Swash
Encyclopedia
Swash, in geography
Geography
Geography is the science that studies the lands, features, inhabitants, and phenomena of Earth. A literal translation would be "to describe or write about the Earth". The first person to use the word "geography" was Eratosthenes...

, is a turbulent layer of water that washes up on the beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

 after an incoming wave
Ocean surface wave
In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Waves in the...

 has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes depending on the type of beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

 (see Figure 1 for beach types). Greater swash generally occurs on flatter beaches. The swash motion plays the primary role in the formation of morphological features and their changes in the swash zone. The swash action also plays an important role as one of the instantaneous processes in wider coastal morphodynamics.
There are two approaches that describe swash motions: (1) swash resulting from the collapse of high-frequency bores (f>0.05Hz) on the beachface; and (2) swash characterised by standing, low-frequency (f<0.05Hz) motions. Which type of swash motion prevails is dependant on the wave conditions and the beach morphology and this can be predicted by calculating the surf similarity parameter εb (Guza & Inman 1975)
Where Hb is the breaker height, g is gravity, T is the incident-wave period and tan β is the beach gradient. Values εb>20 indicate dissipative conditions where swash is characterised by standing long-wave motion. Values εb<2.5 indicate reflective conditions where swash is dominated by wave bores.

Uprush and backwash

Swash consists of two phases: uprush (onshore flow) and backwash (offshore flow). Generally uprush velocities are greater but of shorter duration compared to the backwash. Onshore velocities are at greatest at the start of the uprush and then decrease, whereas offshore velocities increase towards the end of the backwash. The direction of the uprush varies with the prevailing wind, whereas the backwash is always perpendicular to the coastline. This asymmetrical motion of swash can cause longshore drift
Longshore drift
Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...

 as well as cross-shore sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

.

Swash morphology

The swash zone is the upper part of the beach between backbeach and surf zone
Surf zone
As ocean surface waves come closer to shore they break, forming the foamy, bubbly surface we call surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves continue to move in, and they run up onto the sloping front of the beach, forming an uprush of...

, where intense erosion occurs during storms (Figure 2). The swash zone is alternately wet and dry. Infiltration (hydrology)
Infiltration (hydrology)
Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes...

 (above the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

) and exfiltration (below the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

) take place between the swash flow and the beach groundwater table. Beachface, berm, beach step and beach cusps are the typical morphological features associated with swash motion. Infiltration (hydrology)
Infiltration (hydrology)
Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes...

 and sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

 by swash motion are important factors that govern the gradient of the beachface.

Beachface

The beachface is the planar, relatively steep section of the beach profile that is subject to swash processes (Figure 2). The beachface extends from the berm to the low tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

 level. The beachface is in dynamic equilibrium with swash action when the amount of sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

 by uprush and backwash are equal. If the beachface is flatter than the equilibrium gradient, more sediment is transported by the uprush to result in net onshore sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

. If the beachface is steeper than the equilibrium gradient, the sediment transport is dominated by the backwash and this results in net offshore sediment transport. The equilibrium beachface gradient is governed by a complex interrelationship of factors such as the sediment size, permeability, and fall velocity in the swash zone as well as the wave height and the wave period. The beachface cannot be considered in isolation from the surf zone
Surf zone
As ocean surface waves come closer to shore they break, forming the foamy, bubbly surface we call surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves continue to move in, and they run up onto the sloping front of the beach, forming an uprush of...

 to understand the morphological changes and equilibriums as they are strongly affected by the surf zone and shoaling wave processes as well as the swash zone processes.

Berm

The berm is the relatively planar part of the swash zone where the accumulation of sediment occurs at the landward farthest of swash motion (Figure 2). The berm protects the backbeach and coastal dunes from waves but erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 can occur under high energy conditions such as storm
Storm
A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...

s. The berm is more easily defined on gravel beaches and there can be multiple berms at different elevations. On sandy beaches in contrast, the gradient of backbeach, berm and beachface can be similar. The height of the berm is governed by the maximum elevation of sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

 during the uprush. The berm height can be predicted using the equation by Takeda and Sunamura (1982)
where Hb is the breaker height, g is gravity and T is the wave period.

Beach step

The beach step is a submerged scarp at the base of the beachface (Figure 2). The beach steps generally comprise the coarsest material and the height can vary from several centimetres to over a metre. Beach steps form where the backwash interacts with the oncoming incident wave and generate vortex. Hughes and Cowell (1987) proposed the equation to predict the step height Zstep
where 'ws' is the sediment fall velocity. Step height increases with increasing wave (breaker) height (Hь), wave period (T) and sediment size.

