JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C05001, doi:10.1029/2010JC006232, 2011

A vortex force analysis of the interaction of rip
currents and surface gravity waves
B. Weir,1 Y. Uchiyama,2 E. M. Lane,3 J. M. Restrepo,1 and J. C. McWilliams2
Received 8 March 2010; revised 20 December 2010; accepted 7 January 2011; published 3 May 2011.

[1] We use the vortex force formalism to analyze the effect of rip currents on theirown wave forcing. The vortex force formalism allows us to decompose the wave
forcing into the nonconservative flux of momentum due to wave breaking and theconservative vortex force. Following Yu and Slinn (2003), we consider rip currents
initially generated by alongshore variation of wave breaking due to a perturbation of abarred bottom topography. This variation is reduced in magnitude by two current effects
on waves: wave ray bending and the flux of wave energy by currents. We computethe change in wave energy caused by these two effects on their own and use this toshow that their relative magnitude scales with the square of the ratio of the length towidth of the rip current. Both effects increase the wave height over the channels of thelongshore bar, which leads to more wave breaking and counterbalances its longshore
variation due to bottom refraction. In comparison to wave breaking, the change in thevortex force is negligible. Next, we show how the reduction in wave breaking is similar
to an enhanced bottom friction. We then analyze the dependence of this relationship on
the breaking parameterization, angle of incidence of the waves, and bottom drag law.
Citation: Weir, B., Y. Uchiyama, E. M. Lane, J. M. Restrepo, and J. C. McWilliams (2011), A vortex force analysis of theinteraction of rip currents and surface gravity waves, J. Geophys. Res., 116, C05001, doi:10.1029/2010JC006232.

1. Introduction[2] Rip currents are strong, seaward flows formed bylongshore variation of the wave‐induced momentum flux.
They are responsible for the recirculation of water accumulated on a beach by a weaker and broader shoreward flow.
Rip currents were first recognized as an object of scientificinterest by Shepard [1936]. Their field observation dates backto Shepard et al. [1941], Shepard and Inman [1950], andMcKenzie [1958], who noted that rip currents were largely asurface phenomena fed by convergence of the longshore
current, and that they usually fluctuate in time, complicating
the collection of detailed observational evidence.
[3] There are a number of possible physical scenarios
accounting for the existence and regular spacing of rip currents. The first is due to Bowen [1969] and Bowen and Inman[1969], who attributed rip currents to the interaction ofincoming swell with standing edge waves. However, in theabsence of headlands there is no clear explanation for thepresence of standing edge waves. Rip currents may also arise
as a hydrodynamical instability caused by the refraction ofwaves by currents. Investigation of this generation mechanism
1
Department of Mathematics, University of Arizona, Tucson, Arizona,USA.
2
Institute of Geophysics and Planetary Physics, University ofCalifornia, Los Angeles, California, USA.
3
Hydrodynamics Group, National Institute for Water and AtmosphericResearch, Christchurch, New Zealand.

Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JC006232

began with LeBlond and Tang [1974] and was expanded uponby Dalrymple and Lozano [1978], Falqués et al. [1999],
Yu [2006], and Hasan et al. [2009]. In this paper we consider
longshore variation of the wave‐induced momentum flux due
to refraction by a barred bottom topography with an imposed
longshore perturbation [Sonu, 1972; Noda, 1974].
[4] The effect of wave‐current interaction on model predictions of rip currents dates back to the work of LeBlond andTang [1974], who noted that the flux of wave energy bycurrents decreases/increases the wave energy over the peaks/
channels of the longshore bar. This reduces the longshore
variation of wave breaking, and hence the strength of the rip
currents. Yu and Slinn [2003, hereafter YS03] showed thatwave ray bending has a comparable, if not stronger, effect on
the wave energy, and that the combined effect of both current
effects on waves (CEW) qualitatively changes the steadystate and instability mechanism of rip currents. When CEW
are neglected, the instability is a result of the lengthening of
the rip currents due to advection. Far to sea, the depth is large
and the dissipation due to bottom friction is low. The unstableoscillations of these rips are similar to those of a Bickley jet[Haller and Dalrymple, 2001]. When CEW are included theflux of momentum from waves to currents due to wave
breaking decreases significantly, which impedes the seaward
growth of the rips. The instability of these currents usually
features the ejection of vorticity generated in the longshore
trough, but can have many forms [Kennedy and Zhang,2008]. The above is detailed in greater generality by Haas
et al. [2003], whose numerical and experimental results
include the effect of depth variation as well.

C05001

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WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS[5] The wave‐induced momentum flux is commonlydescribed using the radiation stress (RS) first developed by
Longuet‐Higgins and Stewart [1960, 1961, 1962, 1964] andgiven a general derivation by Hasselmann [1971]. In thispaper we use the vortex force (VF) formalism first developed by Craik and Leibovich [1976]. McWilliams et al.
[2004] used the VF formalism to develop a complete
description of the conservative dynamics of waves andcurrents. The relationship between the RS and VF wasestablished by Lane et al. [2007], who demonstrated how
these two formalisms are compatible for the wave‐current
interaction problem at large spatiotemporal scales. The VF
formalism is particularly attractive because it decouples theconservative wave effects on currents (WEC) as a vortex
force and a Bernoulli head.
[6] A primary goal of this paper is to apply the VF formalism to the dynamics of rip currents and show that evenfor this complex flow, the description of the WEC is relatively simple. The comparison of the VF and RS formalisms
has potential implications to modeling more complex flows.
Our analysis also complements the work of YS03 with three
new geophysical insights. First, that the effect of wave raybending on the wave energy relative to the flux of wave
energy by the current velocity scales as the square of the ratioof the length to width of the rip. Next, that the feedback loopon the currents due to CEW is similar to an enhanced bottom friction. Finally, that when the swell is narrow banded,
it is sufficient to consider wave breaking parameterizations
that are power laws, and that as the exponent increases, sodoes the strength of the enhanced bottom friction.
[7] In section 2 we present the basic rip current problem,
which includes defining the bottom topography we use
for all numerical experiments, the model equations, and thetwo model types: one which includes CEW and the otherwhich does not. In section 3 we apply the VF formalism tothe analysis of the CEW. This includes an explanationof how increasing the angle of incidence of the waves
decreases the magnitude of the CEW, and an analysis of thedependence of our results on the choice of wave breaking
parameterization and bottom drag law. In the appendix wedemonstrate the equivalence of the VF and RS analysis of
rip currents. This reinforces the similar findings of Laneet al. [2007] for a case with nonconservative forces on the
waves and currents.

