Barred beaches - Universiteit Twente · 2020. 4. 6. · Barred beaches Kathelijne M. Wijnberg*,...

Barred beaches

Kathelijne M. Wijnberg*, Aart Kroon

Department of Physical Geography, Institute for Marine and Atmospheric Research, Utrecht University, P.O. Box 80115,

3508 TC Utrecht, The Netherlands

Received 23 November 1999; received in revised form 23 June 2000; accepted 24 January 2002

Abstract

Seven different bar types are distinguished to provide a framework for comparing morphodynamic studies conducted in

different areas. Five types occur in semiprotected or open coast settings, of which two are intertidal and three are subtidal. Two

types occur in highly protected settings. The occurrence of a certain bar type is generally determined by the wave energy and tidal

range, although the nearshore slope may also be a differentiating boundary condition. The theory behind the generation, evolution

and decay of bars has evolved most for the subtidal bars in the semiprotected and open coast settings, for which three types of

competing mechanisms have been formulated (breakpoint, infragravity waves, self-organisational). Most research has focused on

these processes on the time scale of storm events and post-storm recovery. However, to understand the longer-term behavior of bar

systems, knowledge of the role of relaxation time andmorphologic feedback is needed as well. At present, such knowledge is very

limited. We think it can best be obtained from the analysis of long time series of morphology and forcing conditions, rather than

from intensive field experiments. In case of a feedback-dominated response (self-organisational), we expect to find no correlation

between the time series of external forcing and the morphologic response. In case of a relaxation time-dominated response, we do

expect to find such a correlation, albeit filtered. This discussion is illustrated by a case study of the Dutch coast.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Coastal morphology; Nearshore bar; Feedback; Relaxation time

1. Introduction

Sandy beaches are among the most widely studied

types of shore in process-oriented coastal geomorpho-

logic research. Nearshore bars in both the intertidal

and subtidal domains are common features on these

types of coast (Table 1). Nearshore bars exist under a

wide range of hydrodynamic regimes, ranging from

microtidal to macrotidal and from swell-dominated to

storm-dominated wave environments. All nearshore

bars are affected by the flow field induced by the

incident waves. Because of their close relationship

with wave-induced flow fields, nearshore bars are also

referred to as wave-formed bars (Greenwood and

Davidson-Arnott, 1979). The incident waves shoal,

break, reflect, and refract on the nearshore topogra-

phy, generating complex flow fields, including asym-

metric oscillatory flow, undertow, horizontal cell

circulation, edge waves, and swash–backwash mo-

tion. The local tidal range and the wave climate de-

termine the frequency and intensity with which

nearshore bars along individual beaches are affected

by the various types of flow field.

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0169 -555X(02 )00177 -0

* Corresponding author. Fax: +31-30-2540604.

E-mail addresses: [email protected] (K.M. Wijnberg),

[email protected] (A. Kroon).

www.elsevier.com/locate/geomorph

Geomorphology 48 (2002) 103–120

Notwithstanding their common occurrence and the

considerable existing research on these features, near-

shore bars are still poorly understood. The prime

concern of this paper is to review the present knowl-

edge of wave-formed bars using the framework of the

morphodynamic system approach. This approach

helps to highlight an area of research that is poten-

tially fruitful for deepening our understanding of

nearshore bars and their behavior.

A nearshore morphodynamic system consists of

three components: the water motion component, the

sediment transport component, and the morphology

component (Fig. 1). Energy input into the nearshore

morphodynamic system, through waves, winds, and

tides, induces a flow field in the nearshore zone that is

modulated by the nearshore morphology. This flow

field induces sediment transport, which will result in

bathymetric change if transport gradients exist. This

modification of the morphology will change the

modulation of the incoming energy, and consequently,

the nearshore flow field. The boundary conditions for

the nearshore morphodynamic system are imposed by

the long-term evolution of the coast on the one hand,

through the composition of the substrate and the

geometric setting (offshore and nearshore slope,

shoreline orientation), and, on the other hand, by the

geographic location and geometry of the sea or ocean

basin (prevailing weather systems, fetch, tidal wave

characteristics).

From the above perspective, it is apparent that the

nearshore system can be studied on two levels. On

the most aggregated level, the focus is on the general

expression of the morphodynamic system as a func-

tion of a set of boundary conditions, i.e., the aim is

to identify different bar types with different behavior

in relation to environmental parameters (cf. bar type

X with dynamics Y only occurs on coasts with tidal

range A and nearshore slope B). On the more detailed

level, the morphodynamic relationships in the system

itself are studied for each specific bar type. Of

course, the physics of water motion and sediment

transport is the same in every nearshore zone, but

Table 1

Examples of locations of nearshore bar observation

Geographic location References

Atlantic Ocean

(USA, Europe)

Strahler, 1966; Sonu and van Beek, 1971; Sonu, 1973; Nordstrom, 1980;

Sallenger et al., 1985; Froidefond et al., 1990; Larson and Kraus, 1992; Liu and Zarillo, 1993

Baltic Sea Pruszak et al., 1997

Beaufort Sea Short, 1975

Black Sea Zenkovich, 1967

Chukchi Sea Short, 1975

Gulf of St. Lawrence Owens and Frobel, 1977; Greenwood and Mittler, 1984

Gulf of Mexico Davis and Fox, 1975

Indian Ocean (Australia) Wright and Short, 1984

Lake Michigan Davis et al., 1972; Weishar and Wood, 1983

Mediterranean King and Williams, 1949; Bowman and Goldsmith, 1983; Guillen en Palanques, 1993;

Barusseau et al., 1994; Aminti et al., 1996

North Sea King and Barnes, 1964; Aagaard, 1990; Ruessink and Kroon, 1994; Wijnberg and Terwindt, 1995;

Dette et al., 1996; Simmonds et al., 1997

Pacific Ocean (USA, Japan) Shepard, 1950; Hunter et al., 1979; Sunamura, 1988

Sea of Japan Sunamura, 1988

Tasman Sea Chapell and Eliot, 1979; Wright and Short, 1984

Fig. 1. Conceptual diagram of nearshore morphodynamic system of

a sandy coastal system. WM=water motion, ST= sediment trans-

port, M=morphology.

