Difference between revisions of "Swash"
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| + | {{Definition|title=Swash|definition= Propagation of thin sheets of seawater up and down the beach, after the collapse of waves on the beach face. Swash is the decelerating uprush phase and backwash is the accelerating downrush phase. }} | ||
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| − | Swash | + | [[File:BaldockFig2b.jpg|thumb|450px|right|Fig. 1. Swash on a reflective beach, Avoca Beach, NSW, Australia. In the background, a wave collapses on the beach; the [[Wave run-up|swash uprush]] reaches the top of the beach berm. Photo credit: Dr Hannah Power, University of Newcastle, NSW, Australia.]] |
| − | While easily observed and experienced by anyone walking along the beach, measuring the hydrodynamic characteristics of swash motion is not easy at all because of its small scale and highly unsteady nature. Swash motion is driven by waves collapsing on the beach which supply initial momentum to the wave uprush along with an amount of suspended sediment. This complex boundary condition, given the irregular character of the incident wave field, is not the only challenge for modelling the swash dynamics. The | + | ==Beach face== |
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| + | When incident waves collapse on the beach face, a thin seawater layer rushes up the beach in the swash zone situated between the surf zone and the dry beach (Fig. 1). The swash zone is often a steeper part of the beach (especially in the case of coarse-grained sediment) bordered by a beach berm at the landward side. Swash motion is a crucial link between the hydrosedimentary processes in the surf zone and the morphology (sand volume, shape) of the backshore. | ||
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| + | Swash motion can be quite strong. Maximum uprush velocities up to 3 m/s have been measured on steep sandy foreshores and up to 2 m/s on gently sloping foreshores, while backwash velocities can attain more than 2 m/s<ref>Houser, C. and Barrett, G. 2010. Divergent behavior of the swash zone in response to different foreshore slopes and nearshore states. Mar. Geol. 271: 106–118</ref>. Uprush suspended sediment concentrations on steep and dissipative beaches can approach or exceed 100 g/l. The net sediment transport of a single swash event can cause fluctuations in beach level (accretion or erosion) by up to several centimeters. The net bed level change over an entire tidal cycle is often not much larger (less than an order of magnitude) than the change by individual swash events<ref>Puleo, J.A., Lanckriet, T. and Blenkinsopp, C. 2014. Bed level fluctuations in the inner surf and swash zone of a dissipative beach. Mar. Geol. 349: 99–112</ref>. | ||
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| + | While easily observed and experienced by anyone walking along the beach, measuring the hydrodynamic characteristics of swash motion is not easy at all because of its small scale and highly unsteady nature. Swash motion is driven by waves collapsing on the beach which supply initial momentum to the wave uprush along with an amount of suspended sediment. This complex boundary condition, given the irregular character of the incident wave field, is not the only challenge for modelling the swash dynamics. The up and down flowing fluid sheets induce pore water pressure gradients with alternating upward and downward seepage that promotes destabilization of the sediment bed and transport of suspended liquefied sediments<ref>Horn, D.P. 2002. Beach groundwater dynamics. Geomorphology 48: 121-146</ref><ref>Kranenborg, J.W.M., Pauli, T., Jacobsen, N.G., van der Werf, J.J., Dionisio Antonio, S., Campmans, G.H.P., Reniers, A.J.H.M. and Hulscher, S.J.M.H. 2023. Measurements and modeling of pore-pressure gradients in the swash zone under large-scale laboratory bichromatic waves. Journal of Geophysical Research: Oceans 128, e2022JC019577</ref>. The capability of process-based models to simulate these fine-scale processes is very limited. The contribution of swash motion to maximum water uprush (see [[wave run-up]]) and to beach accretion and erosion processes is therefore mainly based on empirical relationships. General trends can be fairly well described by these relationships, but field data are widely scattered around these trends. See the article [[Swash zone dynamics]] for a more detailed discussion. | ||
On [[Shoreface profile#Dissipative and reflective beaches|dissipative coasts]] swash processes are dominated by [[infragravity waves]]. | On [[Shoreface profile#Dissipative and reflective beaches|dissipative coasts]] swash processes are dominated by [[infragravity waves]]. | ||
| + | ==Ballistic swash model== | ||
