Journal
Journalof
ofCoastal
CoastalResearch
Research
SI 64
pg -- pg
917
921
ICS2011
ICS2011 (Proceedings)
Poland
ISSN 0749-0208
Verification of the Rip Currents predictive model using the tube image
detecting techniques
J.Y. Lee† and J.L. Lee§
† Department of Civil and
Environmental Engineering
Sungkyunkwan University, Suwon
440-746, Korea
[email protected]
§Department of Civil and
Environmental Engineering
Sungkyunkwan University, Suwon
440-746, Korea
[email protected]
ABSTRACT
LEE, J.Y. and LEE, J.L., 2011. Verification of the Rip Currents predictive model Using the tube image detecting
techniques. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium),
. Szczecin, Poland, ISSN 0749-0208
At Haeundae Beach in Busan which is located in the south east part of South Korea, more than 100 people were
swept away by the fast-moving seaward current on August 13th and 15th in 2009 and on July 29th and 30th in
2010 and were later rescued. In predicting the wave-induced current and consequent tube movement for public
safety, the coastal flows and waves are produced at each time step and grid point by means of systematic
interfacing of hydrodynamic and wave models (Lee, 2001). To provide a user-friendly simulation tool for endusers, the forecasting system has been built in a software package called HAECUM. Outputs from the system are
viewed as graphs of tube positions with combined current vectors for ease of decision-making by emergency
management officials. The tube-wave interaction is taken into account and the traces of swim tube are simulated
by using a Lagrangian random walk (Chorin, 1978; Lee, 1994). In this study, we use Lee’s approach (Lee, 1993)
in estimating the surface onshore currents due to wave breaking.
ADITIONAL INDEX WORDS: HAECUM; Haeundae beach; random walk; rip current; tube movement;
numerical simulation
INTRODUCTION
Rip currents, which are narrow currents initiated close to the
shoreline, flow strongly in a seaward direction through the surf
zone and beyond. Rip currents are an integral component of
nearshore circulations along many of the world’s beaches, and
represent an important mechanism for the seaward transport of
water and sediment (Cook, 1970; Inman et al., 1971; Aagaard et
al., 1997). They also have a pronounced effect on nearshore
morphology (Sonu, 1972; Greenwood and Davidson-Arnott, 1979;
Short, 1979, 1992; Wright and Short, 1983, 1984), and present a
major hazard to beach users (Lushine, 1991; Short and Hogan,
1994). Rip currents are of considerable interest, both scientifically
and publicly. However, our current understanding of their spatial
and temporal behavior is limited. Unfortunately, a lack of suitable
field data has precluded the application of a the morphodynamic
approach to natural rip current systems.
Rip currents occur when waves approach the beach in parallel
lines or when strong winds push water onshore. Water rushes
directly up the beach and the undertow returns it directly back
down the beach slope. The undertow is unable to return all the
water before the next wave (or wind) pushes it back up the beach.
The piling up of water cannot continue, so the excess water will
flow up or down the beach until it approaches an imperfection, at
which point the excess water flows back to the sea in a narrow,
concentrated rip current.
Rip current has recently become a well-known technical term in
Korea. At Haeundae Beach in Busan, more than 100 people were
swept away by the fast-moving seaward current, on Aug, 2009 and
on Aug, 2010 and were later rescued. Haeundae beach in Busan is
located in the south east region of South Korea. The
holidaymakers, who were using tubes to ride the waves near the
shore were carried away seawards, floating as far as 50 ~ 100
meters away from the shore where swimming is restricted for
safety reasons. Coast guards rescued the victims with boats and jet
skis. A similar situation has occured more than 5 times this
summer, and 106 people were saved at 2009 and 140 people were
saved in 2008 for the same reason.
