Coastal Erosion
There are two common definitions of coastal erosion. It is often
defined as the loss or displacement
of land along the coastline due to the action of waves, currents,
tides, wind-driven
water, waterborne ice, or other impacts of storms. In this case, landward
retreat of the shoreline, measured to a given spatial datum, is described over
a temporal scale of tides, seasons, and other short-term cyclic processes.
Alternatively, it is defined as the process of long-term removal of sediment and rocks
at the coastline, leading again to loss of land and retreat of the coastline
landward. Coastal erosion may be caused by hydraulic
action, abrasion, impact and corrosion by
wind and water, and other forces, natural or unnatural.
A. Coastal processes
1. Hydraulic action
Hydraulic
Action occurs when waves striking a cliff face compress
air in cracks on the cliff face. This exerts pressure on the surrounding rock,
and can progressively splinter and remove pieces. Over time, the cracks can
grow, sometimes forming a cave. The splinters fall to the sea bed where they are subjected
to further wave action.
2. Attrition
Attrition occurs when waves cause loose
pieces of rock debris (scree) to collide with each other, grinding and chipping each
other, progressively becoming smaller, smoother and rounder. Scree also
collides with the base of the cliff face, chipping small pieces of rock from
the cliff or have a corrasion (abrasion) effect, similar to sandpapering.
3. Solution
Solution is the process in which acids
contained in sea water will dissolve some types of rock such as chalk or
limestone.
4. Abrasion
Abrasion, also known as Corrasion,
occurs when waves break on cliff faces and slowly erode it. As the sea pounds
cliff faces it also uses the scree from other wave actions to batter and break
off pieces of rock from higher up the cliff face which can be used for this
same wave action and attrition.
5. Corrosion
Corrosion or
solution/chemical weathering occurs when the sea's pH (anything below pH 7.0)
corrodes rocks on a cliff face. Limestone
cliff faces, which have a moderately high pH, are particularly affected in this
way. Wave action also increases the rate of reaction by removing the reacted
material.
B. Factors that influence
erosion rates
1. Primary factors
The
ability of waves to cause erosion of the cliff face depends on many factors.
The hardness
(or inversely, the erodibility) of sea-facing rocks is controlled by the rock
strength and the presence of fissures, fractures, and beds of non-cohesive
materials such as silt and fine sand.
The rate
at which cliff fall debris is removed from the foreshore depends on the power
of the waves crossing the beach. This energy must reach a critical level to
remove material from the debris lobe. Debris lobes can be very persistent and
can take many years to completely disappear.
Beaches
dissipate wave energy on the foreshore and provide a measure of protection to the
adjoining land.
The
stability of the foreshore, or its resistance to lowering. Once stable, the
foreshore should widen and become more effective at dissipating the wave
energy, so that fewer and less powerful waves reach beyond it. The provision of
updrift material coming onto the foreshore beneath the cliff helps to ensure a
stable beach.
The
adjacent bathymetry, or configuration of the seafloor, controls the wave energy
arriving at the coast, and can have an important influence on the rate of cliff
erosion. Shoals and bars offer protection from wave erosion by causing storm
waves to break and dissipate their energy before reaching the shore. Given the
dynamic nature of the seafloor, changes in the location of shoals and bars may
cause the locus of beach or cliff erosion to change position along the shore.
Coastal
erosion has been greatly affected by the rising sea levels globally. There has
been great measures of increased coastal erosion on the Eastern seaboard of the
United States. Locations such as Florida have noticed increased coastal
erosion. In reaction to these increases Florida and its individual counties
have increased budgets to replenish the eroded sands that attract visitors to
Florida and help support its multibillion-dollar tourism industries.
2. Secondary factors
- ·
Weathering
and transport slope processes
- ·
Slope
hydrology
- ·
Vegetation
- ·
Cliff
foot erosion
- ·
Cliff
foot sediment accumulation
- ·
Resistance
of cliff foot sediment to attrition and transport
- ·
Human
Activity
3. Tertiary factors
- ·
Resource
extraction
- ·
Coastal
management
C. Control methods
There
are three common forms of coastal erosion control methods. These three include:
soft-erosion controls, hard-erosion controls, and relocation.
1. Hard-erosion controls
Hard-erosion
control methods provide a more permanent solution than soft-erosion control
methods. Seawalls and groynes serve as semi-permanent infrastructure. These
structures are not immune from normal wear-and-tear and will have to be
refurbished or rebuilt. It is estimated the average life span of a seawall is
50–100 years and the average for a groyne is 30–40 years. Because of their
relative permanence, it is assumed that these structures can be a final
solution to erosion. Seawalls can also deprive public access to the beach and
drastically alter the natural state of the beach. Groynes also drastically
alter the natural state of the beach. Some claim that groynes could reduce the
interval between beach nourishment projects though they are not seen as a
solution to beach nourishment. Other criticisms of seawalls are that they can
be expensive, difficult to maintain, and can sometimes cause further damage to
the beach if built improperly.
Natural
forms of hard-erosion control include planting or maintaining native
vegetation, such as mangrove forests and coral reefs.
2. Soft-erosion controls
Soft erosion
strategies refer to temporary options of slowing the effects of erosion. These
options, including Sandbag and beach nourishment, are not intended to be long
term solutions or permanent solutions. Another method, beach scraping or beach
bulldozing allows for the creation of an artificial dune in front of a building
or as means of preserving a building foundation. However, there is a U.S.
federal moratorium on beach bulldozing during turtle nesting season, 1 May – 15
November. One of the
most common methods of soft erosion control is beach nourishment projects.
These projects involve dredging sand and moving it to the beaches as a means of
reestablishing the sand lost due to erosion. In some
situations, beach nourishment is not a suitable measure to take for erosion
control, such as in areas with sand sinks or frequent and large storms.
3. Relocation
Relocation of infrastructure
any housing farther away from the coast is also called managed retreat. The
natural processes of both absolute and relative sea level rise and erosion are
considered in rebuilding. Depending on factors such as the severity of the
erosion, as well as the natural landscape of the property, relocation could
simply mean moving inland by a short distance or relocation can be to
completely remove improvements from an area. Typically, there has been low public support for
“retreating.