K.M. Wijnberg, A. Kroon / Geomorphology 48 (2002) 103–120104

apparently different terms in the nonlinear equations

describing flow–topography interaction and sediment

transport are dominant for different sets of boundary

conditions.

In this paper, barred beaches are first addressed on

the most aggregated level. An overview is presented

of the full range of nearshore bar types reported in the

literature. Morphologic characteristics and environ-

mental setting of the various bar types are summar-

ised. Next, nearshore bars are addressed on the more

detailed level of the morphodynamic relationships,

focusing on the most widely studied types of near-

shore bar, namely, the bars on semiprotected to fully

open coasts. These relationships relate to time scales

on which bars have been observed to respond, that is,

the time scale of a storm event and the post-storm

recovery phase. This is followed by a discussion of

the dynamics of nearshore bars on longer time scales,

which result from the integrated effect of many storm-

recovery sequences. Issues discussed are the effects of

relaxation time and morphologic feedback. This dis-

cussion will be illustrated by a case study of the Dutch

coast.

2. Types of wave-formed nearshore bars

Nearshore bars are dynamic features which may

change their plan view shape, amplitude or cross-

shore position in response to changing wave condi-

tions. This results in what is apparently a continuum

of bar morphologies. For example, the plan view

shape of bars may be linear, crescentic, or undulating

in a more irregular manner (Figs. 2, 3, and 4). The

orientation of nearshore bars varies from shore-paral-

lel to shore-perpendicular. In addition, nearshore bars

can occur in a cross-shore ordered series of essentially

shore-parallel bars, in which case the set of bars is

referred to as a multiple or multi-bar system.

It is not a trivial task to set up a classification

scheme to cover the wide variety of nearshore bar

shapes. A classification of nearshore bars in terms of

distinctive bar types should provide a framework for

making sensible comparisons between morphody-

namic studies conducted in different areas. Generally,

criteria used to classify nearshore bars are based on

morphological and locational characteristics, such as

plan view shape and cross-shore position in the

nearshore zone (e.g., Greenwood and Davidson-

Arnott, 1979; Chapell and Eliot, 1979; Wright and

Short, 1984; Lippmann and Holman, 1990). These

criteria are implicitly or explicitly assumed to reflect

the processes that control the bar behavior. At present,

no worldwide applicable, universal classification of

wave-formed nearshore bars is available because there

is still an ongoing debate on the mechanisms that

govern nearshore bar formation and development.

The vast majority of studies on nearshore bars deal

with bars on semiprotected and open coast beaches,

i.e., beaches facing oceans, seas, or vast inland lakes

like the Great Lakes in North America. Some of these

beaches may be sheltered from large waves from

certain directions, but all are exposed to moderate to

high wave conditions from at least one directional

sector. A limited number of studies report on wave-

formed nearshore sand bars in highly protected set-

tings such as back-barrier lagoons, estuaries, and

sheltered embayments like Cape Cod Bay (Nilsson,

1972). These differences in environmental setting

seem to warrant a distinction between bars formed

on semiprotected and open coast beaches and bars

formed in highly protected settings.

Because of the bias in the literature, this paper will

pay attention mainly to bars on semiprotected and

open coast beaches. A classification of bars in this

type of setting is presented in the next section. Based

on only scarce literature, two types of bars are

described for the highly protected setting.

2.1. Bars on semiprotected and open coasts

We use locational characteristics as the main attrib-

ute for describing bar types in the semiprotected and

open coast setting, and morphology as the subordinate

attribute (Table 2). The positioning of a bar in the

nearshore strongly reflects water depth characteristics,

which in turn reflects a typical flow field regime for

that location. On the highest level, intertidal bars are

distinguished from subtidal bars. Bars in the intertidal

zone are affected by swash processes and currents

induced by the filling and emptying of trough features

as the tide rises and falls. These processes are absent

in the subtidal zone. In the subtidal zone, the flow

field induced by the shoaling and breaking of waves is

assumed to be dominant. Tides and winds are

assumed to act only as modifiers of this flow field,

K.M. Wijnberg, A. Kroon / Geomorphology 48 (2002) 103–120 105

e.g., through changes in the water level (tidal cycle,

wind set-up/set-down), through adding additional

longshore current vectors, or through the effect of

winds on wave breaking. On a more refined level,

bars will experience a range of hydrodynamic pro-

cesses that depend on tidal range and wave climate.

For instance, a bar located in the lower intertidal zone

of a macrotidal beach will experience both intertidal

and subtidal processes. Bar morphology may then

express the flow field mode that dominates in a given

situation.

2.1.1. Morphologic characteristics and environmental

setting of intertidal bars

Two morphological subtypes of intertidal near-

shore bars can be distinguished: ‘‘slip-face ridges’’

and ‘‘low-amplitude ridges.’’ We abandoned more

commonly used terminology, such as ‘‘swash bar’’

or ‘‘ridge-and-runnel topography,’’ because various

authors applied that terminology with different inter-

pretations (see Orford and Wright, 1978).

Slip-face ridges are intertidal bars with a well-

defined, landward-facing slip-face. They include the

Fig. 2. Time exposures pictures of the surf zone near Noordwijk, The Netherlands (September 9, 1995); the white bands are preferred locations

of wave breaking and as such reflect the plan shape pattern of the underlying bar topography: (a) oblique image, (b) rectified image; tickmark

spacing is 250 m.


