Elf Rink 2002

Coastal Engineering 45 (2002) 149 – 167
www.elsevier.com/locate/coastaleng
Hydrodynamics and sediment transport in the swash zone:

a review and perspectives
Berry Elfrink a,*, Tom Baldock b,1
a
DHI-Water and Environment, Agern Allé 11, 2970 Hørsholm, Denmark
b
Department of Civil and Environmental Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BU, UK
Abstract
The dominant hydrodynamic forcing and resulting sediment transport mechanisms in the swash zone are reviewed,
combined with a discussion of future measurement and modelling requirements. The importance of swash zone processes in the
overall behaviour of the nearshore littoral zone are identified, with the paper subsequently focusing on aspects that directly
relate to sediment transport on beaches. The review considers analytical and numerical modelling work, and both field and
laboratory data. The hydrodynamics of the swash zone are first discussed with reference to the surf zone boundary, swash
characteristics, forcing mechanisms and shoreline oscillations. Subsequent sections consider more micro-scale processes such as
the internal flow kinematics, turbulence, the bed boundary layer and infiltration/exfiltration through permeable beds. The
second part of the paper outlines the fundamental mechanics of sediment transport in the swash zone, followed by a more
detailed discussion of sediment transport modes induced by the dominant hydrodynamic conditions. Modelling concepts are
reviewed, and the strengths and weaknesses of different approaches identified. A final summary and future modelling
perspectives conclude the paper. D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Swash zone; Hydrodynamics; Sediment transport; Beaches; Run-up; Review; Beach groundwater
1. Introduction since it results from the small difference between two

large opposing quantities (Osborne and Rooker, 1999;
Sediment transport mechanisms in the swash zone Hughes et al., 1997b). Nevertheless, the hydrodynam-
have traditionally received less attention than those in ics and sediment transport in the swash zone are
the surf zone, partly due to the difficulty in performing important for a variety of reasons: firstly, an important
high-quality field measurements of sediment transport part of the littoral sediment transport occurs in the
and partly due to the complexity of the processes swash zone (see Fig. 1 for a definition sketch). Sedi-
themselves. For example, net onshore or offshore ment concentrations are often high in the swash zone,
transport in the swash zone is difficult to quantify, and may typically be several orders of magnitude
higher in the swash zone than in the inner-surf zone
*
(Osborne and Rooker, 1999; Beach and Sternberg,
Corresponding author. Tel.: +45-4516-9200; fax: +45-4516- 1991). Secondly, swash run-up influences engineering
9292.
E-mail address: bre@dhi.dk (B. Elfrink).
design (Kobayashi, 1999) and also leads to the ero-
1
Address from July 1st, 2002: Department of Civil Engineer- sion of cliffs and dunes. Thirdly, swash zone mechan-
ing, The University of Queensland, St. Lucia, QLD 4072, Australia. ics determine to a high degree the mechanism of
0378-3839/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 8 3 9 ( 0 2 ) 0 0 0 3 2 - 7
150 B. Elfrink, T. Baldock / Coastal Engineering 45 (2002) 149–167
beach recovery after storms. Swash hydrodynamics to sediment transport on beaches; more general dis-
additionally determine the interaction between marine cussions of nearshore processes are given by Bagnold
processes (inner surf zone) and terrestrial processes (1963), Horikawa (1988), Hardisty (1990), van Rijn
(coastal water table, coastal dunes) and therefore (1993), Dean (1994) and Komar (1998). In the follow-
influence much of the nearshore littoral zone. For ing, four key aspects of the swash are considered:
example, the swash zone plays an important role in
the ecology of the intertidal zone (McArdle and (i) Boundary conditions
McLachlan, 1992) and groundwater levels in subae- (ii) Swash zone characteristics, forcing mecha-
rial littoral beaches and low-lying islands (Nielsen, nisms and shoreline oscillations
1999). Finally, the swash zone may define legal (iii) Internal flow kinematics and turbulence
boundaries (Morton and Speed, 1998). (iv) Bed boundary layer, percolation and ground-
However, in the last few years, swash zone pro- water
cesses have received increasing attention. A recent
review of field measurements is given by Butt and 2.1. Swash zone boundary conditions
Russell (2000). In the present paper, the dominant
swash hydrodynamic and sediment transport mecha- The hydrodynamics of the swash zone are largely
nisms are described, followed by a discussion of the governed by the boundary conditions imposed by the
modelling concepts applied in the swash zone. Section inner surf zone and the underlying beach (Fig. 1).
2 of the paper focuses on swash zone hydrodynamics, These boundary conditions include the wave or bore
while recent developments in the description sediment height, wave frequency, waveshape, spectral band-
transport mechanics are described in Section 3. Sec- width, orbital velocities, currents, turbulence, beach
tion 4 discusses concepts for modelling sediment slope and beach composition. Boundaries of secon-
transport in the swash zone, with final conclusions dary importance may include fixed shoreward (and
and some perspectives for future work in Section 5. longshore) boundaries (structures, cliffs or dunes) and
atmospheric forcing, which are not considered further
here. Longshore variations in the beach planform
2. Swash zone hydrodynamics which may influence surf and swash hydrodynamics
over broad space –time scales, leading to morpholog-
The sediment dynamics and sediment transport in ical/hydrodynamic feedback are beyond the scope of
the swash zone are predominantly governed by the this paper (see Coco et al., 2000 for a description of
swash hydrodynamics. This section of the paper there- recent advances). In addition, the beach composition
fore concentrates on hydrodynamics that directly relate (grain size, permeability, degree of saturation) is
Fig. 1. Definition sketch for the nearshore littoral zone (swash zone width exaggerated).
B. Elfrink, T. Baldock / Coastal Engineering 45 (2002) 149–167 151
largely expected to influence the hydrodynamics at low-frequency motions include shear waves (Oltman-
micro-scales (e.g. infiltration/exfiltration and boun- Shay et al., 1989) and recently identified nongravity
dary layers); these are discussed in detail in Section wave motions (Holland and Holman, 1999). Different
2.4. Consequently, since the global beach slope influ- generation mechanisms for these low-frequency
ences the surf zone conditions (e.g. Iribarren and motions (also frequently termed surf beat, Munk,
Nogales, 1949; Battjes, 1974), the dominant boundary 1949; Tucker, 1950) are well known and are generally
condition for the swash zone may be considered to be related to variations in the incident short wave height
the hydrodynamics of the inner surf zone. This sub- or wave grouping (Longuet-Higgins and Stewart,
section therefore reviews the surf conditions expected 1962, 1964; Gallagher, 1971; Symonds et al., 1982;
at the seaward boundary of the swash zone, with the Schäffer, 1993; Lippmann et al., 1997). However,
resulting characteristics of the swash zone and swash some questions still remain as to which mechanisms
kinematics (including the influence of the local bed dominate under different surf zone conditions (see
slope) discussed in Sections 2.2 and 2.3. Baldock et al., 2000 for a recent review).