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| + | In the case of monochromatic non-breaking incident waves (usually [[infragravity waves]]), the frictionless cross-shore vertical run-up <math>R</math> on a beach with constant slope <math>\beta</math> can be determined analytically, see the article [[Waves on a sloping bed]]. | ||
| + | Such non-broken waves are reflected at the shoreline and form with the incident waves a pattern of standing waves. In the case of oblique wave incidence, these reflected waves can for a pattern of [[edge waves]]. | ||
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| + | Here we consider the more usual case of dissipative or partially reflective beaches, where incident waves collapse on the beach face and rush up as a bore, see figure Uprush. | ||
| + | The velocity <math>V</math> of a collapsing wave front can be estimated from the bore formula<ref>Ritter, A. 1892. Die Fortpflanzung der Wasserwellen. Zeitschrift des Vereines Deutscher Ingenieure 36: 947-954</ref> <math>\; V=2\sqrt{gD_0} \, ,</math> where <math>D_0</math> is the height of the collapsing wave (see [[Dam break flow]]). | ||
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| + | [[File:Swash.jpg|thumb|center|700px|Fig. 2. Schematic representation of swash uprush on the beach face. The vertical scale is strongly exaggerated. The swash excursion <math>X_s</math> extends landward from the location where the incident wave bore collapses. The rapid uprush on the beach face slope causes the collapse of the incident wave bore. ]] | ||
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| + | Swash motion can be described with reasonable accuracy with a simple model based on ballistic theory<ref name=H95>Hughes, M.G. 1995. Friction factors for wave uprush. Journal of Coastal Research 13: 1089– 1098</ref><ref name=PH>Puleo, J.A. and Holland, K.T. 2001. Estimating swash zone friction coefficients on a sandy beach. Coastal Engineering 43: 25– 40</ref>. A short introduction to the ballistic model is given below. | ||
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| + | The horizontal position of the bore front is described by the trajectory <math>x(t)</math> up and down the beach (Fig. 2). The time <math>t=0</math> corresponds to the wave collapse, that generates at <math>x=0</math> a bore with speed <math> dx/dt = V \approx 2\sqrt{gD_0}</math>. If friction and infiltration are neglected, the trajectory <math>x(t)</math> follows the ballistic equation | ||
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| + | <math>\Large\frac{d^2 x}{dt^2}\normalsize = - g \beta \, , \quad x(t) = - \large\frac{1}{2}\normalsize g \beta t^2 + V\, t \, , \qquad (1)</math> | ||
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| + | where <math>\beta</math> is the slope of the beach face (<math>\beta << 1</math>). At the end of the uprush, <math>dx/dt=0</math>. The uprush duration is thus given by <math>T_u \approx V / g \beta </math>. In the frictionless case, the uprush duration <math>T_u</math> and backwash duration <math>T_b</math> are equal; the swash period <math>T_s = T_u+T_b</math> is thus about twice the uprush duration. The horizontal run-up is given by <math>X_s = \large\frac{1}{2}\normalsize g T_u^2 = \Large\frac{V^2 }{2g \beta}\normalsize = \Large\frac{2 D_0}{\beta}\normalsize</math> and the vertical run-up by <math>R = \beta X_s = 2 D_0</math>. | ||
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| + | The uprush loses energy by turbulent overturning at the bore front and by bed friction behind the front <ref>Puleo, J.A., Holland, K.T., Plant, N.G., Slinn, D.N. and Hanes, D.M. 2003. Fluid acceleration effects on suspended sediment transport in the swash zone. J.Geophys.Res. 108: C11 doi:10.1029/2003JC001943</ref>. The velocities in the boundary layer of uprush and backwash have approximately a logarithmic profile with a friction coefficient of the order of <math>c_D \approx 0.02-0.025</math> <ref> Raubenheimer, R., Elgar, S, and Guza, T. 2004. Observations of swash zone velocities: A note on friction coefficients. J.Geophys.Res. 109: C01027,1-8</ref><ref name=K13>Kikkert, G.A., Pokrajac, D., O'Donoghue, T. and Steenhauer, K. 2013. Experimental study of bore-driven swash hydrodynamics on permeable rough slopes. Coastal Eng. 79: 42-56</ref><ref>Inch, K., Masselink, G., Puleo, J.A., Russell, P. and Conley, D.C. (2015) Vertical structure of near-bed cross-shore flow velocities in the swash zone of a dissipative beach. Continental Shelf Res. 101: 98-108</ref>. | ||
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| + | A more accurate expression is found for the run-up by including a quadratic friction term in the ballistic equation (1), | ||
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| + | <math>\Large\frac{d^2 x}{dt^2}\normalsize = - g \beta - c_D \big(\Large\frac{dx}{dt}\normalsize \big)^2 \, . \qquad (2)</math> | ||