2. The Model Equations[8] We consider a rectangular basin and a bottom topography that is constant over all time. The waves, whichapproach the shore from far out to sea at a given angle, areassumed to have small wave steepness and a spectrum witha single peak. We assume that the waves are well describedby a monochromatic wave with the root‐mean‐square wave
height and frequency corresponding to the spectral peak. We
use the ray equations [Mei, 1989] for the waves, and theequations for the Eulerian current u follow from those
governing the total flow by averaging over several wave
periods as by McWilliams et al. [2004]. We neglect the
Coriolis force. Though potentially important, we ignore
wave set up and set down effects, and suppose that thepressure P is in hydrostatic balance so that P = rg(z − z),where z is the (constant) mean sea elevation, g is the con-

C05001stant of gravity, and, for simplicity, the water has a constantdensity r = 1000 kg m−3. The rip currents are driven by theflux of momentum from the waves due to wave breaking
and dissipated by bottom drag.
[9] In order to analyze the CEW, we consider two wave‐
current interaction models based on those by Uchiyama
et al. [2009]: (1) WEC+CEW, which includes CEW, and(2) WEC‐only, which does not. In both models, we firstallow the waves to reach a steady state without CEW. For
the WEC‐only model, the WEC are held constant in time atthis state, whereas for the WEC+CEW model the WEC
evolve in time, dynamically forcing the currents. The surface
gravity wave equations are presented for the WEC+CEW
interaction model only, with the understanding that the
WEC‐only equations are the same with the terms involving
the current velocity u omitted.
[10] The horizontal coordinates of the fluid domain are(x, y), where x is aligned with the cross‐shore direction, andy the longshore. The vertical coordinate z is aligned opposite
to the direction of gravity. We use an east coast configuration so that increasing x corresponds to increasing distancefrom the shore. The domain is 768 m by 768 m and assumed
to be periodic in the alongshore direction. We assume thatthe nearshore boundary is reflectionless, and there is no net
outflow at the offshore boundary. All vectors are horizontal
and independent of the depth. For a given vector v = (v1, v2),
we denote the vector equal in magnitude and 90° counterclockwise as v? = (−v2, v1). We use this definition with thegradient vector r as well, so thatr?  v ¼@v2 @v1

@x
@y

ð1Þ

is the vertical component of the curl of v.
[11] The bottom topography depends on the cross‐shore
coordinate x as5h0 ð xÞ ¼ 1:5 exp  2 ð x  xs  db Þ2
dbþ 1:35f1 þ tanh½0:0253ð x  xs Þg1coshðð x  xs ÞÞ;þ 0:0032 x þ logcoshðxs Þ(see Figure 1a). This topography is based on that used by
Lippmann et al. [1999] for the 11 October 1990 DELILAH
field experiments at Duck, North Carolina. The logarithmic
term is introduced to eliminate any difficulties with thedepth approaching zero, and its steepness is determined by
the parameter a = 0.02 m−1. The inner surf zone is definedas the region shoreward of xs = 150 m, so that db = 80 m isthe distance from this zone to the bar. For x < xs, we artificially increase the numerical viscosity, representing thesituation when dissipation dominates all other forces. We
impose longshore variation of wave breaking by adding
the perturbation




2
5
y exp  2 ð x  xs  db Þ2 ;pð x; yÞ ¼ " cos

db

ð2Þ

so that h = (1 + p)h0 (see Figure 1b). The parameter l is
the wavelength of the perturbation, and unless specified

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C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

Figure 1. (a) The alongshore mean and (b) the three‐dimensional view of the bottom topography.
The bar has a longshore perturbation whose magnitude is " = 0.1 and wavelength is l = 256 m.
otherwise we use l = 256 m. We refer to the minima of thelongshore bar as channels and the maxima as peaks.
2.1. The Surface Gravity Waves[12] The wave number k = (k1, k2) is the gradient of the
wave phase, and is irrotational. The equation for conservation of wave crests is@kþ rðu  k þ Þ ¼ 0;@t

ð3Þwhere the frequency s satisfies the dispersion relation
2 ¼ gk tanh kh

ð4Þ

and k = ∣k∣ is the modulus of the wave number.
[13] The wave action A is related to the wave energy E
and amplitude A by the identities
A¼

and Br and g are model parameters fit to the data. The
parameter Br measures the percentage of the wave face thatis broken, and g the saturation of wave energy, such thatwhen Hrms = gh all waves are assumed to be breaking. In
section 3.6 we discuss the dependence of model predictions
on other choices of "b.
[14] In the numerical simulations below, we assume that
far out to sea the waves have a height of 1 m and a period of
10 s, and we use the breaking parameters Br = 1.3 and g =0.38. In Figure 2 we plot the steady state wave variables
for these choices, the WEC‐only model, and the longshore
uniform bottom topography h0. The steady state represents a
constant wave period, and the balance of the increase in wave
height due to shoaling and the decrease due to breaking.
2.2. The Rip Currents[15] The equations for the Eulerian current u areE1¼
gA2 :
 2

