Beach evolution is a natural process occurring along shorelines where sea, lake, or river water erodes the land. Beaches form as sand accumulates over centuries through recurrent processes that erode rocky and sedimentary material into sand deposits. River deltas contribute by depositing silt carried from upriver, accreting at the river's outlet to extend lake or ocean shorelines. Catastrophic events such as tsunamis, hurricanes, and storm surges accelerate beach evolution.
Accretion and erosion
Sudden and rapid processes
Tsunamis and hurricane-driven storm surges
Tsunamis can cause significant erosion and sediment displacement. They can strip away years of accumulated sand from beaches and devastate coastal vegetation. These powerful waves can flood inland areas far beyond the typical high-tide mark. Additionally, the swift currents associated with the inundating tsunami can demolish homes and other coastal structures.
A storm surge is an onshore gush of water associated with a low pressure weather system. Storm surges can cause beach accretion and erosion. Historically notable storm surges occurred during the North Sea Flood of 1953, Hurricane Katrina, and the 1970 Bhola cyclone.
Volcanism and earthquakes related sea-level changes Both geological events and the climate can change (progressively or suddenly) the relative height of the Earth's surface to the sea-level. These events or processes continuously change coastlines.
Volcanic activity can create new islands. For example, Surtsey Island in Iceland which has a diameter of 800 meters (2,600 ft), was created between November 1963 and June 1967. The island emerged from undersea vents that are part of the Vestmannaeyjar submarine volcanic system. Although the island has since partially eroded, but it is expected to last another 100 years.
Some earthquakes can create sudden variations of relative ground level and change the coastline dramatically. Structurally controlled coasts include the San Andreas Fault zone in California and the seismic Mediterranean belt (from Gibraltar to Greece).
The Bay of Pozzuoli, in Pozzuoli, Italy experienced hundreds of tremors between August 1982 and December 1984. The tremors, which reached a peak on October 4, 1983, damaged 8,000 buildings in the city center and raised the sea bottom by almost . This rendered the Bay of Pozzuoli too shallow for large craft and required the reconstruction of the harbor with new quays. The photo at the upper right shows the harbor before the uplift while the one on the bottom right shows the new quay.
Gradual processes The gradual evolution of beaches often comes from the interaction of longshore drift, a wave-driven process by which sediments move along a beach shore, and other sources of erosion or accretion, such as nearby rivers.
Deltas Deltas are nourished by alluvial systems and accumulate sand and silt, growing where the sediment flux from land is large enough to avoid complete removal by coastal currents, tides, or waves.
Most modern deltas are formed during the last five thousand years, after the present sea-level high stand was attained. However, not all sediment remains permanently in place: in the short term (decades to centuries), exceptional river floods, storms or other energetic events may remove significant portions of delta sediment or change its lobe distribution (pattern in which sediments are deposited across Delta, forming distinct, lobe shaped structures) and, on longer geological time scales, sea-level fluctuations lead to the destruction of deltaic features.
Subsidence and uplift related sea-level changes Subsidence is the motion of the Earth's surface downward relative to the sea level due to internal geodynamic causes, It can occur naturally or due to human activities. The opposite of subsidence is uplift, which increases elevation.
Venice is probably the best-known example of a subsiding location. Built suspended over a coastal lagoon, it experiences periodic flooding when extreme high tides or surges arrive; St Mark's Square is built only 55 centimeters above sea level. This phenomenon is caused by the compaction of young sediments in the Po River delta area, magnified by subsurface water and gas exploitation. Man-made works to solve this progressive sinking have been unsuccessful.
Mälaren, the third-largest lake in Sweden, is an example of deglacial uplift. It was once a bay on which seagoing vessels were once able to sail far into the country's interior, but it ultimately became a lake. Its uplift was caused by deglaciation: the removal of the weight of ice-age glaciers caused rapid uplift of the depressed land. For 2,000 years as the ice was unloaded, uplift proceeded at about /year. Once deglaciation was complete, uplift slowed to about annually, and it decreased exponentially after that. Today, annual uplift rates are or less, and studies suggest that rebound will continue for about another 10,000 years. The total uplift from the end of deglaciation may be up to .
Beach management
Integrated coastal zone management minimizes the negative human impacts on coasts, enhances coastal defense, mitigates the risk associated with the sea level rise and other natural hazards.
The beach erosion is a type of bioerosion which alters the coastal geography through beach morphodynamics. There are numerous incidents of modern recession of beaches, mainly due to the longshore drift and coastal development hazards related to human activities.
Solutions range from "do nothing" to "Move beach seaward" approach which uses the elements of hard and soft engineering. The interventionist methods, such as "Move beach seaward", combine the hard engineering methods such as constructing structures (accropodes) with the soft engineering methods such as sand dune stabilization. These intervention are aimed at prevention of beach erosion caused by longshore drift and coastal development hazards, as well as facilitation of beach evolution and expansion.
