Compilation of notes for Coasts:
Material is eroded from the shore when: (a) gravity pulls material off of high areas, a process known as mass
wasting; and (b) the shore is attacked by relatively rare high-energy winds or waves. Mass wasting occurs when
the base is eroded away (a process called “undercutting”) and then the upper portion falls down.
Most erosion occurs during storms.
The common calm conditions that prevail most of the time do little to erode the shore, and in many cases cause it
As a result, erosion of the shoreline tends to be episodic on the scale of weeks or months.
Sediment to a coast is provided by two sources: (a) rivers, as shown in this photo, carry sediment and deposit it
near the shore in a deposit called a delta; and (b) erosion of shorelines made up of loose sediment, like this
Sediment is removed permanently when it is carried so far offshore during large storms that it cannot be returned
during calmer times.
Sometimes sediment falls into submarine canyons and is lost.
Humans can induce beach erosion by robbing beaches of their sediment supply when dams are built on rivers.
The dams trap the sediment that would otherwise be resupplying the beaches.
This problem is especially severe
in the western USA.
For example, the Matilija Dam, in California, starved Surfer’s Point Beach of sediment and
caused the beach to be lost.
This is what Surfer’s Point beach looked like after the dam was built.
Due to pressure from the community
(including surfers) the dam is presently slated for removal.
Coasts characterized by steep slopes, such as are found on the Pacific and northern Atlantic coasts of the USA,
are usually eroding.
Erosion occurs as a process of: (a) slope undercutting; (b) mass wasting; (c) sand
distribution. Coasts characterized by gentle slopes, such as found along the southern Atlantic and Gulf coasts,
can be either eroding or accreting (building), and often do both seasonally within a single year.
coasts material is largely being relocated, not lost from the coast system.
The sediment size on a beach depends on what kind of rock or sediment the coast is made of.
Coasts made of
hard rock tend to be rocky, while those made of soft sedimentary rocks or sediments tend to be sandy.
The sediment size on a beach also depends on the wave energy.
High-energy waves stir up, entrain, and remove
finer sediment, leaving behind a beach made of pebbles and rocks.
Low-energy waves remove only the finest silt
and clay, and leave a sandy beach behind.
Waves are characterized by a wavelength (λ), amplitude, and velocity.
Geologists sometimes also use period, which is the time between wave crests, and is simply 1/λ. The height of a
wave is the distance from the trough to the peak, and is twice the amplitude.