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Rock Groins
Rock groins (often just called groins) are coastal engineering structures made of large rocks or concrete, built perpendicular to the shoreline. Their main purpose is to interrupt water flow and limit the movement of sediment, which helps to reduce beach erosion and trap sand moving along the coast due to longshore drift.
Purpose and Function
- Sediment Trapping: Groins trap sand that is being transported by longshore currents. This results in a wider beach on the updrift side of the groin.
- Erosion Control: By trapping sand, groins can help protect the shoreline from further erosion.
- Beach Stabilization: Groins are often used to maintain artificial or nourished beaches by keeping the new sand in place.
Construction and Materials
- Typically made from:
- Boulders or rocks (rock groins)
- Concrete slabs
- Wood or steel (less common now due to durability issues)
- Installed perpendicular to the coast, usually in a series spaced along a stretch of beach.
Environmental Considerations
- Down-drift Erosion: While groins trap sand on one side, they can starve the down-drift side of sediment, causing increased erosion further along the coast.
- Habitat Disruption: May affect marine life and natural sediment movement.
- Often require environmental assessments before installation.
Common Locations
- Ocean beaches (e.g., U.S. East Coast, California)
- Lake shores
- Areas where property or infrastructure is at risk from coastal erosion.
Alternatives and Complements
- Breakwaters (offshore)
- Seawalls (parallel to shore)
- Beach nourishment (adding sand)
- Living shorelines (natural vegetation and structures)
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