A large range of scales and types of fluid motion The transition between a saturated surf zone and
may be present in the inner surf zone, which will unsaturated surf zone may be approximately defined
subsequently govern shoreline oscillations and swash using the surf similarity parameter or Iribarren num-
hydrodynamics. These may include short (high-fre- ber, no=b/M(Ho/Lo) (Iribarren and Nogales, 1949;
quency) waves (sea, swell), long waves, edge waves, Battjes, 1974), where b, Ho and Lo are the beach
shear waves, cross-shore and longshore currents, tur- slope, deep water wave height and deep water wave-
bulence and vortices (see Battjes, 1988; Hamm et al., length, respectively. Although originally developed
1993; Svendsen and Putrevu, 1996 for general com- for monochromatic waves by Battjes (1974), follow-
prehensive reviews). To a first approximation, the scale ing the work of Guza and Thornton (1982) the
of the dominant wave motion in the inner surf zone is Iribarren number has been widely applied to natural
determined by whether the surf zone is saturated (short random waves using the spectral deep water signifi-
wave heights depth limited) or unsaturated, i.e. local cant wave height and peak wave period. For mono-
wave height independent of depth (Goda, 1975; Wright chromatic waves, spilling breakers and a saturated
and Short, 1984; Raubenheimer and Guza, 1996). In surf zone typically occur for no<0.5, whereas plunging
the first instance, typical of mildly sloping beaches, the breakers and non-saturated conditions tend to occur
hydrodynamics in the inner surf zone may be expected for no > 0.5 (e.g. Battjes, 1974), with slightly different
to be dominated by non-breaking low-frequency values probably applicable for random waves. Note,
waves (frequently termed infragravity waves) (Huntley however, this does not imply that the swash saturates
et al., 1977; Guza and Thornton, 1982; Wright et al., at similar values of no (see Section 2.2). Conse-
1982). In contrast, unsaturated surf zone conditions quently, for no>0.5, significant bore run-up is likely
frequently show a dominance of short wave energy in to occur in the swash zone (Madsen et al., 1997).
the inner surf zone (Bradshaw, 1980; Wright and Short, Whether infragravity or short wave energy dominates
1984; List 1991), with short wave bores reaching the the swash therefore depends on the relative magni-
shoreline (Hibberd and Peregrine, 1979; Kobayashi et tudes of short and long wave energy in the inner surf
al., 1989; Hughes, 1992; Baldock et al., 1998; Masse- zone, which in turn is dependent on the offshore wave
link and Hughes, 1998). conditions, and hence to a large extent on wave
Surf zone infragravity energy is typically highly groupiness (see List, 1991 for a discussion on wave
correlated with short wave energy (Elgar et al., 1992; groupiness in the nearshore region). However, on
Herbers et al., 1995a; Ruessink, 1998), consistent with natural beaches, infragravity energy typically domi-
generation by short waves. Infragravity energy may nates in the swash zone at low Iribarren numbers
propagate both cross-shore (leaky waves) (Suhayda, (Guza et al., 1984) and the reasons for this are
1974; Guza and Thornton, 1985; Holland et al., 1995; discussed further in Section 2.2.
Raubenheimer and Guza, 1996) and be refractively Surf zone modelling is well advanced, with a range
trapped (edge waves) (Huntley et al., 1981; Oltman- of different types of models available to predict
Shay and Guza, 1987; Herbers et al., 1995b). Other hydrodynamic conditions close to the shoreline. Three
broad types of model are commonly used in the modelling and the advection of turbulence across the
coastal environment; probabilistic models (e.g. Miz- seaward swash boundary (Hughes et al., 1997a).
uguchi, 1982; Mase and Iwagaki, 1982; Dally and
Dean, 1986), parametric models (Battjes and Stive, 2.2. Swash zone characteristics, forcing mechanisms
1985; Thornton and Guza, 1983; Lippmann et al., and shoreline oscillations
1996; Baldock et al., 1998) and time domain models
(Hibberd and Peregrine, 1979; Packwood and Pere- Although the swash zone moves over the whole of
grine, 1981; Kobayashi et al., 1989, 1998; Madsen et the intertidal zone, the local swash zone may be
al., 1991, 1997; Roelvink and Brøker, 1993; Van defined as the region of wave run-up and run-down
Dongeren et al., 1994; Kennedy et al., 2000). around the mean water level, i.e. excluding set-up
The probabilistic and parametric models generally (Holman, 1986). For monochromatic waves this region
only provide information on wave heights and wave is well-defined, whereas for real sea states maximum
height distributions in the nearshore, whereas the run-up, run-down and set-up vary constantly with time.
time – domain models allow a detailed representation Here, therefore, the swash zone is loosely defined as
of non-linear wave motions and kinematics. However, the region in which the beach face is intermittently
despite considerable recent advances in turbulence exposed to the atmosphere, possibly over both long
modelling in the surf zone (e.g. Svendsen and (minutes) and short (seconds) time scales. Perhaps it
Putrevu, 1994; Ting and Kirby, 1996; Lin and Liu, should be noted that based on this definition, much
1998c), both new models and data for their calibration experimental data ascribed to the swash zone in fact
are required to provide an adequate description of relates to the inner surf zone, particularly kinematic
turbulence in the inner surf and swash zones (Thorn- data. Research in the swash zone has predominantly
ton et al., 2000). A more detailed discussion of focused on characterising the behaviour of the moving
turbulence in the surf and swash zones and some shoreward edge, i.e. cross-shore shoreline (swash)
recent swash zone data are given by Petti and Longo oscillations (e.g. Miche, 1951; Battjes, 1971; Guza
(2001). Similarly, the influence of cross-shore and and Bowen, 1976; Huntley et al., 1977; Guza and
longshore currents and vorticity (shear waves) on the Thornton, 1982; Holman and Sallenger, 1985; Mase,
seaward swash zone boundary remain to be inves- 1988; Yeh et al., 1989; Hughes, 1992; Holland et al.,
tigated in detail, although modelling and understand- 1995; Raubenheimer and Guza, 1996; Ruessink et al.,
ing has progressed sufficiently to make this feasible in 1998; Baldock and Holmes, 1999; Holland and Hol-
the near future (e.g. Ozkan-Haller and Kirby, 1999; man, 1999; Puleo and Holland, 2001). This subsec-
Chen et al., 2000). tion therefore focuses on shoreline oscillations, with
In conclusion, the hydrodynamics at the seaward research into the internal flow kinematics in the re-
swash zone boundary can be well modelled at wave mainder of the swash zone reviewed in Section 2.3.
scales, probably providing sufficient detail and accu- Two broadly different types of swash oscillations
racy to predict the overall swash characteristics have been identified by theoretical, field and laboratory
required for sediment transport models. Probabilistic studies, consistent with the forcing at the seaward
and parametric models which provide wave height and swash boundary (Section 2.1). Typically, swash mo-
wave period in the nearshore are probably applicable tions may result from non-breaking standing waves
for simple engineering applications, with further detail (leaky or edge waves) or broken short waves (bores).
