Continual wave breaks in the same location alongshore
suspends beach sediments that are transported by currents.
The continuous turbulent action and cross-shore movement of sand grains creates a sedimentation deficit that
produces the small alongshore trough. The penultimate
forward movement of the wave thus moves up the
beachface as laminar flow, often with several hydraulic
jumps as successive waves break and send new swash
on top of the preceding swash flow, in piggyback fashion.
Bathers entering the surf are often surprised by the presence of the plunge step, especially when the water is
murky, because it may be of decimeter scale, the deeper
parts of the trough collecting organic debris that is
unpleasant to walk through. The plunge step is best developed in low-tidal range beaches with a steep foreshore
slope (Davis, 1985), such as commonly seen along Florida
Atlantic coast beaches but is also common along gravel
beaches. When setup is very high, the plunge step site
may transfer up the beachface to the swash berm or even
the storm berm. Direct measurement of step morphology
is rare on account of the high-energy conditions localized
along this zone.
Bar formation and migration
Surf zone currents transport sediment onshore, alongshore, and offshore to build (sand) wave-formed nearshore
bars and troughs occupying the surf zone (e.g., Davis,
1985; Komar, 1997; Greenwood, 2005). Wave-formed
bars occur as symmetrical or asymmetrical undulations
along the upper shoreface profile in intertidal and
subtidal environments (Greenwood, 2005). Barred
profiles are in general associated with large values of wave
steepness and wave height-to-grain size ratios and are
associated with waning stages of shoaling and dissipation
of wave energy (Wright et al., 1979; Greenwood, 2005).
Bar formation has been related to a number of specific
hypotheses that involve convergence of sediment transport, viz., (1) breakpoint hypotheses, (2) infragravity wave
hypotheses, and (3) self-organization hypotheses. Wave
breaking, for example, is thought to induce a seaward
transport of sediment that is, respectively, entrained by
roller or helical vortices under plunging or spilling breakers (e.g., Zhang, 1994). Alternatively, convergence of
sediment at the breakpoint may be related to onshore
transport associated with increasing asymmetry and skewness of high-frequency incident waves and offshore transport through setup-induced undertow (e.g., Thornton
et al., 1996). Bar frequency of occurrence and geomorphic
position may be produced by the interaction of sandy sediments with infragravity waves that are low frequency
(greater 30 s period) waves produced by sets of higher
and lower waves that are enhanced by wave breaking
across the surf zone. These waves can be standing or
progressive-produced as a result of energy dissipation during breaking and frequently related to groupiness
(Roelvink and Broker, 1993; Reussink, 1998), amplitude
modulation of the incident wave field. As a rule, the longer
the infragravity wave period, the more widely spaced the
bar(s).
Rip currents and channels
Rip currents are narrow, usually fast seaward-flowing currents that penetrate the surf zone, often in a rip channel that
flows to deeper water. Rips are a mechanism for returning
water back to the sea that piles up alongshore in setup
(e.g., Pilkey et al., 2011; Leatherman, 2013). These fastmoving shore-perpendicular currents transport eroded
beach sediment seaward to deep water during high seas.
These fast-moving cross-shore currents often occur
through gaps in offshore bars but are also caused by
depressions in the beach, irregular bottom topography,
obstructions in the surf zone such as rock outcrops or
groins, or longshore currents in circulating cells (e.g.,
Pilkey et al., 2011). Rip currents, a major hazard to swimmers, are responsible for most beach rescues and drownings (Short, 1999; Leatherman, 2013). Edge waves,
a form of infragravity wave (Guza and Inman, 1975;
Reussink, 1998), influence the alongshore spacing of rip
currents and channels, which are typically 200–300 m
apart on open ocean beaches. Bars and troughs that form
on the intertidal part of the beach are known as ridges
and runnels, with sometimes as many as a dozen or more
occurring on intertidal beaches with high tidal ranges.
Individual bars may have their own swash zones and dry
beach on the crests until they are covered by the rising tide.
Large-scale morphodynamic beach processes
Equilibrium beach (cross-shore) profiles are produced by
steady wave forcing during seasonal cycles. Accretionary
(summer) and erosional (winter) beach profiles are expressions of seasonal cycles of wave energy, as elucidated in
the classic works of Shepard (1950) and Bascom (1954).
Accretionary beaches, which are produced by swell waves
with a low wave height (generally <1 m) and a period of
8–12 s, produce a wide and well-developed backbeach
with a relatively narrow foreshore (e.g., Davis and
FitzGerald, 2004).
Large storm events result in a disequilibrium profile
where sand may be permanently lost to deep water
(Dean and Dalrymple, 2002). Erosional or storm beaches,
on the other hand, are temporary morphodynamic states
that are characterized by a profile that is generally flat
and featureless with a narrow or nonexistent backbeach
(Shepard, 1950; Hallermeier, 1981). Most or all of the
beach thus occur in the foreshore position (e.g., Davis
and FitzGerald, 2004). Due to storms, waves are larger
and more energetic in winter than summer. Long periods
of stormy weather, such as El Niño winters, erode beaches
to the underlying cobbles or bedrock and deposit sand far
offshore in deep water, leaving the beach in disequilibrium, as seen, for example, along the California coast.
The winter beach is denuded of sand by storm waves,
leaving heavier cobbles behind as lag deposits. Wave-cut
platforms underlying the mobile sediments are often
52
BEACH PROCESSES
suspends beach sediments that are transported by currents.
