Within the span of a week, the Gulf built a beach in front of most of the rock wall used to close Katrina Cut.
The wall was constructed in 2010 using money provided by BP during the oil spill. The goal was to close a mile-wide gap that opened in the west end of Dauphin Island during Hurricane Katrina.
While a stretch of several hundred yards in the middle of the wall still does not have sand in front of it, there is a beach in front of about two thirds of the mile-long wall.
Scientists expressed surprise after viewing pictures of the new beaches taken by the Press-Register a few days after the passing of Tropical Storm Debby. The change in the beach was obvious and remarkable, as little sand had built up against the rocks in the two years since the wall was completed.
On the Gulf side of the wall, the sand is flush with the top of the rocks and slopes down to the water eight feet below. On the Mississippi Sound side though, there is no sand behind the rock wall, which drops abruptly into the water.
Scott Douglass, a coastal engineer with the University of South Alabama, said that beaches on Dauphin Island have a history of changing dramatically during major storms. In some cases, the beaches erode a great deal, while in other storms, they grow much wider.
Debby, he said, appears to have added sand to the island.
“Those pictures are really interesting,” said Douglass, after viewing the images while in Spain for a coastal engineering conference. “I think perhaps some long-period waves in the latter half of the storm, as it moved away, could have done this.”
In technical terms, the “long-period waves” mentioned by Douglass are waves that are several seconds apart. As Debby traveled through the Gulf toward Florida, the storm generated waves from 5 to 20 feet tall that were about eight seconds apart according to a federal data buoy stationed 12 miles south of Orange Beach.
“That is actually something that I’ve seen before on Dauphin Island. When a storm is moving away, we get long-period waves, which naturally build up beaches,” Douglass said. “This may have been the perfect storm to build up these beaches in that the sand predominately moved north from the offshore.”
Douglass said sand would naturally begin to build up along the rest of the rocks over time.
“I think the sand will stay there, and I think the middle will fill in. I don’t think it is unpredictable at all,” Douglass said. A series of sandbars built up offshore of the rock wall over the last year. Douglass said the storm had caused those bars to “collapse” onto the wall.
He said the beach in front of the wall was higher than the natural beach to the west because the rocks were higher than the natural beach.
“The sand just ran up as high as it could. On a normal beach, it would have overrun the island, but in this case, it just kept climbing up the wall,” Douglass said.
The height difference between the natural beach and the beach in front of the wall raises questions about the long-term fate of that part of the island.
During storms, waves wash over the top of the island from the Gulf side to the Mississippi Sound, often eating out troughs from one side to the other. That’s what breached the island during Katrina.
Waves washed over the island during Tropical Storm Debby, as well. Visiting the island after the storm, it was easy to see the path the water took around the western edge of the rock wall. The sand there is about two feet lower than the sand in front of the rocks.
Douglass said the possibility of the island breaching at the edge of the rocks in a hurricane needs to be studied. He declined to predict what would happen in the next big storm.
“I think what I would say is that barrier islands always breach at the lowest spot,” Douglass said