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| + | This nonlinear equation has an analytical solution, if the thickness <math>D</math> of the bore is assumed constant in space and time. The solutions for the uprush period <math>T_u</math> and the run-up <math>R</math> are <ref name=H95/> | ||
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| + | <math>T_u = \sqrt{ \Large\frac{D}{g \beta c_D}\normalsize} \arctan K , \quad R = \Large\frac{\beta D}{2 c_D}\normalsize \ln (1 + K^2 ) , \quad K = 2 \sqrt{\Large\frac{c_D D_0}{\beta D}\normalsize} \, . \qquad (3) </math> | ||
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| + | With <math>c_D=</math>0.025, a beach slope <math>\beta=</math> 0.1, and with initial and average bore heights <math>D_0=</math>0.5 m and <math>D=</math>0.1 m, respectively, the friction reduces the uprush period <math>T_u</math> from 4.5 s to 2.3 s and the run-up <math>R</math> from 1 m to 0.36 m. For strong friction and gentle slopes, <math>K</math> is generally much larger than 1 and we can approximate <math>\arctan K \approx \pi /2</math>. The uprush period <math>T_u</math> and run-up <math>R</math> are then related by | ||
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| + | <math>R \approx \Large\frac{4 \ln K}{\pi^2 }\normalsize \beta g T_u^2 \, . \qquad (4) </math> | ||
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| + | This relationship is almost the same as for the frictionless case, apart from the factor <math>\ln K</math>, with values typically between 1 and 1.5. Experiments show that the duration of backwash <math>T_b</math> is longer than the duration <math>T_u</math> of uprush by 20-40<math>\%</math> <ref name=HP>Holland, K.T. and Puleo, J.A. 2001. Variable swash motions associated with foreshore profile change. J. Geophys. Res. 106: 4613-4623</ref><ref>Hughes, M.G. and Baldock, T.E. 2004. Eulerian flow velocities in the swash zone: field data and model predictions. J. Geophys. Res. 109, C08009</ref>. | ||
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| + | The run-up Eq. (1) is related to the bore thickness estimates <math>D_0</math> and <math>D(x,t)</math>, which depend on the height and period of the incident waves and on wave dissipation in the surf zone. Because no theoretical expression is available, empirical relationships for <math>R</math> are used in practice, see [[Wave run-up]]. | ||
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| + | ==Swash sedimentation and erosion== | ||
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| + | The sediment suspended in the surf zone and eroded from the lower beach face is deposited at the higher beach face, where the uprush velocity is small. Deposition is enhanced by infiltration, in particular for coarse sediment beaches with average grain size <math>d_{50}</math> larger than 1 mm. Infiltration has two opposite effects on the bed shear stress: a decrease due to reduction of the flow volume and an increase due to thinning of the boundary layer. After reaching the highest point on the beach face, the water retreats along the beach slope in a very thin layer. This so-called downrush or backwash takes sediment from the upper beach face to the lower beach face. The backwash period is generally longer than the uprush period and the velocity is lower. During the late stage of the backwash a thick sheet flow layer is formed which decreases immediately after the arrival of the next arriving bore<ref>Alsina, J.M., van der Zanden, J., Caceres, I. and Ribberink, J.S. 2018. The influence of wave groups and wave-swash interactions on sediment transport and bed evolution in the swash zone. Coast. Eng. 140: 23–42</ref>. | ||
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| + | Exfiltration near the shoreline has two opposite effects: thickening of the boundary layer and related decrease of the bed shear stress versus bed destabilisation trough liquefaction of the bed soil layer<ref>Butt, T., Russell, P. and Turner, I. 2001. The influence of swash infiltration-exfiltration on beach face sediment transport: onshore or offshore? Coastal Eng. 42: 35-52</ref>. Laboratory experiments show that bed shear stresses are similar for impermeable and permeable sandy beaches, whereas higher bed shear stresses are observed for permeable gravel beaches <ref name=K13/>. Field observations suggest that the net effect of infiltration/exfiltration depends on grain size: for medium sediment, it decreases the uprush sediment flux (<math>\approx -10 \%</math>) and increases the downrush sediment flux (<math>\approx 5 \%</math>); for coarse sediment the effect is opposite. | ||