Beach cusps

The beach cusp is a crescent-shaped accumulation of sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

 or gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

 surrounding a semicircular depression on a beach. They are formed by swash action and more common on gravel beaches than sand. The spacing of the cusps is related to the horizontal extent of the swash motion and can range from 10cm to 50m. Coarser sediments are found on the steep-gradient, seaward pointing ‘cusp horns’ (Figure 3). Currently there are two theories that provide an adequate explanation for the formation of the rhythmic beach cusps: standing edge waves and self-organization
Self-organization
Self-organization is the process where a structure or pattern appears in a system without a central authority or external element imposing it through planning...

.

Standing edge wave model

The standing edge wave theory, which was introduced by Guza and Inman (1975), suggests that swash is superimposed upon the motion of standing edge waves that travel alongshore. This produces a variation in swash height along the shore and consequently results in regular patterns of erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

. The cusp embayments form at the eroding points and cusp horns occur at the edge wave nodes. The beach cusp spacing can be predicted using the sub-harmonic edge wave model
where T is incident wave period and tanβ is beach gradient.

This model only explains the initial formation of the cusps but not the continuing growth of the cusps. The amplitude of the edge wave reduces as the cusps grow, hence it is a self-limiting process.

Self-organization model

The self-organization
Self-organization
Self-organization is the process where a structure or pattern appears in a system without a central authority or external element imposing it through planning...

 theory was introduced by Werner and Fink (1993) and it suggests that beach cusps form due to a combination of positive feedback that is operated by beach morphology and swash motion encouraging the topographic irregularity and negative feedback that discourages accretion or erosion on well-developed beach cusps. It is relatively recent that the computational resources and sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

 formulations became available to show that the stable and rhythmic morphological features can be produced by such feedback systems. The beach cusp spacing, based on the self-organization model, is proportional to the horizontal extent of the swash motion S using the equation
where the constant of proportionality f is c. 1.5.

Cross-shore sediment transport

The cross-shore sediment exchange, between the subaerial and sub-aqueous zones of the beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

, is primarily provided by the swash motion. The transport rates in the swash zone are much higher compared to the surf zone and suspended sediment concentrations can exceed 100kg/m³ close to the bed. The onshore and offshore sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

 by swash thus plays a significant role in accretion
Accretion (coastal management)
Accretion is the process of coastal sediment returning to the visible portion of a beach or foreshore following a submersion event. A sustainable beach or foreshore often goes through a cycle of submersion during rough weather then accretion during calmer periods...

 and erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 of the beach.

There are fundamental differences in sediment transport between the uprush and backwash of the swash flow. The uprush, which is mainly dominated by bore turbulence, especially on steep beaches, generally suspend sediments to transport. Flow velocities, suspended sediment concentrations and suspended fluxes are at greatest at the start of the uprush when the turbulence is maximum. Then the turbulence dissipates towards the end of the onshore flow, settling the suspended sediment to the bed. In contrast, the backwash is dominated by the sheet flow and bedload sediment transport. The flow velocity increases towards the end of the backwash causing more bed-generated turbulence, which results in sediment transport near the bed. The direction of the net sediment transport (onshore or offshore) is largely governed by the beachface gradient.

Longshore drift

Longshore drift
Longshore drift
Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...

 by swash occurs either due to beach cusp morphology or due to oblique incoming waves
WAVES
The WAVES were a World War II-era division of the U.S. Navy that consisted entirely of women. The name of this group is an acronym for "Women Accepted for Volunteer Emergency Service" ; the word "emergency" implied that the acceptance of women was due to the unusual circumstances of the war and...

 causing strong alongshore swash motion. Under the influence of longshore drift, when there is no slack-water phase during backwash flows, sediments can remain suspended to result in offshore sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

. Beachface erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 by swash processes is not very common but erosion can occur where swash has a significant alongshore component.

Management

The swash zone is highly dynamic, accessible and susceptible to human activities. This zone can be very close to developed properties. It is said that at least 100 million people on the globe live within one meter of mean sea level. Understanding the swash zone processes and wise management is vital for coastal communities which can be affected by coastal hazards, such as erosion and storm surge. It is important to note that the swash zone processes cannot be considered in isolation as it is strongly linked with the surf zone processes. Many other factors, including human activities and climate change, can also influence the morphodynamics in the swash zone. Understanding the wider morphodynamics is essential in successful coastal management.