In the absence of wave breaking it is a conserved quantity
advected by the absolute group velocity cg = u + Cg, where
the intrinsic group velocity is



2kh
k
Cg ¼
1þ
:
2k
sinh 2kh k

We include dissipation due to shoaling‐induced wave
breaking using the theory of periodic bores first suggested
by LeMehaute [1962], so that the wave action satisfies@A"bþ r  cg A ¼  :
@t


ð5Þ

Unless otherwise specified, we use the semiempirical parameterization of Thornton and Guza [1983], which we referto as TG83a,
pﬃﬃﬃ
3 
H7
"b ¼
g fp Br3 4rms5 ;
 h
16


@u
?
þ u  ru þ gr ¼  uSt þ B  D;@tr  h u þ uSt ¼ 0:


2

and

Hrms ¼ 2A

ð7Þ

The depth‐averaged Stokes drift, which we refer to simply
as the Stokes drift, is the vector field
uSt ¼

Ak
j Aj2 k¼:
h
2kh tanh kh

ð8ÞThe term c (uSt)? is the vortex force, where c = r? · u isthe (vertical) vorticity of the current. The vector field
B¼

"b
k
h

ð9Þ

is the flux of momentum from waves to currents due towave breaking. For the moment, we use the linear bottom
drag lawD¼

where fp is the peak frequency of the waves, Hrms the root‐
mean‐square wave height such that
fp ¼

ð6Þ

f

u
:
h

In order to demonstrate the importance of CEW, we pickvalues of the constant mf that are O(10−3) m s−1. In section 3.8
we examine the appropriateness of this assumption. Since
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WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

C05001Figure 2. The alongshore means of the steady state wave variables: (a) the root‐mean‐square wave
height, (b) the dissipation of wave action due to breaking, (c) the wave number, (d) the frequency,(e) the magnitude of the group velocity, and (f) the magnitude of the Stokes drift. The alongshore components of the wave number, group velocity, and Stokes drift are negligible compared to the cross‐shore
components and are not shown.
we make the rigid lid approximation, the pressure force grz
is simply the hydrostatic pressure contribution.
[16] Since the vector field h(u + uSt) is solenoidal, we
define a pseudo stream function y such thath u þ uSt ¼ r? :

ð10ÞIn terms of the two unknowns y and c, (6) and (7) areequivalent to the two equations
r



1
r
¼
h


@
þ r  u þ uSt@t

þ r?  uSt ;ð11Þ

¼ r?  ðB  DÞ:

ð12Þ

2.3. The Numerical Solution[17] We compute the wave and current variables as follows. First, we advance (3) and (5) forward in time with nocurrents until they reach a steady state. Given an initialvorticity field, we solve (11) for the stream function y, then(10) for the Lagrangian velocity u + uSt. We compute thevorticity at the next step in time using (12). Finally, for the
WEC+CEW model, we advance the waves according to(3) and (5).
[18] We use the corner transport upwind method with themonotonized central difference flux limiter [LeVeque, 2002,
p. 115]. The corner transport upwind method is second‐order accurate in both time and space. The use of a flux
limiter reduces the method to a formally first‐order accurate
method, yet in practice the limited method removes spurious

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C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

oscillations and still produces results that are comparable tosecond‐order accurate methods. We use a uniform grid
spacing of Dx = Dy = 3 m, and a variable time step Dt for
the wave equations (3) and (5) such that the CFL number isalways 0.9. As Haas et al. [2003] did we update the vorticity equation, which is formally decoupled from the wave
equations, after a fixed multiple of the wave time step Dt.
For our particular grid spacing any multiple between one
and the maximum multiple such that the CFL condition issatisfied give nearly identical results. For finer grid spacings, a multiple such that the current equations are updated
every 5 seconds gives accurate results that remain bounded.
We note that one advantage of the VF formalism over the
RS formalism is numerical: the former requires taking one
less derivative in space, and is thus less susceptible tonumerical oscillations.

3. Analysis of Rip Current Effects on Waves[19] The analysis in this section applies to any bottom
topography with the same qualitative properties as our
choice, i.e., monotonically increasing depth from the edge
inner surf zone to the far sea, except at a longshore bar, andvariation in the longshore direction that is small compared tothat in the cross‐shore direction. These assumptions arenecessary so that the resulting currents are rips. Our analysis
requires as well that the wave steepness Ak and the strengthof the currents compared to the group velocity of the waves
are always negligible. We assume that these quantities are
the same or lower order as the longshore variation of thebottom topography, so that Ak = O(") and ∣u/Cg∣ = O(").
[20] We further restrict our attention to waves whoseangle of incidence is near normal. We may then assume¼

k2¼ tan  1;
k1

where is the angle of incidence of the waves far to sea.
Throughout, we will avoid carrying out the formal nondimensionalization and scaling of quantities, with theunderstanding that when we make approximations, it is
the ratio of the two terms in this approximation that have thespecified dimensionless order. To begin, the near‐normal
assumption implies
k¼

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
k12 þ k22 ¼ k1 þ O 2 :because the CEW diminish the flux of momentum from thewaves to the currents due to wave breaking, which is theterm r? · B in the vorticity equation (12) (see Figure 4,
which is a reproduction of Figure 4c and 4c of YS03).
3.1. Longshore Means and Fluctuations[22] For a function f of the horizontal coordinates, wedefine the alongshore mean and fluctuation as
hfi ¼

