Delta-X Field Stories: Measuring Water and Sediment in the Delta

Alligator spotted by Delta-X field team in coastal Louisiana.
Alligator spotted by Delta-X field team in coastal Louisiana. Credit: John Mallard

By John Mallard and Tamlin Pavelsky, University of North Carolina /NEW ORLEANS, LOUISIANA/

Cruising through a bayou during recent fieldwork in the Mississippi River Delta, our boat driver casually pointed out an alligator and zoomed on by without slowing. After seeing us scramble to get out our phones to take a picture, though, he realized that perhaps these scientists from the piedmont of North Carolina were a bit less used to seeing alligators than the locals of southern Louisiana, and obliged us by circling back around for some pictures.

This alligator is one of the many species of animals and plants that depend on the unique habitat provided by the bayous, marshes, lakes, and rivers of southern Louisiana. Equally dependent on them are people; we rely on them to protect inland towns from tropical storms, support recreational and commercial fishing, and provide transportation networks for global shipping. With this vital role of the region in mind, DeltaX is measuring how water and sediment move through the delta to learn how and why marshes are changing in the area.

Our team, Tamlin Pavelsky and John Mallard from the University of North Carolina, was in the delta in early March to install sensors that measure water level in the marshes, bayous, and lakes. DeltaX is measuring the water surface elevation and extent via aircraft (NASA’s AirSWOT and UAVSAR), which allow us to get measurements across a very large area. Our sensors provide a check on these airborne measurements at the points where we install them. This process of checking our airborne measurements against points on the “ground” is called “validation,” and is a crucial part of using airborne measurements.

Of course, fieldwork had to be modified due to COVID. Prior to fieldwork the team quarantined at home and then formed a pod for the trip. Instead of flying down and buying groceries on site, we rented a car and packed two coolers full of food for the week. We wore masks at all times when we were outside of our accommodations. We self-monitored for COVID symptoms every day. Although these precautions added some time to our work, we were extraordinarily grateful to UNC and to NASA for helping us figure out how to work safely and successfully!

Tamlin Pavelsky installs water sensor for Delta-X project in coastal Louisiana. Credit: John Mallard

The sensors we installed are pressure transducers. When underwater, they record the pressure of water pushing on a membrane inside the sensor, and then we use that weight to determine the depth of water above them. The sensors are only about the size of a cigar, but they can record tens of thousands of measurements without running out of battery. The sensors we installed in March will stay in the field through Fall, when we’ll return to retrieve them and download the data.

Our work was done from a small motorboat whose flat hull was specially designed to work in shallow water. On a typical day we would meet our hired boat driver at a local boat ramp to put the boat in the water just after breakfast and be back at the boat ramp by 3 p.m. after installing 5-9 sensors. On board, we had about a dozen 10’ lengths of PVC taking up a whole side of the boat, along with our sensors, tools, and food and snacks for the day.

Caption: John Mallard pushing PVC pipe into the mud to install a sensor for the Delta-X project.
John Mallard pushing PVC pipe into the mud to install a sensor for the Delta-X project. Credit: Tamlin Pavelsky

We would navigate to pre-determined locations to install sensors, which were usually on the edge of the water, and the driver would gently run the bow aground to keep us in place. Then we pushed a PVC pipe into the mud as far as we could by hand and finished the job with the post pounder.

We needed to get them deep enough into the mud so that a slit cut in the side of PVC was below the water line and would allow water into the pipe. We tied the sensor to the cap of the pipe using Kevlar cord that is highly resistant to wear, and hung it inside the pipe so that it was below the water surface. At each location we took measurements of the height of the pipe, the length of the cord, and depth of the water so that we could later translate the depth of water measured above the sensor to the actual depth of water at that point. After installing the sensor, we marked the location with a handheld GPS device and moved on to the next site.

On some days we only traveled a few miles and had to take the boat out of the water multiple times to drive to different boat ramps, but on our last day we put in at a single ramp and traveled more than 70 miles across the lakes and bayous of Terrebonne Bay. The weather was sunny, low 70s, and we had lunch on a narrow beach by an inlet separating the Gulf of Mexico from the bay. We watched birds and dolphins fishing for their own lunch of small fish and crustaceans in the inlet. It was a wonderful break at the end of a successful field trip. After a challenging year with so many disruptions of our work, among so many other things, it was such a relief to feel like we’re starting to be able to “get back to work”!