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| + | Entrainment of the bed top layer (1-2 cm) contributes significantly to the uprush and downrush sediment fluxes <ref>Miles, J., Butt, T. and Russell, P. 2006. Swash zone sediment dynamics: A comparison of a dissipative and an intermediate beach. Mar. Geol. 231: 181-200</ref><ref name=M09>Masselink, G., Russell, P., Turner, I and Blenkinsopp, C. 2009. Net sediment transport and morphological change in the swash zone of a high-energy sandy beach from swash event to tidal cycle time scales. Marine Geol. 267: 18-35</ref>. Most of the sediment deposited by the uprush is remobilized by the backwash. Large sediment concentration and sediment fluxes have been measured at the beginning of the uprush and last stage of the backwash. The highest deposits may remain unaffected because of scour lag; the backwash has already retreated below these deposits when it reaches velocities that are high enough for remobilization. The highest uprush deposits form a beach berm, see Fig. 2. Under extreme storms the beach berm evolves into a ridge on the upper beach <ref>Bendixen, M., Clemmensen, L.B. and Kroon, A. 2013. Sandy berm and beach-ridge formation in relation to extreme sea-levels: A Danish example in a micro-tidal environment. Marine Geol. 344: 53-64</ref>. | ||
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| + | Field experiments by Masselink et al. <ref name=M09/> showed that net accretion or erosion can be very different for successive individual swash events. They found that one single swash event could contribute significantly to the net accretion or erosion occurring over a complete tidal cycle. This illustrates the difficulty of simulating swash-induced accretion or sedimentation with process-based numerical models. | ||
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| + | Holland and Puleo presented field evidence for the following scenario of negative morphodynamic feedback to the swash process <ref name=HP>Holland, K.T. and Puleo, J.A. 2001. Variable swash motions associated with foreshore profile change. J. Geophys. Res. 106: 4613-4623</ref>. If the swash period <math>T_s</math> is shorter than the average period <math>T</math> of the incoming waves, the backwash transports beach sediment to the surf zone. In such a situation the net result of swash is beach erosion. The beach face is lowered and the slope decreases. However, according to Eq. (3), a decrease of the beach slope induces an increase of the uprush period. If the swash period <math>T_s = T_u +T_b</math> is longer than the average wave period <math>T</math>, the backwash collides with the next uprush. This reduces offshore transport by the backwash; the net result of swash is now beach accretion. When the beach face accretes, the slope increases and the swash period decreases. An equilibrium slope of the beach face slope then may result such that the periods <math>T_s </math> and <math>T</math> are of similar duration. | ||
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| + | Observations show that swash periods are not very different from the average period of incident waves at many reflective beaches. However, as mentioned before, long-term equilibrium does not only depend on swash processes, but also on surf processes (wave breaking on the beach) during storm periods <ref> Masselink, G. and Puleo, J.A. 2006. Swash-zone morphodynamics. Cont. Shelf Res. 26: 661-680</ref>. | ||
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| + | The above feedback mechanism does not account for the dependence of beach slope on grain size. However, such a dependence may be expected, because swash on coarse-grained beaches is more strongly influenced by infiltration and exfiltration. Coarse sediment settles more easily at the end of the uprush than fine sediment; equilibrium with downslope transport by backwash therefore requires a steep beach slope. Hence, coarse-grained beaches (gravel beaches, for instance) typically have a steeper slope than fine- or medium-grained beaches. | ||
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| + | The formation of beach cusps is intimately linked to the swash process. This is dealt with in the article [[Beach cusps]]. | ||
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| + | ==Infragravity swash== | ||
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| + | On dissipative beaches incident short waves lose most of their energy by breaking in the surf zone <ref> Ruessink, B.G. 1998. Bound and free infragravity waves in the nearshore zone under breaking and nonbreaking conditions. J. Geophys. Res. 103: 12,795-12,805</ref>. Near the shoreline, only the longest waves survive, the so-called [[infragravity waves]]. Infragravity waves arise mainly from nonlinear interactions between short waves with different wavelengths and frequencies <ref>Hasselmann, K. On the nonlinear energy transfer in a gravity-wave spectrum, part 1. General theory. J. Fluid Mech. 12: 481-500</ref>, see the article [[Infragravity waves]]. They carry only a small part of the wave energy on the lower shoreface (typically of the order of 1<math>\%</math> or less), but their relative importance increases strongly in the surf zone. Empirical formulas for run-up of infragravity waves are given in the article [[Wave run-up]]. | ||