1
L

Z L

f ð x; yÞ dy;

and e
f ¼ f  h f i;0

where L = 768 m is the domain length in the longshore
direction. Since the fluctuation of the bottom topography isO("), where " is the amplitude of the perturbation to thelongshore bar, the fluctuation of f is also O("). Hence, up toO(") the alongshore means of the wave variables are thesolution to the problem in the cross‐shore variable x only,
depicted in Figure 2. When d is nonzero, the irrotationalityof the wave number ensures that to O("), the longshore
component k2 of the wave number is an O(d 2) constant, as is
the angle of incidence.
[23] When currents are weak compared to waves, thecurrents do not change the longshore means of the wave
variables. Nevertheless, they do change the fluctuations of
the wave variables. As noted by YS03, this change can bequite substantial, and as a result, cause a noticeable reduction in the magnitude of the WEC. Below, we derive
identities for the change in the wave variables due to thecurrents, then use these identities to analyze the CEW andtheir dependence on the length and width of the rip, theincoming angle of the waves, the choice of breaking
parameterization, and the bottom drag law. We use theshallow water approximation kh → 0 for mathematical convenience, but the assumption is not essential to the results.
[24] Since all longshore fluctuations are O("), they satisfy
the approximate product rulee  h f ie
fgg þe
f hgi:

Henceforth, we use ≈ to denote equivalence up to the maximum of O("2), O("d), and O(d 2). We make the additional
definition that for a function f of the wave variables, its difference due to CEW is defined as
Df ¼e
f WECþCEW  e
f WEC-only  f WECþCEW  f WEC-only ;For a given value of ", we consider all values of d ranging
from 0 up to ". When " = 0.1, this corresponds to angles ofincidence between 0° and about 6°. This ensures that thealongshore means of the current variables are small compared to their alongshore fluctuations, i.e., that the current isdominated by a rip current flow and not a longshore flow.
The wave variables, on the other hand, are predominantly
the values of their longshore means, " being a measure of
their deviation away from these means.
[21] In Figure 3 we show the vorticity fields of the WEC‐
only (Figure 3a) and the WEC+CEW (Figure 3b) models fora bottom friction of mf = 0.002 m s−1 at t = 80 min. This is a
reproduction of Figure 2 of YS03 for the TG83a wave
breaking model. The most striking effect of the inclusionof CEW is the shortening of the rip currents. This occurs

where f WEC‐only is the steady state value of f for the WEC‐
only interaction model, and likewise for WEC+CEW. The
second equality follows since currents are lower order thanwaves. Hence, D also satisfies the same approximate productrule as the longshore fluctuations.
3.2. The Doppler Shift and Wave Ray Bending[25] We begin our analysis with the changes in the wavefield. From the equation for the wave number (3), we see thatfor the steady state waves
D ¼ Dðu  kÞ  uhk1 i:

ð13ÞThis change is the Doppler shift and it is depicted in Figure 5(when there are no CEW, the steady frequency is the constants0 = 0.62 s−1 everywhere, set at the offshore boundary).

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C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

Figure 3. Comparison of the instantaneous vorticity fields for the (a) WEC‐only and (b) WEC+CEW
models at t = 80 min with mf = 0.002 m s−1. Notice the considerable difference in the seaward (positivex) extent of the contours from Figure 3a to Figure 3b. The WEC‐only current is unstable and oscillates sinusoidally at the rip head, while the WEC+CEW current is stable.
[26] We refer to “wave ray bending” as the changes in the
wave number, along with the subsequent changes
pﬃﬃﬃﬃﬃ in the
wave energy and breaking. Since D(sk−1) = D gh = 0, we
may use the approximate product rule to compute
Dk E
DE
Dk1  D  u:gh@:ðDk2 Þ  
@x
gh @y

Dk2 ¼

Z LD
x

The equation for Dk2 then follows from the irrotationality of
the wave number k. It is
D  E @uparameterized by the alongshore variable y, such that Dk2 =0 at x = L, the offshore boundary. Its solution isð15Þ[27] The resulting change in the longshore component of
the group velocity is
DCg2 

ð14Þ

If Ds is alongshore uniform, then Dk2 ≡ 0. Hence, alongshore variation of the Doppler shift is the cause of wave raybending. Equation (14) is an ordinary differential equation

 E @u
dx′:
gh @y

¼

D gh EDk2 ;
D gh E Z L D  E @u


x

gh @y

dx′;

ð16Þ

whereas the cross‐shore component of the grouppvelocity
is
ﬃﬃﬃﬃﬃ
left unchanged by CEW, since Cg1 ≈ −Cg = − gh. When
there are no CEW, refraction by the bottom topography

Figure 4. Comparison of the forcing due to wave breaking on current vorticity r? · B for the (a) WEC‐
only and (b) WEC+CEW models.
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We define DWRBHrms and DCFHrms as the corresponding
changes in the wave height.
[30] Since wave ray bending causes wave rays to concentrate over rip channels, rather than the peaks in thelongshore bar, it forces wave energy from the peaks toward
the channels. In our numerical solutions, the effect of wave
ray bending on the wave energy (Figure 7a) is approximately two and a half times stronger than that of the current
flux alone (Figure 7b). If l is a typical length scale of the rip
currents and w is a typical width, then using (16) in (18) anddividing by (19) we see that the ratio DWRBE/DCFE scalesas (l/w)2.
3.4. The Stokes Drift and Vortex Force[31] Apart from the wave breaking momentum flux,
the only other WEC we consider is the vortex force. Up toO("2), the term representing the vortex force in the vorticity
equation (12) isr  uSt

