2529
Vol. 5
Indian Journal of Science and Technology
No. 4 (Apr 2012)
ISSN: 0974- 6846
The modelling of rip channel in creation of rip currents
Ali Ghorbani and Amirhosein Rasulyjamnany
Department of Civil Engineering, University of Guilan, Iran
[email protected]; [email protected]
Abstract
Rip currents are generally strong shore– normal flows that originate within the surf zone and are directed seaward
through the breakers. Rip currents typically reach speeds up to 1 m/s and some have been reported 2 m/s. These
currents from areas near the coastline begin and develop in surf zone. Many factors could cause this current that could
mention a perpendicular rip channel the shore. In this study, the rip channel was modelled by Mike 21 software, BW
model and the effects of rip channel on creation of rip currents were studied. At first a shore with constant slope was
modelling and then, creating rip channel at centre of the model, modelling take place. Crating rip channels, a powerful
returning current toward offshore was created at the middle of shore. At the point (600 m, 850 m) return current velocity
have a maximum level (1/4 m/s velocity) which changes at difference times showing that rip current is a instantaneous
flow.
Keywords. Surf zone, shore, Rip current, Wave, Bathymetry, Rip channel
Introduction
waves. Scientific investigations of wave and current
Rip current is a narrow, powerful current of water interactions along the coast have demonstrated that rip
running perpendicular to the beach, out into the ocean. currents are likely present on most beaches every day as
These currents may extend 61 to 762 m lengthwise, but a component of the complex pattern of near shore
they are typically less than 9 m wide.
circulation (Fig.1).
Coastal scientists have been investigating rip
As waves travel from deep to shallow water, they
currents for more than 75 years. Generally, this rip eventually break near the shoreline. As waves break
current research has been conducted by several along the coast, they generate currents that flow in both
methods: through field observations and measurements; the offshore (away from the coast) and the alongshore
laboratory measurements and wave tank experiments; directions. The currents flowing away from the coast are
and computer and numerical modelling. The mechanics called rip currents.
of rip current development are complex, and involve
Complex interactions between waves, currents, water
interactions between waves and currents, waves and levels, and near shore bathymetry result in the generation
water levels, waves and the shape of the near shore of rip currents. Rip current systems form an integral part
bottom (bathymetry), as well as wave-wave interaction. of near shore circulation patterns that include both the
Rip currents can occur along any coastline with breaking alongshore and cross-shore (onshore/offshore) water
motion. Along all coastlines, near shore circulation cells
Fig.1. A snap shot of a rip current in Monterey Bay, CA. The rip
may develop when waves break strongly in some
current is the dark patch. There is intense wave breaking on
locations and weakly in others. Quite often the weaker
both sides of the rip currents with little breaking within the
and stronger wave breaking patterns are evident on
deeper rip channel, where bubbles are advected seaward
beaches with a sand bar and rip channel developed in the
(MacMahan & Thornton, 2005).
near shore zone. A rip current forms as the narrow, fastmoving section of water travels in an offshore direction.
Rip currents can also result from a wave’s natural
variability or when a current travelling along the shoreline
encounters a structure such as a groin or jetty and is
forced offshore.
Rip current strength and speed is variable, and the
velocities of some of the daily rip currents may be too
slow to be a threat to many swimmers. However, the
inherent variability of rip currents makes them especially
dangerous to unwary or uninformed beachgoers. Rapid
fluctuations or pulses in wave groups can quickly
generate rip currents with extreme velocities that have
been measured up to 8 feet per second.
In summary, a classic rip current consists of 3
components: i) a rip feeder which carries water along the
beach close to the shoreline; ii) a rip neck which is a
Research article
Indian Society for Education and Environment (iSee)
“Rip currents”
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A.Ghorbani & A.Rasulyjamnany
Indian J.Sci.Technol.
2530
Vol. 5
Indian Journal of Science and Technology
No. 4 (Apr 2012)
ISSN: 0974- 6846
coastal line which can affect on breaking wave pattern
changes and It is due to the movement of water current
from a cross column toward water channels.
d ~
~ ~
d
(U i M j  Si j )   g ( h  ~ )
 Ri
d xj
dx i
(1)
Fig. 2.
Where i,j=1,2, and Mj is the total mean momentum
flux per unit area, Ui is the depth averaged velocity (ui=
mi /(p(h+η)), Sij are the radiation stress, η is the mean
water level and h the still water depth, and Ri are the
stresses. Considering the cross-shore (x) momentum
balance equation, and assuming the waves are
normally incident and bottom contours straight and
parallel, changes in cross-shore radiation stress, Sxx,
are balanced by the hydrostatic pressure gradient,
narrow and fast offshore flow formed by the meeting of
two feeder currents; and iii) a rip head (Fig. 2.)
