Shore-Zone Processes and LandformsShore-Zone Processes and Landforms

Shoreline: demarcation between land and waterShore (shore zone): area affected by wavesBeach: a narrow strip of land, washed by waves or tides.Coast: geographic region landward of oceanic shore

Components of A Typical BeachComponents of A Typical Beach

Beach a narrow strip of land washed by waves or tidesForeshore is the area between low tide and high tideBackshore is the area between high tide and sea cliff or inland vegetation lineBeach face is the steepest part of ForeshoreBerm is a horizontal bench of storm sediment

WavesWaves

• Ordinary Waves are caused by WINDOrdinary Waves are caused by WIND– Waves are produced when wind drag causes the Waves are produced when wind drag causes the

surface water of oceans/lakes to rise and fall surface water of oceans/lakes to rise and fall

- Waves get refracted on approaching shoreline- Waves get refracted on approaching shoreline

Lake Michigan, a glacially over-deepened trough

Discussion: Fetch

Richard Dana: Two Years Before the Mast

Parts of A WaveParts of A WaveWaves are caused by _____________?

Fetch – Southern Ocean, Straits of Magellan

(Amplitude)

Swell, Wave Trains, DecaySwell, Wave Trains, Decay• Ocean waves originate in stormy, windy areas. When they meet they add and subtract, so that the Ocean waves originate in stormy, windy areas. When they meet they add and subtract, so that the

swell swell is organized into sets of waves, wave trains, of different wavelengths, speeds and amplitudesis organized into sets of waves, wave trains, of different wavelengths, speeds and amplitudes

• Period is the time it takes two crests to passPeriod is the time it takes two crests to pass

• P = P = ff (wavelength, speed) (wavelength, speed)

• Amplitude is wave heightAmplitude is wave height• Long period, low amplitude waves last longerLong period, low amplitude waves last longer

Oscillatory and Translatory MotionOscillatory and Translatory Motion

Translatory motion re-suspends sediment

Wave energy to the shoreWave energy to the shore• Wind energy transferred to water surface• Efficient orbital motion cycles energy KE <=> PE

• Considered “deep” until depth < 1/2 λ

Wave baseTranslatory Wavesrear up, slow downdeposit sedimentOscillatory

Wave RefractionWave RefractionStill a small ‘longshore component

swashbackwash

Kids at the beach

Longshore Currents (Swash and Longshore Currents (Swash and Backwash)Backwash)

Deposition Spits, Hooks, and Baymouth Deposition Spits, Hooks, and Baymouth BarsBars

Longshore current causes longshore drift

Hooks: landward curve of spitsHooks: landward curve of spits• The origin of hooks has many theories• Ocean wave refraction from opposite shore of

Baymouth• Onshore storm surge from Hurricanes • Strong onshore winds hurricanes & Nor'easters• Tidal: High tide carries sediment into bay, deposits

new sediment at higher elevations. Low tide water level lower; ebb flow re-suspends only the portion deposited in deep water: high deposits safe

Storm SurgeInverted Barometer: each 1 mb drop raises sea surface 1 cmHurricanes drop the barometric pressure a lot

Combined w/ onshore wind and high tideResults in deep Storm Surge

Tidal BulgesTidal Bulges

Affect of the Sun Much less

Semidiurnal: 2 high @ low ~ 6 hrs

The Effect of Tides On Shorelines The Effect of Tides On Shorelines – – River Hebert in Nova ScotiaRiver Hebert in Nova Scotia

Source: Clyde H. Smith/Peter Arnold, Inc.

Tidal BoreAnecdote: local tidal bore

Low Tide at Bay of FundyLow Tide at Bay of Fundy

Source: William E. Ferguson

Natural period of oscillationAnalogy: waves in bath tubWaves in swimming pool

Rising Tide at Bay of Fundy Rising Tide at Bay of Fundy

Source: William E. Ferguson

High Tide

Bay of Fundy natural period about 12 hoursHigh tide to high tide period about 12 hours

Wind as geomorphic agentWind as geomorphic agent• Strong onshore winds build Strong onshore winds build

parabolic dunes along the shore, parabolic dunes along the shore, bedload sand moves short bedload sand moves short distances via saltationdistances via saltation

• Grains above high tide Grains above high tide

Primary CoastsPrimary Coasts• shaped by non-marine processes, by changes in the land form.

• in much the same condition as it was when sea level was stabilized after the last ice age

• Primary coasts are created by erosion, deposition, or tectonic activity

• were formed as the sea level rose during the last 18,000 years, submerging river and glacial valleys to form bays and fjords.

• An example of a primary coast is a river delta, which forms when a river deposits soil and other material as it enters the sea. BUT, if the delta is swept away, a secondary coast is created.

River DeltasRiver Deltas• a landform that is formed at the mouth of a

river.

• flows into an ocean, sea, estuary, lake.

• formed from the deposition of the sediment carried by the river as the flow enters the sea or lake and competence drops.

