Shore-Zone Processes and Landforms
Shoreline: demarcation between land and water
Shore (shore zone): area affected by waves
Beach: a narrow strip of land, washed by waves or tides.
Coast: geographic region landward of oceanic shore
Components of A Typical Beach
Beach a narrow strip of land washed by waves or tides
Foreshore is the area between low tide and high tide
Backshore is the area between high tide and sea cliff or inland vegetation line
Beach face is the steepest part of Foreshore
Berm is a horizontal bench of storm sediment
Waves
• Ordinary Waves are caused by WIND
– Waves are produced when wind drag causes the
surface water of oceans/lakes to rise and fall
- Waves get refracted on approaching shoreline
Discussion: Fetch
Lake Michigan, a glacially over-deepened trough
Richard Dana: Two Years Before the Mast
Waves are caused by _____________?
Parts of A Wave
Fetch – Southern Ocean, Straits of Magellan
(Amplitude)
Swell, Wave Trains, Decay
• Ocean waves originate in stormy, windy areas.
When they meet they add and subtract, so that
the swell is organized into sets of waves, wave
trains, of different wavelengths, speeds and
amplitudes
• Period is the time it takes two crests to pass
• P = f (wavelength, speed)
• Amplitude is wave height
• Long period, low amplitude waves last longer
Oscillatory and Translatory Motion
Translatory motion re-suspends sediment
Wave energy to the shore
• Wind energy transferred to water surface
• Efficient orbital motion cycles energy KE <=> PE
• Considered “deep” until depth < 1/2 λ
l
Wave base
Oscillatory
Translatory Waves
rear up, slow down
deposit sediment
Wave Refraction
Still a small ‘longshore component
backwash
swash
Kids at the beach
Longshore Currents (Swash and
Backwash)
Deposition Spits, Hooks, and Baymouth
Bars
Longshore current causes longshore drift
Hooks: 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 resuspends only the portion deposited in deep
water: high deposits safe
Storm Surge
Inverted Barometer: each 1 mb drop raises sea surface 1 cm
Hurricanes drop the barometric pressure a lot
Combined w/ onshore wind and high tide
Results in deep Storm Surge
Tidal Bulges
Semidiurnal: 2 high @ low ~ 6 hrs
Affect of the Sun Much less
The Effect of Tides On Shorelines –
River Hebert in Nova Scotia
Tidal Bore
Anecdote: local tidal bore
Source: Clyde H. Smith/Peter Arnold, Inc.
Low Tide at Bay of Fundy
Natural period of oscillation
Analogy: waves in bath tub
Waves in swimming pool
Source: William E. Ferguson
Rising Tide at Bay of Fundy
Bay of Fundy natural period about 12 hours
High tide to high tide period about 12 hours
High Tide
Source: William E. Ferguson
Wind as geomorphic agent
• Strong onshore winds
build parabolic dunes
along the shore, bedload
sand moves short
distances via saltation
• Grains above high tide
Primary 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.
P 247-248
River 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.
“Gilbert Delta”, 1/3 underwater,
topset,foreset and bottomset beds
Wave-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
D.
Tide-dominated Deltas
• Strong Tidal influence
• Ebb and flow of tides distributes sediment offshore,
much of delta is underwater
• Distributaries become trumpet-shaped
Mississippi Delta
Ganges Delta
Secondary 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 Erosion
High Pressure water and air forced into rock
Source: Criag Tuttle/The Stock Market
Crashing Surf, Oregon Coast
The role of beach slope
• A steep slope is easier to erode, a gentle
slope does not erode rapidly
More of the waves energy is
directed perpendicular to a steep slope
Shore Platforms
• Postglacial rise in sea level is complicated
by rebound.
• Rise often cuts a new platform on old
hillside
• Water rises, erodes higher
• Then rebound raises the land, surf zone
cuts into lower part
• Shear cliffs result
Erosional Coastal Landforms
Skipped 426 R to 431
Submerged Shallow
Area Bends Waves
Sea Caves on Cape Kildare, Prince Edward Island, Canada
Source: John Elk/Bruce Coleman
Deposition of A Tombolo
A Tombolo Landward of A Sea Stack – Big Sur,
California
Source: Cliff Wassmann
Coral Reefs
Here fringing reefs and Atolls
Charles Darwin
coral larvae are planktonic
Discussion: Atolls and Island Chains
Discussion: Corals, zooxanthelae, low quartz & clay sediment, light, injury
Rip currents
• Return flow from longshore currents
Wave Energy
Rip Current
Longshore
Current
Copyright © Rob Brander 2002
Barrier Islands and Lagoons
Theory 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. sealevel 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.
Source: Breck P. Kent
Barrier Island and Lagoon, nr.Cape Hatteras
Theory 2: Storm surge causes large
ripples 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 I
DeBeaumont (1845). believed wind piled
sandbar sand up at low tide,
built areas higher than high tide level,
forming barrier islands.
Barrier
island
formation
II
• Based on
work on
pluvial Lake
Bonneville by
G.K.Gilbert
Barrier island formation III
14000 ya sea level rising
Sand dunes surrounded
Tidal Inlet Created by Hurricane
Waves (North Carolina)
A barrier in its natural state,
broad and low, subject to
washover by storms.
Source: Associated Press/Raleigh News & Observer, Chris
Seward
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 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