New radar analysis method can improve river safety in winter

Researchers at the University of Alaska Fairbanks have developed a method that allows radar to detect open water zones and other changes in Alaska's frozen rivers in early winter. The approach can be automated to provide up-to-date hazard maps and is applicable across the Arctic and sub-Arctic.

Many Alaskans, especially in rural parts of the state, use rivers as ice routes in the winter to travel between communities or for recreation, hunting and fishing. Open water areas in river ice can be dangerous.

The new method is described in an article published March 13 in the journal Remote Sensing of Environment.

Remote sensing scientist Melanie Engram of the Water and Environmental Research Center at the UAF Institute of Northern Engineering led the research.

Co-authors include Franz Meyer of the UAF Geophysical Institute; Dana Brown, Sarah Clement and Katie Spellman of the UAF International Arctic Research Center; and Allen Bondurant, Laura Oxtoby and Christopher Arp of the Water and Environmental Research Center.

“Arctic warming has changed the way rivers freeze and affected river travel in rural areas in winter due to later freezes, open water zones in midwinter, and earlier breaks,” the authors write.

Previous research by others has focused on just one or two river reaches in Canada and temperate climates in Lithuania.

Engram and her UAF colleagues used synthetic aperture radar data from 12 sections on eight rivers in Alaska to create a river ice classification system that can be used in northern high latitudes from October to January.

The period ends in January, as river users have typically shared open water locations by then. The river ice also becomes more complex later in the winter. Other SAR-based river ice classifications focus on spring ice during breakup.

“This can be customized and automated for all rivers in northern latitudes to provide up-to-date maps of open water zones,” Engram said. “It’s not just designed for Alaska.”

Synthetic aperture radar can penetrate clouds and other atmospheric conditions such as haze, fog and rain. This is because SAR operates in the microwave region of the electromagnetic spectrum, which has longer wavelengths than visible light.

SAR technology is widely used for environmental monitoring, agriculture, disaster management and defense.

Engram and the team refined and validated their data processing to reduce the classifications to four: ice, open water, less safe ice, and less safe open water.

To do this, they worked with two types of radar data: vertical-vertical and vertical-horizontal.

In vertical-vertical mode, the electromagnetic wave of both the transmitted and returned radar beams exhibit peaks and troughs, similar to the rise and fall of ocean waves.

In the vertical-horizontal orientation, the emitted electromagnetic wave is similar to ocean waves, but the wave returning from the target object is oriented laterally, similar to a snake.

This is important because the different combinations can reveal different data characteristics.

The data is also affected by the angle at which the radar beam itself is aimed at a target. Different perspectives can provide different perspectives and therefore different information.

Engram used data from the European Space Agency's Sentinel-1 satellite. This data is archived at the UAF's Alaska Satellite Facility.

The researchers then compared the SAR data with aerial images, the field of view of dozens of cameras on land, aerial photos, observations on the ice, and reports from community members who uploaded observations to the observer portal.

“We had coastal cameras all over the state and they were taking a photo of the river every day,” Engram said. “And we consulted with communities and asked, 'What's important to you?'”

Engram chose sections of eight rivers: Colville, Noatak, Tanana, Yukon, Kantishna, Innoko, Copper and Kuskokwim, listed here in descending order of latitude.

The team selected sites with different river volumes, widths, channel types and glacial mud content. They also chose sites in both the tundra and boreal forests, as well as with varying permafrost conditions nearby.

“With this ice classification we try to differentiate between ice and open holes in the ice,” Engram said. “A lot of studies have been done, especially in Canada, looking at different types of ice. We didn't do that. We just rode on ice against open water.”

The research was funded by the National Science Foundation's Navigating the New Arctic and Arctic Observing Network under the UAF Fresh Eyes on Ice program and by NASA's ROSES Citizen Science for Earth Systems program under the Community Eyes on River Ice project.

Engram praised the Alaska Satellite Facility, which hosts the data.

“We’re really happy that scientists have access to this data, not just at the University of Alaska Fairbanks but around the world,” she said. “The Alaska Satellite Facility has made SAR data much more useful for every type of scientist. You don’t have to be a SAR specialist.”