For many years, and even to this day, when people in environmental circles discuss the level of flood protection coastal wetlands can provide, it’s likely they say, “Three miles of coastal wetlands can reduce storm surge by a foot.”
The fact is that number is pulled from a 1963 U.S. Army Corps of Engineers report that studied seven storms in south Louisiana between 1909 and 1957.
More recent studies show that the level of storm surge varies based on the type of landscape the water travels over, the size and speed of the storm, and other factors.
A new four-year study to start in January is aimed at better quantifying the amount of protection different types of coastal wetlands can provide.
Much of coastal Louisiana restoration is based on the following ideas — lack of sediment from the river systems led to wetland loss, increasing the possibility of flooding and making area residents feel less secure.
“When I say that, it sounds like science of the obvious,” said Robert Twilley, executive director of the Louisiana Sea Grant College Program.
However, he said, it’s never really been scientifically tested.
To test this, he said, a team of researchers is going to run computer models starting with the 1860s map of the Terrebonne and Atchafalaya water basins. These basins were chosen because the Atchafalaya Basin has continued to get sediment over the years, while the Terrebonne Basin has not because the flow of sediment down Bayou Lafourche via the Mississippi River was greatly restricted in the early 1900s.
The 1860s maps show that the Houma and Morgan City areas have almost the exact same wetland buffer between the town and the open water.
The assumption is that as the sediment is turned off from Bayou Lafourche, there will be changes seen in that marsh buffer in each area, Twilley said.
Several computer models will be used to do this historical work and project into the future. One model shows how sediment can build land and will be used to show how the lack of sediment affects the landscape. Another model will study storm surge flooding to evaluate the flooding risk as the landscape changes via the loss of sediment. The goal is to compare wetland loss to increased flooding in those areas.
“Calibrating that value is actually pretty important,” Twilley said.
Another goal is to get away from that adage that 3 miles of coastal wetlands will reduce 1 foot of storm surge. Instead, he said, researchers will be able to see just how much wetland, and what type of wetland, is needed to provide flood protection.
Twilley and the other researchers have started gathering needed information starting with the 1800s. Because there was no satellite imagery at that time, it’s going to be a bit of a history hunt, Twilley said. This is going to include descriptions written down by the surveyors at the time about forests and wetlands in an area.
All the data will be used to make a computer model. Researchers will then be able to see if land loss along the coast proceeded the way the computer model predicted. If there is a close resemblance, then the model can be used with confidence to predict what future restoration projects will be able to achieve.
“We always use these models to look into the future, but if you can’t hindcast, how well can you forecast?” he asked.
The project will run for four years, but as information becomes available, it will be released to the National Science Foundation and the public.
Amy Wold covers environmental issues for The Advocate. Follow her on Twitter, @awold10.