Coastal Storms & Effects on Coastal Erosion
Ocean waves are always eroding coastlines to some extent: shifting beaches, molding seastacks and cliffs, dragging stones and sand and driftwood. But they can accomplish the most dramatic work when compelled by storms. The shape of a shore can change overnight in the face of a big disturbance like a hurricane.
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Wave Power
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Seastacks off the Oregon coast, an area subject to regular winter storms, betray wave erosion. Waves slamming into a coastline can be remarkably powerful. Breakers produced by big swells, which may travel huge distances across ocean basins, might strike with forces of 30 tons per square meter. The speed of spray in the wave's swash--its landward, incoming portion--can easily exceed 70 miles per hour. The sediment carried by the wave abrades against the coast material.
There are other dimensions to wave erosion, too. On rocky coasts, air moves into fissures and cracks as the swash hits, then expands when the water falls back; this pressure change can weaken the surrounding rock. Seawater salt dissolves certain rocks, and also deposits in nooks and crannies to additionally loosen them.
Waves & Coasts
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As G.H. Dury notes in An Introduction to Environmental Systems (1981), waves shape coastlines--and coasts affect the shape of waves. The ocean can both add to and deplete shore sediment. A broad, gently sloping offshore shelf typically dissipates wave energy, while a steep one usually concentrates the force of the water. In the former situation, waves are often "constructive"--that is, they deposit sand and build up a coastline. The stronger, more concentrated waves are "destructive," actively eroding the land they crash against.
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Constuction & Destruction
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Some coasts see seasonal cycles of constructive and destructive waves, dependent on offshore atmospheric storm factories. For example, relentless winter storms born in the Gulf of Alaska send southwesterly waves onto the rugged Pacific Northwest coast, shearing away beaches and transporting sand to offshore bars. In summer, lower energy constructive waves build up the beach depositions again.
Hurricanes
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The most powerful oceanic storms--and the largest storms on Earth--are hurricanes, where swirling masses of fast-rotating winds build around low-pressure centers. Their landfall often induces heavy coastal erosion. In the United States, for example, hurricanes and other big storms can affect a shoreline in numerous ways: eroding beaches, dunes, wetlands and cliffs, overwashing sand dunes and inundating and breaching barrier islands.
Buffers
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Mangrove forests can buffer inland areas from incoming coastal storms. Large swaths of coastal wetland or mangrove forest can be effective buffers against big storms. Forty percent of America's coastal wetlands are found in Louisiana, where salt marshes, tidal flats and swamps absorb much of the initial impact of incoming hurricanes, tropical disturbances and winter storms from the Gulf of Mexico, historically protecting inland areas.
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References
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- Photo Credit Ocean Wave image by Joelyn Pullano from Fotolia.com oregon coast image by Jeffrey Sinnock from Fotolia.com mangrove aux antilles image by Danielle Bonardelle from Fotolia.com
