In a groundbreaking development, researchers from NASA and Virginia Tech have utilized satellite data to measure the height and speed of potentially hazardous flood waves traveling along U.S. rivers. This pioneering study, which tracked three significant flood waves, underscores the potential of space-based observations to aid hydrologists and engineers, particularly those in communities that lack extensive flood control infrastructure such as levees and floodgates. The waves observed were likely triggered by extreme rainfall and the loosening of an ice jam.
Unlike ocean waves, which are driven by wind and tides and generally move at a steady pace toward the shore, river waves—often referred to as flood or flow waves—are temporary surges that can stretch for tens to hundreds of miles. These waves, typically caused by rainfall or seasonal snowmelt, play a crucial role in transporting nutrients and organisms downstream. However, they can also be hazardous when extreme conditions, such as prolonged downpours or dam breaches, result in flooding.
One of the researchers involved in this study, Cedric David, a hydrologist at NASA’s Jet Propulsion Laboratory in Southern California, emphasizes the importance of understanding river dynamics. He notes that while ocean waves are familiar to many through activities like surfing and sailing, rivers serve as the planet’s arteries, and further understanding of their behavior is vital.
The key to this research was the Surface Water and Ocean Topography (SWOT) satellite, launched in 2022 as a collaboration between NASA and the French space agency CNES. This satellite employs a sensitive instrument known as the Ka-band Radar Interferometer (KaRIn), which maps the elevation and width of water bodies by bouncing microwaves off the surface and measuring the return time of the signals. This technology enables the satellite to survey nearly all of Earth’s surface waters, both fresh and salty.
The lead author of the study, Hana Thurman of Virginia Tech, delved into the satellite’s data to identify river height anomalies that could indicate the presence of moving waves. Among the notable observations was a significant wave on the Yellowstone River in Montana, which occurred in April 2023. This wave, a striking 9.1 feet (2.8 meters) tall, traveled toward the Missouri River in North Dakota with a dramatic 6.8-mile-long (11-kilometer-long) peak and a more extended tail. This observation highlights the high spatial resolution of the KaRIn instrument.
Further analysis using optical imagery from the European Space Agency’s Sentinel-2 satellite revealed that this wave likely resulted from an ice jam breaking apart upstream, releasing accumulated water. In addition to this, two other waves were identified as being triggered by rainfall runoff. One of these waves was spotted on the Colorado River south of Austin, Texas, in January 2024. This wave, which was associated with the largest flood of the year on that section of the river, measured over 30 feet (9 meters) tall and extended 166 miles (267 kilometers) long. It traveled at a speed of about 3.5 feet (1.07 meters) per second for over 250 miles (400 kilometers) before discharging into Matagorda Bay.
The third wave originated on the Ocmulgee River near Macon, Georgia, in March 2024. This wave rose over 20 feet (6 meters) and stretched more than 100 miles (165 kilometers), moving at approximately a foot (0.33 meters) per second for more than 124 miles (200 kilometers).
These observations are not just academic exercises; they have practical implications. By understanding the shape and speed of these flow waves and how they change over long stretches of river, researchers can better answer critical questions such as the speed at which a flood might reach a specific area and whether existing infrastructure is at risk.
Traditionally, engineers and water managers have relied on stream gauges to measure river waves. These gauges record water height and estimate discharge at fixed points along a river. In the United States, networks of stream gauges are maintained by agencies like the U.S. Geological Survey, but in other parts of the world, such networks are less dense. The satellite data provided by SWOT can complement these ground-based measurements by filling in the gaps. According to George Allen, a hydrologist and remote sensing expert at Virginia Tech, if stream gauges are like toll booths that monitor cars as they pass by, the SWOT satellite acts like a traffic helicopter taking comprehensive snapshots of the highway.
The consistency of wave speeds determined by SWOT with those calculated using gauge data alone demonstrates the satellite’s potential to monitor waves in river basins that lack gauges. This capability is crucial for scientists tracking changing flood patterns worldwide. As SWOT orbits the Earth multiple times daily, it is expected to observe around 55% of large-scale floods at some stage in their development. Cedric David notes that with SWOT’s data, scientists can potentially flag emerging dangerous floods, marking a significant advancement from merely observing rivers from their banks to gaining a comprehensive view from space.
The SWOT satellite project represents a collaborative effort, with NASA and CNES leading the initiative and contributions from the Canadian Space Agency (CSA) and the UK Space Agency. NASA’s Jet Propulsion Laboratory, managed by Caltech in Pasadena, California, oversees the U.S. component of the project. For the flight system payload, NASA provided the KaRIn instrument, a GPS science receiver, a laser retroreflector, a two-beam microwave radiometer, and instrument operations. CNES, with the support of the UK Space Agency and Thales Alenia Space, provided the satellite platform, ground operations, and several key instruments.
This exciting development in river wave monitoring could revolutionize how we predict and respond to flood events, offering a new layer of data to enhance the safety and preparedness of communities worldwide. By leveraging the advanced capabilities of satellites like SWOT, we gain a broader and more detailed understanding of our planet’s dynamic water systems, paving the way for improved environmental management and disaster response strategies.
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