In the case of this summer, rescue workers covering Busan’s
beaches are on the highest alert for repeated rip currents, which
together form a strong channel of water flowing seaward from the
shore. Nearly 100 people at Haeundae Beach have needed to be
rescued over the past July and August after being dragged into
deeper waters by the currents, which can be as fast as 2.5 meters
per second. Fortunately, no casualties were reported but the
authorities stated that, given that the ideal weather conditions for
rip currents are persisting, they could occur again at any time at
Haeundae Beach.
The latest rip current at Haeundae occurred around noon on
31th July, dragging 10 swimmers as far as 50 meters away from
the shore. The swimmers were immediately pulled out of the
water by rescue personnel. On 30th July, as shown in Figure 1,
another reverse current at Haeundae swept away 20 beachgoers,
taking them as far as 30 meters from the beach. The first strong rip
current of the year occurred in Haeundae on 29th July. Of the 26
people affected, one was hospitalized for an excessive intake of
salt water. Rescuers are paying particular attention to the currents
in front of the Paradise Hotel as this is where currents frequently
occur.
Journal of Coastal Research, Special Issue 64, 2011
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Verification of the Rip Currents predictive model Using the tube image detecting techniques
Figure 1. Rip Current generation at Haeundae beach (30th
July, 2010)
Maritime experts explained that a rip current is created when
wind and waves push water toward the shore. This water is often
forced sideways by the oncoming waves. The water streams along
the shoreline until it finds an exit back to the sea, creating a rip
current on the way out. Last year, a total of 42 rip currents were
recorded on Busan beaches. In response, the Korean
Meteorological Administration plans to set up monitoring
equipment to assess weather conditions and issue rip current
warnings on Busan’s beaches from this year.
In this paper, we apply tube image detecting techniques using
the numerical simulation which was installed for Haeundae Beach,
Korea. Using these techniques, we verify the rip current
forecasting model. This model is based on the Particle Image
Velocimetry (PIV) method which is a non-intrusive optical
measurement technique allowing the two-dimensional crosssection of the entire flow field to be instantaneously captured. To
provide a user-friendly simulation tool for end-users, the
forecasting system has been built in a software package called
HAECUM(HAEundae CUrrent Model).
Using this GUI system, HAECUM, we can prevent dangerous
rip currents accident before they occur.
SITE DESCRIPTION
The studies are carried out for investigating the rip-current
occurring at the nearest sea to Haeundae Beach. The beach is
located at the southeastern part of the Korean peninsula and has
the latitude of 35.16 and the longitude of 129.17. Because of the
large population of Busan city, the beach always has many visitors
throughout the year. Especially, during the summer vacation
season, over one million people visit the beach on a peak day.
However, the beach is extremely small with a length of only 2 km,
as shown in Figure 2.
The very high density of visitors results in the arrangement of
many accommodation buildings behind the beach. The buildings
hinder the seaward wind direction mainly at night, and this can
decrease the incident wave height. This is because the beach has
been exposed to higher wave height and beach erosion. The
average width of the beach is within about 50 m.
Because of the small size of the beach, the high density of
people and the requirement to act as a city park, the occurrence of
rip current demands a higher level of attention from the
authorities’. Moreover, the presence of several oversized sunkenrocks located a short distance from the beach results in a very
uneven morphology change. The very uneven water depth
Figure 2. Rip current occurrence point with frequency
facilitates a distribution of wave breaking. This is because the
wave current due to the hydraulic gradient of the mean water level
change would be directed seaward by the almost identical current
nearby.
Haeundae Beach has an average seafloor inclination of 1/15 to
1/30 and a wave-dominated pocket shape with a softer inclination
along the eastern side. According to the tidal characteristics
observed by the Busan Harbour tidal observation station,
Haeundae Beach has an average sea level of 64.9 cm, a spring
tidal range of 123.8 cm and a neap tidal range of 86.0 cm (Table
1). In addition, the representative wave directions (θ) are southeast
(SE), south (S) and southwest (SW) with the mean waves reaching
Table 1: Non-harmonic constant data from the Busan Harbour
tidal observation stationg (35˚06´N.129˚02´E) near the research
area.