The continuous turbulent action and cross-shore movement of sand grains creates a sedimentation deficit that
produces the small alongshore trough. The penultimate
forward movement of the wave thus moves up the
beachface as laminar flow, often with several hydraulic
jumps as successive waves break and send new swash
on top of the preceding swash flow, in piggyback fashion.
Bathers entering the surf are often surprised by the presence of the plunge step, especially when the water is
murky, because it may be of decimeter scale, the deeper
parts of the trough collecting organic debris that is
unpleasant to walk through. The plunge step is best developed in low-tidal range beaches with a steep foreshore
slope (Davis, 1985), such as commonly seen along Florida
Atlantic coast beaches but is also common along gravel
beaches. When setup is very high, the plunge step site
may transfer up the beachface to the swash berm or even
the storm berm. Direct measurement of step morphology
is rare on account of the high-energy conditions localized
along this zone.
Bar formation and migration
Surf zone currents transport sediment onshore, alongshore, and offshore to build (sand) wave-formed nearshore
bars and troughs occupying the surf zone (e.g., Davis,
1985; Komar, 1997; Greenwood, 2005). Wave-formed
bars occur as symmetrical or asymmetrical undulations
along the upper shoreface profile in intertidal and
subtidal environments (Greenwood, 2005). Barred
profiles are in general associated with large values of wave
steepness and wave height-to-grain size ratios and are
associated with waning stages of shoaling and dissipation
of wave energy (Wright et al., 1979; Greenwood, 2005).
Bar formation has been related to a number of specific
hypotheses that involve convergence of sediment transport, viz., (1) breakpoint hypotheses, (2) infragravity wave
hypotheses, and (3) self-organization hypotheses. Wave
breaking, for example, is thought to induce a seaward
transport of sediment that is, respectively, entrained by
roller or helical vortices under plunging or spilling breakers (e.g., Zhang, 1994). Alternatively, convergence of
sediment at the breakpoint may be related to onshore
transport associated with increasing asymmetry and skewness of high-frequency incident waves and offshore transport through setup-induced undertow (e.g., Thornton
et al., 1996). Bar frequency of occurrence and geomorphic
position may be produced by the interaction of sandy sediments with infragravity waves that are low frequency
(greater 30 s period) waves produced by sets of higher
and lower waves that are enhanced by wave breaking
across the surf zone. These waves can be standing or
progressive-produced as a result of energy dissipation during breaking and frequently related to groupiness
(Roelvink and Broker, 1993; Reussink, 1998), amplitude
modulation of the incident wave field. As a rule, the longer
the infragravity wave period, the more widely spaced the
bar(s).
Rip currents and channels
Rip currents are narrow, usually fast seaward-flowing currents that penetrate the surf zone, often in a rip channel that
flows to deeper water. Rips are a mechanism for returning
water back to the sea that piles up alongshore in setup
(e.g., Pilkey et al., 2011; Leatherman, 2013). These fastmoving shore-perpendicular currents transport eroded
beach sediment seaward to deep water during high seas.
These fast-moving cross-shore currents often occur
through gaps in offshore bars but are also caused by
depressions in the beach, irregular bottom topography,
obstructions in the surf zone such as rock outcrops or
groins, or longshore currents in circulating cells (e.g.,
Pilkey et al., 2011). Rip currents, a major hazard to swimmers, are responsible for most beach rescues and drownings (Short, 1999; Leatherman, 2013). Edge waves,
a form of infragravity wave (Guza and Inman, 1975;
Reussink, 1998), influence the alongshore spacing of rip
currents and channels, which are typically 200–300 m
apart on open ocean beaches. Bars and troughs that form
on the intertidal part of the beach are known as ridges
and runnels, with sometimes as many as a dozen or more
occurring on intertidal beaches with high tidal ranges.
Individual bars may have their own swash zones and dry
beach on the crests until they are covered by the rising tide.
Large-scale morphodynamic beach processes
Equilibrium beach (cross-shore) profiles are produced by
steady wave forcing during seasonal cycles. Accretionary
(summer) and erosional (winter) beach profiles are expressions of seasonal cycles of wave energy, as elucidated in
the classic works of Shepard (1950) and Bascom (1954).
Accretionary beaches, which are produced by swell waves
with a low wave height (generally <1 m) and a period of
8–12 s, produce a wide and well-developed backbeach
with a relatively narrow foreshore (e.g., Davis and
FitzGerald, 2004).
Large storm events result in a disequilibrium profile
where sand may be permanently lost to deep water
(Dean and Dalrymple, 2002). Erosional or storm beaches,
on the other hand, are temporary morphodynamic states
that are characterized by a profile that is generally flat
and featureless with a narrow or nonexistent backbeach
(Shepard, 1950; Hallermeier, 1981). Most or all of the
beach thus occur in the foreshore position (e.g., Davis
and FitzGerald, 2004). Due to storms, waves are larger
and more energetic in winter than summer. Long periods
of stormy weather, such as El Niño winters, erode beaches
to the underlying cobbles or bedrock and deposit sand far
offshore in deep water, leaving the beach in disequilibrium, as seen, for example, along the California coast.
The winter beach is denuded of sand by storm waves,
leaving heavier cobbles behind as lag deposits. Wave-cut
platforms underlying the mobile sediments are often
52
BEACH PROCESSES