Measuring Water Levels with Microwaves

Transcript

This is Making Waves from NOAA's National Ocean Service. I'm Troy Kitch.

Every few months, a couple people from the Field Operations Division of NOAA's Center for Operational Oceanographic Products and Services—that's CO-OPS for short—drive an hour and a half from their lab in Chesapeake, Virginia, to pay a visit to this really long pier in Duck, North Carolina.

Today, oceanographer Grace Gray and field technician John Stepnowski are making the trip to the pier, part of the Army Corps of Engineers Field Research Facility in Duck. It's a long, slow drive out to the end. They're here to check in on NOAA equipment and instruments that are undergoing field tests in North Carolina's rough surf zone.

[Grace Gray] "We're sitting here on their research pier which is just a little over a half a kilometer long and it's close by and it also happens to be one of the highest wave environments on the East Coast, so it gives us a great variety of conditions to test our equipment in some of the most severe conditions that our equipment would ever be exposed to."

Today, Grace and John are inspecting a new kind of device called a Microwave Radar Water Level Sensor—and it's a big step forward in how water levels are measured. In time, this new device will replace the hundreds of older style of acoustic water level sensors deployed around the nation.

[Grace Gray] "So this is one of our newer water level sensors. It uses microwave radar technology. It bounces a signal off the water to tell you what the level of the water is under the sensor. There's a sensor just like this under this protective hat here and, we have these set up, they're the same type of sensor, but they're set up with different processing scenarios. So right now we're testing out the best processing algorithms to use in the field."

Here's the big picture of how a microwave radar water level sensor works. This is the top of the sensor - where the distance to the water is computed and displayed. Underneath, there's a cone-shaped tube pointed at the water's surface. Microwaves are sent from the device down to the surface of the water through this tube. Then the microwaves bounce back up to the device. By using algorithms — math — the range to the water can be determined. Grace explains why it's such a big change compared to the technology now in wide use, called an Aquatrack water level sensor.

[Grace Gray] "The technology we've used primarily is the Aquatrack water level sensor. It sits down in a stilling well in the water. Where the nice thing about this one is that it sits — as you noticed — up out of the water. There are no parts submerged that need to be cleaned by divers. It's cheaper technology. They have been declared operational and we do have operational water levels in some locations."

So why is the microwave radar sensor only in use in a few of the over 200 water level stations that make up the NOAA National Water Level Observing Network? To answer that, We need to travel back the Field Operations Division to talk with the chief of the office, Kate Bosley. It turns out you can't just swap out an old water level sensor with a new device. It's a slow process that has to be done with a lot of care.

[Kate Bosley] "The reason we test so extensively and do a lot of overlapping, if we were going to change measurement systems we run them for at least a year right next to each other to make sure that everything is exactly on before we take the other one out. So that's exactly what we're doing now with the network transitioning from the acoustic sensor to the microwave. All of those stations will run a year or more, a lot of them will run two depending on the environment before we take the other one out."

[Kate Bosley] "I think people would be surprised at just how much work is involved in keeping a single station running and just how many people have their fingers in it over the course of a year. It's very important if you're going to do sea level rise and really small change measurements that you don't introduce a bias by using a different instrument. That you're sure of that number."

And Kate said that you have to be sure of that number because so many people rely on NOAA's water level data. It's about a lot more than just measuring today's high tide. The microwave radar water level sensor is one component

[Kate Bosley] "It is more than just telling when the tide comes in. It's providing a rich database of not only what's going on right now but it adds to the archive and history so we can look back and then look forward and find trends — sea level rise is one trend — but there are lots of other things. So it's about providing that backbone network of coastal information so people can make decisions."

This data is critical for safe ocean navigation, but Kate said it's also used for many, many other things…In research, to help in building infrastructure like bridges, to measure how water levels change after an area is dredged, to help plan out development in a coastal community so people are better prepared to deal with storm surge and erosion. And, of course, recreation. Millions of people rely on accurate water level data to know when to head out for a day in a boat and the best times to go fishing. The next time you head out to the ocean or the Great Lakes, see if you can spot a NOAA water level station. For bonus points, see if you can tell if the station is using an acoustic or microwave radar water level sensor — or both.