Mean highest water interval (M.H.W.I)
Observed highest high water (Obs.H.H.W)
Mean sea level (M.S.L)
Observed lowest low water (Obs.L.L.W)
Spring range
Neap range
129.8cm
42.2cm
08h02m
-41.0cm
42.2cm
42.2cm
Table 2: Normal incident wave conditions at Haeundae Beach
Representative dir.
H1/3(m)
T1/3(sec)
θ(˚)
SE
S
SW
0.68
0.68
1.00
5.7
5.7
6.0
S45˚E
S
S19˚W
a significant wave height (H1/3) of 0.68 to 1.0 m and having a
wave period (T1/3) of 5.7 to 6.0 sec (Table 2, Figure 3).
MODEL DESCRIPTION
In predicting the wave-induced current and consequent tube
movement for public safety, the coastal flows and waves are
produced at each time step and grid point by means of systematic
interfacing of hydrodynamic and wave models (Lee, 2001). The
flow velocity consists of tide, wave induced current and free
Journal of Coastal Research, Special Issue 64, 2011
918
J.Y. Lee and J.L. Lee
Figure 3. Frequency of occurrence between wave height and
direction at Haeundae beach (Lee et al.,2007)
surface flow due to wave. The tube moves by flow velocity unlike
the sea water particles, as it is somewhat delayed due to particle
interaction (Lee, 1993). If the tubes come out of the water, and
move to the land, the location of the tubes can be altered to the
water dimension by using a mirror image method and if tubes
escape the open boundary to the open sea, they are overlooked.
To provide a user-friendly simulation tool for end-users, the
forecasting system has been built in a software package called
HAECUM(HAEundae CUrrent Model). Figure 4. shows the flow
chart of a numerical model system, HAECUM which has been
built in a modular way with two sub-models; the Wave model
using radiation stress and the Current model. The flow velocity
and water level due to tide induced combined-flow are determined
by solving the depth-integrated equations of mass and motion (Lee
et al., 2004).
The forcing terms in this model include surface pressure,
surface shear stress due to wind, bottom friction, Coriolis effects,
and tide potential. Radiation stresses are fed back into the
circulation model to calculate the wave-induced current. This
model is solved using a fractional step method in conjunction with
approximate factorization techniques leading to the implicit finite
Figure 5. Rip current occurrence points with frequency
difference scheme
Outputs from the system are viewed as graphs of tube positions
with combined current vectors for ease of decision-making by
emergency management officials.
The tube-wave interaction is taken into account and the traces of
swim tube are simulated by using a Lagrangian random walk
(Chorin, 1978; Lee, 1994). In this study, we use Lee’s approach
(Lee, 1993) in estimating the surface onshore currents due to wave
breaking. The tube detection technique using video imaging will
be used as a tool for improving rip current predictive capabilities.
NUMERICAL RESULTS
At Haeundae Beach, rescuers have strengthened the lookout
measures at several locations where rip currents have occurred.
The beach management authorities found that rip currents mainly
occur at three locations at Haeundae Beach as shown in Figure 5.
Several rip currents have been observed at Haeundae Beach in
the past few years. Among them, we focus on the rip currents that
were observed on July 29 and July 30, 2010 and on August 13 and
August 15, 2009. In the case of July 30, 2010, the flow pattern
captured from the CCTV image is shown in Figure 1.
To verify the agreements of the numerical model results and the
true phenomenon, the rip current, we carried out a series of
numerical simulations of the rip current using HAECUM. The
computation wave conditions are listed in Table 3. We carried out
all numerical simulations using the wave data supplied by
Table 3: Computation wave conditions for the HAECUM
Case
1
2
3
4
Figure 4. Flow chart of a numerical model system (HAECUM)
Date
th
August 13 , 2009
August 15th, 2009
July 29th, 2010
July 30th, 2010
H1/3(m) T1/3(sec)
0.65
0.62
1.52
1.07
10.0
10.2
11.8
10.9
θ(˚)
S2˚W
S5˚W
S17˚W
S15˚W
KMA(Korea Meteorological Adiministration).
