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61-439: There are two approaches that describe swash motions: (1) swash resulting from the collapse of high-frequency bores ( f > 0.05 H z {\displaystyle f>0.05\,\mathrm {Hz} } ) on the beachface; and (2) swash characterised by standing, low-frequency ( f < 0.05 H z {\displaystyle f<0.05\,\mathrm {Hz} } ) motions. Which type of swash motion prevails

122-408: A French engineer, Jean-Baptiste Fourier, and a British geologist, Robert Mallet. They studied wave action and sediment transport; however, at that time, the term "longshore drift" was not yet coined. Instead, the principal focus was to understand the processes of waves and their impact on the resuspension and movement of sand and pebbles. The subject was of primary importance because it helped to explain

183-467: 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 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

244-424: A large role in the evolution of a shoreline , as if there is a slight change of sediment supply, wind direction , or any other coastal influence longshore drift can change dramatically, affecting the formation and evolution of a beach system or profile. These changes do not occur due to one factor within the coastal system, in fact there are numerous alterations that can occur within the coastal system that may affect

305-562: A slight “barrier” between the sea and an estuary or lagoon (called peresyp in the Russian tradition of geomorphology ). The second important spit feature is the down-drift end or distal end, which is detached from land and in some cases, may take a complex hook-shape or curve, due to the influence of varying wave directions. As an example, the New Brighton spit in Canterbury, New Zealand,

366-457: Is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand, shingle, shells) along a coast parallel to the shoreline, which is dependent on the angle of incoming wave direction. Oblique incoming wind squeezes water along the coast, generating a water current that moves parallel to the coast. Longshore drift is simply the sediment moved by the longshore current. This current and sediment movement occurs within

427-407: Is alternately wet and dry. Infiltration (hydrology) (above the water table ) and exfiltration (below the water table ) 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) and sediment transport by swash motion are important factors that govern

488-518: Is available in a bay or lagoon system. Tidal inlets can act as sinks and sources for large amounts of material, which therefore impacts on adjacent parts of the coastline. The structuring of tidal inlets is also important for longshore drift: if an inlet is unstructured, sediment may by-pass the inlet and form bars at the down-drift part of the coast. This may also depend on the inlet size, delta morphology , sediment rate and by-passing mechanism. Channel location variance and amount may also influence

549-412: Is called "beach drift", but some workers regard it as simply part of "longshore drift" because of the overall movement of sand parallel to the coast. Longshore drift affects numerous sediment sizes as it works in slightly different ways depending on the sediment (e.g. the difference in long-shore drift of sediments from a sandy beach to that of sediments from a shingle beach ). Sand is largely affected by

610-425: Is characterised by standing long-wave motion. Values ϵ b < 2.5 {\displaystyle \epsilon _{b}<2.5} indicate reflective conditions where swash is dominated by wave bores. Swash consists of two phases: uprush (onshore flow) and backwash (offshore flow). Generally, uprush has higher velocity and shorter duration than backwash. Onshore velocities are at greatest at

671-593: Is dependent on the wave conditions and the beach morphology and this can be predicted by calculating the surf similarity parameter ϵ b {\displaystyle \epsilon _{b}} (Guza & Inman 1975): ϵ b = 4 π 2 H b 2 g T 2 tan 2 ⁡ ( β ) {\displaystyle \epsilon _{b}={\frac {4\pi ^{2}H_{b}}{2gT^{2}\tan ^{2}{(\beta )}}}} in which H b {\displaystyle H_{b}}

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732-541: 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 and recession. However, more often than not, protecting

793-470: Is incident wave period and tan ⁡ ( β ) {\displaystyle \tan {(\beta )}} 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. The self-organization theory was introduced by Werner and Fink (1993) and it suggests that beach cusps form due to

854-508: Is not very common but erosion can occur where swash has a significant alongshore component. 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

915-402: Is the breaker height, g {\displaystyle g} is gravity and T {\displaystyle T} is the wave period. 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

976-433: Is the breaker height, g {\displaystyle g} is gravity, T {\displaystyle T} is the incident-wave period and tan ⁡ ( β ) {\displaystyle \tan {(\beta )}} is the beach gradient. Values ϵ b > 20 {\displaystyle \epsilon _{b}>20} indicate dissipative conditions where swash