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| + | Laboratory and field observations show that the run-up <math>R</math> is bound to a maximum value for high wave heights (<math>H</math> of the order of 5 m or more). This is called 'swash saturation'. This can be due to breaking of infragravity waves, but also to collision of the uprush with the preceding backwash, see the article [[Swash zone dynamics]]. | ||
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| + | While there is strong evidence that short-wave swash stimulates beach accretion, this is less clear for infragravity swash. Observations point to a net offshore directed transport by infragravity swash, especially in the seaward part of the swash zone. However, reliable models for simulating the complicated infragravity morphodynamics in the surf zone are not yet available <ref> Elfrink, B. and Baldock, T. 2002. Hydrodynamics and sediment transport in the swash zone:a review and perspectives. Coastal Eng. 45: 149-167</ref>. | ||
==Related articles== | ==Related articles== | ||
| + | :[[Swash zone dynamics]] | ||
:[[Wave run-up]] | :[[Wave run-up]] | ||
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==References== | ==References== | ||
<references/> | <references/> | ||
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| + | {{author | ||
| + | |AuthorID=120 | ||
| + | |AuthorFullName=Job Dronkers | ||
| + | |AuthorName=Dronkers J}} | ||
[[Category:Beaches]] | [[Category:Beaches]] | ||
| + | [[Category:Physical coastal and marine processes]] | ||
Revision as of 17:54, 5 January 2025
Definition of Swash:
Propagation of thin sheets of seawater up and down the beach, after the collapse of waves on the beach face. Swash is the decelerating uprush phase and backwash is the accelerating downrush phase.
This is the common definition for Swash, other definitions can be discussed in the article
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Contents
[hide]Beach face
When incident waves collapse on the beach face, a thin seawater layer rushes up the beach in the swash zone situated between the surf zone and the dry beach (Fig. 1). The swash zone is often a steeper part of the beach (especially in the case of coarse-grained sediment) bordered by a beach berm at the landward side. Swash motion is a crucial link between the hydrosedimentary processes in the surf zone and the morphology (sand volume, shape) of the backshore.
Swash motion can be quite strong. Maximum uprush velocities up to 3 m/s have been measured on steep sandy foreshores and up to 2 m/s on gently sloping foreshores, while backwash velocities can attain more than 2 m/s[1]. Uprush suspended sediment concentrations on steep and dissipative beaches can approach or exceed 100 g/l. The net sediment transport of a single swash event can cause fluctuations in beach level (accretion or erosion) by up to several centimeters. The net bed level change over an entire tidal cycle is often not much larger (less than an order of magnitude) than the change by individual swash events[2].
While easily observed and experienced by anyone walking along the beach, measuring the hydrodynamic characteristics of swash motion is not easy at all because of its small scale and highly unsteady nature. Swash motion is driven by waves collapsing on the beach which supply initial momentum to the wave uprush along with an amount of suspended sediment. This complex boundary condition, given the irregular character of the incident wave field, is not the only challenge for modelling the swash dynamics. The up and down flowing fluid sheets induce pore water pressure gradients with alternating upward and downward seepage that promotes destabilization of the sediment bed and transport of suspended liquefied sediments[3][4]. The capability of process-based models to simulate these fine-scale processes is very limited. The contribution of swash motion to maximum water uprush (see wave run-up) and to beach accretion and erosion processes is therefore mainly based on empirical relationships. General trends can be fairly well described by these relationships, but field data are widely scattered around these trends. See the article Swash zone dynamics for a more detailed discussion.
On dissipative coasts swash processes are dominated by infragravity waves.
Ballistic swash model
In the case of monochromatic non-breaking incident waves (usually infragravity waves), the frictionless cross-shore vertical run-up R on a beach with constant slope \beta can be determined analytically, see the article Waves on a sloping bed. Such non-broken waves are reflected at the shoreline and form with the incident waves a pattern of standing waves. In the case of oblique wave incidence, these reflected waves can for a pattern of edge waves.