Figure 5. The Doppler shift Ds. For the WEC‐onlymodel, s is the constant s0 = 0.62 s−1.
forces the wave rays to diverge away from the rip channels
toward the peaks of the bar (Figure 6a). When there are
CEW, the longshore component of the group velocitychanges sign, and the wave rays now diverge from the peaksof the bar and converge toward the rip channels (Figure 6b).
3.3. Current Effects on Wave Height[28] In order to analyze the change in wave height, we firstfind the change in wave energy DE, then use the relation
DHrms ¼4DE;ghHrms i

@  St
hu i
@x

:

ð21Þ

Since the waves are directed onshore, the mean Stokes drift
huSti is negative everywhere. The vortex force thus opposes
the seaward advection of vorticity by the Eulerian current.
Furthermore, the current effect on huSti is O("). Thus, anychange in the vortex force at leading order is due entirely tothe change in the steady current vorticity c, and if the wave
breaking momentum flux does not change, the vortex force
remains unchanged as well. In Figure 8 we decompose theLagrangian flux of vorticity r? · (u + uSt) in equation (12)for both the WEC‐only and WEC+CEW models. Even
though changes in r? · B modify the steady vorticity field,
the changes in the vortex force from Figures 8a–8d arewhich follows from the identity rgH2rms = 8E. Applying theoperator D to both sides of (5) shows that@ 
7 D "b EDE
hCg1 iDE þ
@x
2 E
@ 
DCg2 hEi  r  ½ð2u; vÞhEi:
¼
@y

ð17Þ

The factor of two in front of the cross‐shore current u is dueto the Doppler shift, which reinforces the effect of the cross‐shore current.
[29] Since DE is a linear function of the two terms on theright‐hand side of (17), we decompose it as
DE ¼ DWRB E þ DCF E;

where DWRB E is the effect of the first term (wave raybending) and DCFE the effect of the second, which we referto as the current flux of wave energy. We solve for these twoterms using an integrating factor M = M(x) such that
DWRB E ¼
DCF E ¼

1
M hCg1 i

1
M hCg1 i

Z L@ DCg2 hEi dx′
@y

ð18Þ

M r  ½ð2u; vÞhEi dx′;

ð19Þ

M
x

Z L
x

Z L D E 
7 "b
dx′ :
M ð xÞ ¼ exp
x 2 E

ð20Þ

Figure 6. Comparison of the alongshore fluctuations offg for the (a) WEC‐only andthe steady group velocity, C(b) WEC+CEW models. The sign of the alongshore
component Cf
g2 determines whether the wave rays diverge(Figure 6a) or converge (Figure 6b) over the rip channels.
The reference arrow in the top left corner of Figure 6a is
fg ∣ = 0.33 m s−1.
for ∣C

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C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

Figure 7. Change in wave height due to CEW: (a) change due to wave ray bending, (b) change due toflux by the current velocity, and (c) total change.
minimal, the dominant change being in the flux of vorticity
by the Eulerian current from Figure 8c to frame Figure 8d.
3.5. The Wave Breaking Momentum Flux[32] Applying the operator D to (9) and using the TG83amodel parameterization, we may decompose the change inwave breaking momentum flux as D
"b ED k E @7ðDHrms Þ
Hrms
@y
h
 D

@
"b E
Dk2 :
þ
@x h

D r?  B 

@
@
 :
@x
@y

The second assumption being equivalent to w  l, where
w is a typical width and l a typical length of the rip (seeHaller and Dalrymple [2001] for a more systematic treatment of WEC‐only currents given these assumptions). Then
we may use (15) to write
Dk2  Z LDx

E
! dx′:
gh

Since the ratio of the rip current width to length is negligible
compared to 1, the change in wave energy is due entirely towave ray bending. Hence,DHrms ¼ 

4ghCg1 iHrms M

Z LxJð Þ ¼

ð22Þ

Recall that k is irrotational, so r? · k = 0. Thus, knowing
the changes in the wave number and the wave height allows
us to completely determine the change in the generation ofvorticity due to wave breaking.
[33] Now suppose that within the ripv  u;

WEC‐only model plus an extra forcing that is an integral
operator of the vorticity. In general, it is no easier to solve
this equation than the full WEC+CEW model. Instead, we
assume that the rip is self similar in such a way that we may
express the change in wave breaking momentum flux interms of the bottom friction (Figure 9).
[34] In order to find the best expression for D (r? · B) in
terms of a linear friction, we form the functional

Z L D E
1 @!
dx′′ dx′:
M hhi
h @y
x′

Z L Z L h
D r?  B 
0

r? 

200

1=p
uip
dx dy
h

and minimize to find mCEW. We chose the region shorewardof x = 200 m in order to concentrate the minimization on theregion occupied by the rip. If p = 2, then J measures theroot‐mean‐square difference. We pick p = 8 based on thebest visual fit of the WEC+CEW model with the linear
bottom friction coefficient mf and the WEC‐only modelusing the enhanced bottom friction menh = mf + mCEW.
[35] The comparison of the WEC+CEW rip with mf =0.002 m s−1 and the WEC‐only rip with menh = 0.0065 m s−1
is depicted in Figure 10 and the minima of the functional J
is depicted for a range of bottom frictions in Figure 11.
Notice that even when mf = 0, the enhanced bottom friction
is greater than 0.005 m s−1, a value in the stable regime for
the WEC‐only model.
3.6. Wave Breaking Parameterizations[36] Many wave breaking parameterizations including
those proposed by Thornton and Guza [1983], Church andThornton [1993], and Roelvink [1993] have the form"b
¼ CB hfB ðS Þ;h

where CB is a constant andSubstituting these expressions into (22), we may thus
express the WEC+CEW model for the vorticity as the

8 of 16

S¼

Hrms
h

ð23Þ

C05001

C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

Figure 8. Component by component decomposition of the advection of vorticity, for the (a, c, e) WEC‐
only and (b, d, f) WEC+CEW model type.

