PROJECT

Development of a high accuracy, low cost water level sensor

Devepment of a high-accuracy,  low cost water level sensor

Supervisors

Students

  • Alexandra Utley
  • Ryan Donaldson

Project Description

Water level, or sea surface height, measurements are key inputs into a range of coastal and offshore engineering research and industry projects. Historically, water levels have been measured by subsea pressure sensors or above-water tide gauges. Subsea pressure sensors have several critical limitations: 1) unless the exact deployment depth is known (relative to some datum through surveying) the recorded water levels are only usable for analysis that relies on relative water level variations (e.g. measuring wave heights); 2) the data is not available in real time unless somehow cabled to shore or a floating modem; and 3) for waves, the sea surface pressure signal attenuates with depth such that in deeper waters surface waves result in no pressure fluctuations at the sea floor. Tide gauges overcome these limitations by measuring the sea surface height from above when attached to a pylon or other fixed structure whose height can be surveyed. Although this allows the measurement of absolute (datum based) water levels, the requirement for a fixed feature prevents this approach from being used in many areas. An alternative, and more flexible approach, would be to attach a differential GPS (DGPS, cm-scale accuracy) to a small buoy. With a DGPS buoy it would be possible to measure absolute water levels with <5 cm accuracy in any water depth (moored or drifting) without a fixed platform or depth attenuation issues. An added benefit would be the ability to measure low-frequency wave motions currently not possible with accelerometer-based wave buoys due to drift in the sensors. Until recently, DGPS receivers were very costly (~$20k) and large; however, with the widespread use of drones the cost and size of DGPS receivers has been reduced dramatically. Thus, there is an opportunity to develop a novel and broadly applicable water level sensor.

For this project we aim to develop and test a small (~5kg) and low cost DGPS buoy that can be used to record absolute water levels at up to 10 Hz at any location within 20 km of a shore based GPS base station (this restriction could be relaxed/removed with further development). The system would rely on a low-cost (~$1100) DGPS receiver developed for drones that would be placed in a re-purposed, solar powered wave buoy hull (provided as in-kind). Real-time shore communication would be achieved through a Raspberry Pi computer with a 4G modem. The supervisory team has an existing DGPS base station (required to provide differential corrections) that would be established on the UWA campus for testing of the buoy in the Swan River. An existing WA Department of Transport tide gauge in the Swan River at Barrack Street provides an ideal location for testing and direct comparison. Further tests would be conducted at other locations in the river to understand how the buoy behaves with different moorings and in different sea states. The DGPS receiver also has an inbuilt motion unit that will be used to understand how pitch and roll motions of the buoy impact the recorded water level measurements.