When the U.S. Army Corps of Engineers prepares to dredge the Mississippi River each year, someone has to determine where the problems are and how much sediment needs to be removed.
That job falls to the hydrographic survey crew who operate a survey launch both before and after each dredging operation.
“We do a conditions survey first,” said Bill Chelmowski, one of the hydrographic survey crew members. “That data goes back to our coordinators in the office, and they determine where we have cuts that need to be placed.”
Because the Corps of Engineers has been dredging the Mississippi for decades, patterns have developed as to where the problems usually exist.
“Ninety percent of the time, we are going out to the historically bad areas,” Chelmowski said. “We survey those areas and then the dredge knows where to go.”
One of the challenges for the hydrographic survey team is that the level of the river is constantly changing. They need to make measurements that are exact so the dredge crew knows how much sediment to remove from each location.
“We have survey-grade sonar that works with survey-grade GPS,” said Dave Francksen, who works with Chelmowski on the hydrographic survey. “We use this boat as a measuring tool to know exactly where we are and how deep the water is where we are. We establish the water surface level, and by knowing where we are and where the transducers are in the water, we have all the information we need to know the correct depths.”
Not only do they have to calibrate the sonar and GPS every day, but they have to reset them every time they move into a new pool. They run their surveys one mile at a time and also need to establish new values each mile, because the water level could change during the time they surveyed the mile.
The survey launch has an arm running across the boat which extends the width that can be measured. Six measuring devices, or transducers, constantly measure depths and record that information on computer screens aboard the boat. Each transducer shows up on the monitor in a different color so the crew can track each line.
“We have three transducers on each side of the boat,” Francksen said. “The arms on each side extend out, so it is like we are painting with a wider brush.”
The map on the computer screen gives survey crew members a precise view of the river bottom and records where they have surveyed.
“The warmer colors represent the shallower depths and the cooler colors, the blues and purples, represent the deeper depths, so we know where to survey,” Francksen explained. “It’s like a sight for us.”
The survey crew completes three surveys for every dredging operation. They do a conditions survey, a general survey to determine where the problem areas are. Then they do a pre-survey to provide the dredge boat crew with the exact measurements they need.
“This post-dredge survey that we are doing now ensures that the quality of the work that the dredge is doing is proper,” Francksen said. “It looks good in this case. These guys are good, so we know where the good water is, where the channel is.”
The requirement is to provide a 9-foot deep navigation channel for the shipping industry, but the dredge boat takes more sediment than that.
“The dredge is currently digging at 12 feet,” Francksen said. “It’s kind of a balancing act. It’s important to dredge more material out than we have to, because we want to permit navigation, but we don’t want to take too much. We want to be good stewards of the agency’s resources.”
Francksen said the work the survey crew does in the field is supported by many people in the office who analyze the data and create the specific plans for the dredging operation.
“There are a lot of important things happening that are not on this boat,” he said. “We are only one of the more visible parts of a much larger process.”