Monster waves rearrange deep-water reefs
Big waves from hurricanes like Irene in 2011 or Superstorm Sandy last October or even nor’easters like one that lingered in Delaware last week erode shorelines and the beach and dunes.
But it turns out, they also impact deeper waters offshore – shifting sand, creating ripples and gullies and are even powerful enough to permanently change the artificial reef system just off the Delaware Coast, according to new research by Arthur Trembanis, at associate professor at the University of Delaware College of Earth, Ocean and the Environment.
After a really big storm like Sandy “it looks like somebody attacked the [sea] floor with ill intent,” Trembanis said.
At surface level, the waves from Sandy were large and distant – about two football fields, he said. That wide spread gives waves lots of power – enough to destroy the old Indian River Inlet bridge access road in Delaware or demolish boardwalks and buildings in New Jersey and New York.
In deep water where the state’s Redbird Artificial Reef is located – a collection of 997 subway cars dropped over board to make fish habitat or at the tri-state reef project in which the 563-foot long Navy destroyer, the Arthur W. Radford, was sunk for both a fish and dive area – the waves still pack considerable power and the strength to topple, crush or destroy the subway cars, split old fishing boats, and even separate the bow of the Radford from its midship and stern.
Sandy was powerful in its offshore destruction but so was Hurricane Irene, the previous year. And there also was a damaging nor’easter in March, Trembanis said. It each case, the offshore waves were big and their impact was significant, he said.
He and his crew are planning one last trip to the offshore reef sites next month and to assess whether this latest nor’easter changed the bottom profile or altered the reef sites, yet again.
But among the best data Trembanis and his team of students collected was gathered three days before Sandy hit and then, right after.
The pre-Sandy bottom is relatively smooth – almost like a sandy beach. But during Sandy, the deep wave patterns scoured the sand from the subway cars on each side as the remnants of the powerful, surface waves radiated to the bottom. They formed ridges across the open bottom – a pattern Trembanis describes as ripples and they left smooth, eroded patches.
The physics of the ocean wave at the surface has its power in the wave length. Waves that are spread apart, pack more power than waves closer together. It is those distant waves that have the power to penetrate the deeper waters of the reef sites – at about half the length of the surface wave.
It’s a little more complicated because not only do the waves have to form far apart, but they also have to be big. The perfect sets of conditions – with hurricanes like Irene, large nor’easters and superstorms like Sandy – don’t happen all the time but they aren’t as unusual as you might imagine.
The research is important because it helps scientists understand how sediment – the sand and mud and gravel on the sea floor moves and is transported in the ocean system, Trembanis said. But understanding how the ripples form on the sea floor and the types of wave conditions that make them, help scientists improve the models used to study oceanography.
“If we don’t get the ripples right, we can’t get the models right,” Trembanis said.
The patterns, it turns out, almost create a wave action fingerprint on the bottom based on what is happening on the surface, he said.
“It can help us understand what to expect during coastal storms,” he said.
Wave heights just offshore during Superstorm Sandy totaled about 22 feet, compared the the 1991 “Perfect Storm,” where waves were 18 feet, said Carter DuVal, a graduate student working on the deep wave project.
“That’s pretty tall,” DuVal said.
But a nor’easter that came right after Sandy, had offshore waves of 16 feet and the waves from a nor’easter in March were 26 feet, he said.
What Trembanis isn’t certain of is when the bow of the Radford split from the rest of the ship. He suspects it may have happened during Hurricane Irene because the damage was done prior to Sandy.
But he does know that deep waves from Sandy crushed some of the subway cars and formed deep scour pits around them.
“It was moving really fast across the sea floor,” he said, of the waves near the bottom.
In December 2012, they revisited the site and found the area had begun to recover.
And then, they went out again after the March nor’easter and they found more damage with crushed and rotated subway cars.
“It was like the whole seabed had shifted,” he said.
Contact Molly Murray at 463-3334 or mmurray@delawareonline.com. Follow her on Twitter @MollyMurraytnj.