is the coefficient of wave energy saturation, such thatwhen S = 1 all waves are assumed to be breaking. Recall
that we previously defined "b in the TG83a model. For
this definition,
CB ¼

3
pﬃﬃﬃ gBr3  3 ;32 [37] We now consider two alternative wave breaking parameterizations. The first, the TG83b model,f B ðS Þ ¼ S 7 :9 of 16

2
pﬃﬃﬃ
5
3 
H 6
"b ¼
g fp Br3 2rms3 41 
 h
16

3
1
1 þ ðHrms =hÞ2

7
5=2 5;

C05001

WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

C05001Figure 9. Comparison of (a) the total in change r? · B, (b) the change with the current flux of wave
energy neglected, and (c) a rescaled linear friction term, where mCEW = 0.0045 m s−1. In particular, compare the frames in the region shoreward of x = 200 m.

is also proposed by Thornton and Guza [1983]. As a further refinement to TG83b, Church and Thornton [1993] use
the CT93 model,
2
pﬃﬃﬃ
3
3 
H
6
"b ¼
g fp Br3 rms N 41 
h
16

where

3
1
1 þ ðHrms =hÞ2

7
5=2 5;

 

Hrms
1 :
N ¼ 1 þ tanh 8h[38] In Figure 12, we plot the function fB, defined in (23),for each of these three choices of wave breaking parameterization: the TG83a, TG83b, and CT93 models. To facilitate
the comparison, we normalize the function fB so that fB (1) =1 for all three parameterizations. This is equivalent to using
a parameter Br for the TG83a model that is 0.94 times thevalue used for the TG83b and CT93 models. Each of thefunctions fB in Figure 12 are well approximated by a powerlaw (the TG83a model is a power law). For comparison
with the TG83b and CT93 models, we plot fB(S) = S 6 andfB(S) = S14. Not pictured, is the class of wave breaking
parameterizations derived by Roelvink [1993]

power laws, so that fB(S) = Sa, and analyze the dependenceof our results on the parameter a.
[39] The three parameterizations can be ordered fromlargest to smallest values the exponent a as CT93, TG83a,
and TG83b. Since its exponent is the largest, runs using theCT93 model allow more shoaling of the incoming wave
before breaking occurs than the TG83 models (Figure 13a).
When breaking does occur for the CT93 model, it is more
intense (Figure 13b) than for the other models. This can leadto scenarios where the CT93 currents are unstable, yet the
TG83 currents are stable.
[40] In particular, notice that the first term on the right‐
hand side of (22) is multiplied by a factor of the exponenta (in this case a = 7). Hence, as the exponent of a increases,
so does the strength of the current effect on the wave
breaking momentum flux, as does the value of mCEW.
3.7. Angle of Incidence
e for incoming angles of[41] Figure 14 depicts r? · B
1° (a), 3° (b), and 6° (c). We ignore the alongshore meanr? · hBi of the wave breaking momentum flux, since it is
responsible for driving the mean longshore current hvi. The
addition of a mean longshore current to the flow field does
not change the wavefield, since the longshore velocity v
only appears in the wave energy equation (17) as


 
 
 
1
H3
Hrms mHrms n;"b ¼ g fp Br3 rms
1  exp 
4
h
h
h

@v
@ev
hEi ¼ hEi:@y@ywhere m and n are arbitrary nonnegative numbers. In general, for 0 ≤ S ≤ 1, it is sufficient to make the approximation1  exp½S n   1  e1 S n ;corresponding to the first term in the Taylor series of theleft‐hand side. We thus treat these parameterizations as

However, the longshore current does affect the cross‐shore
current u through the advection term v∂u/∂y in (6), and, ine isthis way, modifies the wavefield indirectly. At 1°, r? · Bessentially equivalent to its WEC+CEW value at 0° (seeFigure 4b), while at 6°, r? · B is nearly identical to its
WEC‐only value at 0° (Figure 4a). Thus, as the angle
of incidence of the waves increases, the CEW decreasein magnitude.

10 of 16

C05001

WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

C05001Figure 11. Enhanced bottom friction menh (solid) as a function of true bottom friction mf, and a linear least squares fit(dashed). The slope of the fitted line is approximately 0.66,
and its y intercept is menh ≈ 0.0053 m s−1.

Figure 10. Comparison of rip current velocity of (a) the
WEC‐only model with the enhanced bottom friction menh =0.0065 m s −1 and (b) the WEC+CEW model with mf =0.002 m s−1. The streamlines are plotted at 3 m3 s−1 intervalsof y. The ratio of this number to the spacing between adjacent contours is approximately ∣ry∣ = ∣h(u + uSt)∣. The
current speed is thus inversely proportional to the productof the depth and the contour spacing.

[42] This decrease occurs because the streamlines of therip current tilt to align opposite to the incoming angle of
the waves (Figure 15); the longshore current generated by
the incoming waves causes the feeder flow of the rips to bestronger in the direction of the waves, which then causes therips to be directed away from the direction the waves. As theangle increases, the circulation cells characteristic of rip
currents in Figures 15a and 15d give way to unbounded
streamlines aligned with the alongshore direction in
Figures 15c and 15f. The gradual aligning of the streamlines
with the longshore direction diminishes the magnitude of∂y/∂y, which causes the cross‐shore velocity u and its
longshore derivative ∂u/∂y to decrease in magnitude as well,thus decreasing the magnitude of the CEW.
[43] If the perturbation to the longshore bar has a magnitude " = 0.1, then for waves with an angle of incidence
greater than approximately 6°, the resulting currents resemble the longshore current with an O(") perturbation. We do
not consider this flow here, since it is covered extensively inthe literature on the longshore current [Allen et al., 1996]. In
particular, Uchiyama et al. [2009] apply the VF formalism
to the dynamics of longshore currents.

