The U.S. Army Corps of Engineers is exploring the potential of another water diversion to keep oil flow from coming ashore.
By this weekend, the Corps anticipates the Bohemia Spillway, off the coast of Plaquemines Parish, La., will reach a flow of 800,000 cubic ft. per second.
“Spillway is kind of a misnomer,” as the low area that runs from mile 25 from head of passes to mile 40 is not a manmade water control structure, says Chuck Shadie, chief of the Corps’ watershed division in Vicksburg, Miss. “It is a low spot in the natural bank on the east side of the Mississippi River,” Shadie says. Once the flow rate reaches 800,000 cfs, the bank typically is overtopped, and water pushes eastward and toward the Gulf of Mexico.
The Corps discussed with the Office of Coastal Protection and Restoration the possibility of cutting notches in hopes of increasing the flow rate. “The big concern if we cut notches is that they don’t grow over time,” Shadie says. To date the Corps has not performed the modeling to determine the beneficial effects (if any) of increased flow. However, a rain system moving through the Mississippi and Ohio Valley is expected to add about 2-4 inches of rain, possibly creating additional flow without human assistance. “We will have high river discharges for the next couple of weeks,” Shadie says.
Previously the Corps and the Army’s Engineering Research and Development Center at Vicksburg determined that opening the Davis Pond and Caernarvon freshwater diversion projects to their full capacity would provide no benefits to pushing the oil away from shore. Considerations to model the effects of opening the Bonnet Carre´ Spillway above New Orleans also were abandoned when flow rates in New Orleans dropped below expected ranges.
Part of the challenge of modeling flow rates and effects on the oil spill is that ERDC uses only two-dimensional modeling. “That is for fairly shallow water and shows how water is moving in X and Y directions,” Shadie explains. “We don’t really look at it in terms of how water changes vertically in a water column.”
Two-dimensional modeling is normally sufficient for bay areas, deltas and marshes of the Louisiana coast, which don’t exceed 10-20 ft. depths, Shadie says. However, ERDC doesn’t really have the capability to analyze how oil is moving in deep water. “It’s a different material, and we haven’t modeled how it sinks within the water column,” Shadie says. “We are trying to find out what agencies are.”
Modeling the oil flow will likely require “an ocean circulation model that allows deep currents and large scale circulation patterns in contract to the near-surface models that are wind-driven and used to track surge and waves,” says Ed Link, senior fellow of civil and environmental engineering at the University of Maryland. “That’s an entirely different breed of model.”