Impacts of the Grays Harbor Jetties

The North and South Jetties on either side of the Grays Harbor inlet were contructed in the early 1900s to prevent natural deformation of the inlet by means of sand movement down the coast, or littoral (longshore) drift.

Naturally, sediment exiting the Columbia River and Grays Harbor inlet would move south during the summer and north during the winter. Jetty construction disables this mechanism, forcing modified wave patterns and new accretion/erosion trends.

Image: Brian Voigt

Originally, the purpose of the jetties was to allow for greater navigation into Grays Harbor, a plan that included deepening the ocean floor immediately outide the Harbor's mouth. This goal was achieved, but with unintended consequences. Before jetty construction, a large shoal (sand bar) was present just outside Grays Harbor. After jetty construction, the shoal's sediment was pushed offshore while disturbed wave patterns moved sediment towards land south of South Jetty. The jetty trapped north-moving sediment, halted longshore drift, and resulted in a 3000-foot advance of land at northern South Beach. (This was a great opportunity for human development to advance in the area as well.) In 1909, deterioration of the jetty became an issue as water could now wash over the worn jetty. This began the overall trend of sand movement from north to south, erosion just south of South Jetty and accretion further away from it.

 	The South Jetty disables sediment from naturally moving south to north. Limited sediment access, combined with wave patterns acting on the area, resulted in the erosion of Pt. Chahalis and the formation of Halfmoon Bay in the 1930s -40s.
	Image: Maarten Buijsman

Other Factors Affecting Sediment Movement Along Washington's Coast:

• Dams on the Columbia reduce the amount of sediment reaching the river's mouth, resulting in less sand available to form beaches
• Elimination of sand dunes for aesthetic purposes (better views of the ocean) affects wave behaviour
• Lack of buffers—sand bars offshore South Beach are small
• El Nino and other weather patterns affect winds that affect longshore drift

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