Transcript The-Coast-Slideshow - Chaparral Star Academy

The Coast:
Beaches and Shoreline Processes
Coastal
region constantly changes
Primarily due to waves
Erosion
Deposition
Many
people live in coastal regions
80%
coast
of people in U.S. live within easy access of
Coastal regions
Coast
= extends inland from shore as far as ocean-related features can be found
Coastline = marks boundary between shore and coast
Beach
= deposit of sediment along the shore area (active coastal area); consists
of wave-worked sediment that moves along the wave-cut bench (flat, waveeroded surface)
Beach face = wet, sloping surface between berm and shoreline (aka “low
tide terrace”)
Berm = dry, gently sloping region at the foot of cliffs or dunes
Shore
= between low tide line and the highest elevation on land that is affected
by storm waves (coastline)
Shoreline = water’s edge; migrates with tides
 foreshore = between low tide line and high tide line
 backshore = above high tide line (covered with water only during storms)
Nearshore = between low tide line and low tide breaker line; above wave base
longshore bar/longshore trough
Offshore = deeper water; below wave base
Coast
= extends inland from shore as far as ocean-related features can
be found
Coastline = marks boundary between shore and coast
Beach
= deposit of sediment along the shore area (active coastal area);
consists of wave-worked sediment that moves along the wave-cut bench
(flat, wave-eroded surface)
Beach face = wet, sloping surface between berm and shoreline (aka
“low tide terrace”)
Berm = dry, gently sloping region at the foot of cliffs or dunes
Shore
= between low tide line and the highest elevation on land that is
affected by storm waves (coastline)
Shoreline = water’s edge; migrates with tides
 foreshore = between low tide line and high tide line (aka
“intertidal” or “littoral” zone)
 backshore = above high tide line (covered with water only during
storms)
foreshore
= between low tide line and high tide line (aka “intertidal”
or “littoral” zone)
 backshore = above high tide line (covered with water only during
storms)
Beach
face = wet, sloping
surface between berm and
shoreline (aka “low tide terrace”)
Berm = dry, gently sloping region
at the foot of cliffs or dunes
Nearshore
= between low tide line and low tide breaker line; above
wave base
longshore bar, longshore trough
Offshore = deeper water; below wave base; beyond breakers
Beach Composition
Locally
available material
May be coarse or fine:
Boulders, cobbles from local cliffs
Sand, mud from rivers
Significant biologic material at tropical beaches
Example, Coral reef material, shell fragments
Lithologic
= eroded from cliffs or deposited by rivers; volcanic material
Gravel, sand, silt, clay
Biogenic = shell fragments; coral debris
Beach material is in transit along the shoreline
Sand movement along beach
Perpendicular
to shoreline (toward and away)
Swash (brings in sand) and backwash (takes sand away)
Parallel to shoreline (up-coast or down-coast
Longshore current (moves sand along beach)
Swash and backwash
After
wave breaks, uprush of
water (swash) on beach
Sediment moved toward
land; deposition
Backwash
Water returns to ocean
Sediment drains away
from shore, gravity driven
Light wave activity (low energy
waves)
Swash dominates
Sediment moved
toward shore
Wider beach
(berm)
Fair weather: Summertime
beach
Swash and backwash
Heavy
wave activity
High energy waves
Backwash dominates
Sediment moved away
from shore; erosion
Narrower beach
Sand forms offshore sand bars
Stormy weather
Wintertime beach
Wave Activity and
Beach Characteristics
A-Long-the-Shore Motion
Millions
of tons of sediment moved yearly
Direction of transport changes due to wave
approach
In general, sediment is transported
southward along Atlantic and Pacific coasts
of U.S.
Longshore
current = zig-zag
movement of water
4 km/hr (2.5 mph)
Faster if:
Steeper beach
Higher angle of
incidence of waves
Greater wave heights
Longshore drift (longshore
transport) = zig-zag movement of
sediment due to longshore current
Net movement =
downcurrent
Only within surf zone
(beach), where waves touch
bottom and refract (d <L/2)
Shoreline Features
Erosional
Shores
Well-developed cliffs
Tectonic uplift
Pacific
coast
Depositional
Shores
Sand deposits and
offshore barrier islands
Slow subsidence
SE
Atlantic coast
Gulf coast
Erosional Shores
Wave-cut sea cliffs
Wave-cut bench
Headlands/coves
Sea caves
Sea arches/sea stacks
Marine terraces
Blowholes
Wave
erosion increases with
More shore exposed to open ocean
Smaller tidal range
Weaker bedrock
Erosional Shores
View of the marine terrace just south of Goat Rock.
