4 minute read
Skyway Wave Attenuation Devices
BY LINDSEY RANAYHOSSAINI, STAFF WRITER
The Florida Coast has been barrelled by hurricane after hurricane in recent years, and as those storms have become both more frequent and volatile, finding innovative solutions to improve coastal resiliency is imperative.
One such solution has been installed by the Florida Department of Transportation and Transystems in Bradenton along the south causeway of the Sunshine Skyway Bridge. Wave Attenuation Devices (WADs) are large triangular pyramids constructed from fiber-reinforced concrete that have been placed 50 to 200 feet offshore at two different locations on the west side of the fishing pier access roads.
This $6.3 million project was completed in October 2023 with the goal of not only protecting the Interstate 275 connector – a critical roadway for freight, commuters and tourists – but also promoting seagrass growth for future bridge replacement projects.
Created by Living Shoreline Solutions, WADs are designed to reduce a wave’s energy by absorbing the energy rather than deflecting the wave. Ultimately, this reduction in wave energy helps prevent storm surge damage to coastal roadways, as repeated wave energy can erode the embankment that holds up a road.
Passing The Test
Less than a year after installation, the WADs were put to the test when Hurricanes Helene and Milton made landfall in September and October.
“Hurricanes Helene and Milton tested the wave breaking ability and resiliency of the new (WADs), and they performed as expected,” said Brent Setchell, P.E., district drainage design engineer for FDOT District 1.
“While the fishing pier access road did sustain minor damage after the storms, most of the damage occurred in areas outside of the WADs or was attributable to significant storm surge.”
Furthermore, the devices have contributed to two acres of seagrass growth to date. Prior to their installation, there were almost no patches of seagrass present in the area. The WADs fulfilled their role by allowing seagrass roots to take hold on the seafloor without being swept away by waves.
Setchell said that seagrasses create ideal habitats for fish and other marine life, and the WADs contribute to the habitat.
“Their unique hollow shape with triangular cutouts allow small and medium size fish to hide from prey and freely swim in and out,” Setchell said. “Additionally, each time we have been snorkeling at the WADs we’ve seen schools of bait fish swimming between the WADs. The new seagrass beds and WADs offer excellent nursery habitat for an abundance of marine life.”
How WADs Work
The triangular pyramid shape of the WADs provides structural stability and helps the devices to stay in place after repeated impacts. The inside and base of each WAD is hollow, and the devices feature angled cutouts that allow a wave to enter. The flat side of the devices face the wave direction, providing maximum surface area for initial impact.
When a wave passes through a WAD, it hits the three angled inside walls, reducing the wave’s energy. The two rear-facing sides allow the wave to exit with a significant reduction in velocity. Waves typically die out three feet behind the structures, and a wave’s process through the WADs removes sediment from waves and deposits it on the shoreward side.
The WADs used along the Sunshine Skyway Bridge were built in two different sizes to match the water depth in the areas where they were placed. The smaller WADs are 8.5 feet tall and weigh approximately 8,500 pounds, while the larger WADs are 10.5 feet tall and approximately 13,000 pounds. The devices are held in place by their own weight and are arranged in two rows to force impact with waves for maximum energy dissipation.
These massive structures stick out of the water two or three feet to allow for visibility to boats.
HOW THEY’RE MADE
The WADs were constructed and installed in three phases. Specially designed steel fabrication molds were used for casting, and the fiber-reinforced concrete took about 48 hours to cure.
They were then transported by barge to the desired location and placed by crane in the water at a rate of 10 WADs per hour. A total of 844 devices were deposited.
Though the WADs have proven successful, Setchell said that FDOT does not currently have plans to install them elsewhere.
“While we are very pleased with the early success of the WADs project, we are still assessing their effectiveness and evaluating future projects which may benefit from WADs,” Setchell said.