Here we consider the more usual case of dissipative or partially reflective beaches, where incident waves collapse on the beach face and rush up as a bore, see figure Uprush. The velocity V of a collapsing wave front can be estimated from the bore formula[5] \; V=2\sqrt{gD_0} \, , where D_0 is the height of the collapsing wave (see Dam break flow).
Swash motion can be described with reasonable accuracy with a simple model based on ballistic theory[6][7]. A short introduction to the ballistic model is given below.
The horizontal position of the bore front is described by the trajectory x(t) up and down the beach (Fig. 2). The time t=0 corresponds to the wave collapse, that generates at x=0 a bore with speed dx/dt = V \approx 2\sqrt{gD_0}. If friction and infiltration are neglected, the trajectory x(t) follows the ballistic equation
\Large\frac{d^2 x}{dt^2}\normalsize = - g \beta \, , \quad x(t) = - \large\frac{1}{2}\normalsize g \beta t^2 + V\, t \, , \qquad (1)
where \beta is the slope of the beach face (\beta \lt \lt 1). At the end of the uprush, dx/dt=0. The uprush duration is thus given by T_u \approx V / g \beta . In the frictionless case, the uprush duration T_u and backwash duration T_b are equal; the swash period T_s = T_u+T_b is thus about twice the uprush duration. The horizontal run-up is given by X_s = \large\frac{1}{2}\normalsize g T_u^2 = \Large\frac{V^2 }{2g \beta}\normalsize = \Large\frac{2 D_0}{\beta}\normalsize and the vertical run-up by R = \beta X_s = 2 D_0.
The uprush loses energy by turbulent overturning at the bore front and by bed friction behind the front [8]. The velocities in the boundary layer of uprush and backwash have approximately a logarithmic profile with a friction coefficient of the order of c_D \approx 0.02-0.025 [9][10][11].
A more accurate expression is found for the run-up by including a quadratic friction term in the ballistic equation (1),
\Large\frac{d^2 x}{dt^2}\normalsize = - g \beta - c_D \big(\Large\frac{dx}{dt}\normalsize \big)^2 \, . \qquad (2)
This nonlinear equation has an analytical solution, if the thickness D of the bore is assumed constant in space and time. The solutions for the uprush period T_u and the run-up R are [6]
T_u = \sqrt{ \Large\frac{D}{g \beta c_D}\normalsize} \arctan K , \quad R = \Large\frac{\beta D}{2 c_D}\normalsize \ln (1 + K^2 ) , \quad K = 2 \sqrt{\Large\frac{c_D D_0}{\beta D}\normalsize} \, . \qquad (3)
With c_D=0.025, a beach slope \beta= 0.1, and with initial and average bore heights D_0=0.5 m and D=0.1 m, respectively, the friction reduces the uprush period T_u from 4.5 s to 2.3 s and the run-up R from 1 m to 0.36 m. For strong friction and gentle slopes, K is generally much larger than 1 and we can approximate \arctan K \approx \pi /2. The uprush period T_u and run-up R are then related by
R \approx \Large\frac{4 \ln K}{\pi^2 }\normalsize \beta g T_u^2 \, . \qquad (4)
This relationship is almost the same as for the frictionless case, apart from the factor \ln K, with values typically between 1 and 1.5. Experiments show that the duration of backwash T_b is longer than the duration T_u of uprush by 20-40\% [12][13].
The run-up Eq. (1) is related to the bore thickness estimates D_0 and D(x,t), which depend on the height and period of the incident waves and on wave dissipation in the surf zone. Because no theoretical expression is available, empirical relationships for R are used in practice, see Wave run-up.
Swash sedimentation and erosion
The sediment suspended in the surf zone and eroded from the lower beach face is deposited at the higher beach face, where the uprush velocity is small. Deposition is enhanced by infiltration, in particular for coarse sediment beaches with average grain size d_{50} larger than 1 mm. Infiltration has two opposite effects on the bed shear stress: a decrease due to reduction of the flow volume and an increase due to thinning of the boundary layer. After reaching the highest point on the beach face, the water retreats along the beach slope in a very thin layer. This so-called downrush or backwash takes sediment from the upper beach face to the lower beach face. The backwash period is generally longer than the uprush period and the velocity is lower. During the late stage of the backwash a thick sheet flow layer is formed which decreases immediately after the arrival of the next arriving bore[14].
