Chapter 11 outline
The Coast: Beaches and Shoreline Processes
a) In the US 80% of the population lives near coastal waters
b) Waves cause both deposition and erosion
1. The Coastal Region
a. The coast is the area that extends inland from the coast to as far as coatal feature can
be found
b. The shore is the area between the lowest low tide and the highest elevation on land
that is affected by storm waves
i.
Inside it is the foreshore, the area between the high tide shoreline and low tide
shoreline
ii.
And the backshore, the area above the high tide shoreline and is only covered by
storm waves
iii.
Shoreline is the water’s edge on the coast, and is constantly migrating with the
tides
c. Nearshore extends from the low tide shoreline to the low tide breaker line
i.
The low tide breaker line is the line where wave during the low tide begin to
break
d. Offshore is the area past the lowtide breaker line
e. The beach include both the nearshore and shore and is the entire part of the coast that
experiences changes due to breaking waves, it consists of sediment deposited along a
wave cut bench, or flat wave-eroded surface
f. The area above the shoreline is often called the recreational beach
g. The berm is a dry gently sloping region at the foot of coastal cliffs
h. The beach face is wet and is the sloping surface that extends from the berm to the
shoreline
i.
Longshore bars are sand bars that run parallel separated from the beach face byy the
longshore trough
j.
Wentworth scale is used to measure sediment size
2. Beach Composition and Slope
a. Beaches are composed of whatever is locally available to them
b. Muddy coastline called mud flats are formed typically by rivers being their main
sediment source, giving them fine clay sized particles
c. The coarser the material the steeper the slope of the beach is
3. Movement of sand on the beach
a. Swash is water that runs up the beach face towards the berm
b. Backwash is swash that runs back down the beach face towards the water
c. Light wave activity determines that swash dominates the transport system, so what is
happening to the beach?
i.
This what a summertime beach is
d. Backwash dominates heavy wave activity, which means what happens here?
i.
What a winter time beach is
e. Due refraction wave line up nearly parallel to the shore, causing water and to be
swashed up on the shore at an angle and brought back down perpendicular, creating a
zigzag called longshore current
i.
The transport of sediment in this current is called longshore drift or transport,
this amount is dependent of the equilibrium forces in the ocean
ii.
In the US the longshore drift typically flows southward on both coasts
f. Rip currents are very strong back wash currents that flow out towards the ocean on the
surface and can easily carry people out
i.
80% of lifeguard rescues are from these
ii.
Best to swim side ways
4. Erosional-Type Shores
a. Shores typically classified as either erosional or depositional based on the net
movement of sediment
i.
Erosional beaches typically have wave-eroded and are in areas of tectonic uplift
ii.
Depositional beaches typically have sand deposits and offshore barrier islands
b. Due to wave refraction waves concentrate on the headlands that jut out of continents
c. Waves pound on headlands and they eventually collapse to form wave cut cliffs
d. As they continue to hit the cliffs they may form sea caves at the base of the cliff
e. If waves erode through a head land the form a sea arch
f. Tops of sea arches eventually erode to form sea stacks
g. Uplift of the wave cut bench forms a marine terrace
5. Depositional-Type Shores
h. Coastal wave-erosion is where most transported sediment comes from
i.
i.
It also comes from either inland rock by wind or rivers
A spit is ridge of sediment created from the longshore current, and juts out into deep
water near the mouth of a bay
i.
If the spit continues on to connect to the main land then it forms a bay barrier or
bay-mouth bar
ii.
j.
If the spit connects to an island I forms a tombolo
Extremely long offshore deposits form a barrier island
i.
They are the lands first defense against storm waves
ii.
Contain distinct sections from ocean to land:
i. Ocean beach- beach facing the ocean
ii. Dunes- sand blown inland form dunes
iii. Barrier flat- flat land colonized by many grasses
iv. Salt marsh-flat area after the barrier flat containing fine sediment and
very salt tolerable plants, one of the most productive areas of the Earth
v. Lagoon- water between the land and the barrier island
k. Peat deposits are found under the barrier islands as they move inland due to rising sea
level
l.
