Posted on June 26, 2023
Scientists from the U.S. Army Engineer Research and Development Center Environmental Laboratory (ERDC-EL) recently assisted the U.S. Army Corps of Engineers (USACE) New Orleans District in measuring the turbidity near a dredging operation in the Bayou Rigaud Federal Navigation Channel.
“In simple terms, turbidity is the measure of relative clarity of a liquid,” said Justin Wilkens, ERDC-EL research biologist. “High turbidity can significantly reduce the aesthetic quality of bodies of water. It can harm fish and other aquatic life by reducing food supplies, degrading spawning beds and affecting gill function.”
The Bayou Rigaud Federal Navigation Channel passes within approximately 500 feet of the Louisiana Sea Grant’s Oyster Research and Demonstration Farm (LSG) and the Louisiana Department of Wildlife and Fisheries Michael C. Voisin Oyster Hatchery (MCV).
A cutterhead dredge is currently performing maintenance dredging of the channel. The dredged sediment is transported by pipeline and placed for beneficial use on Fifi Island adjacent to the channel.
“There is concern that dredge induced turbidity plumes — measure of water clarity reported as nephelometric turbidity units, or NTU — and associated total suspended sediments — TSS in milligrams per liter — could reach oyster operations and thus require additional filtering capabilities to protect oyster production” Wilkens said.
As concerns over the potential hazard to oyster production increased, the USACE New Orleans District submitted a Dredging Operations Technical Support (DOTS) Program request to measure the turbidity in the channel through a technical demonstration. The DOTS Program provides environmental and engineering technical support to the USACE Operations and Maintenance navigation and dredging missions.
Wilkens and Shea Hammond, another ERDC-EL research biologist, needed to learn more about the dredge plume prior to it reaching the LSG or MCV facilities. To accomplish this, Wilkens and Hammond deployed the i3XO EcoMapper, an autonomous underwater vehicle (AUV).
“We used the EcoMapper AUV to conduct three surveys in the vicinity of the dredging site,” said Wilkens. “Two of the surveys were conducted down current of the dredge to document the turbidity plume and the third survey was performed up current to document background conditions.”
The surveys concluded that turbidity levels generally decreased with distance from the dredge and were lower near the water’s surface. Researchers noted that the dredge-induced turbidity plume extended downstream for approximately 2,000 feet before consistently returning to background levels. The surveys revealed that the large spatial scale of the plume was likely due to the strong tidal currents moving through the area. The data also showed that the dredge tenders contributed to the elevated turbidity levels.
“Based on our observations, as the dredge moves closer to the oyster hatchery, we anticipate dredge-induced turbidity levels could approach 30 NTU,” Wilkens said. “We used the data collected to communicate the risks of turbidity to the state oyster hatchery, so they could prepare accordingly. The dredging contractor will also use the data to work to minimize potential impacts from the dredge tenders.”
The solution provided by the ERDC-EL team offers several advantages that have a positive impact on the Nation.
By utilizing unmanned autonomous technologies like the EcoMapper, the solution enables more accurate and comprehensive monitoring of turbidity levels. This information is crucial for understanding the impact of dredging activities on water quality and ecosystems, particularly near sensitive areas such as oyster hatcheries.
The technical demonstration provides real-time data on turbidity levels. This allows for proactive measures to be taken to mitigate potential impacts. By identifying the scale and extent of the dredge-induced turbidity plume, dredge tenders can make informed decisions to minimize turbidity levels that are protective of sensitive areas such as the oyster hatchery. This proactive approach saves time and resources by avoiding unnecessary delays or remedial actions.
Another benefit is cost-effectiveness. Deploying AUVs like the EcoMapper offers a more cost-effective alternative to traditional manned monitoring methods. The unmanned vehicles can collect data over extended periods, cover large areas and operate autonomously, reducing the need for extensive human involvement.
Using AUVs also helps maintain safe operational condition in a marine environment. Excessive turbidity can hinder navigation, impact underwater visibility and pose potential risks to personnel and vessels. By providing information on turbidity patterns and suggesting mitigation measures, the solution contributes to the safety of dredging operations.
“Overall, the use of autonomous underwater vehicles offers a more efficient, cost-effective and environmentally conscious approach to dredging operations,” said Wilkens. “By providing real-time data, it enhances decision-making, reduces potential negative environmental impacts and supports the sustainable management of marine resources.”