Coastlines are dynamic by nature, shaped by the push of inland sediment and the pull of ocean tides. A recent study gives a new view of coasts around the world. Researchers used changes in ocean color to show that sediment in the water has declined. That could have significant effects on everything from habitat health to coastal infrastructure.
A new tool uses Landsat, a network of land-focused satellites, to derive how much sediment is in these coastal zones on the basis of the light reflected from the water column. The tool relies on an algorithm developed by Wenxiu Teng as part of his doctoral research at the University of Massachusetts Amherst, where he studies remote sensing and geomorphology. Teng will present the initial findings on 12 December at AGU’s Annual Meeting 2024 in Washington, D.C.
Waves, tides, shifting sea level, river runoff, and other processes affect the amount of sediment that’s near shorelines. And the best way to track that coastal sediment is from space. High-energy coastal zones destroy most ground- or water-based instruments, making shorelines “uniquely suited to remote sensing,” said Brian Yellen, a geomorphologist at the University of Massachusetts Amherst and coauthor on the study. Suspended particles of fine sand, silt, and clay scatter incoming light. And the colors that result can be detected by instruments aboard satellites.
But monitoring from space has its challenges, too. Satellites that measure ocean color are generally designed to record big blobs of blue, including NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard the Terra and Aqua satellites. MODIS captures ocean color on a near-daily basis, but its 1-square-kilometer resolution is also too coarse to record small variations in a 300-meter-wide coastal zone.
“It’s really hard to see a suspended sediment bloom along the coastline [with MODIS] because one pixel covers both land and ocean,” Teng said.
Landsat also monitors color and has a much finer 30-meter resolution, but as the name suggests, it’s optimized to measure the light reflecting off land. Water is a darker target, and researchers interested in using Landsat data to explore the oceans must apply corrections to remove extra noise. The new algorithm corrects for surface reflectance, isolating just the light reflecting off the water column.
The researchers verified the algorithm using coastal studies from 12 locations worldwide where sediment loads were driven by marine, marsh, or river influences. Teng and his colleagues compared water samples taken by teams at a specific date and time against Landsat images taken within 2 hours.
“It’s an incredibly powerful tool.”
Confident in the algorithm, they created a high-resolution mapping tool called Global Coastal Sediments Viewer that can visualize coastal sediments anywhere on Earth. It’s a fast, free online resource powered by Google Earth Engine that can quickly produce a 40-year timeline of coastal sediment history, organized in 5-year bins. The results show sediment trends over time.
Individually, that kind of hyperlocal data “would be the product of a year or two of effort, and you can do it instantly now,” Yellen said. “It’s an incredibly powerful tool.”
Dwindling Deltas
Using the tool, the researchers selected 3,000 sites around the world to track coastal sediment. Three out of four sites showed a decline in total suspended sediment since Landsat launched in 1984, reducing an average of 1.63% per year. Sandy coasts showed the largest decline, with 86% of them dwindling in suspended sediment over the 40-year window. About 81% of muddy coasts and 59% of rocky coasts showed reduced sediment.
Deltas, marshes, mudflats, and wetlands provide a range of services, from boosting biodiversity to protecting cities against storm surges, and all of them need new sediment to keep pace with sea level rise. If new sediment isn’t arriving, the ocean will claw into these landforms. Coastal degradation is especially concerning as coastal floods increase in intensity and frequency as climates change.
Inland engineering such as revetments and reservoirs can stop sediment from reaching the ocean. Coastal infrastructure such as seawalls and jetties disrupts the natural sediment process. And dredging intentionally removes coastal sediment to benefit shipping and navigation. Local land managers can use the new dataset to see how these infrastructure projects may have affected their region’s sediment supply.
“In terms of the utility of what they’re providing, it’s tremendous.”
The new tool is useful, but it has its limits, said Emmanuel Boss, an optical oceanographer at the University of Maine who is not involved in the project. For one, the data are empirical. Taking observations from 12 sites to build a global algorithm will introduce uncertainties that are not well constrained, he said.
Coastal managers should still collect in-person observations, Boss cautioned. But anything that illustrates sediment data for nonspecialists is important. “In terms of the utility of what they’re providing, it’s tremendous,” he said.
The study is a first step and strictly observational. Most sites decreased in sediment, but many others increased. Individual coastal managers now have a tool to evaluate the causes and consequences of local sediment decline.
—J. Besl (@J_Besl), Science Writer