(waveshape, long waves) provided by hybrid para- Cross-shore standing wave swash oscillations are usu-
metric – time –domain models (Roelvink, 1993; John- ally observed at infragravity frequencies (Suhayda,
son and Kobayashi, 1998). However, since there may 1974; Huntley et al., 1977; Guza and Thornton, 1982;
be considerable hydrodynamic interaction between the Holland et al., 1995; Raubenheimer and Guza, 1996),
surf and swash zone, the time –domain models refer- as are shoreline oscillations due to edge waves (Bowen
enced above, which link the surf and swash zones, will and Inman, 1971; Huntley et al., 1981; Guza and
generally provide the most accurate description of the Thornton, 1985; Oltman-Shay and Guza, 1987; Hol-
inner surf zone boundary conditions. Nevertheless, land and Holman, 1999). In contrast, swash motion due
considerable further research is required on turbulence to incident bores which collapse at the shoreline and
propagate up the beach is typically forced by higher where as is the vertical amplitude of the shoreline
frequency short waves (Shen and Meyer, 1963; Wad- motion, x is the angular wave frequency (2pf — where
dell, 1976; Hibberd and Peregrine, 1979; Bradshaw, f is the wave frequency), g the gravitational acceler-
1980; Packwood, 1983; Mase, 1988; Yeh et al., 1989; ation and b the beach slope. Consequently, at swash
Hughes, 1992, 1995; Brocchini and Peregrine, 1996; saturation, Miche’s hypothesis assumes a saturated surf
Madsen et al., 1997; Baldock and Holmes, 1999). zone and, based on the limiting amplitude for mono-
The magnitudes of these two main types of swash chromatic unbroken standing waves, esc1 (Carrier
motion in any particular set of conditions again largely and Greenspan, 1958; Munk and Wimbush, 1969).
appears to be a function of no (e.g. Holman and However, experimentally determined values for es are
Sallenger, 1985; Madsen et al., 1997). However, this generally significantly greater than this; esc3F1
dependence is complicated by the fact that infragravity (Guza and Bowen, 1976), esc1.26 (Battjes, 1974),
wave amplitudes (both in the swash and further off- esc2F0.3 (Van Dorn, 1978), es=23 (Huntley et al.,
shore) are not necessarily related to no, but are gen- 1977). Indeed, for monochromatic waves, the non-
erally linearly related to the offshore wave height dimensional swash amplitude es varies with the non-
(Goda, 1975; Guza and Thornton, 1982; Herbers et dimensional incident wave amplitude ei (ei=aix2(2p)1/2/
al., 1995a). Consequently, the relationship between the gb5/2) as es=ei for ei<1, esc(ei)1/2 for 1<ei<9 and esc3
magnitude of infragravity swash oscillations and no for ei>9 (Guza et al., 1984; Kobayashi et al., 1989).
may be site-specific (Raubenheimer and Guza, 1996; Nevertheless, field and laboratory data qualitatively
Ruessink et al., 1998). Nevertheless, typical field data confirm Miche’s saturation hypothesis and the de-
shows that low-frequency energy dominates swash pendence of saturated swash amplitudes on b2, f 2
oscillations for no<1.5, whereas short wave (sea, swell) (e.g. Huntley et al., 1977; Guza and Thornton, 1982;
energy tends to dominate at higher no (Holman and Guza et al., 1984; Mase, 1988). More recently, Bal-
Sallenger, 1985; Raubenheimer and Guza, 1996; see dock and Holmes (1999), using Shen and Meyer’s
also Butt and Russell, 2000 for a review of field (1963) solution to the non-linear shallow water wave
measurements). Again, this value for no may be site- equations (NLSWE) (see also Hughes, 1992), showed
specific since infragravity energy is dependent on that Eq. (2.1) was also consistent with saturation of
location (mild or steep beaches, broad or narrow the swash due to both monochromatic and random
continental shelves) (Herbers et al., 1995b), and Mad- wave bore run-up, and derived a theoretical value for
sen et al. (1997) found swash oscillations to be short esc2.5, in closer agreement with most of the exper-
wave-dominated for plunging/surging breakers at a imental values. In this instance, swash saturation
value of noc0.7. In addition, for monochromatic short occurs due to the duration (and hence magnitude) of
waves (no infragravity energy), swash oscillations will swash oscillations being controlled by the amplitude
be obviously short wave-dominated, irrespective of no. and frequency of the incident bores, rather than surf
Furthermore, both standing wave and bore-driven zone saturation as proposed by Miche (1951).
swash oscillations on the beach face may themselves For random sea states, the swash motion at both low
become saturated (independent of the incident wave/ and high frequencies is also dependent on the inter-
bore height), but for different reasons. action between successive swash events and the inter-
Miche (1951) proposed that the swash would be action between standing waves and incident bores.
saturated when the incident wave amplitude increased Swash due to larger incident waves may catch the
above the limiting amplitude for non-breaking stand- swash from preceding smaller waves, or the backwash
ing waves on a slope, with additional incident wave from a large uprush event may prevent the uprush of
energy completely dissipated by wave breaking. Satu- later smaller waves (Kubota et al., 1993), leading to a
ration is expected to occur when the non-dimensional merging of swash cycles and a progressive increase in
parameter es reaches some critical value (Iribarren and swash period with decreasing Iribarren number (Hol-
Nogales, 1949; Miche, 1951; Huntley et al., 1977): man, 1986; Mase, 1988). This process of swash –
swash interaction leads to further difficulties in deter-
as x2 mining how the swash forcing mechanism varies with
es ¼ ð2:1Þ
gb2 Iribarren number. For example, both Mase (1988) and
Baldock and Holmes (1999) showed that, due to wave motions, with field data suggesting that low-frequency
grouping remaining in the inner surf zone, short wave motions will dominate for no<1.5 (Holman and
bores could directly induce low-frequency oscillations Sallenger, 1985). However, this is likely to be site-
of the shoreline, even in the absence of low-frequency specific (Raubenheimer and Guza, 1996) and depend-
waves in the surf zone. ent to a large extent on surf zone conditions, partic-
Field and laboratory data therefore indicate that ularly wave groupiness and trapped long wave energy
swash motion may be due to both infragravity standing (Herbers et al., 1995b).
waves and short wave bores, with the dominance of
one form of motion over the other controlled by the 2.3. Internal flow kinematics and turbulence
conditions in the surf zone, which may at least be
distinguished qualitatively by no. However, typically, Less attention has been focused on the internal
low-frequency motions will dominate swash run-up flow kinematics of the swash zone than on character-
spectra for several reasons: high infragravity energy ising shoreline oscillations. This is probably due to the
levels in the surf zone (e.g. Guza and Thornton, 1982; difficulty of making measurements in this highly
Wright et al., 1982), saturation and dissipation of short dynamic zone and the need to clarify the forcing
wave energy in the surf zone (Miche, 1951; Battjes, mechanisms, more readily achieved by measurements
1974; Thornton and Guza, 1983; Raubenheimer and of the shoreline oscillations. However, the develop-
Guza, 1996) and saturation of bore-driven swash ment and validation of sediment transport models for
oscillations at short wave frequencies (Mase, 1988; the swash zone requires both an understanding and
Baldock and Holmes, 1999). detailed measurements of the internal kinematics. This
Longshore swash motions and longshore variations section considers two key aspects of the kinematics;
in cross-shore swash oscillations have been investi- the dominant free stream velocities and turbulence.