Group II bars defined by Greenwood and Davidson-

Arnott (1979), which were named ‘‘swash bars’’ by

Carter (1988). In addition, they include the bar type

occurring in the ‘‘Ridge-and-Runnel/Low-Tide-Ter-

race’’ beach state defined by Wright and Short

(1984). The typical cross-shore scale of slip-face

ridges is of order 101 m. The bar height, defined as

the elevation difference between the crest of the ridge

and the trough area landward of it (also called runnel),

is of the order of tens of centimeters to well over a

meter. Slip-face ridges are elongated features that tend

to line up with the general shoreline configuration.

However, these intertidal ridges may also be shore-

oblique, spit-shaped features (Fig. 5). Slip-face ridges

are often cut by small rip channels, which drain the

shallow trough landward of it. The spacing of these

drainage rips may vary from several tens of meters to

a couple of hundred meters (Short, 1985). Slip-face

ridges may migrate onshore at rates of order 100 m

day � 1 (Owens and Frobel, 1977; Kroon, 1994).

During appropriate conditions, the ridge will become

stranded at the high tide mark where they form a

‘berm’. Slip-face ridges occur on mild slopes of order

0.01–0.03 generally under microtidal to mesotidal

regimes (King and Williams, 1949; Davis et al.,

1972).

Fig. 3. Time exposures pictures of the surf zone of Palm Beach (NSW), Australia (March 17, 1996); the white bands are preferred locations of

wave breaking and as such reflect the plan shape pattern of the underlying bar topography: (a) oblique image, (b) rectified image; tickmark

spacing is 80 m.


Low-amplitude ridges are intertidal bar features

without a slip-face. They generally occur in a cross-

shore sequence of shore-parallel ridges that align with

the general shoreline planform. Many ridges remain

apparently static and only respond to movements of

the still water level as they migrate with spring–neap

tidal movement (Orford and Wright, 1978). This type

of intertidal bar morphology is the same as the ridge-

and-runnel topography described by King and Wil-

liams (1949). It also includes the ‘‘Group II’’ bars

defined by Short (1991) for intermediate wave energy,

meso- and macrotidal beaches, as well as the Group I

bars defined by Greenwood and Davidson-Arnott

(1979). In cross-section, the ridges may be asymmet-

ric in landward direction (Greenwood and Davidson-

Arnott, 1979; Short, 1991). Their height is at most a

few tens of centimeters (Short, 1991) and their cross-

shore width is at most a few tens of meters. They

occur on lower angle slopes than those associated

with slip-face ridges, with beach gradients of about

0.005–0.01. Generally, the smaller beach slopes are

related to a larger tidal range, such that low-amplitude

Fig. 4. Time exposures pictures of the surf zone of Agate Beach (Oregon), USA (April 24, 1994); the white bands are preferred locations of

wave breaking and as such reflect the plan shape pattern of the underlying bar topography: (a) oblique image, (b) rectified image; tickmark

spacing is 250 m.


ridges may be expected under meso- to macrotidal

conditions. In addition, the formation of these ridges

tends to be restricted to sea environments, as opposed

to swell (Short, 1991). Note that low angle slopes may

also occur under microtidal conditions in low wave

energy environments (see Section 2.2).

2.1.2. Morphologic characteristics and environmental

setting of subtidal bars

Subtidal nearshore bars are found both in single-

and multi-bar settings, the latter typically consisting of

two to four bars (Greenwood and Davidson-Arnott,

1979). Subtidal bars are found on nearshore slopes

with gradients of order 0.005–0.03 (Short and

Aagaard, 1993; Ruessink and Kroon, 1994). The

gentler nearshore slopes seem to favor a larger num-

ber of bars, although there is no unique relation

between these two factors (Davidson-Arnott, 1987;

Short and Aagaard, 1993; Wijnberg, 1995). The cross-

shore spacing between bars in multi-bar systems is of

order 101–102 m and increases in the offshore direc-

tion. The height of this bar type is of order 10� 1–100

m. For example, bar heights of up to 3 m have been

reported for the Terschelling coast, The Netherlands

(Ruessink and Kroon, 1994). Bar height tends to

increase with distance offshore (Komar, 1998). How-

ever, in multi-bar systems, this trend often reverses at

some distance offshore (Lippmann et al., 1993; Rues-

sink and Kroon, 1994; Wijnberg and Terwindt, 1995;

Pruszak et al., 1997).

Subtidal nearshore bars may migrate both onshore

and offshore. Near Duck, NC (USA), offshore migra-

tion rates of up to 2.2 m h� 1 were observed (Sal-

lenger et al., 1985). Onshore migration rates are

generally slower, namely, of order 10 � 1 m h � 1

(Sunamura and Takeda, 1984). However, onshore

migration rates may locally be as large as 1 m h� 1

when related to the development of crescentic top-

ography (Sallenger et al., 1985).

Subtidal nearshore bars are most commonly

studied in nontidal to microtidal environments, but

Table 2

Classification of nearshore bars on sandy coasts with indication of environmental setting

Fig. 5. Intertidal slip-face ridges near Noordwijk, The Netherlands:

(a) shore-oblique orientation (May 26, 1995), (b) shore-parallel

orientation (December 28, 1997).


their occurrence is not limited to these settings (e.g.,

Davidson et al., 1997). The influence of tidal range on

subtidal bars is not clear. Wright et al. (1987) suggest

that a larger tidal range may be associated with more

subdued bar–trough topography. However, their con-

clusion is based on variation of bar topography over

spring–neap cycles on a microtidal coast.

The morphological appearance of a subtidal near-

shore bar is highly variable. Based on plan view

characteristics, we distinguish three families of bar

morphology: two-dimensional longshore bars, three-

dimensional longshore bars, and shore-attached bars.

A two-dimensional longshore bar is a uniform,

straight bar oriented parallel to the shoreline. It

resembles the bar morphology in the so-called Long-

shore-Bar-and-Trough state defined by Wright and

Short (1984).

A three-dimensional longshore bar is any non-

straight bar where the onshore protruding parts do

not attach to the shore; that is, these parts are

separated from the beach by a distinct trough feature.