1037-483: Is the sediment fall velocity. Step height increases with increasing wave (breaker) height ( Z s t e p {\displaystyle Z_{\mathrm {step} }} ), wave period ( T {\displaystyle T} ) and sediment size. The beach cusp is a crescent-shaped accumulation of sand or gravel surrounding a semicircular depression on a beach. They are formed by swash action and more common on gravel beaches than sand. The spacing of

1098-416: Is transported by the uprush to result in net onshore sediment transport . 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

1159-580: The Kaitorete Spit or hapua which form at river-coast interface such as at the mouth of the Rakaia River . The Kaitorete Spit in Canterbury, New Zealand, is a barrier/spit system (which generally falls under the definition of barrier, as both ends of the landform are attached to land, but has been named a spit) that has existed below Banks Peninsula for the last 8,000 years. This system has undergone numerous changes and fluctuations due to avulsion of

1220-549: The headland and moderate erosion of the down-drift end of the headland, this is undertaken in order to design a stabilised system that allows material to accumulate in beaches further along the shore. Artificial headlands can occur due to natural accumulation or also through artificial nourishment. Detached breakwaters are shore protection structures, created to build up sandy material in order to accommodate drawdown in storm conditions. In order to accommodate drawdown in storm conditions detached breakwaters have no connection to

1281-454: The sediment transport system in the swash zone is also vital for beach nourishment projects. Swash plays a significant role in transportation and distribution of the sand that is added to the beach. 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. The coastal management at Black Rock, on

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1342-445: The surf zone . The process is also known as littoral drift . Beach sand is also moved on such oblique wind days, due to the swash and backwash of water on the beach. Breaking surf sends water up the coast (swash) at an oblique angle and gravity then drains the water straight downslope (backwash) perpendicular to the shoreline. Thus beach sand can move downbeach in a sawtooth fashion many tens of meters (yards) per day. This process

1403-649: The Waimakariri River (which now flows to the north of Banks Peninsula), erosion and phases of open marine conditions. The system underwent further changes c.  500 years Before Present , when longshore drift from the eastern end of the “spit” system created the barrier, which has been retained due to ongoing longshore transport. The majority of tidal inlets on longshore drift shores accumulate sediment in flood and ebb shoals. Ebb-deltas may become stunted on highly exposed shores and in smaller spaces, whereas flood deltas are likely to increase in size when space

1464-405: The amount of sand to nourish with, especially on the southern part of the beach. 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

1525-401: The backbeach and coastal dunes from waves but erosion can occur under high energy conditions such as storms. 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 during

1586-440: The beaches; they didn't fully understand the mechanics, however. Because of the general scientific knowledge, this was an interesting but somewhat misunderstood phenomenon. The systematic investigation into the coast processes, including those responsible for longshore drift, began in the mid-1800s when scientists tried to explain the processes of sediment movement along coasts. Among the first of such theories were those proposed by

1647-423: The better understanding of the morphodynamics in the swash zone including turbulence, flow velocities, interaction with the beach groundwater table, and sediment transport . However, the gaps in understanding still remain in swash research including turbulence, sheet flow, bedload sediment transport and hydrodynamics on ultra-dissipative beaches. Longshore drift Longshore drift from longshore current

1708-460: The coastal system is inlet ebb-tidal shoals, which store sand that has been transported by long-shore transport. As well as storing sand these systems may also transfer or by pass sand into other beach systems, therefore inlet ebb-tidal (shoal) systems provide good sources and sinks for the sediment budget. Sediment deposition throughout a shoreline profile conforms to the null point hypothesis ; where gravitational and hydraulic forces determine

1769-405: The coastline processes has continued to evolve through a succession of developments that began many years ago. Erosion of coasts and sediment transport was known in ancient times, mostly in those parts of the world where dramatic changes of shores take place. However, these early observations were largely anecdotal. Fishermen, sailors and locals would note that sand and gravel seemingly "moved" down

