Analysis of Rip Current Characteristics Using Dye Tracking Method

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Article Analysis of Rip Current Characteristics Using Dye Tracking Method

Hyun Dong Kim 1,* and Kyu-Han Kim 2,*

1 Department of Civil Engineering, Osaka University, Suita Yamadaoka 2-1, Osaka 565-0871, Japan 2 Department of Civil Engineering, Catholic Kwandong University, Saimdang, Gangneung 641-28, Korea * Correspondence: [email protected] (H.D.K.); [email protected] (K.H.K.)

Abstract: Rip currents are strong water channels flowing away from the shoreline. They can occur on any shore with breaking waves. Rip currents play a significant role in changing the topography of shallow water regions by transporting large amounts of bed material offshore. Moreover, they pose a significant danger for people living in nearshore zones and surfers and cause hundreds of deaths annually worldwide. Therefore, rip current generation characteristics have been investigated to prevent casualties. In this study, a GPS drifter survey was chosen as the investigation method; however, a few drawbacks were discovered, such as low accuracy due to the GPS drifter becoming trapped in the surf zone. Therefore, drones and dyes were used to overcome the drawbacks of drifter methods. The results of dye tracking and the 3D wave-induced current numerical simulation were compared; the velocity and formation of the rip current were found to be relatively similar. With the technological advancements and invention of new survey equipment, the survey techniques also evolve, and this paper shows that the disadvantages of the GPS-based Lagrangian method can be overcome using a dye-mounted drone, which observes the rip current easily and accurately. Citation: Kim, H.D.; Kim, K.-H. Analysis of Rip Current Keywords: rip current; drones dye tracking; bathymetry; topography; field investigation Characteristics Using Dye Tracking Method. Atmosphere 2021, 12, 719. https://doi.org/10.3390/ atmos12060719 1. Introduction

Academic Editors: Rip currents are fast seaward water currents that flow in a narrow channel from the Halina Kowalewska-Kalkowska and surf zone [1]. As the waves approach the shore, the depth of water becomes shallower and Anna Cedro produces breaking waves; these are a form of energy-carrying wave current. As the wave size increases, the amount of seawater that enters increases, and the waves break at the Received: 27 April 2021 surface as the waves with longer wavelengths push toward the shore. When no waves are Accepted: 30 May 2021 approaching the shore at an angle and meet at a spot where the breaker is low, a strong Published: 3 June 2021 and quick seaward flow, called the rip current, develops because the accumulated seawater cannot flow as a longshore current anymore [2]. Publisher’s Note: MDPI stays neutral Rip currents are a significant cause of deaths in coastal regions. The damages caused with regard to jurisdictional claims in by rip currents are increasing every year, and they affect the United States and Australia published maps and institutional affil- more compared to other countries as they have a greater number of large beaches as well iations. as tourists [3]. An increasing number of accidents caused by rip currents are reported every year, indicating the pressing need for a more accurate analysis of rip current characteristics. A numerical rip current prediction can be conducted regularly and optimized using data about rip current features obtained from field observations. In addition, the availability of Copyright: © 2021 by the authors. a large amount of field observation data is expected to improve the accuracy of existing Licensee MDPI, Basel, Switzerland. prediction technologies. This article is an open access article Since predicting the rip current generation with exact location is very difficult and distributed under the terms and unexpected rip currents can cause sudden casualties, it is still argued that establishing conditions of the Creative Commons adequate countermeasures against the damage is difficult [4]. Although researchers are Attribution (CC BY) license (https:// delving into ways of obtaining the location of rip currents in real-time, it is a challenging creativecommons.org/licenses/by/ problem due to limited technology. Moreover, the generation of rip currents is influenced 4.0/).