Longshore variability may range from regular cres-

centic patterns, which may be skewed (cf. Rhythmic-

Bar-and-Beach state defined by Wright and Short,

1984), to highly irregular and complex patterns (cf.

‘‘non-rhythmic 3D bar’’ state defined by Lippmann

and Holman, 1990). Length scales of longshore var-

iability are of order 102–103 m, where larger scales

tend to be related to bars located farther offshore (e.g.,

Sonu, 1973; Bowman and Goldsmith, 1983). Due to

the limited understanding of the mechanisms control-

ling the wide variety of three-dimensional patterns, we

could not justify a distinction between different types

of three-dimensionality.

The shore-attached bars are, as expressed by their

name, bar features that are attached to the shore.

Generally, they are located just below the low-tide

water level. Their morphology is to a large extent

defined by well-developed rip and feeder channels

(see Fig. 3b, a shore-attached bar is present in the

forefront of the oblique image and on the left side in

the rectified image). The rip channels are oriented

perpendicularly or obliquely to the shore (cf. ‘‘Trans-

verse-Bar-and-Rip’’ state defined by Wright and

Short, 1984; and the bar–rip systems described by

Hunter et al., 1979). In these systems, bars are almost

parallel to the shore, but they attach to the shore at one

end and terminate by a rip channel at the other end. A

measure for the longshore length scales of the shore-

attached bars is rip spacing, which is generally of

order 101–102 m (e.g. Hunter et al., 1979; Wright and

Short, 1984). It should be noted that the place where

the bar attaches to the shore may partially emerge at

low tide, inducing a low-tide shoreline with a some-

what cuspate appearance (Hunter et al., 1979).

The three bar types described above largely resem-

ble three of the states of the Wright and Short (1984)

bar-beach state model. Wright and Short found a

correlation between the occurrence of these states

and the value of the Dean parameter X (X =Hb/xsT,

with Hb = breaker height [m], xs = sediment fall veloc-

ity [m s � 1], T =wave period [s]). Therefore, the

occurrence of our three types of plan shape morphol-

ogy should probably correlate with X, too. If so, two-

dimensional longshore bars would tend to be related

to higher values of X than three-dimensional long-

shore bars, which should be related to higher X values

than shore-attached bars. Since the value of X is most

sensitive to breaker height (Short and Aagaard, 1993),

the above sequence can also be interpreted in terms of

decreasing levels of wave energy (Table 2). For

Australian single-bar beaches, distinct X-threshold

values were determined, but the universal validity of

these threshold values is still in question (Short and

Aagaard, 1993).

2.2. Bars on highly protected coasts

The nearshore bars observed in back-barrier

lagoons, estuaries, and sheltered embayments (low

wave energy environments) only occur on gentle

nearshore slopes (Table 2). Two types of bars may

occur: ‘multiple parallel bars’ and ‘transverse bars’

(Greenwood and Davidson-Arnott, 1979). To prevent

confusion with terminology often applied in open

coast bar classification, we refer to the latter bar type

as ‘transverse finger bars’ (after Niederoda and Tan-

ner, 1970).

In plan view, multiple parallel bars are straight to

undulating features that are often oriented parallel to

the shoreline, although they also may occur at an

angle with the shoreline (Nilsson, 1972). The multiple

parallel bars are spaced more or less equidistantly,

with a spacing in the order of 101 m and a longshore

length scale of 100 m (Zenkovitch, 1967; Nilsson,

1972; Greenwood and Davidson-Arnott, 1979). The


number of bars is on the order of 4–10 (Greenwood

and Davidson-Arnott, 1979), but it may be as large as

30 (Nilsson, 1972). In cross-section, this bar type is

generally symmetric, and the height of the ridges, on

the order of 10 � 1 m, is nearly uniform offshore

(Greenwood and Davidson-Arnott, 1979). Some

uncertainty exists about the characteristic tidal range

for this bar type. Greenwood and Davidson-Arnott

(1979) mention a small tidal range as the characteristic

environmental setting, whereas Nilsson (1972) states

that a large tidal range is required. More recently,

Short (1991) observed that multiple low-amplitude

bars that occur on wide, flat intertidal slopes of

macrotidal, low wave energy beaches (‘Group III’

bars) are morphologically similar to the multiple

parallel bars that occur subtidally on low-energy

microtidal beaches. No indication is given whether

these bars are also genetically similar.

The transverse finger bars are linear features

oriented perpendicular to or at a high angle to the

shoreline. They have a typical length scale of 102 m,

where very long bars tend to have forked ends at the

seaward side (Niederoda and Tanner, 1970). The

longshore spacing of these transverse bars ranges

from 101 to 102 m, with bar heights on the order of

10� 1 m (Greenwood and Davidson-Arnott, 1979).

Prerequisites for the occurrence of transverse finger

bars are a small tidal range, small nearshore slopes,

and the low wave energy (Niederoda, 1972; Green-

wood and Davidson-Arnott, 1979).

3. Morphodynamics of intertidal ridges

3.1. Generation of intertidal ridges

The formation of intertidal slip-face ridges is

associated with storm activity (e.g., Davis et al.,

1972). During the storm, the beach erodes, becomes

planar or concave, and the sediment is deposited in the

low-tide area. In the days following a storm, an initial

ridge feature is formed. The exact mechanism for the

initial ridge formation is not known, but swash and

backwash processes seem to play an important role in

combination with the temporary still stand of the

water level around low tide (Kroon, 1994). The land-

ward-directed swash velocity near the bed exceeds the

seaward-directed backwash velocity due to the wave

asymmetry effect. Part of the volume of the swash

may infiltrate into the beach and thus does not flow

seawards as backwash. On beaches with a gentle to

moderate slope, these differences between swash and

backwash transport capacities may result in initial

ridge formation near the water level. On steeper

beaches, this process leads to the direct development

of a berm (e.g., Sonu and van Beek, 1971). Other

studies suggest that these ridges are formed as bars in

the subtidal zone and migrate onshore into the inter-

tidal zone (e.g., Davis et al., 1972; Aagaard et al.,

1998).

