SOUTH PADRE ISLAND — Call it the pelican briefing.
The deaths of browns pelicans on the Joseph “Joe” Gayman Bridge on State Highway 48 between Brownsville and Port Isabel have generated headlines for a decade when cold fronts with gusty north winds hit the region.
But what, exactly, is the aerodynamic dilemma that causes the birds to flop down on the roadway and tangle with traffic?
John Young is a wildlife biologist and environmental specialist working at TxDOT headquarters in Austin.
This month at the 26th annual Winter Wildlife Outdoor Expo he provided some answers.
“I’ve been involved somewhat with some of the research studies that have been going on with pelicans to try to resolve the issue and find out what is causing the pelicans to fall out of the sky on SH 48,” he said. “Really to try to resolve some of the issues of why did the pelican cross the road? How many pelicans have crossed the road? What is this level of mortality that we’re witnessing, and does it have population implications and influence?”
Kept happening
The pelican problem only arises when a winter cold front comes barreling in from the north or northwest, bringing high winds.
It’s also only an issue for the birds at the right time of day. In this case, late afternoon and early evening.
Pelicans, you see, are flying southeast to northwest, paralleling the canal that runs from the Brownsville Ship Channel under the Gayman bridge and empties into the Bahia Grande, where the pelicans roost at night.
This channel was created to allow saltwater to flow into the Bahia Grande after years of the bay being cut off. But what was intended as a sound environmental restoration brought with it new problems.
Some of the pelicans, sometimes a lot of them, never make it.
“We thought at first it was isolated events, but it continued to happen and it was around 2013, 2012, when we met with the Fish and Wildlife Service and we had real concerns for this,” Young said. “We’re not just worried about the pelicans. You have the volunteer rescuers out in the road and in traffic trying to pull pelicans out, and we’re worried about the safety of the traveling public.”
One of the issues is the speed limit along the highway, 75 miles per hour. Many have suggested the state just lower the speed limit, but Young said it’s not so easily done.
“The Texas Department of Transportation has very strict rules that were set by the Legislature and the (Texas Transportation) Commission on traffic speeds, and so we did the traffic speed study on SH 48 and found the traveling public was going between 45 to 90 miles an hour and 85 percent of them were traveling around 70 to 75,” he said.
“And that right there, that 85 percent, is what we’re forced to set the speed limit at,” he added. “Whatever 85 percent of the public is going at, that’s how the speed limit is set.”
Wind tunnel
So what are the physical forces at play here?
The answer to that began with modeling performed in Texas A&M University’s wind tunnel, at least when TxDOT could shoehorn in the testing when the lab wasn’t performing U.S. Department of Defense work, Young said.
“So what happens is you hit the concrete traffic barrier and wind speed increases,” he said. “It’s increasing all the way up to about 15 to 20 feet in the air. So there’s a 25-mile-an-hour wind blowing and pelicans are flying into this and when they hit that wall, that 25-mile-per-hour wind becomes fast and it’s like flying into a wall.”
What was happening was the wind was actually speeding up as it swept over the northern-most rail barrier on the bridge. These safety barriers, since replaced, consisted of a solid concrete barrier on the north, with another in the center of the divided highway, and one more on the south side of the bridge.
As the wind swept over the north barrier, it also created a downdraft just behind it that was actually sucking the birds down onto the roadway.
Young said pelicans normally fly at about 22 mph, and with the wind increasing to above 30 mph as it accelerated crossing the bridge, the birds couldn’t navigate the higher wind speed to make it into the Bahia Grande.
Fluid dynamics
With the results of the wind tunnel modeling in hand, TxDOT turned in-house to its research arm, the Texas Transportation Institute.
Using data from the wind tunnel, scientists created computer models of just what kind of safety barrier would cause the least disruption to pelican flight as the birds traversed the bridge.
Young said TxDOT was limited to eight barrier models — these are important safety features that keep cars and trucks from leaving the roadway and falling into the water — which were already approved designs.
“The Texas Transportation Institute used wind tunnel model results to inform the computer and we ran computational fluid dynamics that this is how wind functions over the bridge as it exists, and then we can play with different types of rails, different types of things, to look at how wind is affected as we change and modify the bridge,” Young said.
“The one thing everybody was saying was true,” he added. “The concrete traffic barriers were creating a problem with the wind. But now the question is, before we go and try and start throwing all these different solutions at it, what can we do that might be the best thing to do?”
The answer turned out to be T2P, a traffic safety barrier consisting of a parapet, then two louvers and an oval steel rail on top. Total height is 44 inches.
“So a little concrete parapet, two steel rails and then a steel pole and that eliminated or reduced and brought down our air speed,” Young said. “The wind is picking up speed now only five feet above the bridge instead of 15 to 20 feet.”
“The area of downdraft is completely gone … we no longer have a pulling, sucking motion going on,” he added.
For the pelicans, the physics have improved, but it’s hard to tell if that is the primary reason pelican deaths have been reduced from their highest levels of years past.
Pelicans are still killed crossing over the bridge, but efforts by the volunteer rescuers, the Pelican Team, and Texas Department of Public Safety and other law enforcement agencies, and Port Isabel firefighters all have contributed to reduced mortality.
In addition, permanent warning signs have been erected by TxDOT to advise motorists to slow down when adverse winter conditions prevail.
“We’re very concerned about the protection of the population and where it goes,” Young added. “We don’t want to see brown pelicans come back on the Endangered Species List, or even on the state endangered species list.”
