed by a reduction to GD. Finally, the data is checked for irregularities before carrying out tidal corrections in the post processing stage.

Current measurements
The current measurements are carried out using a downward-looking acoustic Doppler instrument (Nortek AWAC) that is mounted on the survey vessel. The main advantages of using this type of equipment are:
  • Flexibility of the system: the survey boat with this equipment can easily be moved to any location where measurements are required, for example in the Demerara River.
  • It can stay in one location for continued measurements over a period of time, or the boat can move across a river cross section providing integrated river discharge information. This information is essential for setting up the hydraulic model.
  • The instrument measures detailed velocity profiles, and not a single point velocity only. This may be essential in studying salt and fresh water interaction in the estuaries in the project area.
  Wave measurements  
During the short-term data collection campaign wave measuring instruments are deployed offshore. The focus of this effort is to establish a relationship between:
  • Waves measured at 20 m water depth and waves measured nearer to the shore at 5 m water depth, resulting in a transformation function.
  • Online wave data from NOAA global wave model and waves measured at 20 m water depth.

The following equipment is used:

  • Nortek AWAC Profiler for measuring waves and currents at 20 m water depth.
  • Nortek Aquadopp for measuring waves and currents at 5 m water depth.

Wave Predictions
Wave predictions are prepared continuously by global wave models by NOAA (National Oceanographic and Atmospheric Administration, www.noaa.gov). From NOAA high quality wave predictions are available on line, updated at 6 hourly intervals. The properties of a virtual buoy will be defined based a comparison of the 3 months of wave measurements combined with NOAA model wave data for the same period. Based on this relation the “virtual buoy” will provide wave data on a daily basis that is readily available during the long term monitoring campaign. The main advantage of this approach is that a sustainable system is put in place. Essential data can be acquired online at any time and this does not depend on the cumbersome process of retrieving vulnerable equipment at regular intervals, requiring extensive logistical support.

At the beginning of each day of the Bathymetric Survey, the predicted wave, weather and tide conditions are taken into account.This enables the surveyors to assess the conditions for survey both from a safety point of view and from a technical point of view. Each day forecasts for weather and wave conditions are obtained; the wave data from NOAA wave predictions (www.worldwavedata.com).