The development of an initial ridge feature occurs

over several tidal cycles. At first, the dominant pro-

cesses are still swash and backwash over the crest. At

some point, the height of the ridge is such that the

swash water that overtops the crest is trapped in the

depression landward of the ridge and no longer flows

back to sea as backwash. The water in the depressed

area eventually flows back to sea through small rip

channels. At this stage, the ridge develops a landward-

facing slip-face. An incompletely eroded ridge feature

on the middle or upper intertidal beach (see Section

3.2) may serve as an ‘initial ridge feature’ away from

the low-tide area (Kroon, 1994). Therefore, the build

up of a slip-face ridge may also be observed away

from the low-tide area.

The generation of the multiple low-amplitude

ridges has not been satisfactorily explained. King

and Williams (1949) suggested that the formation of

these bars could be related to the local process of

beach gradient adjustment at temporary still stands of

the water level such as those that occur during low

and high, spring or neap tides. Simmonds et al. (1997)

argue that this is an unlikely mechanism because of

the large variation in the number of ridges observed

on natural beaches. They suggest that the multiple bar

formation could be enforced by cross-shore standing

long waves.

3.2. Evolution of intertidal ridges

Once an initial intertidal slip-face ridge is present,

and provided the wave-energy level remains low to

moderate, the slip-face ridge will migrate onshore or

stabilise (e.g., Davis et al., 1972; Mulrennan, 1992;

Kroon, 1994). Migration of the ridge in the landward

direction occurs as long as the swash can overtop the


ridge crest. Sedimentary structures show that this

onshore migration is a coherent progradation of the

complete slip-face (Dabrio and Polo, 1981). Stabilisa-

tion of the ridge feature may occur when the elevation

of successive maximum high water levels decreases,

which happens when moving from spring tide to neap

tide conditions (Kroon, 1994). This implies that over-

topping of the bar crest by the swash ceases, but

swash and backwash processes are still active on the

seaward slope of the bar. This may result in an overall

increase in elevation of the feature due to accretion on

the seaward side of the ridge and to steepening of the

foreshore. This resembles the beach face development

during accretionary conditions (e.g., Sonu, 1973).

Little is known about the evolution of low-ampli-

tude ridges, other than that they seem to migrate with

spring–neap tidal movement (Orford and Wright,

1978).

3.3. Decay of intertidal ridges

The decay of an intertidal slip-face ridge is related

to high-energy wave conditions (Davis et al., 1972;

Owens and Frobel, 1977; Kroon, 1994). With an

increase in wave energy, the wave set-up becomes

larger and the higher energy conditions are often

accompanied by an additional wind set up. This

implies that processes like undertow may temporarily

become dominant over the intertidal beach, especially

during high tide, which apparently results in a flat-

tening of the beach (Kroon, 1994). However, no exact

mechanism is given that explains the divergence of

transport over the ridge crests.

The decay of low-amplitude ridges is also related

to high-energy wave conditions (e.g., King and Wil-

liams, 1949), but knowledge of the exact mechanism

is lacking.

4. Morphodynamics of subtidal bars

4.1. Generation of subtidal nearshore bars

Over the past decades, a variety of mechanisms

have been proposed to explain the generation of

subtidal nearshore bars. These mechanisms can be

placed into three general groups: breakpoint-related

mechanisms, infragravity wave-related mechanisms,

and self-organisational mechanisms. The first two

types of mechanisms have been most widely explored.

Although fundamental differences exist between these

two mechanisms, both start off from the same concept

that bars form because a template in the flow field is

imprinted on the seabed. The self-organisational

mechanisms, on the contrary, are based on the concept

that the formation of bars occurs completely due to

interaction between nonlinear flow and topography,

without any predefined template in the flow field. So,

starting from a barless and completely smooth profile,

self-organisational mechanisms cannot form bars

while the other mechanisms can. Note that in reality

there will never be a lack of perturbations on a

nearshore profile.

Breakpoint-related mechanisms predict longshore

bar formation (two-dimensional) where short period

incident waves (sea and swell) initially break, that is,

at the breakpoint. All proposed concepts suggest that

bar formation depends in some way on gradients of

processes occurring in the vicinity of the breakpoint

(e.g., King and Williams, 1949; Dyhr-Nielsen and

Sørensen, 1970; Dally and Dean, 1984; Stive, 1986;

Sallenger and Howd, 1989). These gradients may

relate to a single process (e.g., scouring by the action

of plunging breakers) as well as to the transport

processes induced by a composite flow field (e.g.,

cross-shore gradients induced by the combined effect

of undertow and wave asymmetry). The latter

approach has even been extended to include the effect

of long period waves on the nearshore flow field

(Roelvink and Stive, 1989). That study suggested that

two-dimensional bar formation could be related to the

transition of horizontally asymmetric, groupy, non-

breaking waves to vertically asymmetric, surf-beat

modulated breaking waves. Therefore, this hybrid

model is still considered an exponent of bar formation

near the breakpoint. According to the breakpoint

concept, multiple subtidal nearshore bars should form

due to multiple breakpoints related to the reformation

of broken waves. Flume tests showed evidence for

breakpoint mechanisms (Dally, 1987). In the past,

field observations have generally not been conclusive

(Sallenger and Howd, 1989), but more recent obser-

vations are suggestive of bar formation by the break-

point mechanism (Aagaard et al., 1998).