gated to a lesser extent, largely because shore normal The internal flow kinematics in the swash lens may
swash oscillations tend to dominate. However, Olt- be quite different for swash motions dominated by
man-Shay and Guza (1987) and, more recently, Hol- standing long waves or progressive bores (Kemp,
land and Holman (1999) showed that low mode edge 1975; Zelt, 1991). For infragravity standing long
wave energy may be significant in certain field con- waves, Eulerian measurements of the flow velocity at
ditions (30 – 50% of infragravity energy). Holland and a point appear approximately symmetrical with respect
Holman (1999) also identified low-frequency non- to the time of maximum run-up (Hibberd and Pere-
gravity-driven longshore swash motions, possibly grine, 1979; Beach and Sternberg, 1991; Butt and
related to shear waves. Obliquely incident short wave Russell, 1999). In contrast, the velocities induced by
bores arriving at the shoreline will also induce some short wave bores are more asymmetrical, with rapid
longshore component to both the shoreline motion and acceleration at the start of the run-up and a more saw-
swash zone kinematics (Ryrie, 1983; Chadwick, toothed time-history (Kemp, 1975; Kobayashi et al.,
1991a,b; Asano, 1994, 1996; Kobayashi and Karjadi, 1988; Beach and Sternberg, 1991; Hughes et al.,
1996; Elfrink, 1997; Brocchini, 1997; Chen et al., 1997b; Masselink and Hughes, 1998; Puleo et al.,
2000; Van Wellen et al., 2000). This longshore com- 2000). In this instance, flow seaward of the shoreline
ponent of the swash kinematics will clearly lead to position also tends to become offshore-directed prior to
longshore sediment transport in the swash zone (e.g. maximum run-up occurring, leading to diverging flow,
Bodge and Dean, 1987; Kamphuis, 1991; see Section and hence rapid thinning of the swash lens (Hibberd
3), although there appear to be few published measure- and Peregrine, 1979; Larson and Sunamura, 1993;
ments of longshore velocities in the swash. Raubenheimer and Guza, 1996; Hughes et al.,
In summary, recent research has clarified both the 1997b; Baldock and Holmes, 1997). Similarly, flow
forcing mechanisms and resulting shoreline motions depths at point in the swash zone increase gradually to
for a wide range of incident wave conditions. A a maximum around the time of maximum run-up if
mixture of infragravity waves, and other low-fre- unbroken standing long waves occur (Hibberd and
quency motions, combined with higher frequency Peregrine, 1979; Zelt, 1991), whereas incident bores
oscillations due to bores typically occur in shoreline generate a very rapid rise in the depth after the shore-
line passes, with the depth decreasing prior to max- turbulence generated at bore collapse, is expected to be
imum run-up and then subsequently back to zero advected into the swash zone during the run-up (Yeh
during the backwash (Hibberd and Peregrine, 1979; and Ghazali, 1988; Hughes et al., 1997a; Puleo et al.,
Packwood, 1983; Hughes, 1992; Cox et al., 1994; 2000), potentially leading to very high concentrations
Raubenheimer et al., 1995; Hughes et al., 1997b). of suspended sediment (see Section 3). Conversely,
These differences in the swash lens kinematics due to during the backwash, the bed boundary (wall turbu-
standing waves or bores are likely to lead to differences lence) appears to be the main source of turbulence
in sediment transport modes and directions (e.g. Beach generation (Petti and Longo, 2001), potentially
and Sternberg, 1991), discussed further in Section 3. enhanced towards the end of the backwash/next run-
The kinematics in the swash zone are additionally up by the formation of backwash bores (Shen and
characterised by high free stream velocities, both in Meyer, 1963; Hughes, 1992) and hydraulic jumps due
the run-up and backwash, where, due to rapidly to swash – swash interactions (e.g. Osborne and
decreasing water depths, the flow may become super- Rooker, 1999). Surface shear waves may also arise
critical. Infragravity run-up and backwash velocities due to flow separation from the bed towards the end of
in the field have been observed to reach 2 m/s (Beach the backwash (Peregrine, 1974), further increasing
and Sternberg, 1991; Butt and Russell, 1999), and vorticity generation. However, at present, modelling
measured peak run-up velocities due to bores have turbulence in swash zone hydrodynamic and sediment
exceeded 5 m/s (Hughes et al., 1997b), although run- transport models is beyond the state of the art. Never-
up/backwash velocities in the field of 1 –3 m/s are theless, both the advection of turbulence and the
more typical (Hughes, 1995; Masselink and Hughes, generation of turbulence in the bed boundary layer
1998; Blewett et al., 1999). For bore-driven swash, can potentially be incorporated into present time –
Masselink and Hughes (1998) also found that the peak domain models. For example, Elfrink (1997) included
and mean run-up velocities were not significantly a description of the turbulent boundary layer, based on
different from the backwash velocities, although the the work of Fredsøe (1984), into the non-linear shallow
backwash duration was typically greater than the run- water equations and concluded that the effect of the
up duration, consistent with the diverging flow dis- boundary layer is most prevalent during run-down.
cussed above. This difference between the run-up and Modelling turbulence generation by swash – swash
backwash durations is also typically observed in interactions presents a major challenge, but appears
laboratory data (e.g. Larson and Sunamura, 1993; critical to describing the total turbulent kinetic energy
Baldock and Holmes, 1997), which is likely to result levels at the seaward swash boundary. For a more
in negatively (offshore) skewed velocity moments comprehensive review of recent measurements of
towards the seaward limit of the swash zone. As a turbulence and the state of the art, see Longo et al.
result, steady flow energetics (Bagnold, 1963, 1966) (in press).
and Shields-type (see Nielsen, 1992) sediment trans-
port models based on the free stream velocities in the 2.4. Bed boundary layer, percolation and ground-
swash zone may be inherently biased towards offshore water
transport (see Section 4). Consequently, the inclusion
of turbulence in a description of the swash zone In comparison with the surf zone, where the under-
kinematics appears necessary (Hughes et al., 1997a), standing and modelling of the bed boundary layer is
together with a description of the bed boundary layer relatively advanced (Fredsøe, 1984; Grant and Mad-
(see Section 2.4). sen, 1986; Nielsen, 1992; Sleath, 1995; Foster et al.,
Potential sources for turbulence in the swash zone 2000), a good description of the bed boundary layer in
include the inner surf zone, initial bore collapse at the the swash zone is probably beyond the present state of
shoreline, the bed boundary, backwash bores and the art. For example, Hughes et al. (1997a) suggest that
swash – swash interactions. Highly turbulent flow in the swash zone boundary layer may not be similar to
the inner surf zone (Battjes, 1975; Stive, 1980; Svend- either wave- or current-induced bed boundary layers.
