When ex-typhoon Merbok barreled through the Bering Sea in September 2022, it brought high winds and flooding to 1,300 miles of the Western Alaska coastline. But the community-level impacts weren’t only a product of their proximity to the storm’s path. A recent study used data from Typhoon Merbok to show how diverse tides and the shapes of the region’s bays also play a key role in these impacts.
Lead author and University of Alaska Fairbanks professor Steven Dykstra said the inspiration came from his prior research in flood-prone areas in the southeastern United States.
"I saw these funneling shaped basins there and was really curious about, how important are these basins for explaining the different flooding exposures that these communities might be having?" Dykstra said.
The funneling effect of some bays can increase the severity of storm surges as they travel upriver. But to explain the phenomenon, Dykstra said tidal influences need to be removed from the equation. He and his team developed an analytical tool to accomplish that for the first time.
"We have to use a new approach that we developed to look at how the tides are changing through time, and then can remove the tide from the water level and look at what factors are left," Dykstra said.
Dykstra said that water level data is especially sparse in Western Alaska. The National Oceanic and Atmospheric Administration (NOAA) doesn’t operate any permanent monitoring stations along more than 700 miles of coastline stretching from Bristol Bay to Unalakleet on Norton Sound.
"For long-term, high-quality observational data, we have a very large gap there," Dykstra said.
But in the months leading up to Typhoon Merbok, Dykstra said that NOAA had coincidentally placed numerous short-term water level gauges along the Western Alaska coast as part of a standardization effort.
This gave Dykstra’s team the data they needed to apply their novel method to three funneling basins that define the region – Norton Sound, Kuskokwim Bay, and Bristol Bay.
The study – published in the journal Continental Shelf Research – found that the shape and depth of Norton Sound causes storm surge to increase as it pushes inland, while in Bristol Bay storm surge decreases as it pushes inland.
Dykstra said that Kuskokwim Bay falls somewhere in the middle, but that tidal influences are especially important for how a storm hits upriver communities.
"Kuskokwim River and [Kuskokwim] Bay is actually the longest estuary in the United States. The tides move inland further than anywhere else in the U.S., even further inland than the Columbia River or the Hudson River or even Chesapeake Bay," Dykstra said.
While tides decrease in magnitude as one travels further upriver, the funneling shape of Kuskokwim Bay causes storm surge to significantly increase as it travels upriver from the mouth. During Typhoon Merbok, it nearly doubled over a roughly 140-mile stretch.
"The storm surge itself increased by almost 40% from Goodnews Bay to Bethel," Dykstra said.
Having the NOAA water level data from Typhoon Merbok also led Dykstra’s team to another first – a statewide map showing how ocean water levels fluctuate seasonally across Alaska. Dykstra said that that cannot be fully explained by changes in sea ice.
"In the northern Bering Sea the waters are much higher in the summer, and then they decrease throughout the fall. Whereas in the southern Bering Sea, they're low throughout the summer, and then they increase throughout the fall. And this is a change of a foot or a foot and a half on average throughout the year," Dykstra said.
Dykstra hopes that the method developed in the study to isolate storm surge data can help both local communities and the National Weather Service better identify and understand vulnerable areas in Western Alaska.
"They can see a community in Norton Sound is going to be more susceptible to flooding than a community in Bristol Bay," Dykstra said. "They may look like they're both just in Western Alaska, but it's these different basins that are going to have these different effects on storms, and we need to be separating out these different drivers."
Dykstra said that the study is just the first step in understanding funneling basins in Western Alaska. He is now focused on a pilot project looking at turbidity, or cloudiness, of water in Kuskokwim Bay to facilitate water-penetrating lidar analysis of the basin. It’s part of a national, interagency aerial surveying project led by the U.S. Army Corps of Engineers.