Infragravity wave-related mechanisms have the

potential to produce any of the three types of subtidal


bar. Infragravity waves are long period waves (20–

300 s) and their existence is usually attributed to the

natural occurring groupiness of the incident, short

period waves (e.g., List, 1991; Roelvink, 1993; Rues-

sink, 1998). The long wave mechanism that has been

explored most widely, because it was suggested first,

is bar formation due to a second-order drift velocity

associated with standing long waves (Carter et al.,

1973). This mechanism predicts that two-dimensional

bars form under the nodal or antinodal location of

cross-shore standing infragravity waves (leaky waves)

(e.g., Short, 1975) or longshore progressive edge

waves (Bowen, 1980). Three-dimensional patterns

and shore-attached bars may form due the longshore

standing edge wave structures (Bowen and Inman,

1971; Holman and Bowen, 1982). The formation of

multiple subtidal nearshore bars is attributed to the

presence of multiple nodes or antinodes in the cross-

shore direction, related to the presence of leaky waves

or higher mode edge waves. The existence of low-

frequency water motions in the surf zone has been

confirmed on many occasions. However, although

some studies are suggestive of the correctness of the

edge wave mechanism (Bauer and Greenwood, 1990),

no firm evidence has been found for the actual forcing

of subtidal nearshore bars by this mechanism (Holman

and Sallenger, 1993).

A weak point of the second-order drift velocity

concept is the requirement of a single dominant

frequency, which is usually lacking in barred surf

zones. This problem does not exist for a more recently

proposed long wave mechanism, which relates two-

dimensional bar formation to the phase coupling

between the primary orbital motion of partially stand-

ing long waves and groupy short waves (O’Hare and

Huntley, 1994). Theoretically, the mechanism gener-

ates bars for a broad-band spectrum of long waves (as

usually observed in nature), especially when break-

point-forced long waves dominate over bound long

waves (O’Hare and Huntley, 1994). The present

literature, however, does not indicate this to be a

likely condition (Ruessink, 1998).

The conclusion of a lack of a single dominant

frequency in natural surf zones is often derived from

wave run-up spectra measured at the shoreline

(Bowen, 1997). However, theory shows that edge

waves may also be trapped and amplified on long-

shore currents (such that wave energy spectra offshore

will differ from those measured at the shoreline) and

form bars (Howd et al., 1992). Once formed, bars may

trap edge waves themselves (Bryan and Bowen,

1998). However, under realistic conditions concerning

the shape of the longshore current profile and the

presence of tidal modulations of the sea level, this

mechanism is unlikely to produce significant morpho-

logical change unless the current is very strong (Bryan

and Bowen, 1998).

Self-organisational mechanisms encompass a

range of mechanisms that are intended to be capable

of producing all three types of observed nearshore bar

patterns. Mechanisms proposed include nonlinear

interaction between the wave-driven longshore current

and the bed (Hino, 1974; Damgaard Christensen et al.,

1994; Falques et al., 1996) and the nonlinear inter-

action between shoaling, incident waves and the bed

(Boczar-Karakiewicz and Davidson-Arnott, 1987).

The self-organisational mechanisms are attractive

because they exploit the fact that the ubiquitous non-

linearities in sediment transport processes and surf

zone hydrodynamics have a large potential to induce

self-organised behavior. This approach implies that

the influence of the morphology on the local flow

field overwhelms the effect of preexisting structures in

the external hydrodynamic input (Fig. 1). This con-

cept may explain how different bar morphologies can

develop along a given coastline with virtually the

same overall characteristics concerning sediment,

nearshore slope, and wave conditions (e.g., Bowman

and Goldsmith, 1983; Wijnberg, 1995). However, part

of the bar characteristics predicted by the currently

existing nonlinear feedback models are not in line

with observations, such as the orientation of bars

relative to the longshore current (Damgaard Christen-

sen et al., 1994), or the scale of cross-shore spacing of

bars (Hulscher, 1996).

To conclude, no firm field evidence has been found

that justifies the selection of any of the individual

mechanisms as the nearshore bar-generating mecha-

nism. This may indicate that possibly all elements of

the flow field act in concert to form bars, rather than

having one dominant process. For instance, an ele-

ment of the nearshore flow field not mentioned so far

is the oscillatory motion in the far infragravity range,

having its origin, for example, in shear instabilities in

the longshore current (e.g., Bowen and Holman, 1989;

Dodd et al., 1992). These far infragravity motions can


contribute significantly to sediment transport in the

cross-shore direction and thus to the evolution of the

nearshore profile (Aagaard and Greenwood, 1995). It

is also possible that different mechanisms predomi-

nate in different settings (Aagaard et al., 1998). If so,

insight into such setting-dependent forcing mecha-

nisms could provide a useful basis for setting up an

improved bar classification.

4.2. Evolution of subtidal nearshore bars

The majority of proposed bar-generation mecha-

nisms form bars in the surf zone or at the outer limit of

the surf zone, often during fairly energetic conditions

(associated with the ‘dissipative assemblage’ of

Wright and Short, 1984). In nature, these energetic

conditions are generally of limited duration. When

wave energy level drops, bars may migrate onshore

and merge with the beach if wave energy drops

sufficiently (e.g., Wright and Short, 1984), they may

decay in situ (Larson and Kraus, 1992), or they may

get arrested when wave energy drops dramatically

(Short and Aagaard, 1993). Except for the latter case,

the final stage is an unbarred berm profile (associated

with the ‘reflective assemblage’ of Wright and Short,

1984).

In the case of onshore migration under falling wave

energy levels, bars tend to change in type, from the

two-dimensional longshore bar type to the three-

dimensional longshore bar type, to the shore-attached

bar type (e.g., Wright and Short, 1984; Lippmann and

Holman, 1990). Often, wave energy levels do not

drop low enough, or for long enough, to complete this

full sequence. As a consequence, subtidal bars

become permanent features that tend to shift between

two or all three morphologic subtypes.

The debate on the mechanisms that cause the

transitions between morphologic subtypes is as unde-

cided as that on generation mechanisms, as they tend

to be taken as equivalent. The flow template mecha-

nisms presume a dominant influence of the hydro-

dynamic input on the local flow field. This implies

that the variation in the external input will dominate

the influence of the morphology on the local flow

field, and hence, on the transport gradients (Fig. 1).