sen and Madsen, 1984; Svendsen, 1987; Svendsen and Furthermore, the highly variable nature of the flow in
Putrevu, 1994; Ting and Kirby, 1996), combined with the swash zone, with rapidly varying depths and
velocities, presents significant difficulties in advancing 1983), although it may become significant on shingle/
a description of the swash zone boundary layer. In gravel beaches.
addition, boundary layer growth will be different under Nevertheless, infiltration/exfiltration or fluid load-
decelerating (uprush) and accelerating (backwash) ing/unloading on the bed surface will induce a vertical
flows (Nielsen, 1992; Masselink and Hughes, 1998), flow/pressure gradient (lift force) in the upper layers
boundary layer growth lags associated bore fronts of the beach (Baird et al., 1996; Baldock et al., 2001),
(Madsen and Svendsen, 1983) and the backwash potentially increasing/decreasing the effective weight
typically comprises of a very small flow depth and a and effective normal stress of near surface sediment
fluid-sediment ‘‘slurry’’, similar to a single-phase (Martin, 1970; Nielsen, 1992). Infiltration/exfiltration
debris flow (Hughes et al., 1997a). However, available also has the potential to modify the bed shear stress
boundary layer models developed for the surf zone (Turcotte, 1960; Conley and Inman, 1994; Turner,
may be adaptable for the swash zone. For example, 1995), with infiltration leading to a thinning of the
Fredsøe’s (1984) model provides a description of the boundary layer and a greater shear stress, and vice
time-development of the boundary layer and associ- versa for exfiltration. Both effects have the potential
ated bed shear stress, and this model was subsequently to change sediment transport rates (see Section 3),
adapted by Elfrink (1997) to model longshore sedi- with the boundary layer and shear stress changes
ment transport in the swash zone. Nevertheless, to appearing dominant over the vertical lift forces
date, much of the influence of the bed boundary layer (Turner and Masselink, 1998). In summary, therefore,
on swash zone sediment transport has been investi- future hydrodynamic models for the swash zone will
gated qualitatively (see Section 3), by considering how need to account for the flow across the beach surface
the boundary layer may differ during uprush and and the interaction between the beach groundwater
backwash, and in particular, the influence of percola- and surface flows. This again appears possible by
tion, beach groundwater levels and high sediment extending present time – domain models (e.g. Pack-
concentrations. wood and Peregrine, 1980; Packwood, 1983; Kobaya-
Infiltration and exfiltration of water through the shi et al., 1991; Kobayashi and Wurjanto, 1992;
beach surface are expected to varying degrees during Madsen et al., 1997).
run-up and backwash, respectively, dependent on
groundwater levels, the permeability of the beach
material and whether the beach sediment is saturated 3. Sediment transport in the swash zone
or unsaturated (Grant, 1948; Duncan, 1964; Turner
and Nielsen, 1997; Nielsen, 1999). Beach ground- In this section, some important physical mecha-
water levels may vary seasonally, tidally, and at wave nisms will be described that affect the sediment trans-
frequencies (see for example Turner, 1998), and may port in the swash zone. Initially, it is important to
be modelled by BEM and 1D Boussinesq models (Li consider the very different hydrodynamic conditions
et al., 1997; Baird et al., 1998). The degree of during run-up and run-down. The run-up is charac-
saturation of the beach material is subsequently typ- terised by decelerating flow, whereas the flow during
ically dependent on the relative position of the water- run-down gradually accelerates until it reaches a
table outcrop and swash zone but may be highly maximum in the final stage of the run-down. Further-
variable over short durations and even over individual more, infiltration of water into the beach face occurs
swash events (Turner, 1995; Nielsen and Turner, in during run-up, whereas exfiltration occurs during run-
press). Infiltration into the upper layers of the beach down (Masselink and Hughes, 1998; Conley and
due to set-up and run-up also leads to a large-scale Inman, 1994) and Masselink and Hughes (1998) and
circulation within the beach sediment, with exfiltra- Osborne and Rooker (1999) recognised the impor-
tion occurring in the outer surf zone (Longuet-Hig- tance of differences in suspended sediment concen-
gins, 1983). However, infiltration/exfiltration is likely trations at the onset of run-up and run-down,
to lead to minimal changes in the overall swash respectively. Sediment transport in the swash zone
hydrodynamics, since the volume of fluid flow across usually occurs under sheet flow conditions and with a
the beach surface is typically small (see Packwood, flat bed, although temporary anti-dune bed forms
sometimes develop during supercritical flow con- and Russell (1993) noted the importance of low-
ditions at the end of the backwash (Osborne and Roo- frequency motions in the swash zone, and Beach and
ker, 1999). The distinction between bed load and Sternberg (1991) found that under high-energy con-
suspended load is difficult to verify in the field and ditions, the largest suspended sediment concentrations
several studies have proposed formulae for total load occurred on infragravity time scales. Low-frequency
transport without distinguishing the transport mode. motions in the swash zone are associated with negative
However, in this analysis, the distinction between bed vertical and horizontal asymmetry of the flow velocity,
load and suspended load is maintained in order to with the intermittent high-velocity backwash leading
illustrate the different physical mechanisms that are to offshore sediment transport.
dominant for the respective transport mode.
3.2.1. Advective sediment transport
3.1. Bed load transport In many applications, the convective terms in the
transport equation for suspended sediment are
Bed load transport is mainly determined by the neglected, which means that the sediment concentra-
Shields parameter h, which describes the balance tions are determined by local conditions only. Simi-
between a mobilising force (shear) and a stabilising larly, bed load transport is affected by the local bed
force (gravity). The critical value of h, e.g. the value shear stress, which in turn is a non-linear function of
where the motion of sediment particles is initiated, is a the flow velocity. This indicates that the near-bed
weak function of the Reynolds number and is in the skewness and asymmetry are important for the net
order of 0.05. Several formulae for bed load transport bed load transport. During run-up, the shoreward-
(Meyer-Peter and Müller, 1948; Engelund and Fred- directed velocity increases rapidly from zero to a
søe, 1976; and many others), are based on the differ- maximum value. During run-down, the flow acceler-
ence between the actual Shields parameter and its ates gradually until it reaches a maximum speed at the
critical value. The critical Shields parameter is affected end of the run-down. The vertical velocity gradients in
by factors such as bed slope (Fredsøe and Deigaard, the bottom boundary layer are steeper during the
1992; Turner and Masselink, 1998) and infiltration/ rapidly accelerating flow during run-up than during
exfiltration of water through the beach face (Conley run-down, resulting in higher shear stresses and bed
and Inman, 1994; Turner and Masselink, 1998). The load transport rates during uprush. On the other hand,
influence of different swash zone processes on bed the duration of the run-down is generally longer than
load transport is discussed in more detail later. Note the duration of the run-up (Larson and Sunamura,
that a Shields-type transport formula does not account 1993; Baldock and Holmes, 1997; Masselink and
for inertial forces, which may become significant for Hughes, 1998). Horizontal asymmetry (skewness) is
coarse grain sizes due to the high fluid accelerations a measure of the difference between onshore and
during swash run-up (Hardisty, 1990; Baldock and offshore velocity magnitudes, whereas vertical asym-
Holmes, 1997). metry is a measure of the different flow acceleration
during uprush/backwash, typical of a saw-tooth wave
3.2. Suspended load transport profile. Horizontal asymmetry is typically not uni-
formly distributed across the frequency spectrum; in
Several authors have discussed the relative impor- shallow water, observations tend to show a positive
tance of suspended load as compared to bed load. Horn skewness in the high-frequency domain, whereas the
and Mason (1994) found that the bulk of the transport low-frequency components of the spectrum commonly
in the swash zone of shingle beaches occurred as bed exhibit a negative skewness. The observed offshore-
load. Butt and Russell (1999) measured suspended directed swash zone bed load transport on high-energy
sediment concentrations in a high-energy swash zone beaches with saturated surf zones is therefore often
and found that the sudden offshore to onshore velocity related to the predominance of low-frequency motions
transition and turbulence in the swash zone leads to (Butt and Russell, 2000). Under low-energy condi-
subsequent onshore advection of sediment by run-up. tions, or on steep beaches, characterised by unsaturated
Beach and Sternberg (1991), Butt and Russell (1999) surf zones, the high-frequency motions are dominant,
resulting in shoreward bed load transport in the swash 1991; Osborne and Rooker, 1999; Puleo et al., 2000).
