Coasts

CoastsCoastal areas join land and seaCoasts are temporary junctions (of land and sea) that are subject to rearrangement by waves, tides, sea level, biological processes and tectonic activity50% of the worlds population lives on the coastThe term coast includes the shore (where ocean and land meet) as well as marshes, dunes, and cliffs inland of the beach

You are going to coast through this chapterRemember coastal margins?These were the submerged portions of continental crustCoastal regions are constantly subject to change, most dramatically by tides and erosion (short-term effects) and rising or lowering sea level (long-term effect)The coast is an active and dynamic place

Sea level 18,000 years ago during last ice age

Year 2100: Projected (extreme) sea level rise of 200ft

Changes in sea level: eustaticSeveral factors can influence global, or eustatic sea level variationsSea level is lower during periods of glaciation (more water held up in ice) and higher during warm periods (when glaciers are smaller)High rates of seafloor spreading produce larger rises, decreasing the volume of the ocean basin compared to slower rates of spreadingThermal expansion: water occupies a greater volume when warmed

Changes in sea level: localOther factors can influence regional or local sea level changesIsostasy; continental plates weighed down by ice or sediment sink deeper into the aesthenosphere, raising local sea level!Wind, currents, storm surges, El Nio/La Nia can force more (or less) water on the shore

The lithosphere is floating in isostatic equilibrium on the aesthenosphere

The weight of an ice sheet causes the lithosphere to sink deeper into the aesthenosphere

Isostasy and Local sea level changeAESTHENOSPHERELITHOSPHEREOceanic crustContinental crustIce

Isostasy and Local sea level changeAESTHENOSPHERELITHOSPHEREOceanic crustContinental crust

The larger the iceberg, the deeper into the water it sinks!

Local sea level changeThe flooding Mississippi River delta is one example of subsidence or land sinking, caused by an accumulation of several hundred feet of sediments forcing the lithosphere to sink deeper into the asthenosphereThe continent of Antarctica is another example; massive ice sheets weigh down the continent raising local sea levels www.magazine.noaa.gov/storiesmag101.htm

ErosionThe immense power of the waves abrades the coasts and can cause erosion, the removal of coastal materialWhen erosive forces dominate over depositional forces, erosion occursMost rapid on high energy beaches; areas frequently battered by large waves (ex. Maine, southern tip of South America and Africa)When deposition meets or exceeds erosion, the coast is stable or growing, respectively Low energy beaches (ex. NJ to Florida)

ErosionErosion tends to produce a smooth coastline (imagine what erosion does to sea cliffs over time)The sediment eroded away from the coast eventually collects as beaches and may actually protect the shore from the incoming energy (and erosive power) of the waves

Lets go to the beachA beach is a zone of loose particles that covers part of a shore (dunes, vegetation and/or sea cliffs are technically not part of the beach)Beaches result when waves (or rivers) transport sediment usually sand to the shoreBeaches are constantly changing what you see during our field trip to Smith Point is likely different from what your children will see and varies seasonally

Its a shore thingThe material that makes up a beach can range from boulders, cobbles, pebbles and gravel to very fine siltThe black beaches of Hawaii are made of lava!Some beaches consist of shells or coral fragments

Parrotfish Poop = ParadiseParrotfish feed on coral engulfing the animal and its CaCO3 skeleton; they ingest the polyp and excrete the CaCO3, which ends up on local beaches as soft, fine sand between your toes!www.flickr.com/photos/mikesegal/1571667591/http://www.youtube.com/watch?v=8T-gnZevlaY

Beaches have distinct profilesIn general, the flatter the beach, the finer the material from which its madeWater from waves washing onto the beach carries particles onshore (swash); water returning to the ocean (backwash) carries material back to seaThe amount of particles transported by the swash and backwash are not always equal, resulting in sloped beaches

Beaches have distinct profilesThe berm is an accumulation of sediment that runs parallel to shore, and marks the normal limit of sand deposition by wave actionThe peaked top of the highest berm is called the berm crest, and is usually the highest point on the beach; it corresponds to the shoreward limit of wave action during the most recent high tides

Is Oceanography fun? Shore!

Is Oceanography fun? Shore!The steepness of beaches changes with storms and the seasonsSummer and calm periods build up beachesWinter and storms erode and flatten beachesWHY???

Light versus heavy wave activityDuring light wave activity, much of the swash soaks into the beach, and so backwash is reducedResults in a net movement of sand up the beachDuring heavy wave activity, the beach is saturated with water from previous waves, so very little swash soaks into the beach; more particles are moved offshore by backwashResults in a net movement of sand away from the beach

Summer versus winter beachesLight and heavy wave activity alternates with the seasonsLight wave activity dominates during summer, and so a wide, sandy berm developsHeavy wave activity dominates during winter, and transports sediment/particles offshoreA wide berm developed over many months during summer can be destroyed in hours by high-energy winter storm waves

Wide, prominent berm

No (or reduced) bermSand deposited offshore

Smith Point: SummerCourtesy of Prof. Pamela Lynch

Smith Point: SummerCourtesy of Prof. Pamela Lynch

Longshore currentsLongshore currents transport sand along the coastlineRecall that most wind waves approach the shore at an angle, and then refract to break nearly parallel to the shoreWith each breaking wave, swash moves onto the beach at a slight angle, then gravity pulls the backwash straight down (no angle); this zig-zag movement of water along a shore is called a longshore current

cc

Break the grip of the ripRip currents is a dangerously strong current of water flowing offshore with speeds averaging 1-8 feet per second! Water streams along the shore until it finds an exit back to sea, usually a narrow trench between sandbars

To escape a rip current, it is extremely important to swim parallel to shore as you cannot outswim the rip tide current and you will eventually exhaust yourself (and drown) in the process. The rip current is found only along a narrow portion of shorelineTopography and surging seas determine whether a rip tide will occur; can be visualized by rough, brown water offshore

Longshore currentsLongshore currents transport sediments away from the beach and towards regions downwindJetties, groins, and breakwaters serve to intercept, or stop the transport of sediment away from beachesLongshore current

Barrier IslandsDepositional coasts (coasts where depositional forces dominate over erosive) may develop narrow, exposed sand bars that run parallel to, but are separated from, land; known as barrier islandsFire Island, NY; Atlantic City, NJ; Ocean City, MD; and Miami Beach, FL are barrier islands!

Barrier IslandsBarrier islands protect the coast behind them, but are very unstable themselves

What about Long Island?Net transport of sediment is from east to west

A state park your great-grandparents never hadSince the Fire Island lighthouse was constructed 100 years ago, nearly 5 miles of barrier island have been added to the west forming what is now Robert Moses State Park!

www.pbase.com/image/66549546

Hurricane Paths on Long Island

The importance of dunesDune plants prevent erosion by trapping sediments in their rootsRemoval (or destruction) of beach vegetation, and the hardening of shorelines (bulkheads) increases erosionPlant rhizomes can extend 20 feet from the plant!Courtesy of Prof. Pamela Lynch

And dont forget about overpopulation!http://www.snopes.com/politics/quotes/guamtip.asp

****************************************