They are never too young to be drafted as a field assistants… Eh Trish?
The unique geology of the Yucatan means that hydrologists like my wife typically study water flowing through the flooded cave systems. But she also advises some of the many great archaeology projects in the area (The Maya had to get their drinking water from somewhere…right?) so we don’t spend all of our time wrapped in neoprene. Though I have to say that when she does do surface work, she always seems to find the stinkiest stuff around to balance all that canned-air we would otherwise be breathing:
Smells like science!
We did eventually make our way to a wider section where the water was moving swiftly. The original plan was to simply anchor one of the flow sensors on a rock and drop it in. But after mucking about at the site for a while, we were puzzled to see hardly any displacement on our little tilt meter though the surface flow was quite strong. Then we realized that the water at our feet also felt warmer than it did on the surface, and it dawned on us that this close to the ocean, the tide was coming in underneath the outgoing fresh water. The best solution I could come up with on the spot was to put the pivot joint on top of one of the pendulum rods: raising the flow meter to a higher position in the water:
We will have to wait for the data to see if we’ve given the logger enough height and The temperature should tell us if the logger has been trapped in rising salt water. I would not be surprised if we end up using an entirely different arrangement next time, perhaps with some kind of floating buoy system so we can go back to a pendulum arrangement where the buoyancy is easier to control. Sometimes all you get from your first deployment, is an understanding of how to do the next one…
Further in, we could put the logger right on the stream bed which had been scraped clean by the high water velocity. You can tell from the video (above) that we are well past 10cm/sec where vortex shedding starts to be a problem.
The next day we set out at a different river, working our way much further inland to (hopefully) avoid the salt water intrusion. We also planted a pressure unit to capture water level data near the archaeological dig site. Given how exposed these sensors are, I can only hope that the local fishermen don’t decide our little bots should spend the rest of their days decorating someones coffee table.
The water in these coastal discharges was racing by comparison to the gentle subterranean flows we usually work in, so I expect allot of turbulent noise in the signal. On top of that the removal of their salt-water ballast mass (used on pendulum installations) moved the center of mass enough that I had to put them in upside down. That’s just the kind of things you run into when you are doing something for the first time. With all the things we learned from the open ocean deployments, I’m really excited to see the Pearls moving into another new environment.
Addendum 2016-07-18
Well we finally made it back up to the north coast, to replace those upside-down units we left back in March:
Happy to report that the first deployment gave us good data, and I think this next crop will do even better, provided they don’t get hit by a passing boat propeller…
Addendum 2017-06-24
Believe it or not the unit was still buoyant, and gave us reasonable data.
After nearly a year we returned to discover that the stream-bed unit pictured above had gone AWOL, and the remaining two flow sensors were heavily encrusted with criters of all kinds. We’d seen bio-growth before on reef deployments, but never to this extent. To avoid further losses, the replacement sensors have been painted black and deployed in “stealth mode” locations.
Addendum:
During these deployments we stayed in the fishing village of Chiquila. I spotted this lovely graffiti during a morning run:
Underwater Arduino Data Loggers 