zone. During unsaturated surf zone conditions, a strong
The stabilising force on sediment particles is de- backwash vortex frequently develops at the seaward
pendent on several factors. Firstly, the sediment grain- edge of the swash zone during flow reversal, with the
size; the coarser the sediment, the larger the weight presence of the backwash vortex associated with the
and the stabilising force. On natural beaches, grain formation of a beach step (Matsunaga and Honji,
size frequently increases shoreward, with the coarsest 1980; Takeda and Sunamura, 1983; Larson and Suna-
material often found at the seaward edge of the swash mura, 1993). A beach step is defined as a small
zone, which is sometimes additionally characterised morphological feature found on steep, coarse grained
by the presence of a beach step (see below). Secondly, sand or gravel beaches (Bauer and Allan, 1995;
the slope of the swash zone directly affects the Larson and Sunamura, 1993). Beach steps are mainly
stability of sediment particles on the bed, with the observed on beaches with a poor sorting of sediment,
effective weight of the sediment particle higher during and their size is determined by the mean grain size and
uprush than during down rush. Fredsøe and Deigaard the variation of the grain size across the shore
(1992) showed that, in the case of obliquely incident (Hughes and Cowell, 1987). Larson and Sunamura
waves, longshore particle motion in itself will result (1993) studied the flow characteristics at the beach
in an additional offshore movement due to the bed step in the laboratory and found that supercritical flow
slope effect. Finally, the infiltration and exfiltration of conditions occur at the beach step during the final
water through the beach face affects the stability of phase of the run-down. A hydraulic jump occurs just
the sediment particle via changes in the effective seaward of the beach step and a strong backwash
weight and the bed shear stress (see Sections 2.4 vortex develops. Due to the backwash vortex, the flow
and 3.3). velocity near the bed is directed onshore before the
new wavefront arrives, making the transition between
3.2.2. Convective sediment transport high offshore/onshore velocities more gradual.
The importance of convective sediment transport is The presence of the backwash vortex at the beach
small compared to advective transport in cases where step leads to additional upward flow and the con-
the hydrodynamic conditions vary gradually in the vection of sediment into the upper parts of the water
flow direction. In sediment transport models for the column (Larson and Sunamura, 1993). For example,
surf zone, the convective transport is often neglected very high concentrations of suspended sediment
because that allows the solution of the transport have been observed in the field at the seaward limit
equation for suspended sediment in only one (vertical) of the swash zone (Hughes et al., 1997b), which is
direction, which is computationally much less time- subsequently advected into the swash zone during
consuming than the complete solution. In Deigaard et the following uprush. Consequently, due to the
al. (1986), the omission of the convective terms was locally strong cross-shore gradients in suspended
partly alleviated by adding the product of the time- sediment concentrations associated with the back-
averaged Lagrangian flow velocity and the time- wash vortex, convective sediment transport is impor-
averaged sediment concentration. tant in the swash zone. Neglecting it may lead to
However, in the swash zone, strong lateral gra- underestimation of suspended sediment transport
dients in sediment concentration occur due to the during run-up.
interaction between incident bores and the preceding
backwash. On natural beaches, it can often be 3.3. Effect of infiltration/exfiltration
observed that an incident bore is slowed down con-
siderably, or even moves offshore, if the flow velocity The interaction between the flow in the swash zone
in the backwash exceeds the speed of the bore (super- and the coastal water table has been analysed in several
critical flow conditions). The resulting flow separation studies (Turner and Nielsen, 1997; Turner and Masse-
under the bore is associated with strong vortices near link, 1998; Turner, 1995; Butt et al., 2001). Downward
the bed that lead to very high sediment concentrations and upward pressure gradients during run-up and run-
over the entire water column (Beach and Sternberg, down cause percolation of water through the beach
face. The effects of infiltration and exfiltration on for increasing sediment grain size, since the vertical
sediment transport in the swash zone can be summar- flow velocities and the resulting additional bed shear
ised as: (1) reduction of backwash volume and dura- stress become larger for coarser sediments, and a
tion, (2) increase and decrease of the effective weight relatively larger part of the sediment transport occurs
of sediment particles, and (3) increase and decrease of close to the bed.
the shear force on sediment particles.
The reduction in backwash volume and duration 3.4. Sediment grading
will lead to slightly lower mean and maximal flow
velocities during run-down. However, this effect is On natural beaches, a very sudden change in sedi-
expected to be of minor importance on sandy beaches ment characteristics can frequently be observed at the
as the vertical flux through the beach face is small seaward edge of the swash zone. The coarsest material
compared to the horizontal flux in the swash zone. is usually found around the lowest down rush level
However, on shingle and pebble beaches, this effect (Bascom, 1980; Hughes and Cowell, 1987), with the
may become important. The infiltration and exfiltra- deposition of coarse material at the beach step by
tion is also associated with additional pressure forces avalanching (Larson and Sunamura, 1993). However,
on sediment particles (see Section 2.4). Turner and at present, few models provide a good description of
Masselink (1998) showed that the critical Shields the cross-shore variation in sediment size, particularly
parameter may vary significantly due to the altered in the swash zone where the hydrodynamic and sedi-
effective weight. However, they found that the effect of ment transport processes have a strong sorting effect
the enhanced bed shear stress was more important than on the sediment particles (Holmes et al., 1996).
the altered effective weight and concluded that infiltra- Larson and Sunamura (1993) found that the backwash
tion/exfiltration processes support onshore sediment vortex plays a dominant role in this sorting mecha-
transport in the swash zone. nism, with finer fractions transported up the beach by
The infiltration during run-up is associated with a the wave run-up or transported into the surf zone
downward transfer of momentum from the outer flow during run-down.