When rivers carry too much sediment for longshore current to carry they form at delta
at their mouth
i.
Normally fertile and have periodic flooding
ii.
They grow through distributaries, or branching channels that deposit sediment,
and form a bird’s foot formation
m. Beach compartments consist of three distinct caompartments
i.
Rivers that supply sand
ii.
The beach itself transporting sand
iii.
And submarine canyons where sand is drained away
n. Human alterations, like dams or concrete lining, to rivers can cause beach starvation by
limiting the amount of sediment the beach receives
i.
One solution is to remove the objects (never happens due to the objects being
created for some purpose)
ii.
Or there is beach nourishment or replenishment which is where you pay to bring
sediment in from other sources (but very expensive)
o. Erosional beaches mainly have:
p. Depositional beaches mainly have
6. Classification of coasts
a. Francis Shepard is considered “the father of marine geology” and created a system
which divides coasts into either primary or secondary coasts
b. Primary coasts are formed by non-marine processes and are relatively young
c. Secondary coasts have aged to the point where marine processes are the dominant
character of the coast
7. Emerging and Submerging Shorelines
a. Shorelines are defined relative to their to their current sea level
b. If the shoreline is emerging above sea level it is called an emerging shoreline, and if it is
submerging below sea level it called a submerging shoreline
c. Marine terraces are common to emerging shorelines due to them forming when a wavecut bench is exposed
d. Stranded beach deposits can also show that the shoreline used to be higher than sea
level
e. Submerging shoreline have drowned beaches, submerged dune topography, and
drowned river valleys
f. The cause of submerging and emerging shoreline is tectonic activity and worldwide sea
level change
i. he most dramatic sea level change is by tectonic activity though
g. change in sea level worldwide is called eustatic change
i.
can be caused by lakes opening up into the ocean, melting glaciers, tectonic
activity, ect.
ii.
If all ice was to melt the sea level would change by 60 meters
h. For every 1*C change the sea level changes 2 meters
i.
Most coasts have both emerging and submerging characteristics
j.
Carbon dioxide has increased by 30% in the last 200 years and temperature has
increased by 0.6*C, causing a eustatic rise of 50-72 centimeters
8. Characteristics of US coasts
a. 70% of the world’s sandy beaches are eroding, and 90% of US sandy beaches are
eroding
b. The Atlantic coast
i.
has an average of 0.3 meters rising every century
ii.
79% of the Atlantic coast is eroding
iii.
eroding on average 0.8 meters every century
iv.
Tidal range of less than 1 meter to more than 2 meters
c. The Gulf of Mexico
i.
eroding at 1.8 meters every century
ii.
63% of the coast is eroding
iii.
Rising 0.3 meters every century
iv.
Tidal range of less than a meter
d. Pacific coast
i.
Tidal range of 1 to 2 meters
ii.
Erosion rate of .005 meters per year
iii.
30% is eroding
iv.
Average rate of erosion is 2.4 meters per year
9. Hard stabilization
a. Structure built on a beach to prevent the erosion of a coast or the movement of
sediment is called hard stabilization
b. A groin is structure built perpendicular to the coast and traps sand moving in the
longshore current
i.
Commonly made out of rip-rap or a large bulky rocks
ii.
Though the groin counters erosion upstream, downstream in is still taking place
so another groin must be built and so on creating a groin field
iii.
This usually creates many erosional problems
c. Jetties are similar to groins except they protect a harbor entrance from waves
i.
Do to how long they are they cause more pronounced upstream deposition and
downstream erosion
d. Breakwaters are hard stabilization created parallel to the shore
i.
Normally they just move the location of deposition to the location of the
breakwater
e. The most destructive hard stabilization is the sea wall
i.
Its destructiveness comes from the waves bouncing off the wall and releasing
the energy back eroding the beach
f. One alternative to hard stabilization is to limit construction on erosion prone areas
g. Another is relocation which moves landmarks in danger of erosion to a safer location