Landform Geography Coastal Processes and Landforms July 24, 2013.

Landform Geography

Coastal Processes and Landforms

July 24, 2013

Oceans and Seas

• Oceans – largest bodies of water: Pacific, Atlantic, Indian, Southern, and Arctic

• Seas – next largest water bodies: Black, Mediterranean, Barents, etc.

• Gulf – next largest, usu. opens to larger water body: Mexico, Alaska, Guinea, etc.

• Bay – smaller still: Fundy, Biscay, etc.

Oceans and Seas

Water as Solvent

• Salinity – concentration of dissolved solids in seawater – global seawater salinity 34-37 parts per thousand (‰)

• Brine – water with >35‰ salinity

• Brackish water - <35‰ salinity

Shaping the Coastline

• Key to shaping coastline is movement of water

• Eustatic Change - changes in water level in ocean – due to tectonic uplift or hydro cycle variation

• Land above sea level forms river valleys that extend to sea level – when sea level rises, valley floods, as with Chesapeake Bay or Delaware Bay

• Ria – river valley flooded by rising sea level

• Fjord – glacial valley flooded by rising sea level

Eustatic Sea Level Changes

North American Coastline Through Time

Lowest sea levels occurred during glaciations when water was tied up in glaciers (130k & 19k bp) – highest sea levels in interglacial periods (120k bp)

Tides

• Regular, predictable oscillations of sea level – due to gravitation of moon (56%) & Sun (44%)

• High tide on side of Earth facing moon and on side away from moon – ellipsoid shape

• Long, narrow bays usually have highest tides – up to 16 meters in Bay of Fundy in eastern Canada

Waves

• Oscillations in water due to force of friction from wind blowing across its surface

• Waves travel horizontally, but most movement of water is vertical, up-and-down

• Near coast seafloor slopes upward – wave base intersects ocean floor – wave pushes water up as it slows – waves pile up from behind - wave height exceeds 7X wavelength, and forms a breaker

• Only horizontal movement of water from breaker to beach, called surf – erosional agent

Wave Compression

Tsunami• Caused by undersea earthquakes with vertical

displacement, volcanic eruptions, or landslides• Vertical displacement of water causes fast wave

with long wavelength – no harm at sea, but massive as it hits coastline

• March 2011– Pacific plate subducts under North American plate which snaps upward – massive tsunami hits Indian Ocean

2011 Japan Earthquake

Littoral Processes

• Transport & deposition of sediment in shore zone• Longshore current – forms when wave hits beach

at oblique angle – water deflects downwind, parallel to beach

• Longshore drift – process of longshore current eroding & carrying sediment down shore

• Beach drift – zig-zag motion of sediment down beach due to swash & backwash

• Littoral Drift – longshore & beach drift together

Littoral Drift

Coastal LandformsErosional Coastlines

• Breaking waves have great power to erode

• Headland – promontory that juts into ocean or sea – made of resistant rock

• Waves slow & pivot around headlands – erosive power of waves concentrated on headland

• Retrogradation – retreat of coastline due to erosion

• Wave-Cut Bluff – basic erosional landform of coastlines – near-vertical cliff at water’s edge

Coastal Erosional Landforms

Depositional Coastlines

• Progradation – process of coastline extending outward into water through deposition

• Beach – dynamic transition from sea to land– Offshore – under water, where waves break– Foreshore – rise & fall of tides– Offshore Bar – between offshore & foreshore– Beach Ridge – at high water line– Backshore – flat, only covered in storms

Beach Cross Section

Spits and Baymouth Bars

• Longshore current carries sediment down beach

• Current slows upon reaching bay – sand deposited as a Spit extending out into bay – current in bay turns spit toward land in hook shape

• Baymouth Bar – spit extends across bay, isolating it from ocean – bay now called Lagoon

• Tombolo – longshore currents from 2 directions meet – sand extends out to island or sea stack

Depositional Landforms

Barrier Islands

• Elongated bars of sand that form parallel to shore

• Likely formed from sand deposited on cont. shelf during last glaciation – waves & wind shaped sand

• Lagoons w/mudflats form behind barrier islands

• Mudflats develop into vegetated salt marsh

Coral Reefs

• Coral polyps excrete external skeletons of calcium carbonate (limestone) – Coral Reefs

• New reefs form on top of old, dead reefs

• 30º N - 25º S latitude, water warmer than 20º C

• 3 settings:– Fringing reef – on shallows around island– Barrier reef – line of coral parallel to shore– Atoll – semicircular reef around degraded volcanic

island

Development of Atoll

Global Distribution of Coral Reefs

Human Impacts on Coastlines

• 37% of world pop. live <60 mi. from shore (2 billion) – 50% within 120 miles of shore

• In US, 53% of pop. live near coastline

• Coastal Engineering Purposes:– Protecting shore & property from hazards– Stabilizing & nourishing beaches– Maintaining traffic & trade into ports

Mitigating Coastal Hazards

• Raise buildings on stilts so waves roll underneath

• Sea wall – vertical, concrete wall to absorb energy of waves

• Revetment – slope covered with large rocks (rip-rap) to absorb energy of waves

• These work locally, but they aggravate erosion up and down shore from wall due to wave refraction

Beach Nourishment

• Bringing sand to beaches that have eroded – over $336M spent in Florida since 1960s

• How?– Trucking in sand from remote location– Limit loss of sand through groins – low walls built at

right angles to beach – intercept longshore drift

• Jetties – stone or concrete structures to keep channel open, keep sand to side

Sea Wall

Groins and Jetties