into the wave boundary layer. The process is similar to
the mechanism of streaming in the turbulent boundary 3.5. Longshore sediment transport in the swash zone
layer under progressive waves as was investigated by
Brøker (1985) and Deigaard and Fredsøe (1989). Con- Most of the work related to sediment transport in the
ley and Inman (1994) made a careful analysis of flow swash zone deals with aspects of cross-shore sediment
and turbulence characteristics of ventilated boundary transport. Only little attention has been paid on the
layers in the laboratory. They found that turbulence in longshore part of the transport, even though hydro-
case of infiltration is confined to a compact layer close dynamic models for oblique wave run-up have existed
to the bed, whereas it is more evenly distributed across for quite a long time (Ryrie 1983; Chadwick 1991a,b;
the water column in case of exfiltration. The compres- Asano, 1994; Kobayashi and Karjadi, 1996). Kam-
sion of the boundary layer during infiltration leads to phuis (1991) performed laboratory experiments on
enhanced flow velocities and shear stress in the boun- sediment transport due to oblique wave attack and
dary layer, whereas the opposite occurs during exfiltra- found two peaks in the longshore transport rates; one in
tion. Conley and Inman (1994) derived an analytical the surf zone and one in the swash zone. Elfrink (1997)
expression for the change in mean bed shear stress performed laboratory experiments on oblique wave
during a half cycle that fits well with laboratory run-up and found a similar local maximum in the
measurements. Turner and Masselink (1998) per- time-averaged longshore flow velocities near the still
formed direct measurements of instantaneous infiltra- water line. The occurrence of the increased longshore
tion and exfiltration in the field. They applied the flow velocities in the swash zone is related to differ-
expression for altered bead shear stress in a sediment ences in fluid motion between the inner surf and swash
transport model and concluded that seepage forces zone. In the surf zone, the oscillatory part of the flow is
significantly increase onshore sediment transport in directed more or less perpendicular to the wave crests.
the swash zone. The effects are expected to increase In the swash zone, however, the flow direction during
run-up is perpendicular to the wave crest during run- or mechanisms of sediment transport, but simply
up, but perpendicular to the beach orientation during relate the immersed weight transport directly to a
run-down, not taking into account the mean longshore few parameters such as the free stream velocity, bed
current. This effect increases with increasing bed slope, sediment characteristics and bed friction factor.
slope, or rather, surf-similarity parameter. Therefore, However, the bed friction factor itself is difficult to
the importance of this additional longshore transport is determine accurately (Hughes, 1995) and recent work
greater during calm conditions than during storms, considering the influence of friction on the shoreline
when the bulk of the littoral transport occurs in the motion suggests that it varies over the swash cycle
surf zone. Nevertheless, longshore sediment transport (Puleo and Holland, 2001). In addition, a friction
in the swash zone may account for up to 50% of the factor appropriate to the run-up tip may not apply
total longshore transport (Kamphuis, 1991; Van within the swash lens since the boundary layer struc-
Wellen et al., 2000). It may additionally play a dom- ture varies with both space and time.
inant role in the longshore sediment transport and Typically, in order to find a reasonable agreement
shoreline evolution due to artificially generated waves between model and data, the energetics-based models
from, for example, fast ferries (Kirkegaard et al., above required a factor of 2 difference in efficiency
1999). factor between the uprush and backwash. The differ-
ences were attributed to the effects of infiltration/
exfiltration and to differences in the flow conditions
4. Models for sediment transport in the swash zone and suspended sediment concentrations at the onset of
the run-up and run-down. Consequently, given the
In this section, a number of modelling concepts likely importance of convective sediment transport
for sediment transport in the swash zone will be and cross-shore gradients in suspended sediment con-
reviewed and evaluated on the basis of the transport centration noted in Section 3.2.2, the validity of
mechanisms described above. The model concepts energetics-type models in the swash zone must be
show a large variation in the degree of detail at which questioned (Hughes et al., 1997b; Puleo et al., 2000).
different processes are resolved and in the computa- Very recently, Nielsen (2002) has proposed a modified
tional requirements. The available modelling con- Shields parameter for unsteady turbulent flow, which
cepts may be loosely categorised as: (1) empirical includes acceleration effects. This gives much higher
models, (2) simplified deterministic models, and (3) shoreward-directed bed shear stresses during the
detailed deterministic models. Evaluation of sediment uprush phase of the swash cycle than during the
transport models for the swash zone against measured backwash. Calculated transport rates using this param-
data is difficult. Field measurements usually only eter with a constant efficiency factor compare well
cover a small range of hydrodynamic and sediment with the data of Masselink and Hughes (1998),
transport conditions. Laboratory measurements have suggesting that differences in flow acceleration over
other restrictions such as scale effects and inability to the swash cycle are an important factor governing net
reproduce percolation and uniform conditions in case transport rates.
of oblique wave attack. Therefore, our attention is
focused on the different model concepts rather than 4.2. Simple deterministic models
on their respective ability to reproduce measured
data. These models are characterised by a more detailed
description of physical mechanisms, but still show a
4.1. Empirical models considerable degree of simplification and/or parame-
terisation. Luccio et al. (1998) performed careful
Several models have been developed on the basis laboratory experiments of the motion of large bodies,
of some form of the energetics model proposed by such as cobbles in the swash zone. A deterministic
Bagnold (1963, 1966) (e.g. Hughes et al., 1997b; model was developed using the analogy with a dam
Masselink and Hughes, 1998; Puleo et al., 2000). break problem that showed good agreement with
Energetics models do not resolve details of the flow measurements. The model describes the motion of
large bodies on an impermeable bed as a function of the longshore transport rates on a shingle beach. The model
bore height, and cobble characteristics. Kobayashi and also suggests that simple monotonic relationships
DeSilva (1987) and Kobayashi et al. (1988) developed between the swash sediment transport rate and wave
a model for hydrodynamics and bed load transport in parameters (e.g. height, period), typical of empirical
the swash zone. The hydrodynamics in their model surf zone transport models, are unlikely due to swash –
were calculated from the non-linear shallow water swash interactions. Swash – swash interactions may
equations, analogous to the model of Hibberd and also govern broader scale variations in the beach plan-
Peregrine (1979). The hydrodynamic forces acting on form via morphodynamic/hydrodynamic feedback, in
individual sediment particles were separated in drag, particular, the formation of beach cusps and shore-
lift and inertia forces and were calculated from the connected bars (Masselink et al., 1997; Coco et al.,
depth-averaged velocities. The model was able to 2000). However, the impact of these features on beach
explain observed erosional trends in laboratory experi- erosion and accretion remain to be determined.