Coastal planning approaches
Five generic planning approaches involved in coastal defense are:
Abandonment of shore: do nothing, let the natural process takeover. Managed retreat, also called realignment. Hold the shoreline: by using shoreline hardening techniques to create permanent concrete and rock constructions such as groynes. Move beach seaward: by using hard and soft intervention techniques usually in areas of high economic significance. Limited intervention: usually in areas of low economic significance, often includes the succession of haloseres, including salt marshes and sand dunes.
Coastal engineering
Coastal engineering techniques can be classified into two categories: hard engineering methods and soft engineering methods
Hard engineering methods Hard engineering methods are also called "Structural methods". "Move towards the sea" beach accretion can be facilitated by the four main type of hard engineering structures, namely seawall, revetment, groyne or breakwater. Most commonly used hard structures are seawall and series of "headland groyne" (breakwater connected to the shore with groyne).
Main types of structures
Four main types of structures or accropodes are seawalls, groynes, breakwater and revetments. Headland groynes are a combination of breakwater and groyne.
Seawalls
Seawalls re-direct most of the incident energy in the form of sloping revetments, resulting in low reflected waves and much reduced turbulence. Designs use porous designs of rock or concrete objects such as Tetrapods or Xblocs with flights of steps for beach access. Seawall at Cronulla beach, NSW,Armour Units – Random Mass or Disciplined Array, – C.T.Brown ASCE Coastal Structures Specialty Conference, Washington, March 1979; The Design & Construction of Prince St. Seawall, Cronulla, EHW Hirst & D.N.Foster – 8th CCOE, Nov 1987, Launceston, Tasmania for example, uses concrete wall. Submerged seawalls or structures are constructed to create the underwater reefs to slow down wave energy and beach erosion.
Groynes and Headland groyne
Groynes are the walls perpendicular to the coastline. Groynes are generally placed in series and the areas between groups of groynes are called groyne fields. To directs the sand towards the shore targeted for sand accumulation, a shorter groyne turned slightly towards downdrift side of the beach is deployed at updrift end of the beach, a longer groyne at the downdrift end of the beach is deployed, a series of groyne are deployed between the two ends. Groynes are often made of gabion, greenharts, concrete, rock or wood. Material builds up on the downdrift side, where littoral drift is predominantly in one direction, creating a wider and a more plentiful beach. Groynes are cost-effective, require little maintenance and are one of the most common defences.
Headland groyne or Bulkhead breakwaterWhen groyne is built to attach a breakwater to shore, the resulting T-structure is called "headland breakwater", "headland groyne", "bulkhead groyne" or "bulkhead breakwater". Use of groynes and headland groyne, accumulates the sand across the beach but it tend to deplete the sand faster from the downdrift end of the beach. This can be mitigated and sand could be accumulated at the downdrift end of the beach also. This is achieved by having a longer "groyne" or "headland groyne" at the end of downdrift side of the beach. To enhance the sand accumulation, this "headland groyne" could have another series of smaller "headland groyne" jutting out of it pointing towards updrift end of the beach in a way that the smaller "headland groyne" are parallel to the shore and perpendicular to main "headland groyne". This will facilitate gradual natural creation of ayre (sand or gravel filled beach). If there is a near shore island near the downdrift end of the beach and "headland groyne", then this could be turned into a cuspate foreland headland with the use of the gradual natural creation of ayre (gravel filled beach). Main "headland groyne" at the end of downdrift could be further stabilized by a hard engineered detention basin and grassy mangrove salt marsh. Salt marsh could be created with the use of soft engineering approach, such as lose stone sills, while leaving a whole in the sill for a seawater channel. Seawater channel could be a cemented open channel or a pipe buried under the beach. This marsh could be designed to taper into a hard engineered sandy beach. Having inland saltwater marsh between the beach and mainland will lower the cost by eliminating the need for filling up the marshy area with the sand, and the mangroves and grasses in the marsh will facilitate gradual built up of sediments.
Breakwater Breakwater, also called "offshore breakwater", are offshore structure constructed parallel to the shore to alter wave direction and tide energy. The waves break further offshore and therefore lose erosive power. This leads to formation of wider beaches, which further absorb wave energy. A series of breakwaters is often deployed across the beach shore.
Revetment Revetments are slanted or upright blockades, built parallel to the coast, usually towards the back of the beach to protect the area beyond. The most basic revetments consist of timber slants with a possible rock infill. Waves break against the revetments, which dissipate and absorb the energy. The shoreline is protected by the beach material held behind the barriers, as the revetments trap some of the material. Unless other methods are used in combination, surf progressively erodes and destroys the revetment which requires ongoing maintenance.
Other types of structures Riprap / Rock armour Rock armour, also called riprap, is basement placed at the sea edge using local material. This could be the protruding foot of a seawall or revetment to reduce maintenance of those. Longshore drift is not hindered.
Cliff stabilization