Therefore, according to the template mechanisms, a

change in bar type should be explicitly related to a

particular type of hydrodynamic input. For example,

Bauer and Greenwood (1990) related the development

from a two-dimensional bar to a three-dimensional bar

to forcing by an observed standing edge wave.

For self-organisational mechanisms, the dominance

of flow–topography feedback is self-evident because

it is the underlying concept for this mechanism. This

concept implies that the morphologic feedback effect

overwhelms the effect of the variation in the energy

input (Fig. 1). That is, energy input into the nearshore

system is needed to move the sediment, but the pattern

of transport gradients is determined by the preexisting

morphology. For example, longshore currents along a

two-dimensional bar may preferentially amplify irreg-

ularities in the crestline with specific wave lengths

(Deigaard et al., 1999).

It should be noted that the nearshore morphology

may change considerably without changing bar type.

For example, the spatial scale of longshore variability

of three-dimensional bars may evolve over time, and

bars may migrate in the longshore direction. The types

of processes that will govern these changes will differ

for each morphologic subtype, because each subtype

has its own process signature (Wright and Short,

1984). The flow field over two-dimensional longshore

bars will be rather uniform alongshore, and, under

dissipative conditions, vertical circulation will domi-

nate the flow field (Wright and Short, 1984). When

bars become more three-dimensional in shape, weak

to moderate rip circulation will prevail and rips will be

persistent in location (Wright and Short, 1984). The

dominant flow field over shore-attached bars will be a

strong rip circulation, such that horizontal circulation

will dominate the flow field (Hunter et al., 1979;

Wright and Short, 1984).

4.3. Decay of subtidal nearshore bars

Few studies address the decay of bar features.

Decay is not to be confused with the disappearance

of a bar feature due to the welding of the bar to the

beach. Some studies indicate that bar features may

decay under the action of highly asymmetric, non-

breaking waves (Larson and Kraus, 1992; Wijnberg,

1995, 1997; Ruessink, 1998). This seems in line with

the observation that the type of plan shape morphol-

ogy of subtidal nearshore bars is mainly a function of

breaker height (Short and Aagaard, 1993). This

implicitly suggests that bars do not decay in the


breaking wave regime. However, the precise mecha-

nism of in situ bar decay is not yet clear.

5. Integrated, long-term nearshore bar dynamics:

a discussion

As reflected in the previous sections on the mor-

phodynamics of nearshore bars, most studies focus on

short-term morphodynamics which involve the

response of the nearshore morphology to individual

storm events and subsequent post-storm recovery.

Long-term bar system behavior is the result of many

storm-recovery sequences. Therefore, to arrive at

these large scales, the integrated effect of short-term

storm-recovery sequences needs to be understood.

In the simplest case, the response of the nearshore

morphodynamic system is dominated by variation in

the forcing of the system (Fig. 1), and the response is

more or less instantaneous. The relaxation time of the

system is small compared to the duration of the

forcing events. In this case, each forcing sequence is

uniquely related to a particular morphologic response

sequence; i.e., independent from the initial morpho-

logical condition, a given forcing sequence will pro-

duce one and the same morphological sequence.

In reality, the duration of events is often not long

enough to let the bar system reach an equilibrium with

the conditions (e.g., Bauer and Greenwood, 1990;

Holman and Sallenger, 1993). When the relaxation

time is large compared to the duration of the event, the

chronology in the forcing events becomes important

for understanding the evolution of the bar system. In

this case, the initial morphology is relevant for the

morphologic response sequence induced by a given

forcing sequence.

Interestingly, if a nearshore bar system is a feed-

back-dominated system, the initial morphological

condition is important, too, but the chronology of

wave events will only be of minor importance. The

evolution of such a system is largely dependent on the

preexisting morphology instead of on the details of

the forcing sequence. In this case, different forcing

sequences (but statistically similar) starting out on the

same initial morphology can produce similar sequen-

ces of morphologic response.

It is of crucial importance for predictive purposes

to know the extent to which the nearshore system is

dominated by morphologic feedback or by relaxation

time effects. In general, it is difficult to discriminate

between self-organised responses and relaxation time

effects from morphological observations during field

experiments because of their limited duration. A

possible approach to distinguish between relaxation

time effects and self-organised response in bar sys-

tem change is to analyse long time series of mor-

phologic evolution and forcing conditions. In case of

feedback-dominated, self-organised response, we

expect to find no correlation between the forcing

signal and the morphologic response. In the case of

relaxation time affected responses, however, we

expect to find a correlation with the forcing signal,

albeit filtered. The usefulness of this approach can be

illustrated by a case study of the decadal behavior of

multi-bar systems along the Holland coast (The

Netherlands).

Analysis of 28 years of annual bathymetric data

along the Holland coast (JARKUS data set) indicates

that the long-term behavior of multi-bar systems along

this coast exhibits characteristics of feedback-domi-

nated behavior (Wijnberg, 1995). Along the Holland

coast, two multi-bar systems (two to four bars) are

present, which are separated by a set of jetties that

extend about 2 km offshore. Both bar systems exhibit

systematic, cyclic behavior. All bars migrate in a net

offshore direction, with the outer bar decaying off-

shore and a new bar being generated near the shore-

line; this new bar also migrates net offshore, and so on

(e.g., De Vroeg, 1987; Kroon, 1994). This implies that

a given bar system configuration reappears on a

periodic basis. It should be noted that the observed

offshore migration is not an apparent migration due to

longshore migration of obliquely oriented bars. In one

bar system, this time period is about 4 years, whereas

in the other bar system, it takes about 15–18 years to

complete one cycle. This change in cycle period

occurs abruptly across the jetties (Wijnberg and Ter-

windt, 1995). Because both bar systems are forced by

the same sequence of wave conditions, the cyclicity in

the morphologic behavior cannot simply be explained

by a matching cyclicity in the forcing sequence.

