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Coastal Zones and Processes. Coastal zones are important to the United States:
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Coastal Zones and Processes Coastal zones are important to the United States: 30 of the 50 states abut a major body of water (Atlantic or Pacific Ocean, Gulf of Mexico, one of the Great Lakes), and those of 30 states are home to about 85% of the nation’s population. About half of that 85% live in the coastal regions and that number is growing. Chapter 7
Nature of the Coastline • Factors that impact coastlines: • Tectonic setting • Materials present at the shore • Energy of water striking the coast • Active margin – associated with active plate tectonics • Often cliffs found near active margins • Passive margin – far from active plate tectonics • Often a broad shelf, beaches and with sandy offshore beaches
Figures 7.3 a and b, a beach A beach is a gently sloping surface washed over by the waves and covered by sediment The beach face is the portion regularly washed by the waves as tides rise and fall The berm is the flatter part of the beach landward of the beach face
Nature of the Coastline • Beach – a gently sloping surface washed over by the waves and covered by sediment • Sediments are carried by longshore currents and placed on the beach by littoral drift • Waves – generated by wind are small undulations on the ocean surface • Waves move along an ocean and water molecules move only in orbits • Waves disappear below wave base • Orbits are larger nearest the surface and become smaller with depth • Once a wave ‘feels’ the bottom the orbits are interfered with and breaking occurs • Erosion of solid rock along a coast takes place because of milling • Ocean surfaces appear to move in and out relative to the shore because of tides • Tides occur because of the gravitational pull on the oceans by the moon and sun
Figures 7.6 a and b Spring tides: Sun, moon, and earth are all aligned; Neap tides: When moon and sun are at right angles, tidal extremes are reduced
Figure 7.7 longshore currents and their effect on sand movement. Waves push water and sand ashore at an angle
Storms and Coastal Dynamics • Storms are caused by low pressure systems • Low pressure cause a bulge of water to form • Onshore winds, associated with storms, approach the shore with unusually high tides called a surge • The exceptional force of the wind driven waves and surge combine to erode beach faces, dunes above the beach, and sometimes cliffs behind the beach • Hurricanes are power examples of these storms
Emergent and Submergent Coastlines • Long-term sea-level changes often are associated with plate tectonics • Continental margins may uplift and produce an emergent coastline • Large rivers drain water and sediments from the continental interior and form large deltas in adjacent oceans • Loading of the crust produces submergent coastlines • Signs of changing relative sea level • A wave-cut platform will form at coastlines • A number of wave cut platforms is an indication of uplifting land or dropping sea level • A drowned valley can record changes in sea level also
Coastal Erosion and Stabilization • A rise in sea level will cause coastal erosion and will require stabilization • Sand may be needed, from dredging, to replace sand lost to long shore currents • Breakwaters, jetties, or groins act to stabilize beaches • These artificial barriers have erosion and deposition problems associated with them also • Projects to stabilize coastlines and beaches are required because of changes in sea level
Cliff Erosion • Waves and currents act to erode beaches and eventually cliffs • Cliffs are undercut • Cliff face then slumps or slides off into the sea • Cliff faces show landward retreat of the shoreline • Wave refraction works to erode points of land jutting out into the sea • Wave base interaction with the ocean bottom slow the progress of the wave • Wave base encounters the bottom near a jutting point before the coastline
Barrier Islands • Long, low, narrow islands parallel to the coastline • Unstable environments • Easily washed over because of low relief • Sand migrates constantly with the wind • Constantly retreating landward with time
Estuaries • Body of water along a coastline, open to the sea, in which tides rise and fall • Often rivers bring freshwater into a mix with sea water producing a brackish water environment • Water circulation is limited • Estuaries do not tolerate major changes in salinity or chemistry of the water • Heavily impacted by pollution
Construction and Reconstruction in High Energy Environments • Coastal areas are easily damaged by major storms that frequent our oceans and seas • People have traditionally flocked to the coast to live and work • Property values are high along coastlines and at risk • Governments spend large amounts of money to maintain coastal areas • Recognize the coastal hazards: building locations