Atmosphere 2021, 12, 719. https://doi.org/10.3390/atmos12060719 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 12

Atmosphere 2021, 12, 719 2 of 12 delving into ways of obtaining the location of rip currents in real-time, it is a challenging problem due to limited technology. Moreover, the generation of rip currents is influenced by numerous factors such as the size and regularity of waves, tide, wave direction, and shorelineby numerous structure factors [5]. such The as size the and size regulari and regularityty of waves of waves, determine tide, the wave amount direction, of shore- and wardshoreline flow; structure in addition, [5]. The the sizetide and can regularityaffect the ofpower waves of determine the shoreward the amount flow, and of shoreward the wave directionflow; in addition, and shoreline the tide structure can affect decide the power the wa ofves’ the shorewardshoreline approaching flow, and the angle wave [6]. direction These factorsand shoreline make it structure difficult to decide predict the and waves’ track shoreline rip currents approaching and identify angle their [6]. characteristics. These factors makeQuite it difficult fittingly, to predict in light and of trackthese rip realities, currents many and identifyresearchers their have characteristics. developed a risk scoreQuite calculation fittingly, method in light categorizing of these realities,the rip currents many occurrences researchers into have score developed brackets a con- risk sideringscore calculation the state of method sea weather, categorizing bathymetric the rip topography, currents occurrences and their regional into score characteris- brackets ticsconsidering to be able the to statepredict of seasuch weather, currents bathymetric in the future topography, [1,4,6]. In Korea, and their these regional efforts charac-are re- teristics to be able to predict such currents in the future [1,4,6]. In Korea, these efforts are flected in the indigenous development of a rip current alarm system, which provides rip reflected in the indigenous development of a rip current alarm system, which provides rip current predictions in 3-h intervals, by the Korea Hydrographic Oceanographic Agency current predictions in 3-h intervals, by the Korea Hydrographic Oceanographic Agency (KHOA) [7]. (KHOA) [7]. This study presents a method for rip current measurement, which is based on meas- This study presents a method for rip current measurement, which is based on measur- uring the average flow velocity by dropping a GPS drifter following the Coastal Improve- ing the average flow velocity by dropping a GPS drifter following the Coastal Improvement ment Project Design Guidebook of 2010 [8]. Because GPS drifter observation has draw- Project Design Guidebook of 2010 [8]. Because GPS drifter observation has drawbacks of backs of low accuracy, an alternative dye tracking method is then utilized to measure and low accuracy, an alternative dye tracking method is then utilized to measure and locate rip locate rip currents and identify their movement paths. Numerical simulations are carried currents and identify their movement paths. Numerical simulations are carried out, and out, and the results are directly compared with the field observations. the results are directly compared with the field observations.

2. Identifying the Area of Expected Rip Current Occurrences As mentioned above, nearshore bathymetrybathymetry isis oneone ofof thethe factorsfactors affectingaffecting the the genera- gener- ationtion of of rip rip currents. currents. Therefore, Therefore, our our aim aim was was to to collect collect the the latest latest bathymetric bathymetric information information of ofthe the target target sea sea before before conducting conducting an an on-site on-site investigation investigation and and data data analysis analysis ofof riprip current characteristics. Haeundae Haeundae Beach Beach shown shown in in Figu Figurere 1 1was was chosen chosen as asthe the target target sea, sea, and and the requiredthe required investigation investigation and and coastal coastal bathymetric bathymetric change change analysis analysis were were performed. TheThe bathymetric information was then used in the numericalnumerical simulation for obtainingobtaining basic data to identify and predict the frequent rip current occurrence locations, which are called rip current hot spots in this study. The reason forfor selectingselecting Haeundae Beach as the target beach is thatthat KHOAKHOA mainlymainly focused focused on on preventing preventing damages damages due due to to the the frequent frequent rip rip current cur- rentoccurrences occurrences at Haeundae at Haeundae Beach, Beach, and itand is one it is ofone the of main the main beaches beaches where where a large a large number num- of peopleber of people are exposed are exposed to rip currentto rip current danger danger as it has as the it has highest the highest number number of visitors of visitors during duringthe season. the season.

Figure 1. The study location of Haeundae Beach in Busan,Busan, South Korea (images from GoogleGoogle Earth).Earth).