Construction of sea walls has been a common tool to protect developed property, such as roads and buildings, from coastal erosion
Coastal erosion
Coastal erosion is the wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage...

 and recession. However, more often than not, protecting the property by building a seawall
Seawall
A seawall is a form of coastal defence constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation and leisure activities from the action of tides and waves...

 does not achieve the retention of the beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

. Building an impermeable structure such as a seawall
Seawall
A seawall is a form of coastal defence constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation and leisure activities from the action of tides and waves...

 within the swash zone can interfere with the morphodynamics system in the swash zone. Building a seawall
Seawall
A seawall is a form of coastal defence constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation and leisure activities from the action of tides and waves...

 can raise the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

, increase wave reflection and intensify turbulence against the wall. This ultimately results in erosion of the adjacent beach or failure of the structure. Boulder ramparts (also known as revetments or riprap) and tetrapods are less reflective than impermeable sea walls, as waves are expected to break across the materials to produce swash and backwash that do not cause erosion. Rocky debris are sometimes placed in front of a sea wall in the attempt to reduce the wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 impact, as well as to allow the eroded beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

 to recover.

Understanding the sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

 system in the swash zone is also vital for beach nourishment
Beach nourishment
Beach nourishment— also referred to as beach replenishment—describes a process by which sediment lost through longshore drift or erosion is replaced from sources outside of the eroding beach...

 projects. Swash plays a significant role in transportation and distribution of the sand that is added to the beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

. There have been failures in the past due to inadequate understanding. Understanding and prediction of the sediment movements, both in the swash and surf zone, is vital for the nourishment project to succeed.

Example

The coastal management at Black Rock, on the north-east coast of Phillip Bay, Australia, provides a good example of a structural response to beach erosion which resulted in morphological changes in the swash zone. In the 1930s, a sea wall was built to protect the cliff from recession at Black Rock. This resulted in depletion of the beach in front of the sea wall, which was damaged by repeated storms in winter time. In 1969, the beach was nourished with approximately 5000m³ of sand from inland in order to increase the volume of sand on the beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

 to protect the sea wall. This increased the sand volume by about 10%, however, the sand was carried away by northward drifting in autumn to leave the sea wall exposed to the impacts of winter storms again. The project had failed to take the seasonal patterns of longshore drift
Longshore drift
Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...

 into account and had underestimated the amount of sand to nourish with, especially on the southern part of the beach.

Research

It is said that conduct of morphology research and field measurements in the swash zone is challenging since it is a shallow and aerated environment with rapid and unsteady swash flows. Despite the accessibility to the swash zone and the capability to take measurements with high resolution compared to the other parts of the nearshore zone, irregularity of the data has been an impediment for analysis as well as critical comparisons between theory and observation. Various and unique methods have been used for field measurements in the swash zone. For wave run-up measurements, for example, Guza and Thornton (1981, 1982) used an 80m long dual-resistance wire stretched across the beach profile and held about 3cm above the sand by non-conducting supports. Holman and Sallenger (1985) conducted run-up investigation by taking videos of the swash to digitise the positions of the waterline over time. Many of the studies involved engineering structures, including seawalls, jetties and breakwaters, to establish design criteria that protect the structures from overtopping by extreme run-ups. Since the 1990s, swash hydrodynamics have been more actively investigated by coastal researchers, such as Hughes M.G., Masselink J. and Puleo J.A., contributing to the better understanding of the morphodynamics in the swash zone including turbulence, flow velocities, interaction with the beach groundwater table, and sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

. However, the gaps in understanding still remain in swash research including turbulence, sheet flow, bedload sediment transport and hydrodynamics on ultra-dissipative beaches.

Conclusion

Swash plays an important role as one of the instantaneous coastal processes and it is as important as the long-term processes such as sea level rise and geological processes in coastal morphodynamics. Swash zone is one of the most dynamic and rapidly changing environments on the coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...

 and it is strongly linked with the surf zone
Surf zone
As ocean surface waves come closer to shore they break, forming the foamy, bubbly surface we call surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves continue to move in, and they run up onto the sloping front of the beach, forming an uprush of...

 processes. Understanding the swash mechanism is essential for the understanding of formation and changes of the swash zone morphology. More importantly, understanding of the swash zone processes is vital for society to manage coast wisely. There has been significant progress in the last two decades, however, gaps in understanding and knowledge in swash research still remain today.

See also

  • Beach cusp
  • Beach nourishment
    Beach nourishment
    Beach nourishment— also referred to as beach replenishment—describes a process by which sediment lost through longshore drift or erosion is replaced from sources outside of the eroding beach...

  • Coastal management
    Coastal management
    In some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...

  • Longshore drift
    Longshore drift
    Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...

  • Sea wall
  • Sediment transport
    Sediment transport
    Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...

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