Why are beaches strongly affected by erosion

In general terms, a detached breakwater is a coast-parallel structure located inside or close to the surf-zone. As this subject is too extensive for this page, see Detached breakwaters for a detailed discussion on the subject.

Other types of coastal protection that do not protrude into the sea will, however, also cause increased coastal erosion. Seawalls and revetments are typically constructed along coastal sections to provide protection of the coast. Seawalls are alongshore structures built as vertical walls, as sea dikes with smooth sloping stone revetments, as geotubes or as rip-rap concrete blocks or rock. Coasts protected by seawalls are called 'armoured coasts'. The impact of seawalls on coastal erosion depends, among other things, on the location of the structure in the coastal profile, on the littoral drift and on the natural beach evolution [4].

In the case of a naturally accreting coast, the main reason for building a seawall is to protect coastal settlements from storm surge damage or to protect low-lying hinterland from flooding. The seawall is usually fronted by a wide beach and its impact on beach dynamics is negligible. Seawalls are built most often on eroding coasts; their primary function is to stop erosion.

However, the erosion process seaward of the structure continues and may even be enhanced. After some time no beach will be left in front of the seawall. At that stage the toe of the seawall will be scoured by waves and the structure will collapse if the toe is not sufficiently protected [5] Fig. Whether a seawall enhances erosion before the beach has disappeared so-called 'active erosion' is site dependent.

Active erosion increases with decreasing beach width. Three processes play a role:. The erosion of soft cliffs often appears to be very drastic, which is why they have, in many cases, been the first to be protected in an area.

However, before their protection they were the main suppliers of sediments to the littoral cell in question. Consequently, their protection leads to increased erosion at adjacent lower sections of the coastline. The result is that the erosion has been shifted to less resistant areas resulting in higher area losses per year. Another impact of seawalls is the reduction of aeolian transport for the formation and extension of a coastal dune belt [12].

Other relevant articles on this topic are Seawall , Revetments and Seawalls and revetments. Decrease of fluvial sand supply to the coastal zone is a common cause of coastal erosion. Reduction of fluvial sand supply can result from different human interventions:. Thousands of dams have been constructed on rivers worldwide, creating reservoirs which retain a large part of the sediment discharge from the catchment areas Fig. The promontory propagated until and then began to erode.

Sand mining in rivers is a major cause of coastal erosion in many countries. Sand mining in a river lowers the river bed, causes bank erosion and reduces the supply of sand to the coast. There are five components in the sediment balance for a degrading river section, schematically represented in Fig. Many rivers consist of a steep upper part, the mountain part, and a gently sloping lower part, where the river crosses the coastal plain. Sand extraction in the upper part of the river causes lowering of the river bed and a similar lowering of the water level, hence no changes in the sediment transport capacity.

Thus the sand extraction in the upper part of the river is almost entirely balanced by local bed degradation, and has hardly any immediate impact on the supply of sand to the coast. Sand mining in the lower part of the river at some distance from the river mouth causes a local lowering of the river bed. However, the water level will not lower as much, which results in a local decrease in the flow velocity and in the sediment transport capacity.

The river bed depression will gradually be filled in from upstream supply and will travel towards the coast. When the impact of sand mining reaches the coast there may be an accumulated deficit in available river bed material corresponding to several decades sediment supply from the catchment. This means that an immediate halt in the sand mining will have hardly any remedial effect on the supply of sand to the coast, as the entire river bed has to rebuild before the original supply is re-established.

Sand mining close to the river mouth causes an immediate decrease in the supply of sand to the coast, and halt of the sand mining in this situation will quickly cause recover of the supply of sand to the coast. These impacts of sand mining on coastal sediment supply are observed in many rivers, for example for rivers in Sri Lanka [14]. Coastal erosion is not the only impact of river sand mining. Other impacts also have to be taken into consideration:.

Hence, river sand mining requires an integrated approach taking into account all the impacts. This calls for close collaboration between river authorities and coastal authorities. Small sandy bays enclosed between headlands have in general a crescentic shape, which is due to wave diffraction at the headlands and wave refraction in nearshore shallow water see Shallow-water wave theory.

However, the shape and position of the shoreline depends not only on the wave climate, but also on sand supply to the bay. There are two possible sources see Fig. The overall transport mechanisms in a crescent-shaped bay can be characterised as follows. The supply of sand from the upstream bay Q B will pass the headland and cross the bay via a bar.