The results of the simulations are shown from Figure 6. to
Figure 9. The arrows in the figure represent the calculated wave
induced current vectors. At the same time, the tube-wave
interaction is taken into account and the traces of swim tube are
simulated by using a Lagrangian random walk. The black dots in
the figure signify swim tubes. The tubes move by flow velocity
unlike sea water particles, as they are somewhat delayed due to
Journal of Coastal Research, Special Issue 64, 2011
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Verification of the Rip Currents predictive model Using the tube image detecting techniques
particle interaction(Lee, 1993). The squares in the figures
represent the place where the rip currents phenomenon occurred at
that time and the numbers expressed in the figure refer to the
lookout tower address at Haeundae Beach.
Compared with the real locations where the rip currents
occurred, we judged that our numerical model together with the
parameters used in the simulation is reasonable and appropriate.
Figure 8. Simulation results for seaward drifting of swim tubes
due to rip currents (Case 3; 2010.7.29; Wave height:152cm,
Wave period:11.8sec, Wave angle:197°)
Figure 6. Simulation results for seaward drifting of swim tubes
due to rip currents (Case1; 2009.8.13; Wave height:65cm,
Wave period:10sec, Wave angle: S2˚W )
Figure 9. Simulation results for seaward drifting of swim tubes
due to rip currents (Case 4; 2010.7.30; Wave height:107cm,
Wave period:10.9sec, Wave angle:195°)
Figure 7. Simulation results for seaward drifting of swim tubes
due to rip currents (Case 2; 2009.8.15; Wave height:62cm,
Wave period:10.2sec, Wave angle: S5˚W )
CONCLUSION
When surface waves break on a beach, wave energy is lost to
the turbulence generated in the process of breaking, and wave
momentum is transferred into the water column thus generating
nearshore currents. There are two current systems of which the
flow structures are predominantly horizontal, alongshore currents
caused by obliquely incident waves and cell-like circulations,
which can occur when waves are nearly at normal incidence.
Often described as narrow, jetlike, and seaward directed flows, rip
currents are part of these cellular circulations, fed by the
converging alongshore flows close to the shoreline. Rip currents
can cause the seaward transport of beach sand, and hence have a
direct impact on beach morphology. On the other hand, the
circulations may produce sufficient exchange of nearshore and
offshore water, thus providing a flush of the nearshore region
affecting the across-shore mixing of heat, nutrients, chemicals,
and biological species.
In the past few years, rip currents have been observed at three
points on Haeundae Beach, as shown in Figure 5. In this study, we
apply tube image detecting techniques using the numerical
simulation which was installed for Haeundae Beach, Korea. Using
these techniques, we verify the rip current forecasting model. We
carried out a series of numerical simulations to verify the
Haeundae current model, HAECUM. Compared with real
locations where rip currents occurred, we judged that our
numerical model together with the parameters used in the
simulation is reasonable and appropriate.
Although the forecasting system developed provides good
estimations of the occurrence, location and intensity of rip currents,
several improvements need to be implemented. First, since a
precise bathymetry is a requirement to obtain accurate wave
conditions on the beach, a video based system would be useful to
obtain this information at a lower cost.
The intensive field experiments carried out at Haeundae Beach
from July 27th to August 20th, 2010 allowed the collection of data.
These in-situ measurements data will be used to validate our
modeling approach.
Journal of Coastal Research, Special Issue 64, 2011
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J.Y. Lee and J.L. Lee
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ACKNOWLEDGEMENT
This work was funded by the Korea Meteorological
Administration Research and Development Program under Grant
CATER 2010-1184.
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