1830-665: The creation of a port in Timaru, New Zealand in the late 19th century led to a significant change in the longshore drift along the South Canterbury coastline. Instead of longshore drift transporting sediment north up the coast towards the Waimataitai lagoon, the creation of the port blocked the drift of these (coarse) sediments and instead caused them to accrete to the south of the port at South beach in Timaru. The accretion of this sediment to

1891-405: The cusps is related to the horizontal extent of the swash motion and can range from 10 cm to 50 m. 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 . The standing edge wave theory, which

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1952-433: The development in the concept of "longshore currents," which in turn transport sediment along the coast. These currents then became recognized as the main agent of longshore drift. An important concept which emerged during this generation was that of the "drift-aligned" beach. It explained how beaches get to form as a result of prevailing wind and wave directions and that on one side of the beach deposition takes place, while on

2013-419: The distribution and impact of longshore drift. Some of these are: The sediment budget takes into consideration sediment sources and sinks within a system . This sediment can come from any source with examples of sources and sinks consisting of: This sediment then enters the coastal system and is transported by longshore drift. A good example of the sediment budget and longshore drift working together in

2074-456: The dominant drift direction and shoreline do not veer in the same direction. As well as dominant drift direction, spits are affected by the strength of wave-driven current , wave angle and the height of incoming waves. Spits are landforms that have two important features, with the first feature being the region at the up-drift end or proximal end (Hart et al., 2008). The proximal end is constantly attached to land (unless breached) and may form

2135-425: The effects of longshore drift on the coastline but in other cases have a negative impact on long-shore drift ( ports and harbours ). Groynes are shore protection structures, placed at equal intervals along the coastline in order to stop coastal erosion and generally cross the intertidal zone . Due to this, groyne structures are usually used on shores with low net and high annual longshore drift in order to retain

2196-420: The gradient of the 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 level. The beachface is in dynamic equilibrium with swash action when the amount of sediment transport by uprush and backwash are equal. If the beachface is flatter than the equilibrium gradient, more sediment

2257-645: The impact of longshore drift on a tidal inlet. Arcachon lagoon in southwest France is an example of a tidal inlet system, which provides large sources and sinks for longshore drift sediments. The impact of longshore drift sediments on this inlet system is highly influenced by the variation in the number of lagoon entrances and the location of these entrances. Any change in these factors can cause severe down-drift erosion or down-drift accretion of large swash bars. This section consists of long-shore drift features that occur unnaturally and in some cases (e.g. groynes , detached breakwaters ) have been constructed to enhance

2318-421: The morphological features of any coast. However, while much is covered, the complete significance of such mechanisms was yet to be fully realised. In the early years of the 20th century, longshore drift became much more refined in its explanation through oceanographers and coastal engineers. They realized that the angle of wave approach to the coast is of paramount importance to sediment transport. This then led to

2379-406: 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 3 cm above

2440-454: The net sediment transport (onshore or offshore) is largely governed by the beachface gradient. Longshore drift by swash occurs either due to beach cusp morphology or due to oblique incoming waves 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 . Beachface erosion by swash processes

2501-400: 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

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2562-460: The oncoming incident wave and generate vortex. Hughes and Cowell (1987) proposed the equation to predict the step height Z s t e p {\displaystyle Z_{\mathrm {step} }} Z s t e p = H b T w s , {\displaystyle Z_{\mathrm {step} }={\sqrt {H_{b}Tw_{s}}},} where w s {\displaystyle w_{s}}

2623-432: The oscillatory force of breaking waves , the motion of sediment due to the impact of breaking waves and bed shear from long-shore current. Because shingle beaches are much steeper than sandy ones, plunging breakers are more likely to form, causing the majority of longshore transport to occur in the swash zone , due to a lack of an extended surf zone. The concept of longshore drift or transportation of sediment parallel to

2684-488: The other side, erosion does. While the mechanics were becoming more apparent, the interrelationship of the forces in play still proved quite problematic for those trying to manage coasts. Numerous calculations take into consideration the factors that produce longshore drift. These formulations are: These formulas provide a different view of the processes that generate longshore drift. The most common factors taken into consideration in these formulas are: Longshore drift plays

