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The research, conducted by USGS, Washington State Department of Ecology, and Oregon State University, and funded by the U.S. Army Corps of Engineers and the Northwest Association of Networked Ocean Observing Systems (NANOOS), focused on the impact of two engineered inlets along an 80-kilometer stretch of wave-dominated coastline in the Pacific Northwest between 2014 and 2023. Jetties are commonly built to stabilize channels and improve navigation through inlets, but they can also disrupt the natural movement of sand along the coast, known as longshore sediment transport. The nine-year study period revealed a regional reversal of net sediment transport direction, with areas on opposite sides of the inlets experiencing distinct patterns of erosion and deposition.

By coupling long-term beach and nearshore monitoring with advanced numerical modeling techniques, the researchers could pinpoint the driving forces behind these changes. A wave model, calibrated with real-world data, suggested that variations in wave direction on seasonal to multi-annual timescales were responsible for reversing the net direction of sediment transport. This reversal led to significantly contrasting patterns of beach erosion and growth on either side of the inlets.

“It's nice seeing nine years of our survey data being used in combination with models to understand the beach erosion patterns we're seeing in the Pacific Northwest,” said USGS Oceanographer Andrew Stevens, lead author of the study.

Of particular note is the timing of these changes, which coincided with a shift in the Pacific Decadal Oscillation (PDO), a major climate pattern that influences weather and ocean conditions across the Pacific Ocean. The researchers suggest that multi-decadal climate variability, as reflected in the PDO, plays a critical role in driving longshore sediment transport and coastal erosion. This points to the possibility that similar patterns of sediment dynamics could occur in other regions affected by large-scale climate oscillations.

The study highlights the importance of understanding how climate variability interacts with human-made coastal structures and helps to inform management of sediment resources around engineered inlets. As sea level rises and coastal areas face increasing pressure from climate change, resource managers need the best available science to develop strategies that ensure safe navigation for people and goods while maintaining stable coastlines and healthy coastal ecosystems.