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2.1. Survey and Measurements for the Numerical Simulation Data 2.1. Survey and Measurements for the Numerical Simulation Data To understand the general topographical changes of the targeted ocean over time, To understand the general topographical changes of the targeted ocean over time, the the coastal topography and bathymetry along Haeundae Beach’s surf zone (width ~100– coastal topography and bathymetry along Haeundae Beach’s surf zone (width ~100–120 m) 120 m) were investigated to analyze the evolution of Haeundae Beach over the years from were investigated to analyze the evolution of Haeundae Beach over the years from 2011, 2011, 2014, and 2019. For the topography of sandy beach, the Virtual Reference Station 2014, and 2019. For the topography of sandy beach, the Virtual Reference Station method, method, which is a real-time kinematic surveying technique, was conducted over an area which is a real-time kinematic surveying technique, was conducted over an area of 1.4 km in of 1.4 km in length and 2 m in depth [9], and for the ocean bathymetry, a detailed sea floor length and 2 m in depth [9], and for the ocean bathymetry, a detailed sea floor topographical topographical survey was conducted using a multibeam echo sounder. This equipment survey was conducted using a multibeam echo sounder. This equipment uses the swath uses the swath coverage method. Each measurement covers tens of multiple times of the coverage method. Each measurement covers tens of multiple times of the water depth and water depth and uses the Seabat 7125 Multibeam Sonar, which has a frequency of 200 kHz uses the Seabat 7125 Multibeam Sonar, which has a frequency of 200 kHz with 256 beams. with 256 beams. The beach profiles were observed and recorded at 50 m intervals, and The beach profiles were observed and recorded at 50 m intervals, and among them, three amongspecific them, beach three profiles specific showing beach the profiles most show criticaling differences the most critical were compared, differences as were shown com- in pared,Figure as2. Byshown comparing in Figure these 2. By bathymetry comparing surveys these bathymetry near the surf surveys zone from near 10 the April surf (2011), zone from7 May 10 (2014),April (2011), and 17 7 May June (2014), (2019), and it was 17 June found (2019), that it the was overall found depth that the and overall bathymetry depth andhad bathymetry changed continuously had changed due continuously to the continuous due to the nourishment continuous and nourishment sediment and transport, sedi- mentbecause transport, the continuous because beachthe continuous erosion left beach the governmenterosion left ofthe Korea government with only of theKorea quickest with onlyoption, the quickest i.e., nourishment, option, i.e., to nourishment, protect the beach to protect from the erosions beach from and disappearing.erosions and disap- As a pearing.result, the As beacha result, bathymetry the beach bathymetry changes irregularly changes irregularly due to the due stacked to the artificial stacked sedimentartificial sedimentsupply at supply the beach at the in beach certain in certain locations; locations; however, however, the overall the overall bathymetry bathymetry of Haeundae of Hae- undaeBeach Beach does notdoes change not change critically critically over timeover astime shown as shown in Figure in Figure2. The 2. latest The latest bathymetric bathy- metricinformation information (17 June (17 2019,June 2019, which which is the is exact the exact date date of the of fieldthe field survey), survey), was was used used for for the thenumerical numerical simulations. simulations.

FigureFigure 2. 2. SurveyedSurveyed bathymetry bathymetry and and beach beach transect transect along along Haeundae Haeundae Beach Beach (A (A-2011,-2011, BB- -2014,2014, C-2019).-2019).

TheThe grain grain size size analysis analysis of of marine marine sediment sediment is isconsidered considered a basic a basic tool tool in inmarine marine envi- en- ronmentalvironmental research. research. The The sand sand grain grain size size distribution distribution of the of thebeach beach acts actsas a asrun-up a run-up for the for the waves and affects its morphology and slope, ultimately impacting the formation of waves and affects its morphology and slope, ultimately impacting the formation of rip rip currents. Therefore, twenty-one sand samples were collected using Van Veen grab, currents. Therefore, twenty-one sand samples were collected using Van Veen grab, con- constituted by two jaws, to collect superﬁcial sediments. The sampling criteria and instru- stituted by two jaws, to collect superficial sediments. The sampling criteria and instru- ments selected for marine sediments are generally functional for obtaining an undisturbed ments selected for marine sediments are generally functional for obtaining an undisturbed sample. Because the textural characteristics of sediments and their distribution are key sample. Because the textural characteristics of sediments and their distribution are key factors in marine environmental research, great attention should be given to the accuracy factors in marine environmental research, great attention should be given to the accuracy of grain size distribution. The grain size analysis of marine sediments started with pre- of grain size distribution. The grain size analysis of marine sediments started with pre- treatment. The saline content was removed by immersing the sample in distilled water, treatment. The saline content was removed by immersing the sample in distilled water, and the coarse and ﬁne fractions were separated by wet sieving [10,11]. The sampling and the coarse and fine fractions were separated by wet sieving [10,11]. The sampling points are indicated by red and blue dots in Figure3. For determining the size of sand pointsparticles are in indicated millimeters by (mm),red and a phiblue scale dots was in Figure used, and3. For the determining analyzed sand the characteristics size of sand particleswere then in usedmillimeters as sediment (mm), input a phi parameters scale was inused, the and numerical the analyzed simulations. sand characteristics The sand grain were then used as sediment input parameters in the numerical simulations. The sand

