Groins and Jetties

Successive groin structures along the coast south of Gateway National Recreation Area, New Jersey.

NPS photo.

Groins are shore perpendicular structures, used to maintain updrift beaches or to restrict longshore sediment transport. By design, these structures are meant to capture sand transported by the longshore current; this depletes the sand supply to the beach area immediately down-drift of the structure. In response, down-drift property managers often install groins on adjacent properties to counteract the increased erosion, leading to a cascading effect of groin installation.

Jetties are another type of shore perpendicular structure and are placed adjacent to tidal inlets and harbors to control inlet migration and minimize sediment deposition within the inlet. Similar to groins, jetties may significantly destabilize the coastal system and disrupt natural sediment regimes. Such effects can be seen near the construction of three jetties at the Mouth of the Columbia River adjacent to Lewis and Clark National Historic Park, Oregon and Washington. Here, the South Jetty extends nearly three miles seaward, and in combination with the North Jetty, disrupts the ebb tidal flow and longshore current near the river mouth (USACE, 2012). As a result, the morphology of nearby park beaches has changed. Hardened structures such as this exist nearby many National Seashores, ultimately altering the natural processes within park unit boundaries.

Inlet stabilization adjacent to Assateague Island National Seashore, Maryland and Virginia.

Photo by Jane Thomas, UMCES/IAN Image Library.

Jetties and groins can be constructed from a wide range of materials, including armorstone, precast concrete units or blocks, rock-filled timber cribs and gabions, steel sheet pile, timber sheet pile, and grout filled bags and tubes. Sea level rise increases the possibility of flanking or submergence of these structures. Landward retreat of the adjacent beach and dune line may leave the structure’s landward attachment point exposed. This increases the likelihood of additional maintenance costs and development of new features.

Related Links

• Assateague Island National Seashore, Maryland and Virginia—[Geodiversity Atlas] [Park Home]
• Gateway National Recreation Area, New York and New Jersey—[Geodiversity Atlas] [Park Home]
• Lewis and Clark National Historic Site, Oregon and Washington—[Geodiversity Atlas] [Park Home]

• NPS—Coastal Geology
• NPS—Beaches and Coastal Landforms
• NPS—Geology
• NPS—Oceans