3.8. Bottom Drag[44] In this section we consider three bottom dragparameterizations: two of which are nonlinear and take into
account both the current and wave velocities and Rayleighbottom friction, which depends linearly on the current
velocity.
[45] In the presence of waves and currents, the mostcommon model for the bottom drag on the current [Sleath,1984] ist b ¼ cf jutot jutot ;where utot is the total velocity due to waves and currents,
and the overline denotes averaging over several wave periods.

Figure 12. Dependence of the TG83a (solid), TG83b(dashed), and CT93 (dot‐dashed) wave breaking parameterizations on the coefficient of wave energy saturation S =Hrms/gh. For comparison, S6 (squares) and S14 (circles) arealso plotted. The functions have been normalized so thatfB (1) = 1 for every parameterization.

11 of 16

C05001

C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

Figure 13. Alongshore means of (a) wave height and (b) wave breaking for three different wave breaking parameterizations: TG83a (solid), TG83b (dashed), and CT93 (dot‐dashed).

There are many choices for the bottom friction coefficient cf.
Two examples arecf ¼

1 þ lnðz0 =hÞ2or 1=3ks
cf ¼ 0:01525:
H

The first is derived by Soulsby [1997] as a best possible fit fora number of experimental results. The second is the Manning‐
Strickler law. Here = 0.4 is the von Kármán constant, and z0
is the bed roughness length. We use z0 = d50/12 = 10−3 m andtake the apparent roughness length (or, Nikuradse roughness
length) as ks = 0.0125 m. In general, the two forms for cf arequite similar, so we restrict our attention to the Manning‐
Strickler law.

[ 46 ] The bottom friction parameterization of Soulsby[1995] represents the combined influence of waves andcurrents as a function of the individual current t c and wavet w shear stresses. It is"t b ¼ t c 1 þ 1:2

jt w jjt c j þ jt w j3:2 #:

ð24Þ

[47] The individual shear stresses are defined ast c ¼ cf juju;

and

1 
2
t w ¼ fw uwv;b2

e for the WEC+CEW model typeFigure 14. Longshore fluctuations of the forcing of vorticity r? · B
and an angle of incidence (a) = 1°, (b) = 3°, and (c) = 6°. Compare Figure 14a with Figure 4b
and Figure 14c with Figure 4a. The alongshore mean r? · hBi (not pictured) is the forcing for thelongshore current vorticity and increases with .
12 of 16

ð25Þ

C05001

C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

Figure 15. Dependence of the current streamlines on the angle of incidence of the waves.

where fw is an empirical wave friction factor[49] When the angle of incidence of the waves is almost
normal, and the currents are weak compared to the waves,
we use the approximation

 wv 0:52
u
fw ¼ 1:39 b
z0

and uwv
b is the wave orbital bottom velocity. For a monochromatic wave, it is
uwv
b ¼

A
;
sinhðkH Þ

which is the velocity such that for the linear wave theory,the velocity at the bottom z = −H would be
uwv
x  ~t Þ;b cosðk  ~where ~
x and ~t are the spatiotemporal coordinates of the wave
variables, i.e., the variables x and t before the wave averaging.
[48] A second nonlinear parameterization of bottom stressthat we consider was proposed by Feddersen et al. [2000]
"



juj
t b ¼ cf ð1:16Þ þ wv
urms

2 #1=2

2

jutot jutot 

In reality, there is an asymmetry between the cross‐shore
and longshore components of this frictional term, with thefactor of the cross‐shore component being 4/p. Following Dodd [1994] and Özkan‐Haller and Kirby [1999] we
use the symmetric form instead, as it tends to give similar
results, and in practice the uncertainty in the bottom friction
coefficient makes this simplification even more germane. The
longshore mean of the factor (2/p) uwv
b , depicted in Figure 16,
varies over space and time. A common approximation is totake its steady state WEC‐only value averaged over both
spatial directions (dashed line). We may then writet b ¼  f u;

where we have defined the linear bottom friction coefficient as

uwv
rms u;

f ¼

where uwv
rms is the root‐mean‐square wave orbital velocity,such that when the waves have a single spectral peak1 wv
uwv
rms ¼ pﬃﬃﬃ ub :
2

2 wv
u u:
 b

21
L

Z L
0

hcf uwv
b i dx;where, as before, <·> is the longshore mean and L isthe domain length in the cross‐shore direction. This is theRayleigh bottom friction model. For the WEC‐only model,

13 of 16

C05001

WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS

C05001

particular bottom drag law (Figures 17d–17f). Both Özkan‐
Haller and Kirby [1999] and Thornton and Guza [1986]
suggest that this agreement holds even in the extreme casewhen the currents are strong and a nonlinear model is
expected to be more appropriate.

4. Conclusions

Figure 16. The alongshore mean of the factor (2/p) cf uwv
b
for the Manning‐Strickler law (solid) and its cross‐shore
average (dashed). The Soulsby [1997] bottom drag coefficient is not pictured.
the Soulsby and Feddersen results are nearly identical to theresults for a constant bottom friction of mf = 0.002 m s−1 (seeFigures 17a–17c). We see in fact that the approximation of mf
in Figure 16 slightly overestimates the bottom friction, since
the most important values of the friction are in the neighborhood of the trough shoreward of the bar. The negative
feedback loop on the currents formed by adding CEW makes
the WEC+CEW model results even less dependent on the[50] We have shown that rip currents reduce the flux of
momentum from waves to currents due to wave breaking.
The bending of wave rays by currents and current flux forces
wave energy into the channels of the longshore bar. This
reduces the longshore variation of the flux of momentum
due to wave breaking. We detailed precisely the calculationof the change in the wavefield due to wave ray bending
and due to the current flux of wave energy. This calculation
gives the added geophysical insight that the ratio of these
two forces scales as the square of the ratio of the typicallength to width of the rip currents. This confirms the findingof YS03 that neglecting the wave ray bending in rip current
calculations is not appropriate, since it is the dominant current effect on the wave energy. We then showed that thevortex force is equivalent to the cross‐shore flux of vorticity
by the mean Stokes drift, and in the absence of other WEC, it
is unchanged by CEW. The CEW form a negative feedback
loop on the currents. We defined a parameter mCEW thatmeasures the best fit of this dissipative force with linear
bottom friction. We found that the enhanced friction, thesum of this fictitious friction and the true bottom friction,
had a minimum greater than 0.005 m s−1. For the physicalscenario we considered numerically, the WEC‐only model is