"Perched on the top of the terrace is a large rock,
floating like a ship on the level surface." Below the
terrace, a modern-day wave cut platform and sea stacks
are being formed by the pounding surf. Photo copyright
Barrie Rokeach 2001
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Wave-cut sea cliffs
Wave-cut bench
Headlands/coves
Sea arches/sea stacks
Marine terraces
Depositional Shores
Beach
Spit
Bay
barrier/bay-mouth bar
Tombolo
Barrier island
Long,
narrow offshore
deposits parallel to shore
Most developed due to rise
of sea level about 18,000
years ago
Common along East and
Gulf coasts of U.S.
Protect mainland from high
wave activity
Delta
Beach
compartments
Primarily deposited by longshore drift
Depositional Shores
Barrier Islands
Defense against rising sea level
and high-energy storm waves
Many developed during global
rise in sea level associated with
the last Ice Age ~18,000 years
ago.
a) NC Outer
Banks
b) South TX
coast
(Padre
Island, etc.)
c) Tom’s River,
NJ
Barrier Islands
ocean beach (summer vs. winter)
dunes (wind; stabilized by grasses; protect lagoon during storms)
barrier flat (sand blown further during storms; bio-succession)
high salt marsh (to high spring-tide line)
low salt marsh (mean sea level to high neap-tide line; bio-productive)
lagoon
New marshland created by overwash; island migrates
Deltas
Stream empties into ocean
Fan-shaped deposit of sediment
Distributary channels
MS River delta, Nile River delta
SAC-San Joaquin River delta
Sacramento-San Joaquin River Delta
Beach Compartments
3 components:
Characteristic of depositional shores.
rivers & coastal erosion =
source of sediment to
beaches
beach = sand in motion due to
longshore transport
submarine canyons = sink
where sediment drains away
from beach
Beach Starvation
Human activities alter balance
of natural system in beach
compartments, e.g., dams
deprives beach of source
material.
Upstream = narrow, rocky beach
Downstream = wide, sandy beach
Shorelines
Emerging shorelines = land rises or sea level drops
marine terraces (exposed wave-cut benches), ancient sea cliffs
stranded beach deposits
Submerging shorelines = land subsides or sea level rises
drowned beaches
submerged dune topography
drowned river valleys
Sea Level Changes
Sea level dropped globally by about 120 m (400 ft) as the last glacial
advance removed water from the oceans and transferred it to glaciers on
land. About 18,000 years ago, sea level began to rise as the glaciers
melted and water returned to the seas.
Tectonic movements
uplift or subsidence
e.g., Pacific coast
Isostatic adjustment
subsidence or rebound
e.g., Hudson Bay, Gulf of
Bothnia (900 ft)
Eustatic changes
seawater volume or ocean
capacity
e.g., seafloor spreading
rates, ice ages
Euastatic Sea Level Changes
Measured relative sea
level rise at NYC.
Tide-gauge data from NYC
show in increase in sea
level of 40 cm (16 in) since
1850. While some of this
rise is due to local effects,
most is due to thermal
expansion of warmer
ocean water and the
retreat of small ice caps
and glaciers.
Factors affecting U.S. coasts
Map shows coastal bedrock type, mean spring-tide range, degree of
exposure, and average direction of longshore drift. Map also shows average
rate of erosion (-) or deposition (+) between 1979 and 1983 in m/yr.
Atlantic Coast
In general, a submerging coastline
with erosion dominant (avg. 0.8 m/yr
or 2.6 ft/yr).
BUT:
Open exposure to storm waves;
barrier islands protect some areas.
Tide ranges from <1 m (Florida) to >2
m (Maine).
Bedrock varies; affects shoreline
features.
Glacial features from NY northward
and rising sea level since Ice Age;
rebound in Maine.
Pacific Coast
In general, an emerging coastline;
avg. erosion (0.005 m/yr).
Open exposure to storm waves.
Tectonic uplift.
Tide ranges between 1-2 m
Bedrock mostly sedimentary; easy to
erode.
Beach starvation.
Hard Stabilization
Groins/groin fields - perpendicular to
shoreline
Jetties - perpendicular to shoreline to protect
harbor
Breakwaters - parallel shoreline
Seawalls - landward of berm
Hard Stabilization
1931
1949
Seawalls and beaches
When a seawall is build along a beach to
protect property:
a)
A large storm can remove the beach
from the seaward side of the wall and
steepen its seaward slope.
b)
Eventually the wall is undermined and
falls into the sea.
c)
The property is lost. . .
d)
. . .as the oversteepened beach slope
advances landward in its effort to
reestablish a natural slope angle.
Replenishment (beach nourishment)
Relocation