2745-410: The property by building a seawall does not achieve the retention of the beach. Building an impermeable structure such as a seawall within the swash zone can interfere with the morphodynamics system in the swash zone. Building a seawall can raise the water table , increase wave reflection and intensify turbulence against the wall. This ultimately results in erosion of the adjacent beach or failure of

2806-541: 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

2867-409: The sediments lost in storm surges and further down the coast. There are numerous variations to groyne designs with the three most common designs consisting of: Artificial headlands are also shore protection structures, which are created in order to provide a certain amount of protection to beaches or bays. Although the creation of headlands involves accretion of sediments on the up-drift side of

2928-415: The self-organization model, is proportional to the horizontal extent of the swash motion S using the equation λ = f S , {\displaystyle \lambda =fS,} where the constant of proportionality f is c . 1.5. The cross-shore sediment exchange, between the subaerial and sub-aqueous zones of the beach, is primarily provided by the swash motion. The transport rates in

2989-431: The settling velocity of grains in a seaward fining sediment distribution. Long shore occurs in a 90 to 80 degree backwash so it would be presented as a right angle with the wave line. This section consists of features of longshore drift that occur on a coast where long-shore drift occurs uninterrupted by man-made structures. Spits are formed when longshore drift travels past a point (e.g. river mouth or re-entrant) where

3050-442: The shore by wave action has evolved considerably with time. Early observations related to sediment displacement can be traced back to coastal communities, but the formal scientific understanding of this started crystallizing in the 19th and early 20th centuries. While such early perceptions were imprecise, this evolution has encouraged a gradually more sophisticated understanding of the processes occurring at coastlines. Understanding of

3111-458: The shoreline, which lets currents and sediment pass between the breakwater and the shore. This then forms a region of reduced wave energy, which encourages the deposition of sand on the lee side of the structure. Detached breakwaters are generally used in the same way as groynes, to build up the volume of material between the coast and the breakwater structure in order to accommodate storm surges. The creation of ports and harbours throughout

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3172-498: 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 as well as cross-shore sediment transport . The swash zone is the upper part of the beach between backbeach and surf zone , where intense erosion occurs during storms (Figure 2). The swash zone

3233-422: 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

3294-404: 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 is sometimes placed in front of a sea wall in the attempt to reduce the wave impact, as well as to allow the eroded beach to recover. Understanding

3355-594: The swash zone are much higher compared to the surf zone and suspended sediment concentrations can exceed 100 kg/m close to the bed. The onshore and offshore sediment transport by swash thus plays a significant role in accretion and erosion 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

3416-400: The uprush. The berm height can be predicted using the equation by Takeda and Sunamura (1982) Z b e r m = 0.125 H b 5 / 8 ( g T 2 ) 3 / 8 {\displaystyle Z_{\mathrm {berm} }=0.125H_{b}^{5/8}(gT^{2})^{3/8}} where H b {\displaystyle H_{b}}

3477-446: The wave height and the wave period. The beachface cannot be considered in isolation from the surf zone 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. 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

3538-438: The world can seriously impact on the natural course of longshore drift. Not only do ports and harbours pose a threat to longshore drift in the short term, they also pose a threat to shoreline evolution. The major influence, which the creation of a port or harbour can have on longshore drift, is the alteration of sedimentation patterns, which in turn may lead to accretion and/or erosion of a beach or coastal system. As an example,

3599-495: Was created by longshore drift of sediment from the Waimakariri River to the north. This spit system is currently in equilibrium but undergoes alternate phases of deposition and erosion. Barrier systems are attached to the land at both the proximal and distal ends and are generally widest at the down-drift end. These barrier systems may enclose an estuary or lagoon system, like that of Lake Ellesmere / Te Waihora enclosed by

3660-672: 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 . 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 λ = g π T 2 tan ⁡ ( β ) , {\displaystyle \lambda ={\frac {g}{\pi }}T^{2}\tan(\beta ),} in which T {\displaystyle T}

3721-427: Was nourished with approximately 5000m of sand from inland in order to increase the volume of sand on the beach 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 into account and had underestimated

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