Rip current can be identified by:
-a channel of churning, choppy water
-an area having a notable difference in water color
-a line of foam, seaweed, or debris moving steadily
seaward
-a break in the incoming wave pattern
Structure of rip currents
Rip currents are formed when the waves and water
are accumulated near the coast and, then, they suddenly
turn back toward the sea. So, the velocity of rip currents
is variable and may, during some minutes, be increased
by interring of bigger waves or instabilities of the
circulation of water. Whatever the rip currents have more
distance to each other they have more velocity, and
whatever the rip currents have less distance it indicates a
less velocity.
Essential structure of rip current is others side of
breaking zone and is determined by fungous propagation
or circulation. This part, in which the velocity and
resistance of rip current is weakened significantly, is
known as the head of rip current. Another part of rip
current is known as the jet of rip current that it is located
in the breaking zone and is defined by slender strip of
water with high velocity. A rip current is more dangerous
when it has more velocity and more rotation. The current
feeders, also, become convergence and appear parallel
before forming the jet of rip current.
Theory of rip currents
All dynamical models of rip currents are forced by
alongshore variations of wave height that result in
alongshore variation in wave-induced momentum flux,
termed radiation stress by Longuet-Higgins and Stewart
(1964). A convenient starting point is the depthintegrated, horizontal momentum balance equation that
are averaged over many wave groups representing
stationary wave condition (Phillips,1977).
The perception of rip current ordering is very
important and determinant to foresee its occurrence. Rip
currents can be determined by depth changes in the
Research article
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dsxx
d~
  g (h  ~)
dx
dx
(2)
Where ρ is the density of seawater and g is the
gravitational acceleration. Applying linear wave theory in
shallow water, Sxx=(3/2)E, where E is wave energy,
results in wave set-down outside the surf zone as the
wave energy increases due to shoaling, and set-up inside
the surf zone as the waves break and the energy
decreases (Bowen,1969).
The vertical imbalance between the cross-shore
pressure gradient and wave forcing produces an offshore
return flow within the surf zone between the bed and the
trough that often is referred to as undertow (Dyhr-Nielsen
& Sorensen,1970). In the tree-dimensional case of
alongshore variations in wave height of normally incident
waves on an alongshore uniform beach, the larger waves
generate larger set-down/up, which creates alongshore
pressure gradients both outside and inside the surf zone.
Considering the alongshore (y) momentum equation,
forcing is given by
Fy 
d~
 g (h  ~ )
dy
dy
dS yy
(3)
Where Syy=E/2 in the shallow water. Out of the surf
zone, alongshore pressure gradients are balanced by the
along shore gradients in radiation stress. However, inside
the surf zone, the gradients in the alongshore radiation
stress and pressure act together to produce a flow of
water from the regions of high waves to the regions of low
waves. Approximating wave breaking as proportional to
the total depth of water nH=γ(h+η), and E=1/8ρgH2, the
alongshore momentum balance in the surf zone is given
by:
d ( h  ~ )
d~
(4)
Fy   1  gy 2 ( h  ~ )
 g ( h  ~ )
8
dy
dy
Starting with models on alongshore homogeneous
beaches, Dalrymple and Lozano (1978) imposed
alongshore wave height variations to show that refraction
by the outgoing rip current causes the waves to impinge
on the beach obliquely, generating convergent long shore
“Rip currents”
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A.Ghorbani & A.Rasulyjamnany
Indian J.Sci.Technol.
2531
Vol. 5
Indian Journal of Science and Technology
currents, which then flow offshore as a rip current. The
result is a self-sustaining rip current, although the initial
perturbation on the wave heights is not addressed.
Alongshore variation of wave height can also be the result
of wave-wave interactions.
Bowen (1969) was the first to show that alongshore
perturbations in bathymetry result in alongshore
variations in wave height, which generate rip currents. A
number of models have followed with various
refinements, which include improved nonlinear bottom
shear stress and turbulent momentum mixing (Noda,
1974). More recent models, although forced by
monochromatic waves, allow for time dependence,
include nonlinear Bossiness Q waves in which wavecurrent interaction is incorporated (Sorensen et al.,1998),
and have a quasi-3d formulation (Haas et al., 2003).
Wave-current interaction is included by Yu and Slinn
(2003), who find that the rip current produces a negative
feedback on the wave forcing to reduce the strength and
offshore extent of the flow. Complex flow patterns result
with instabilities formed at the feeder currents with the
unsteady rip flow characterized by vortex shedding.