Exfiltration near the shoreline has two opposite effects: thickening of the boundary layer and related decrease of the bed shear stress versus bed destabilisation trough liquefaction of the bed soil layer[15]. Laboratory experiments show that bed shear stresses are similar for impermeable and permeable sandy beaches, whereas higher bed shear stresses are observed for permeable gravel beaches [10]. Field observations suggest that the net effect of infiltration/exfiltration depends on grain size: for medium sediment, it decreases the uprush sediment flux (\approx -10 \%) and increases the downrush sediment flux (\approx 5 \%); for coarse sediment the effect is opposite.
Entrainment of the bed top layer (1-2 cm) contributes significantly to the uprush and downrush sediment fluxes [16][17]. Most of the sediment deposited by the uprush is remobilized by the backwash. Large sediment concentration and sediment fluxes have been measured at the beginning of the uprush and last stage of the backwash. The highest deposits may remain unaffected because of scour lag; the backwash has already retreated below these deposits when it reaches velocities that are high enough for remobilization. The highest uprush deposits form a beach berm, see Fig. 2. Under extreme storms the beach berm evolves into a ridge on the upper beach [18].
Field experiments by Masselink et al. [17] showed that net accretion or erosion can be very different for successive individual swash events. They found that one single swash event could contribute significantly to the net accretion or erosion occurring over a complete tidal cycle. This illustrates the difficulty of simulating swash-induced accretion or sedimentation with process-based numerical models.
Holland and Puleo presented field evidence for the following scenario of negative morphodynamic feedback to the swash process [12]. If the swash period T_s is shorter than the average period T of the incoming waves, the backwash transports beach sediment to the surf zone. In such a situation the net result of swash is beach erosion. The beach face is lowered and the slope decreases. However, according to Eq. (3), a decrease of the beach slope induces an increase of the uprush period. If the swash period T_s = T_u +T_b is longer than the average wave period T, the backwash collides with the next uprush. This reduces offshore transport by the backwash; the net result of swash is now beach accretion. When the beach face accretes, the slope increases and the swash period decreases. An equilibrium slope of the beach face slope then may result such that the periods T_s and T are of similar duration.
Observations show that swash periods are not very different from the average period of incident waves at many reflective beaches. However, as mentioned before, long-term equilibrium does not only depend on swash processes, but also on surf processes (wave breaking on the beach) during storm periods [19].
The above feedback mechanism does not account for the dependence of beach slope on grain size. However, such a dependence may be expected, because swash on coarse-grained beaches is more strongly influenced by infiltration and exfiltration. Coarse sediment settles more easily at the end of the uprush than fine sediment; equilibrium with downslope transport by backwash therefore requires a steep beach slope. Hence, coarse-grained beaches (gravel beaches, for instance) typically have a steeper slope than fine- or medium-grained beaches.
The formation of beach cusps is intimately linked to the swash process. This is dealt with in the article Beach cusps.
Infragravity swash
On dissipative beaches incident short waves lose most of their energy by breaking in the surf zone [20]. Near the shoreline, only the longest waves survive, the so-called infragravity waves. Infragravity waves arise mainly from nonlinear interactions between short waves with different wavelengths and frequencies [21], see the article Infragravity waves. They carry only a small part of the wave energy on the lower shoreface (typically of the order of 1\% or less), but their relative importance increases strongly in the surf zone. Empirical formulas for run-up of infragravity waves are given in the article Wave run-up.
Laboratory and field observations show that the run-up R is bound to a maximum value for high wave heights (H of the order of 5 m or more). This is called 'swash saturation'. This can be due to breaking of infragravity waves, but also to collision of the uprush with the preceding backwash, see the article Swash zone dynamics.
While there is strong evidence that short-wave swash stimulates beach accretion, this is less clear for infragravity swash. Observations point to a net offshore directed transport by infragravity swash, especially in the seaward part of the swash zone. However, reliable models for simulating the complicated infragravity morphodynamics in the surf zone are not yet available [22].
