Transcript coast - Net Start Class

COASTLINES
Aquatic Science
coast: the zone extending from the
ocean inland, as far as the environment
is immediately affected by marine
processes
• Broad categories of coasts based on
long period plate tectonic processes:
• 1. Pacific type (active): mountainous,
close to plate collisions
• 2. Atlantic type (passive): mostly coastal
plains instead of mountains, far away from
plate collisions
Types of coasts based on short-term
coastal dynamics:
1. Primary Coasts: formed by nonmarine processes
– Erosion coasts
– A. fjords: glaciers carve out land and it then
floods with sea water
B. drowned river valley: river flow erodes a
valley that floods (Chesapeake Bay)
–Sedimentation coasts
– A. delta: material carried by
rivers flow into ocean, are
deposited and accumulate in a
triangular shape.
– Volcanic coasts: coasts of Hawaiian islands
– Tectonic coasts (fault coasts): plate
collisions push some of seafloor above water
or a fault opens and ocean floods a new area
2. Secondary Coasts: formed and significantly changed by
wave action and other marine processes.
Wave-erosion coasts: pounding by waves wears away the
coast
A. Waves pound the coast, forcing water and air into
cracks. Air in these cracks is compressed and then
expands when the wave subsides. When the air re-expands,
it can crack rocks, dislodge rock fragments, and enlarge
fractures.
B. Abrasion: rocks hurled by waves and sediments in waves
that have traveled hundreds of miles grinding and rounding
rocks.
These erosive forces can create certain features on
secondary coasts
A. Sea cliffs: steep cliffs caused by undercutting
B. Sea caves: cut into areas of softer rock in sea cliffs and may
develop “blow-holes”
C. The irregular coast formed by erosion has bays
(inlet/indentation in the shoreline) separated by
headlands (points of land jutting into the ocean).
These headlands get attacked from multiple sides because of
wave refraction.
wave refraction: caused by waves approaching the shore at
an angle.
1. When they reach shallow water, the part of the wave
nearest shore slows down first bending the wave.
2. The rest of the wave moves at full speed and becomes
parallel to the shore.
3. Due to refraction, wave impact is concentrated against the
sides and ends of headlands (the most protruding parts) and
strike it from all 3 sides.
Headlands become eroded to form offshore features such as:
a. sea arches: formed by erosion of headlands (when
two caves unite on opposite sides)
b. sea stacks: formed when arches fall, leaving isolated
remnant of a headland
c. shoreline straightening: when a coastline is straightened
by erosion. A shoreline will straighten over time due to:
- wave refraction (attacks anything protruding out)
- deposition of sediment into bays
Marine-deposition coasts: when sea action causes ocean
sediments to accumulate in one place If slope is steep,
sediments will drain to deep water. If slope is not very
steep, sediment will build beaches.
Sediment is transported along the coast by wave and current
action called longshore drift.
Beaches: zones of unconsolidated (loose) particles that cover
part or all of a shore.
A. Form when sediment (usually sand) is transported to a place
suitable for deposition. Ex. calm spots between headlands,
shores sheltered by offshore islands, etc.
B. most familiar feature of secondary coasts
C. landward limit: vegetation, seawall (built at shoreline to
separate land from water), permanent dunes, or sea cliffs
D.seaward limit: where sediment movement offshore ceases
(depth approx. 33 feet)
E. composition and slope: composition of beaches can
determine the shape.
1. smaller particles on beach = smaller slope.
- The smaller particles interlock and do not allow water to
percolate down.
- Water rushes on the beach (swash), carrying particles and
since no water percolates down, the same amount of water
carries particles seaward during backwash. Result:
gradual slope
2. Large particles do not fit together tightly, allowing water to
drain beneath them.
-On-rushing water disappears into the beach (less backwash to
carry particles seaward).
-The largest particles tend to build up along the back of the
beach due to being thrown by large waves. Result: steeper
slope
The finer the particles, the flatter the beach.
Key features of a beach:
A. berm: accumulation of sediments parallel to the shore,
marking limit of sand deposition by wave action. Berm
crest indicated by the marks left during most high tides.
B. backshore: from berm to farthest point sand has been
deposited = inactive part of beach with windblown dunes
and grasses
C. foreshore: seaward from berm and marks active zone of
beach
Large-scale features of secondary coasts
A. Sand spits:
1. most common feature
2. form where long shore current slows as it clears a
headland and approaches a quiet bay
3. slower current in bay is unable to transport
sediment, so sand and gravel are deposited in a line down
current of a headland (often have a curve at the tip)
B. Bay mouth bar: protects the bay from waves
1. forms when a sand spit closes off a bay by attaching to a
headland adjacent to the bay
2. inlet: the passage through the bay mouth
C. Tombolo: a ridge of sand that connects an island to the
mainland
D. Barrier Islands:
1. Ridges of sand that parallel the shoreline and extend
above sea level
2. Mainly along Atlantic and Gulf coasts
3. Many are densely populated (Atlantic City, Miami
Beach, Galveston)
4. separated from the mainland by a lagoon: long, shallow
body of seawater isolated from the ocean
5. Originate in several ways:
a. spits sever from mainland
b. from old sand dunes that were on the shore during
lower sea levels (ice age)
Coasts built by marine organisms: coasts that
emerge due to biological activity
A.Coral reefs
B. Accumulation of oyster shells
C. Marine plants that hold sediment and reduce force
of waves Mangroves, sea grasses, etc.)
Human interference in coastal processes
A. groins: a barrier built at a right angle to the beach to trap
sand that is
moving parallel to the shore
B. breakwater: constructed parallel to the shoreline to create a
calm
water area to protect a marina. Result: too much sand
accumulation
and marina gets filled in.
C. seawalls: massive barriers (granite), intended to prevent
waves from reaching the areas behind the wall. Result:
erosion on seaward side and total loss of the beach or
collapse of the wall.
D. jetties: to prevent a harbor or inlet from being blocked by
Groin
Seawall
Jetties