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graingrain size size distribution distribution showed showed an an average average particle particle diameter diameter ranging ranging from from 0.305 0.305 to to 1.209 1.209 size distribution showed an average particle diameter ranging from 0.305 to 1.209 mm, and mm,mm, and and fine fine (~0.25 (~0.25 mm) mm) to to medium medium (~0.5 (~0.5 mm) mm) sand sand was was dominant dominant in in the the overall overall compo- compo- ﬁne (~0.25 mm) to medium (~0.5 mm) sand was dominant in the overall composition. sition.sition.

FigureFigure 3. 3. Grain GrainFigure characteristics characteristics 3. Grain characteristics of of the the sand sand sampling of sampling the sand along along sampling Haeundae Haeundae along Beach.Haeundae Beach. Beach.

AllAll the the collected collectedAll images images the collected of of Haeundae Haeundae images Beac Beac of Haeundaeh hwere were processed processed Beach werewith with processedorthometric orthometric with correc- correc- orthometric correc- tionstions using using the the followingtions following using steps: thesteps: following (1) (1) Take Take imag steps: imageses (1) of of TakeHaeundae Haeundae images Beach Beach of Haeundae with with Ground Ground Beach Control Control with Ground Control PointsPoints (GCPs). (GCPs). (2)Points (2) Continue Continue (GCPs). taking taking (2) Continuemeasurem measurem takingentsents between between measurements the the GCPs GCPs between from from each each the image GCPsimage for from for each image distortiondistortion correction. correction.for distortion (3) (3) Proceed Proceed correction. with with the the (3) correction Proceedcorrection with of of distorted distorted the correction images images of by distortedby matching matching images the the by matching measuredmeasured GCPs GCPsthe [12]. [12]. measured At At least least GCPsfour four GCPs [GCPs12]. Atmust must least be be fourincluded included GCPs in mustin each each be image image included to to perform perform in each dis- imagedis- to perform tortiontortion correction; correction;distortion however, however, correction; to to increase increase however, the the accuracy accuracy to increase of of the the the distorted distorted accuracy images, images, of the adistorted atotal total of of 174 images,174 a total of GCPsGCPs were were surveyed surveyed174 GCPs from from were onshore onshore surveyed and and fromoffshore offshore onshore areas areasand for for image offshore image distortion distortion areas for correctionimage correction distortion as as correction shownshown in in Figure Figureas 4: shown4: 87 87 GCPs GCPs in Figure from from 4onshore :onshore 87 GCPs an an fromd doffshore, offshore, onshore separately. separately. and offshore, The The separately.inherent inherent average average The inherent average horizontalhorizontal error errorhorizontal in in this this process process error was in was this approximately approximately process was approximately8.5 8.5 to to 13.7 13.7 cm. cm. 8.5 to 13.7 cm.