Figure 17. Current streamlines at (a–c) t = 50 min for the WEC‐only model and (d–f) steady state values
for the WEC+CEW model, for the linear bottom friction with mf = 0.002 m s−1 (Figures 17a and 17d), thenonlinear model of Soulsby [1995] (S95, Figures 17b and 17e), and the nonlinear model of Feddersen
et al. [2000] (F00, Figures 17c and 17f).
14 of 16

C05001

stable at this value of the bottom friction. As the angle ofincidence of the waves increases, the CEW decrease inmagnitude. For an O(0.1) perturbation to the longshore bar,
the CEW are negligible when the angle of incidence exceeds
6°. These results hold for any wave breaking parameterization whose dependence on the coefficient of wave energy
saturation S = Hrms/gh is well approximated by a power law.
Based on our analysis, this property is generic to breaking
parameterizations for narrow‐banded swell. The exponentof this power law determines both the strength and stabilityof the predicted rips.

[53] The Lagrangian bottom drag does not change thedissipation of the current sinceh ifu þ uStr?  DL ¼ r? 
 h

1
f
¼
r?  U  u þ uSt  r? h þ Oð Þ:hhUsing the shallow water approximation of (8) we find
 2  ?
1 St
1 A2 
A  k rh?
u  r? h ¼
;

k

r
h
¼

h
h 2ðkhÞ22h2 k khwhere

Appendix A: Equivalence of the VF and RS
Formalisms[51] We consider the two RS model types from YS03:“interaction,” where the wave variables are dynamic, and“no interaction,” where the wave variables are the steadystate values computed without CEW. Both model types use
the current equations

 2
 2 !
A  k ?  1
1¼ O  A  2  0:01;
 2h2 k  2
h
8

@U
1
þ U  rU þ rF ¼ t  DL ;
@t


ðA1Þ

and the term ∣rh/kh∣ is small by assumption, as it measuresthe variation of the bottom topography [Mei, 1989]. Hence,the differences between the Lagrangian and Eulerian bottom
drags are lower order.
[54] Using equations (3) and (5), we find that the divergence of the radiation stress tensor isr  ðhUÞ ¼ 0;

ðA2Þt ¼ B þ r’;

and the velocity field U is used in the wave equations (3)and (5). The WEC t is the divergence of the radiationstress tensor Sij. It is defined as
1 @Si1 @Si2þ;
i ¼ 
h @x@ywhere ’ is a function of the wave variables. When there isno interaction,’ ¼ g ^ ¼ gi ¼ 1; 2:

ðA3Þ


 
2Cg
A 2Cg ki kj

1
Sij ¼
þ
ij ;
2
c k2
c

1  2’ ¼ g ^  uSt  :
2



ðA4Þ

dij is the Kronecker delta function, Cg = ∣Cg∣, and c = s/k
is the phase velocity. Upon substitution we find





1
Akh
kr  Cg A þ A Cg  rk þ r:hsinh 2kh

Equations (A1) and (A2) are thus equivalent to equations (6)and (7) with a change in the pressure term, which the flow
velocity is independent of, because we neglect wave set up.
[55] Finally, for the interaction equations, we must showthat using the Lagrangian velocity U rather than the Eulerianvelocity u does not affect the wave variables. This follows
since the magnitude of the group velocity dominates themagnitude of the Stokes drift, i.e.,uSt
1 Hrms 2 1 2i¼
   0:02;
Cgi 8
h
8

The bottom drag isU
DL ¼ f ;hwhere we have used the “L” to denote it’s dependence on
the Lagrangian current U, and not the Eulerian current u.
[52] Now suppose U = u + uSt, so that (A2) is equivalent
to (7). Suppose further that the vorticity cSt = r? · uSt is
O(n) compared to the current vorticity c, where n is a small
parameter. Under this assumption, the curl of (A1) is@þ r  u þ uSt
@t

A2 k
;
2 sinh 2kh

where ^ is the quasi‐static response of the waves. When
there is interaction,

where the radiation stress tensor is ¼

C05001WEIR ET AL.: VORTEX FORCE ANALYSIS OF RIP CURRENTS
¼ r?  t þ DL :−2

The parameter n = mf /(sh) is typically O(10 ).

i ¼ 1; 2:Thus, YS03’s interaction model is equivalent to our
WEC+CEW model, and their no interaction model equivalent to our WEC‐only model.
[56] Acknowledgments. This research is supported by the National
Science Foundation under grant DMS‐0723757 and the Office ofNaval Research under grant N00014‐04‐1‐0166. J.M.R. acknowledges
the Institute for Mathematics and its applications, where part of this work
was performed.

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Hall, 603 Charles E. Young Dr., E., Los Angeles, CA 90065‐1567, USA.
J. M. Restrepo and B. Weir, Department of Mathematics, University of
Arizona, Tucson, AZ 85721‐0089, USA. (bweir@math.arizona.edu)

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