ments but was found insufficient to predict beach
accretion. The model did not include the effects of 4.3. Deterministic models for future research
percolation, which was expected to be responsible for
the inability of the model to reproduce beach recovery. The sediment transport models described in the
Chadwick (1991a,b) adopted the model of Ryrie previous sections do include some aspects of a detailed
(1983), which is an extension of the work of Hibberd deterministic approach and have a certain validity
and Peregrine (1979) and Packwood (1983). This range. However, none of them seem to resolve all
model includes oblique wave attack by solving the potentially important details of the flow and sediment
longshore flow equations de-coupled from the cross- transport, such as the wave boundary layer, percola-
shore flow. Chadwick (1991a,b) applied the sediment tion, flow separation at the beach step and full 2D or
transport model of Mc Dowell (1989), which relates the 3D suspended sediment concentrations. For example,
transport of coarse particles, i.e. shingle/gravel, to the the models developed by Kobayashi and DeSilva
Manning number and the bed shear stress. Elfrink (1987), Kobayashi et al. (1988) and Elfrink (1997)
(1997) developed a model for longshore sediment have quite a detailed description of the outer flow, but
transport in the swash zone based on the 2D non-linear the effects of seepage are not included. The model of
shallow water equations, including the effects of the Turner and Masselink (1998) resolves the effects of
turbulent boundary layer as described in Fredsøe seepage, but does not resolve the outer flow and the
(1984) and a description of the surface roller. Sediment sediment transport in any detail. No models appear to
transport was calculated according to Engelund and include the effects of the backwash vortex on sediment
Fredsøe (1976), Deigaard et al. (1986) and Elfrink et al. transport in the swash zone.
(1996). Turner and Masselink (1998) presented a sedi- However, important future improvements may be
ment transport model for the swash zone and focused possible using Computational Fluid Dynamics models
mainly on the effects of infiltration/exfiltration. The (see Ferziger and Periæ, 1999 for a general review of
flow through the beach face was modelled by means of CFD techniques) that, so far, have mainly been
1D model for groundwater flow in the beach. The applied in the surf zone. The models are based on
model was calibrated upon field data and it was con- Navier – Stokes solvers, allowing the flow field to be
cluded that seepage through the beach face signifi- resolved in great detail. Based on this approach
cantly increases onshore sediment transport in the Christensen (1998) and Christensen and Deigaard
swash zone. More recently, Van Wellen et al. (2000) (2001) developed a large eddy simulation (LES)
presented a deterministic model for cross-shore and model that in principle will be able to resolve the
longshore transport in the swash zone, based upon dominant characteristics of hydrodynamics and sedi-
Shen and Meyer’s (1963) solution of the NLSWE ment transport in the swash zone. The model com-
and a Shields-type transport formulae. Longshore sedi- bines a Navier – Stokes solver with a free surface
ment transport predictions from the model compare model and a subgrid scale model (SGS). The large
well with laboratory data from Kamphuis (1991) and turbulent structures are simulated directly, while the
are consistent with field data of total (surf and swash) smaller turbulent scales are accounted for by the SGS-
model. However, although the swash zone was gravity frequencies, or bores, generally at sea/swell
included in the model, the resolution was too coarse frequencies. However, although the dominant mode of
to predict the swash zone mechanics in any detail. swash oscillation can be defined qualitatively in terms
Refining the grid in the swash zone may enable of no, the actual nature of the swash zone, as well as
further progress in this area, but at the expense of the absolute magnitude of swash oscillations in terms
increased computational time. of no, appears site- and surf zone-specific. Never-
Other work which has applied CFD techniques to theless, low-frequency motions typically dominate
the surf zone includes Lemos (1992), Lin and Liu swash run-up spectra due to high infragravity energy
(1998a,b) and Bradford (2000). In these instances, the levels in the surf zone, saturation and dissipation of
flow field was assumed to be two-dimensional, and a short wave energy in the surf zone and saturation of
Reynolds Averaged Navier –Stokes (RANS)-type tur- bore-driven swash oscillations at short wave frequen-
bulence model (e.g. k –e model) was applied. How- cies. Further work is required to quantify the relative
ever, again, the resolution was insufficient to model magnitude of the cross-shore motions discussed above
the wave boundary layer, with the focus on the and longshore swash motions forced by edge waves
dynamics taking place at the water surface and in and obliquely incident bores.
the surf zone roller. These models therefore have to be The gross characteristics of the internal flow kine-
improved significantly if they are to be used to study matics are also relatively well-understood, and are
the near-bed dynamics in the swash zone. More again dependent on the type of swash motion. Infra-
promisingly, a recent RANS-based model developed gravity swash motions tend to result in more sym-
by Christensen et al. (2000) was set up on a non- metrical flows relative to maximum run-up time than
orthogonal grid, thereby allowing for finer resolution bore-induced swash flows, where backwash durations
at the bed and enabling the study of the wave are typically greater than run-up durations. Diverging
boundary layer, and potentially the swash zone. flow and a rapidly thinning swash lens is therefore a
Unfortunately, all the RANS-based models pres- feature of bore-driven swash, but both infragravity
ently predict turbulence levels that are generally two to and higher frequency swash oscillations will fre-
three times larger than those measured in the surf zone. quently lead to supercritical flow in the backwash.
The first question is therefore whether a general Further work is, however, required to verify and
turbulence model, like the k – e model, can be used in test models for swash zone kinematics. In addition,
the surf zone without modification, since the large although potential sources of turbulence are well
turbulence levels, to some extent, destroy the solution. known, descriptions of turbulence in the swash zone,
Only then can an extension to the swash zone be and the development of suitable models has barely
realistically considered. started. Similarly, despite good conceptual models of
the influence of infiltration/exfiltration and beach
groundwater on the boundary layer and swash hydro-
5. Conclusions dynamics, quantitative measurements and modelling
of boundary layer processes are a prime requirement
Recent research has significantly increased both for future work. The micro-scale processes (turbu-
process knowledge and modelling ability in the swash lence/boundary layers) may be best investigated
zone. Boundary conditions at the seaward limit of the through laboratory and numerical work, combined
swash zone are diverse but well-understood, and with further investigations of macro-scale processes
given appropriate input conditions, can be predicted (cross-shore/longshore motions and internal kinema-
by a range of models. As a result, the main swash tics) from field sites with a diverse range of surf
hydrodynamic forcing mechanisms have been identi- zones, beach slopes and sediment.
fied, although some details remain open to question. Many important aspects of sediment transport in
Cross-shore swash or shoreline oscillations resulting the swash zone have been analysed and several good
from this forcing can be broadly divided into two conceptual models have been developed that include
different types, depending on whether the dominant one or more of these aspects. However, no model yet
forcing is due to standing waves, typically at infra- exists that includes all potentially important aspects,
such as bed slope, infiltration/exfiltration, inertial sent preprints of papers prior to publication and to
forces on coarse grains, bore-generated turbulence, Prof. T. Asano for his review comments.
flow separation near beach steps and sediment suspen-
sion and convection. Advanced CFD models are in
principle able to resolve the most important mecha-
nisms simultaneously. However, these models have References
mainly been applied in the surf zone and not yet in
the swash zone. The relative importance of the different Asano, T., 1994. Swash motion due to obliquely incident waves.
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Asano, T., 1996. Sediment transport in swash zone under obliquely
likely to be site-specific, complicating model verifica-
incident waves. Proc. 25th Int. Conf. on Coastal Eng., ASCE,
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Coastal Engineering 45 (2002) 149 – 167

Hydrodynamics and sediment transport in the swash zone:

1. Introduction since it results from the small difference between two

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