Figure 4. The Ground Control Point (GCP)-surveyed points along Haeundae Beach. FigureFigure 4. The 4.GroundThe Ground Control Control Point (GCP)-survey Point (GCP)-surveyeded points pointsalong alongHaeundae Haeundae Beach. Beach. 2.2. Numerical Simulations 2.2. Numerical2.2. Simulations Numerical Simulations A numerical simulation was performed to identify the rip current flows by reproduc- A numerical simulationA numerical was simulation performed was to identify performed the rip to current identify flows the rip by currentreproduc- flows by repro- ing the current flows from waves near Haeundae Beach. Although various numerical ing the currentducing flows the from current waves flows near from Haeu wavesndae near Beach. Haeundae Although Beach. various Although numerical various numerical models are available for reproducing current flows from waves, a 3-D Wave-Induced Cur- models are availablemodels for are reproducing available for current reproducing flows from current waves, flows a from3-D Wave-Induced waves, a 3-D Wave-Induced Cur- Cur- rent Numerical Model for predicting beach deformation was used to recreate and analyze rent Numericalrent Model Numerical for predicting Model forbeach predicting deformation beach was deformation used to recreate was used and to analyze recreate and analyze the rip current patterns using wave-induced current flows. This model was developed by the rip currentthe patterns rip current using patterns wave-induced using wave-induced current flows. current This model flows. was This developed model was by developed by Watanabe and Maruyama [13], and it calculates wave-induced current using the calcu- Watanabe andWatanabe Maruyama and [13], Maruyama and it calcul [13], andates it wave-induced calculates wave-induced current using current the usingcalcu- the calculated latedlated wave wave deformation deformationwave deformation by by the the equation equation by the equationof of equilibrium equilibrium of equilibrium [13]. [13]. A A flow flow [13]. chart Achart flow of of the chart the described described of the described model modelmodel process process isprocess is shown shown is in shownin Figure Figure in 5. Figure5. 5.

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Initial Topography

Wave Field Calculation

Current Field Calculation

Local Sediment Calculation

Bottom Shear Stress Change Topography Change due to Water Depth Change Calculation

: calculation time

Yes Final Topography

FigureFigure 5.5. FlowFlow chartchart ofof thethe 3-D3-D Wave-InducedWave-Induced CurrentCurrent NumericalNumerical ModelModel processprocess forfor predicting predicting beachbeach deformation.deformation.

TheThe modelmodel usedused inin thisthis researchresearch isis aa partpart ofof thethe 3-D3-D Wave-InducedWave-Induced Current Current Model Model for for predictingpredicting beachbeach deformationdeformation usingusing thethe 2-dimensional2-dimensional planeplane monolayermonolayer equation.equation. Wave-Wave- inducedinduced currents currents can can be be described described using using two two equations equations toto representrepresent the the xx andand yy directions:directions: continuitycontinuity equationequation (Equation(Equation (1))(1)) andand motionmotion equationequation (Equations (Equations(2) (2) and and (3)). (3)). TheseThese equationsequations includeinclude thethe externalexternal forceforce of of the the 2-D 2-D monolayer monolayer shallow-water shallow-water equation, equation, which which comprisescomprises aa wave-inducedwave-induced radiation radiation stress stress component. component.

∂η̅ ∂ ∂ + + ((hℎ + + η)U̅) + + (h +(ℎ η + )V =̅) 0 = 0 (1)(1) ∂t ∂x ∂y ̅ ∂U + ∂U + ∂U +∂η + − + = 0 (2) + U + V + g + F − M + R = 0 (2) ∂t ∂x ∂y ∂x x x x ̅ ∂V + ∂V + ∂V +∂η + − + = 0 (3) + U + V + g + F − M + R = 0 (3) ∂t ∂x ∂y ∂y y y y where Rx, Ry, Mx, My, Fx, and Fy can be obtained from Equations (4)–(9). Rx and Ry are the where Rx, Ry, Mx, My, Fx, and Fy can be obtained from Equations (4)–(9). Rx and Ry are radiation stress terms, Mx and My are the viscous terms, and Fx and Fy are the bottom re- the radiation stress terms, Mx and My are the viscous terms, and Fx and Fy are the bottom sistance terms. The study field of Haeundae was divided into micro-grids for finite differ- resistance terms. The study field of Haeundae was divided into micro-grids for finite entiation, and each grid point was defined using the water depth (ℎ), average water level differentiation, and each grid point was defined using the water depth (h), average water (̅), and radiation stress () values as shown in Equations (4)–(9). The average water level (η), and radiation stress (sij) values as shown in Equations (4)–(9). The average water flowsflowsU and andV are are defined defined between between two two adjacent adjacent grids grids using using the the staggered staggered grid grid model. model. 1 = 1 ∂S ∂+ Sxy (4) R = (ℎ + ̅) xx+ (4) x ρ(h + η) ∂x ∂y = + (5) ∂ ∂U ∂ ∂U Mx = ε + ε (5) ∂x ∂x 1 ∂y ∂y = 1 (6) F = (ℎ + τ̅B) (6) x ρ(h + η) x 1 + (7) (ℎ1 + ̅∂)Syx ∂Syy R = + (7) y ρ(h + η) ∂x ∂y = ∂ ∂V + ∂ ∂V (8) M = ε + ε (8) y ∂x ∂x ∂y ∂y