P 247-248

“Gilbert Delta”, 1/3 underwater, topset,foreset and bottomset beds

Wave-dominated DeltasWave-dominated Deltas• much of the sediment emanating from the

river mouth is deflected along the coast line by longshore currents, forming spits, barrier bars and lagoons, etc.

• Deltas of this form, such as the Nile Delta, tend to have a characteristic delta shape .

Tide-dominated DeltasTide-dominated Deltas• Strong Tidal influence• Ebb and flow of tides distributes sediment offshore,

much of delta is underwater• Distributaries become trumpet-shaped

Ganges Delta

Mississippi Delta

Secondary CoastsSecondary Coasts

• produced by marine processes, such as the action of the sea or by creatures that live in it.

• Secondary coastlines include sea cliffs, barrier islands, mud flats, coral reefs, mangrove swamps and salt marshes.

Coastal ErosionCoastal Erosion

Source: Criag Tuttle/The Stock Market

High Pressure water and air forced into rock

Crashing Surf, Oregon CoastCrashing Surf, Oregon Coast

The role of beach slopeThe role of beach slope• A steep slope is easier to erode, a gentle

slope does not erode rapidly

More of the waves energy isdirected perpendicular to a steep slope

Shore PlatformsShore Platforms

• Postglacial rise in sea level is complicated Postglacial rise in sea level is complicated by rebound.by rebound.

• Rise often cuts a new platform on old Rise often cuts a new platform on old hillsidehillside

• Water rises, erodes higherWater rises, erodes higher

• Then rebound raises the land, surf zone Then rebound raises the land, surf zone cuts into lower partcuts into lower part

• Shear cliffs resultShear cliffs result

Erosional Coastal LandformsErosional Coastal Landforms

Submerged Shallow Area Bends Waves

Skipped 426 R to 431

Sea Caves on Cape Kildare, Prince Edward Island, CanadaSea Caves on Cape Kildare, Prince Edward Island, Canada

Source: John Elk/Bruce Coleman

Deposition of A TomboloDeposition of A Tombolo

A Tombolo Landward of A Sea Stack A Tombolo Landward of A Sea Stack – – Big Sur, Big Sur, CaliforniaCalifornia

Source: Cliff Wassmann

Coral ReefsCoral Reefs

Charles Darwin coral larvae are planktonic

Here fringing reefs and Atolls

Discussion: Corals, zooxanthelae, low quartz & clay sediment, light, injury

Discussion: Atolls and Island Chains

Rip currentsRip currents

• Return flow from longshore currents

Copyright © Rob Brander 2002

Wave EnergyLongshore

CurrentRip Current

Barrier Islands and LagoonsBarrier Islands and Lagoons

Source: Breck P. KentBarrier Island and Lagoon, nr.Cape HatterasBarrier Island and Lagoon, nr.Cape Hatteras

Theory 1Theory 1, Bloom p 437 left, paragraphs 3 and 1. On trailing (passive margin) coasts, the Atlantic Stage 6000 years ago caused a high transgression, i.e. sea-level was higher than today. During that time, transgression eroded more land, increased offshore sand supply … that eventually became too large to move. Rebound and cooling lowered sea-level, leaving offshore mounds of sand that became strung out and connected by longshore drift.

Theory 2: Storm surge causes large Theory 2: Storm surge causes large ripples that climb the shallowsripples that climb the shallows

• Ridges exposed at low tide, survive after the storm surge ends

One theory for Barrier Bar growth

Bloom 437: "Some authorities thought that wave action built barriers above sea-level."

Barrier island formation IBarrier island formation I

DeBeaumont (1845). believed wind piled sandbar sand up at low tide,

built areas higher than high tide level, forming barrier islands.

• Based on work on pluvial Lake Bonneville by G.K.Gilbert

Barrier Barrier island island

formation formation IIII

Barrier island formation IIIBarrier island formation III14000 ya sea level rising

Sand dunes surrounded

Tidal Inlet Created by Hurricane Tidal Inlet Created by Hurricane Waves (North Carolina)Waves (North Carolina)

Source: Associated Press/Raleigh News & Observer, Chris Seward

A barrier in its natural state,broad and low, subject to washover by storms.

Why are lagoons muddy?Why are lagoons muddy?

• In seawater, clay flocculates into round bunches, heavier, settles in slow water

• Bloom p. 439 (paraphrased): Clay along shore suspended, carried into lagoon by high tide. Lagoons are shallow, so mud can settle quickly.

• Fraction falls and sticks in lagoon, rest carried out to ocean again, where water is deep

• Most offshore mud bunches still suspended next incoming tide, another chance to settle out and stick to the lagoon bottom.

Tidal Mangroves, Florida CoastTidal Mangroves, Florida Coast

Source: S. J. Krasemann/Peter Arnold, Inc.

Prop roots grow seaward, new growth rises from root, root "pilings" cause interference, velocity drops, suspended sediment falls, land extends seaward