20 Coasts, Beaches and Shorelinesbrosenhe/Oceanography/17_Coasts-Beaches.pdf · • Stormier...
Transcript of 20 Coasts, Beaches and Shorelinesbrosenhe/Oceanography/17_Coasts-Beaches.pdf · • Stormier...
1
Coasts, Beaches and Shorelines
Jurassic Coast, England (SW)
“Coast”
“Coast” and “Shore”
2
Coast and Shore• Shore - between low tide and highest
elevation affected by storm waves• Coast - from shore to farthest inland
ocean features
Coast and Shore
• What are ocean features?
Shoreline and Beach• Shore: backshore, foreshore,
nearshore, offshore– Berm, beach face, longshore bar,
longshore trough• Coast – not necessarily effected by
wave action
3
Beach
• Actively changing• Composition
– What is locally available?• Coral and carbonate sand in the Bahamas• Lithogenous sediment in Louisiana, Gulf Coast• Lava in Hawaii
• Shape– Determined by composition and energy (waves, tides)
• Fine sand, more gentle slope• Coarse material, steeper slope
Beach
• Coarse Grain – steep slope
Alaska
Beach
• Fine Grain – gentle slope
Puerto Rico
4
Swash Zone of the Beach
• Swash and backwash– Sediments moved up and down beach
face– Smaller, low energy waves move sand
up beach face (depositional to berm)• Summertime beach
– Larger, high energy waves move sand off shore (erosive to berm)• Wintertime beach
Sand Movement parallel to Beaches
• Longshore current
Sand Movement parallel to Beaches
• Longshore current– Longshore drift or transport– Parallel to shore– Increasing strength
• More wave energy• Higher wave frequency• Steeper beach• Greater angle
– Southward along both U.S. coasts (most of the time)
– Main sediment transport along coasts
5
Rip “Tide”
• Localized currents return swash zone water to deeper water
Rip Current
• Location determined by bathymetry built by swash and longshore drift
Shore Types• Erosional Beaches
– Evident of local sea level fall– Erosion and transport offshore
• Depositional Beaches– Gradually subsiding coast lines– Large sediment source elsewhere
• Both erosion and deposition happen on both types of beach!
6
Beach Erosion and Deposition
Erosional Beaches
Depositional BeachBarrier Islands
Bay mouth bar
Sand spit
Tombolo
Lagoon
River Deltas
7
Beach Features
Beach Features
http://www.shorebirdworld.org/fromthefield/FLsurvey.htm
Beach Features
http://www.cografyaogretmeni.org/galeri/details.php?image_id=22
8
Beach Features
http://www.cografyaogretmeni.org/galeri/details.php?image_id=22
Beach Features
http://www.etribes.com/node/36793
Beach Features
http://verseguru.com/media/?gallery/state=South%20Ari%20Atolhttp://geology.uprm.edu/Morelock/morphol.htm
9
Seasonal Beach Features
• Stormier (winter) beaches take sand offshore – sandbars
• Gentler slope in summer (calm weather)
Coastline Types
• Deltas– Delivery of lithogenous sediment from river
systems “bends” lithosphere, creates space– Sedimentation keeps pace with sea level
Deltas
• Why do deltas only form where large rivers intersect the oceans? Why not where every river enters the ocean?
10
Fjords
• Fjords– Valleys carved by glaciers– Filled by rising sea level
Mangrove Coasts
• Vegetation stabilized coasts in the tropics• Sedimentation within the roots of large
expanses of mangroves
Barrier Islands
• Temperate latitudes, passive margins
11
Coastal Classification
Barrier Island Beaches
• Progression response to sea level
1887 1996
Barrier Island Beaches
• Recreationally important• Building on barrier islands has increased• How do we protect these investments?
12
Beach Restoration and Protection
• Maintain deposition– Sediment source
• What happens when nearby rivers are dammed?
• Prevent or slow erosion– Steady state is desirable
• What structures intensify beach erosion?• What about rising sea level?
Beach Barrier Management
• Beach groins
Beach Barrier Management
• Jetties and groins
13
Beach Barrier Management
• Breakwater
Beach Barrier Management
• Breakwater and dredge system
Beach Barrier Management
• Seawall
14
Key Concepts
• Beach classification• Beach movement• Sea level rise• Protection of barrier islands
– Natural systems or resorts?