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1 = (9) (ℎ + ̅) Using the above-mentioned model, the following parameters were used to calculate the wave-induced currents and identify the rip current hot spot: experimental range: 1500 m × 1300 m; the number of grids: 150 × 130 = 19,500; grid size: dx = dy = 10 m. The normal significant wave height and period in Haeundae are 1.4 m and 10 s, respectively, as shown in Figure 6B [14]. The same method and grid configurations were used to calculate the Atmosphere 2021, 12, 719 6 of 12 real-time wave in the same time zone as that of the rip current observation of the input conditions to compare the numerical simulations from this research with those of Deguchi et al., (2008) [14]. Furthermore, the results of the numerical model used in this study were compared with those from the numerical model of Kim et al.,1 (2013)B [15]; it used the same Fy = τy (9) model to predict longshore and rip currents with respectρ( hto+ theη) seasonal change in the directions of waves Usingand examine the above-mentioned the direction of model, wave-induced the following currents. parameters were used to calculate Because differentthe wave-induced time periods currents with a different and identify wave theand ripbathymetric current hot data spot: were experimental used range: for different test1500 scenarios m × 1300 (A, mB,; and the numberC), the results of grids: can 150 be ×only130 compared = 19,500; gridfor the size: rip dx cur- = dy = 10 m. The rent generationnormal location. signiﬁcant While comparing wave height the and rip periodcurrent in generation Haeundae tendencies are 1.4 m and at 10Hae- s, respectively, as undae Beach, itshown was observed in Figure that6B[ 14longshor]. The samee currents method were and developed grid conﬁgurations from the west were to used to calculate the east, generatingthe real-time strong rip wave currents in the in same front time of the zone beach as that at the of the hot rip spot current location observation of the of the input present study. conditionsBuoy and dye to compare tracking theobservations numerical were simulations then performed from this researchat the predicted with those of Deguchi hot spot. Figureet 6 al., shows (2008) the [14 hot]. Furthermore,spot prediction the points results obtained of the numerical using rip model current used model in this study were verification withcompared the current with observed those from bathymetric the numerical topography. model of KimThe location et al., (2013) of the [15 ];hot it used the same spot obtained usingmodel this to numerical predict longshore model was and similar rip currents to that obtained with respect in a toprevious the seasonal study change in the by Deguchi et al.directions [14]; hence, of waves the observations and examine herein the direction were made of wave-induced at the same location. currents.

Figure 6. ResultsFigure of 6. the Results numerical of the simulations numerical simulations to predict hot to spotpredic oft rip-current hot spot of formation rip-current at Haeundaeformation at Beach Hae- (A): Present research, (Bundae): Duguchi Beach et ( al.,A): 2008Present [13 ],research, (C): Kim (B et): al.,Duguchi 2013 [ 14et]. al., 2008 [13], (C): Kim et al., 2013 [14]).

3. Field InvestigationBecause different time periods with a different wave and bathymetric data were used The rip currentfor different characteristics test scenarios were (A,analyz B, anded by C), examining the results the can movement be only compared range, di- for the rip current rection, and flowgeneration velocity location.of the rip Whilecurrents comparing using a GPS the ripdrifter current designed generation to measure tendencies the at Haeundae surf zone circulationBeach, it[16]. was The observed rip current that longshore hot spot currentsat Haeundae were Beach developed was fromidentified the west to the east, through a numericalgenerating simulation strong ripin comparison currents in frontwith ofprevious the beach studies at the [17,18]. hot spot Ten location GPS of the present drifters were droppedstudy. Buoy at the and rip dye current tracking hot observationsspots and tracked were thento analyze performed the rip at current the predicted hot spot. generation pattern,Figure including6 shows the hotaverage spot flow prediction velocity, points maximum obtained instantaneous using rip current flow modelve- veriﬁcation locity, and direction.with the Simultaneously, current observed the previously bathymetric mentioned topography. dye The tracking location method of the was hot spot obtained using this numerical model was similar to that obtained in a previous study by Deguchi et al. [14]; hence, the observations herein were made at the same location.