Therefore, it seems inevitable to conclude that, in this

case, morphologic feedback plays a dominant role in

the response of the nearshore bar system.

A useful approach for further analysis of a feed-

back-dominated system is to identify the essential


feedback mechanisms. A simple model including

these mechanisms can then be used to explore the

stability of multi-bar system behavior. The feedback

in a multi-bar system consists of local flow–topog-

raphy interaction over individual bars but also

includes interaction between bars. Bars interact by

affecting the nearshore flow field in a global sense

(e.g., wave breaking over a bar reduces the wave

height experienced by a more landward located bar)

and by sediment exchange.

The above types of feedback can be illustrated by

the Holland coast case study. The cyclic behavior of

the Holland coast bar systems is essentially a cross-

shore redistribution of sediment (Wijnberg, 1995).

Fig. 6. Nearshore bathymetry along the North-Holland coast, The Netherlands (JARKUS database) in 1967 and 1973.


Therefore, the sediment of the decaying outer bar is

apparently transported onshore and this sediment

somehow needs to be accommodated in the inner

nearshore zone, inducing morphologic change. The

sediment exchange interaction may also be of rele-

vance in the formation of the new bar near the shore-

line. Possibly, the inner bar does not form until

sufficient sediment can be eroded from the beach. If

so, we might expect to find a relation between inner

bar generation and long-term beach accretion, which

along the Holland coast tends to occur by intertidal

slip-face ridge stranding (Kroon, 1994). In that case, a

matching long-term cycle in intertidal slip-face ridge

behavior should be present. For example, in certain

stages of the long-term, subtidal bar cycle, intertidal

ridges might be less prone to erosion by storm events

(global flow–topography feedback). Another concept

for the formation of the new inner bar is that a slip-

face ridge near the low-tide level transforms into a

subtidal bar (local flow– topography feedback).

Because of its location near low-tide level, the slip-

face ridge will experience subtidal processes for a

large amount of the time. If this is true, this implies

that a continuum exists between the subtidal and

intertidal bar forms, because not only may bars

migrate from the intertidal into the subtidal zone, as

suggested by some authors (Section 3.1), but also the

other way around.

The Holland coast case study also illustrates the

necessity of considering large stretches of coast for

longer-term coastal behavior. For example, the abrupt-

ness of the change in bar cycle period across the

jetties, which is an essential indicator of the dominant

role of feedback, would have been missed if only a

single kilometer of coast had been considered. In

addition, the longshore coherence and regularity in

the cyclic behavior becomes especially apparent when

considering large stretches of coast. For example, the

net offshore progression of nearshore bars occasion-

ally gets out of phase in the longshore direction (i.e.,

some parts of the bar system temporarily move faster

than others), resulting in ‘outer bar’– ‘inner bar’

attachments (Fig. 6). As a result, the bar behavior

observed in two neighbouring stretches of coast can

locally deviate considerably over a period of many

years (Wijnberg and Wolf, 1994). On the large scale

and long term, however, the bar system behavior

appears to be very systematic and coherent and the

bar attachment events are only details of the full bar

system dynamics (see Wijnberg and Terwindt, 1995).

On the basis of the analysis of long-term morpho-

logic data, combined with small-scale process knowl-

edge, conceptual bar behavior models can be

formulated. To further explore the realm of long-term

bar dynamics, such models should be translated into

simple simulation models (e.g., Plant et al., 1999).

These models may build on first principle physics (cf.

Hulscher et al., 1993), or use paramaterised sediment

transport relationships that are derived from field

experiments (e.g., Ruessink, 1998).

6. Conclusions

The sandy coast morphodynamic system is able to

express itself with a variety of nearshore bar types.

Based on the literature, seven types of bars are

distinguished (Table 2). These different expressions

of the nearshore morphodynamic system are associ-

ated with differences in boundary conditions concern-

ing wave energy, nearshore slope, and tidal range.

The specific morphodynamic relationships that

govern the formation and evolution of the different

bar types have been the subject of considerable

research to date. Most research has focused on under-

standing the processes and morphologic response on

the time scale of storm events and post-storm recov-

ery. Understanding the long-term behavior of bar

systems, however, also requires knowledge of more

generic properties of the nearshore morphodynamic

system such as the extent to which relaxation time

and morphologic feedback affect the bar behavior.

Such knowledge is probably best obtained from the

analysis of long time series of morphology and

forcing conditions, rather than from intensive field

experiments.

Fortunately, some monitoring programs for mor-

phologic and hydrodynamic data already exist and

have created valuable long-term data sets, such as the

JARKUS data set (The Netherlands), the Duck data set

(NC, USA), the Lubiatowo data set (Poland), or the

ARGUS video data set (USA, UK, The Netherlands,

Australia) (Hamm, 1997). These and hopefully addi-

tional, new monitoring programs will help increase

our understanding of the intriguing phenomenon of

nearshore bars along sandy shorelines.


Acknowledgements

K.M.W.’s work on this paper was funded by the

EU-sponsored Marine Science and Technology Pro-

gramme (MAST-III), as part of the PACE-project

under contract number MAS3-CT95-0002. The work

was co-sponsored by the Andrew Mellon Foundation.

A.K.’s work on this paper was partly funded by the

EU-sponsored Marine Science and Technology Pro-

gramme (MAST-III), as part of the SAFE-project

under contract number MAS3-CT95-0004. Gerben

Ruessink (Utrecht University) is thanked for his useful

comments on the manuscript. Further, all images of

barred beaches in this paper originate from the ARGUS

video image database of Rob Holman (Oregon State

University). We are grateful for access to this great

database of nearshore bars. Finally, we thank the

reviewers, Brian Greenwood and Robin Davidson-

Arnott, for their useful suggestions to improve the

manuscript.

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