ESCI Coasts

Coasts 

  • Where the land meets the ocean 

  • Habitable part of Earth is a very thin layer 

Ocean bathymetry 

  • Coast: where the ocean meets the land 

  • Continental shelf + continental slope (assuming this is a convergent boundary of continental and oceanic crust) 

Active vs. Passive Margins 

  • Active margins are on plate boundary 

  • Narrow continental shelf, cliffs, steep dropoff 

  • Passive margins are far from plate boundary 

  • Broad beaches and sandy offshore islands 

Sediment 

  • Submarine canyons – flow of mud and debris from coast to deep ocean 

  • Sediment delivered from land by rivers - I.e. Mississippi River delivers 93,000,000 tons of sediment to Gulf of Mexico each year 

  • River sediment builds deltas 

  • River splits into channels and distributes sediment across a wider area (fan-like structure that changes over time) 

  • Cliff erosion also produces sediment – waves and currents erode near-shore land 

  • Cliffs and blunts cerated by undercutting 

  • Cliff face then slumps or slides off into the sea 

  • Cliff faces show landward retreat of the shoreline 

Tides 

  • Small variations in local sea level due to gravitational effects of both the Sun (solar) and the Moon (lunar) 

  • If moon is on the same or opposite side of the Sun (new moon or full moon), solar and lunar tides overlap for extra-large tides  

  • Tidal range (amplitude) varies spatially – global average is 1m tidal range 

  • Bay of Fundy has highest tides in the world (bay btwn Maine, New Brunswick, and Nova Scotia) 

  • Tides move too slowly to erode or transport sediment 

  • Affects water level (obviously) 

Waves 

  • Small undulations on the water surface that are generated (largely) by wind 

  • Crests with troughs in between 

  • Wave height/amplitude 

  • In shallow water, waves drag on seafloor and slow down 

  • Move sediment on seafloor 

  • Become steeper and steeper, until eventually break 

  • Waves deposit sediment during fair weather 

Beaches 

  • Gently sloping depositional surfaces washed over by waves and covered by sediment 

  • Beach system actually starts 1-2km offshore as depth becomes shallower and therefore starts affecting waves  

  • Beaches change seasonally 

Storms 

  • Storms are caused by low-pressure systems, which cause a bulge of water to form 

  • Onshore winds, associated with storms, approach the shore w/ unusually high tides called a storm 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 

Rocky coast 

  • Occurs when erosion is greater than accretion 

Wave energy 

  • Focused at headlands 

  • Wave refraction works to focus energy at 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 

  • Sometimes we build on top of barrier islands which is. Questionable! 

Longshore current 

  • Waves coming at an angle to the coast leading to sediment transport 

Breakwaters 

  • Change location of wave breaking 

  • Can help protect beaches 

Bank armoring 

  • Another way to temporarily protect shoreline 

Changes in relative sea level change the coastline 

  • Land subsidence – absolute sea level goes up (due to more water in ocean and/or lower density of ocean water) 

  • Land uplift – absolute sea level goes down (due to less water in ocean and/or higher density of ocean water) 

  • Amount of land submerged/exposed depends on its slope 

Emergent + submergent coastlines 

  • Long-term sea-level changes are often associated with plate tectonics or climate change 

  • Emergent: Continental margins may uplift and produce an emergent coastline 

  • Submergent: loading of the crust produces submergent coastlines 

  • Signs of changing relative sea level 

  • Emergent: A wave-cut platform will form at coastlines and indicate uplifting land or dropping sea level 

  • Submergent: A drowned valley is associated with a rise in sea level 

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 [… - check slides] 

  • Heavily affected by pollution 

Glaciers’ effect on global sea level 

  • Presence of glaciers decreased global sea level 

  • Continuous progressive increase in sea level since the last ice age 

Glaciers’ effect on local sea level 

  • Glaciers are heavy – cause compression of crust, peripheral fore-bulge next to compression 

  • Rebounding of crust can cause drastic, relatively fast (few thousand year) changes in sea level 

Sea level shifts 

  • Threat of future flooding 

  • Projected major sea level rise over the next 100 years 

US Population 

  • 85% of US population lives in a coastal state 

  • 40% of US population lives in a coastal county, with this number increasing  

  • Coastal areas support terrestrial and marine flora and fauna 

  • Coastal areas are dynamic and threatened by climate change 

Worldwide 

  • 3bil people live within 200km of a coastline 

Flooding 

  • Sunny-day flooding – Hampton, NH – flooding just from high tides