The results and analysis of modelling
At first shore with constant slope is modelled. In this
modelling the velocity and height changes is the same
and don’t create any rip current along the shore. Then
shore with rip channel is modelled. One of main reasons
of creation of rip currents is normal rip channel on the
shore. Consequently, here we study this characteristic
and its effects on creation of rip current. To study this
characteristic, initially, a shore without rip channel and
with 3.3 % slope is considered and model making is done
for it. In the next step, a shore with 3.3 %slop with a rip
channel at the middle of it is considered and the
modelling is performed for it (Fig. 3). Within two
No. 4 (Apr 2012)
ISSN: 0974- 6846
Fig. 4. Snapshot of Instantaneous velocity perpendicular
to shore (u) in the times of t=7, 16, 19 min for figures of
a, b, c, respectively.
b)
Fig. 3. Model setup for the rip channel test.
Contour plot of the bathymetry
c)
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Fig. 5. Velocity changes perpendicular to shore, the line (- - )
represent the velocity in the coordination of (896 m , 570 m) and
the line of ( - - ) represent the velocity in coordination of (850 m
, 600 m), the line (
)
700
750
800
850
900
1.5
1
0.5
0
-0.5
-1
-1.5
distance ( m )
Fig. 8. Velocity changes (u) at out of beach centre in the
seventh minute. The blue and red lines represent shore with
rip channel and shore with constant slope, respectively.
Fig. 6.Velocity changes (u) at centre of channel
in range 700-900 m
Fig. 9. Height changes of water along the shore
where with the length of x = 850 m
The surface rollers are shown in white. Due to the
increased depth and due to depth refraction by the rip
channel, incipient breaking is seen to occur comparatively
close to the shore along the centreline. Here, the setup is
quite small and the larger setup appearing away from the
rip channel gives an alongshore gradient in the mean
water surface forcing a current towards the centre line.
The flow from the two sides join to form a rip current
and two symmetrical circulation cells appear. It is seen
that three nearly circular areas are located m, in (896
570 m), (896 m, 630 m) and (850 m, 600 m) which have
the velocity higher than 1.2 m per second in direction of
Research article
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ISSN: 0974- 6846
Fig. 7. Velocity changes (u ) at centre of beach with
constant slope in range 700-850 m
u ( m/s )
presented model in this study, domain ranges are 1200m
* 1200m, the height of regular waves is 2.8m, its period is
7.9 s (the total time of model making is 20 second).
Contour plots of the shore- perpendicular velocity
(shown by u) are shown in the Fig.4. These pictures were
taken at times of 7,15,19 minutes after starting of the
simulation when the rip current is created by two circular
cells. Fig.4, a, b, c show that the rip current face to
increase of speed at first and then decrease of speed.
These changes say that the rip current is an
instantaneous current and shows that the speed inside of
this current has oscillation in every time.
No. 4 (Apr 2012)
the offshore. These velocities threatened the life of any
swimmers. Given the critical nature of these points,
speed changes for shore with constant slope at point (850
m , 600 m)and shore with rip channel at point (850 m ,
600 m)and (896 570 m),shown at Fig. 5. Speed in rip
current is higher than shore with stable slope. Maximum
speed in this point is equal to 1.4 and 0.1 m/s,
respectively speed in shore with constant slope has little
changes but in rip channel to be many changes because
exist rip current.
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Since the speed changes in center of channel have
the most critical mode so speed changes (u) in centre of
channel at 700-900 m are shown in Fig. 6 that this figure
show speed in the fifth minute. In this period, speed
changes are very high at near the shore and whatever go
to sea, return current speed increases but in 850 m
length, The slope of the curve will change and the return
current speed will increase. The speed changes in this
range are less than the areas that is in near shore. In Fig.
7, speed changes in centre of beach with constant slope
are shown in the fifth minute. By comparing these two fig
6 and 7, it is clear that speed changes in both the beach
are similar but have different values. But these changes
in out of beach centre have similar values (Fig. 8).
Rip channel causes the interaction between incoming
waves and toward offshore currents and also causes a
wave height decrease in the centre of rip channel to its
sides ( in the distance less than 200 m from the centre of
rip channel), as shown in Fig. 9. But the wave height at
the centre of the channel is variable to the areas far from
the centre of rip channel (in the distance more than 200 m
from the centre of rip channels) in different times, as in
the time of t = 16 min and t = 17 min, changes of the
height of wave in the centre are respectively less and
more than areas which are far from the centre of channel.
Conclusion
Rip currents change sediment transport system and
shore currents, also is considered a threat to swimmers.
Rip currents creation have two circular currents and one
rip neck that goes to offshore. The return current speed at
rip neck is more than the other part and also more than
the shore without rip channel. With movement from shore
to offshore, the return speed increases at first and then
decreases again.
References
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investigations. J. Geophys. Res. 74, 5467–5478.
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No. 4 (Apr 2012)
ISSN: 0974- 6846
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“Rip currents”
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Indian J.Sci.Technol.