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Shore Drift

A constant river of sand and gravel flows along Puget Sound beaches. Shore drift or "littoral drift" can move materials from eroding bluffs and streams to shorelines miles away. Weather and waves pick up particles in one area and drop them off in another area.

 

Materials
on the Move

How shore drift works...

Waves come ashore diagonally, as their backwash flows perpendicular to the beach. Sediment particles travel in a zigzag path as they are moved along the beach. Movement can shift or increase with tides, storms, and seasons.

Sources & Sinks

Shore drift along Puget Sound is not a straight conveyor belt. Drift is interrupted by headlands, inlets, and sharp bends in the shore contours. The shoreline is divided into "drift sectors." Each drift sector contains its own pick-up (source) and drop-off points (sinks). Feed sources along Puget Sound include eroding bluffs, rivers, and streams. Sinks appear as spits, bars, tombolos, beaches and tideflats.

Waves carry materials from eroding bluffs (source) and move them along the beach (drift path) to a spit, bar, tombolo, or beach (sink).  
 

Blocking Drift Can Damage Beaches

A stream of sediments moves along the shores, building Puget Sound beaches. If the stream of sediments is blocked by marinas or groins, beaches downdrift can erode and beach habitat can be damaged. Bulkheads and seawalls can also prevent materials from entering the sediment stream and can cause erosion. One person's bulkhead can cause another person's beach to erode.

Related Links

Coastal Change: Natural Processes, USGS.
How coastal lands and sediments are in constant motion. 

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Comments? E-mail: Shellyne Grisham