Feb
07
2010

Vital signs in an estuary: monitoring underwater waves

Underwater, or “internal” waves, unlike the familiar wind-generated surface waves, occur due to density stratification often generated by coastal tides. These internal wave can lead to redistribution of nutrients and minerals. Internal waves can also cause vertical “velocity shear”, intensifying the vertical mixing process within the water column and bringing suspended particles and nutrients to the surface. Understanding and tracking these internal waves is another way to monitor the vital signs of an estuary.

Internal waves
Internal wavesCourtesy CMOP

CMOP successfully launched its new autonomous underwater vehicles (AUV) to help scientists gain a better understanding of the Columbia River estuary. One of the first studies to use these vehicles will be directed at internal waves. Craig McNeil, oceanographer from the Applied Physics Laboratory at the University of Washington and CMOP investigator, is using AUV’s to study the generation and propagation of internal waves in the Columbia River estuary and plume. He's interested in the physics of internal waves and mixing near the sea surface and the sea floor.

McNeil said,

“Scientists speculate that some bottom following internal waves have closed circulations that traps water and biology. The AUVs will help us sample these waves so we can better understand these complex mixing mechanisms.”

One upcoming experiment will study the dynamics of the freshwater plume as it spreads out over the denser saltwater of the coastal ocean. Of particular interest is to compare measured observations with theoretical predictions. McNeil will program the vehicles to travel into the advancing plume and navigate through the plume front. This will allow CMOP to study the progression of internal waves that are known to be generated at the advancing plume front and determine their propagation speed.

Watch researchers deploy the AUV: Watch Craig, Troy, and Trina deploy the AUV in the Columbia River.
Watch researchers deploy the AUV: Watch Craig, Troy, and Trina deploy the AUV in the Columbia River.Courtesy CMOP

Before those measurements could take place, McNeil needed to test the vehicles’ capabilities in the field. Along with oceanographer Trina Litchendorf and field engineer Troy Swanson, McNeil tested the vehicle in Lake Washington over the winter months. By spring the team was ready to take it through its paces in the Columbia River estuary.

They traveled to Astoria, Oregon, and met up with CMOP’s field team for the vehicle’s first mission in the river. They decided initial tests would be conducted during slack tide due to the limits of the vehicle in strong currents. The mission was based on tidal cycle information supplied by CMOP’s cyber-team. The expected velocities during slack tide would be less than 0.5 m/s or about 1 knot, which was in the acceptable range for the vehicles

The vehicle was deployed near the first transponder set by the team in the North Channel of the Columbia River. There it performed a compass calibration and proceeded to its first designated waypoint. To make sure it was on track, McNeil monitored the vehicle’s position with a device called the Ranger. The Ranger's transponder receives status updates from the vehicle.

Water temperature map: This figure shows a map of water temperature recorded by the CTD on the AUV during its first mission in the North Channel of the Columbia River estuary westward of the Astoria Bridge.
Water temperature map: This figure shows a map of water temperature recorded by the CTD on the AUV during its first mission in the North Channel of the Columbia River estuary westward of the Astoria Bridge.Courtesy CMOP

The results of the mission were a success. The vehicle traveled upon its designated coordinates and collected salinity and temperature data. Now the team has a better understanding of how to control the vehicle’s navigation in the river, which means it will be able to perform longer missions.

McNeil and his team will now use the AUVs to study various physical processes in the Columbia River estuary, including internal waves, currents, and mixing of various biogeochemical components of the water; all of these adding to our understanding of the estuary’s vital signs.

More photos of the AUV deployment: More photos of the AUV deployment
More photos of the AUV deployment: More photos of the AUV deploymentCourtesy CMOP

Your Comments, Thoughts, Questions, Ideas

Racheal's picture
Racheal says:

Going to the north, and south to far is colder. Tropical fish live near the equator. My favorite kind of fish live in Japan.

posted on Sun, 05/02/2010 - 2:29pm
Anonymous's picture
Anonymous says:

Awsome! haha this is cool what is it with salt waters?

posted on Wed, 06/23/2010 - 3:50pm
Anonymous's picture
Anonymous says:

Super ultra mega weird! I HATE tsunamis and vocanos and earthquakes.They are terrible and i never want to see one whare i live...EVER!

posted on Wed, 06/23/2010 - 3:56pm
Connor Anderson's picture
Connor Anderson says:

MY comment for this article is that it really teaches ypu about water

posted on Thu, 07/22/2010 - 2:23pm

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