Related articles
References
- Jump up ↑ Houser, C. and Barrett, G. 2010. Divergent behavior of the swash zone in response to different foreshore slopes and nearshore states. Mar. Geol. 271: 106–118
- Jump up ↑ Puleo, J.A., Lanckriet, T. and Blenkinsopp, C. 2014. Bed level fluctuations in the inner surf and swash zone of a dissipative beach. Mar. Geol. 349: 99–112
- Jump up ↑ Horn, D.P. 2002. Beach groundwater dynamics. Geomorphology 48: 121-146
- Jump up ↑ Kranenborg, J.W.M., Pauli, T., Jacobsen, N.G., van der Werf, J.J., Dionisio Antonio, S., Campmans, G.H.P., Reniers, A.J.H.M. and Hulscher, S.J.M.H. 2023. Measurements and modeling of pore-pressure gradients in the swash zone under large-scale laboratory bichromatic waves. Journal of Geophysical Research: Oceans 128, e2022JC019577
- Jump up ↑ Ritter, A. 1892. Die Fortpflanzung der Wasserwellen. Zeitschrift des Vereines Deutscher Ingenieure 36: 947-954
- ↑ Jump up to: 6.0 6.1 Hughes, M.G. 1995. Friction factors for wave uprush. Journal of Coastal Research 13: 1089– 1098
- Jump up ↑ Puleo, J.A. and Holland, K.T. 2001. Estimating swash zone friction coefficients on a sandy beach. Coastal Engineering 43: 25– 40
- Jump up ↑ Puleo, J.A., Holland, K.T., Plant, N.G., Slinn, D.N. and Hanes, D.M. 2003. Fluid acceleration effects on suspended sediment transport in the swash zone. J.Geophys.Res. 108: C11 doi:10.1029/2003JC001943
- Jump up ↑ Raubenheimer, R., Elgar, S, and Guza, T. 2004. Observations of swash zone velocities: A note on friction coefficients. J.Geophys.Res. 109: C01027,1-8
- ↑ Jump up to: 10.0 10.1 Kikkert, G.A., Pokrajac, D., O'Donoghue, T. and Steenhauer, K. 2013. Experimental study of bore-driven swash hydrodynamics on permeable rough slopes. Coastal Eng. 79: 42-56
- Jump up ↑ Inch, K., Masselink, G., Puleo, J.A., Russell, P. and Conley, D.C. (2015) Vertical structure of near-bed cross-shore flow velocities in the swash zone of a dissipative beach. Continental Shelf Res. 101: 98-108
- ↑ Jump up to: 12.0 12.1 Holland, K.T. and Puleo, J.A. 2001. Variable swash motions associated with foreshore profile change. J. Geophys. Res. 106: 4613-4623
- Jump up ↑ Hughes, M.G. and Baldock, T.E. 2004. Eulerian flow velocities in the swash zone: field data and model predictions. J. Geophys. Res. 109, C08009
- Jump up ↑ Alsina, J.M., van der Zanden, J., Caceres, I. and Ribberink, J.S. 2018. The influence of wave groups and wave-swash interactions on sediment transport and bed evolution in the swash zone. Coast. Eng. 140: 23–42
- Jump up ↑ Butt, T., Russell, P. and Turner, I. 2001. The influence of swash infiltration-exfiltration on beach face sediment transport: onshore or offshore? Coastal Eng. 42: 35-52
- Jump up ↑ Miles, J., Butt, T. and Russell, P. 2006. Swash zone sediment dynamics: A comparison of a dissipative and an intermediate beach. Mar. Geol. 231: 181-200
- ↑ Jump up to: 17.0 17.1 Masselink, G., Russell, P., Turner, I and Blenkinsopp, C. 2009. Net sediment transport and morphological change in the swash zone of a high-energy sandy beach from swash event to tidal cycle time scales. Marine Geol. 267: 18-35
- Jump up ↑ Bendixen, M., Clemmensen, L.B. and Kroon, A. 2013. Sandy berm and beach-ridge formation in relation to extreme sea-levels: A Danish example in a micro-tidal environment. Marine Geol. 344: 53-64
- Jump up ↑ Masselink, G. and Puleo, J.A. 2006. Swash-zone morphodynamics. Cont. Shelf Res. 26: 661-680
- Jump up ↑ Ruessink, B.G. 1998. Bound and free infragravity waves in the nearshore zone under breaking and nonbreaking conditions. J. Geophys. Res. 103: 12,795-12,805
- Jump up ↑ Hasselmann, K. On the nonlinear energy transfer in a gravity-wave spectrum, part 1. General theory. J. Fluid Mech. 12: 481-500
- Jump up ↑ Elfrink, B. and Baldock, T. 2002. Hydrodynamics and sediment transport in the swash zone:a review and perspectives. Coastal Eng. 45: 149-167
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