If a river also contributes Q R to the littoral budget, this material will be transported downdrift into the bay, partly along the shoreline and partly onto the bar. Many of the seawalls of Sea Gate crumbled, and more than 25 homes were lost. Sea Level Rise Beaches are also threatened by sea level rise. Sea levels have been gradually rising for many years, drowning some beaches completely. New Moore Island, for example, was a small, uninhabited island in the Bay of Bengal.

Both India and Bangladesh claimed the island, which was little more than a strip of sandy beach. In March , rising sea levels drowned the island completely. New Moore Island is now a sandbar. Development Although the natural forces of wind and water can dramatically change beaches over many years, human activity can speed up the process.

Dam s, which block river sediment from reaching beaches, can cause beaches to retreat. In some places, large quantities of sand have been removed from beaches for use in making concrete. Development threatens the natural landscape of beaches. People develop homes and businesses near beaches for many reasons.

Beaches are traditional tourist destinations. Places like the U. Businesses, such as charter boat facilities, restaurants, and hotels, are built on the beach. People also enjoy living near beaches. Beachfront property is often very highly valued. Homes in the Hamptons are some of the most expensive in the United States. Development can crowd beaches. As more buildings and other facilities are built, beaches become narrower and narrower.

The natural, seasonal movement of beach sediment is disrupted. Communities spend millions of dollars digging, or dredging, sand from one place to another in order to keep the beach the same all year.

Disappearing beaches are bad for coastal facilities. Natural beaches reduce the power of waves, wind, and storm surge s. Without these barrier beaches, waves and storm surges crash directly into buildings. In , a storm swept away more than homes in the Hamptons. On Kauai, one of the islands in Hawaii, more than 70 percent of the beach is eroding, partly because of construction of seawalls and jetties, and from clearing out stream mouths.

Geologist s say Oahu, another Hawaiian island, has lost 25 percent of its shoreline. Beach Pollution Many beaches, especially in urban areas, are extremely polluted. Waves wash up debris from the ocean, while drainage pipe s or rivers deposit waste from inland areas. Some of this waste includes sewage and other toxic chemicals. After strong storms, some beaches are closed.

The amount of bacteria , raw sewage , and other toxic chemicals is hazardous to human health. Sometimes, it takes days or even weeks for the toxic waters to wash out to sea. Beach pollution also includes garbage, such as plastic bags, cans, and other containers from picnics.

Medical waste , such as needles and surgical instruments, has even washed up on beaches. All beach pollution is harmful to wildlife. Birds may choke on small bits of plastic. Marine mammal s such as sea lions may become tangled in ropes, twine, or other material. Floating plastic may prevent algae or sea plants from developing.

This prevents animals that live in tide pool s, such as sea anemones or sea stars, from finding nutrient s. Protecting Beaches Reducing pollution is an important way to protect beaches. Visitors should never leave trash on the beach or throw it in the ocean. Beachgoers should also leave wildlife alone—including birds, plants, and seaweed.

Taking shells or live animals from the beach destroys the habitat. People can also protect beaches from excess erosion. Limiting beachfront development can be an important step in protecting the natural landscape of beaches. In some places, machinery is used to dredge sand from the seabed just offshore and return it to the beach. Miami Beach, in the U. Best Beaches Environmental scientist Dr. Stephen P. Leatherman is known as Dr. Every year, Dr.

Beach makes a list of the top 10 beaches in the United States. Beach judges beaches based on 50 criteria, including sand softness, wind speed, water temperature, presence of runoff, public safety, rip currents, and pollution. Read about Dr. Beach and the science of beaches here. Beachs Top 10 for 1. Coronado Beach, California 2. Kahanamoku Beach,Hawaii 3.

East Hampton, New York 4. George Island State Park, Florida 5. Hamoa Beach, Hawaii 6. Coast Guard Beach, Massachusetts 7. Waimanalo Bay Beach Park, Hawaii 8. Cape Florida State Park, Florida 9. Beachwalker Park, South Carolina Xia, V.

Bex and P. Midgley eds. Geoscience Australia is committed to support Australia's capability to managing the impact of natural hazards, including coastal erosion. Geoscience Australia:. Coastal Erosion. What is coastal erosion? Coastal erosion can be either a: rapid-onset hazard occurs very quickly, a period of days to weeks slow-onset hazard occurring over many years, or decades to centuries.

What causes coastal erosion? Where does coastal erosion occur in Australia? Single storm events have caused coastal erosion, such as that associated with East Coast Low storms in and that damaged beachfront properties in Sydney. A series of large storm events in led to even more extensive damage to coastal properties and infrastructure in this region. Slow onset: The Twelve Apostles along the Great Ocean Road are a result of landscape change and coastal erosion over millenia.

There were originally 12 limestone features with 8 now remaining.