3. Field Investigation The rip current characteristics were analyzed by examining the movement range, direction, and flow velocity of the rip currents using a GPS drifter designed to measure the surf zone circulation [16]. The rip current hot spot at Haeundae Beach was identified through a numerical simulation in comparison with previous studies [17,18]. Ten GPS drifters were dropped at the rip current hot spots and tracked to analyze the rip current generation pattern, including the average flow velocity, maximum instantaneous flow Atmosphere 2021, 12, 719 7 of 12 Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 12

velocity, and direction. Simultaneously, the previously mentioned dye tracking method also executedwas using also drones executed and dyes using for drones flow velocity and dyes and for direction ﬂow velocity observations and direction consid- observations ering the limitationsconsidering of the the GPS limitations drifter surv of theeys GPSand drifterfor improving surveys the and investigation for improving accu- the investigation racy. The performancesaccuracy. The of performancesthe GPS drifter of and the GPSdye driftertracking and method dye tracking were compared method were by compared by matching the matchingflow velocity the ﬂowand direction velocity andresults. direction results.

3.1. Investigation3.1. of Investigation the Rip Current of the Using Rip CurrentGPS Drifters Using GPS Drifters The buoy surveysThe buoy at Haeundae surveys atBeach Haeundae were conducted Beach were from conducted June 1st from to September June 1st to September 5th of 2019, which5th of is 2019, the period which when is the periodthe beach when is open the beach for tourists is open during for tourists summer. during The summer. The vicinity of thevicinity rip current of the hot rip spot current was hotderi spotved from was derivedthe numerical from the simulations numerical shown simulations in shown in Figure 6, and Figuresix trials6, andof rip six current trials of survey rip currents were surveys conducted were using conducted five GPS using drifters ﬁve GPSper drifters per trial. Each trialtrial. was Each conducted trial was on conducted a different on date. a different A widely date. used A Lagrangian widely used method, Lagrangian method, recommendedrecommended by the Korean by Ministry the Korean of Ocean Ministry and ofFisheries, Ocean and was Fisheries, used to measure was used rip to measure rip currents, and currents,the results and are the plotted results in areFigure plotted 7 [8]. in Figure7[8].

FigureFigure 7. GPS 7. GPS drifter drifter survey survey location location and and tracking tracking (background (background image image obtained obtained from from Google Google Earth Earth [6]). [6]). A total of six field surveys were conducted from June 17 to September 5 of 2019; A total ofhowever, six field surveys the wave were heights conducted were substantial from June (over17 to September 1 m) during 5 theof 2019; third how- (24 July 2019) and ever, the wavefourth heights (25 Julywere 2019) substantial trials making (over it1 impossiblem) during forthe the third divers (24 orJuly observers 2019) and to continue with fourth (25 Julythe 2019) survey trials using making GPS drifter.it impossible Throughout for the this divers study, or it observers was found to that continue the survey was only with the surveypossible using when GPS thedrifter. wave Throughout height is lower this thanstudy, 1 m,it forwas reasons found ofthat safety. the survey As a result, Figure7 was only possibleshows when the GPS the trackingwave height of the is first lower (17 Junethan 2019), 1 m, secondfor reasons (18 June of safety. 2019), fifthAs a(26 July 2019), result, Figureand 7 shows sixth the (5 September GPS tracking 2019) of trials the first that (17 could June be 2019), observed; second however, (18 June the 2019), drifter could not go fifth (26 July 2019),beyond and the sixth surf zone(5 September and returned 2019) to trials the shore. that could The GPS be observed; drifter indicated however, that the average the drifter couldflow not velocity go beyond of all the trials surf was zone 0.4 and ms− returned1, and each to the value shore. is listed The GPS in Table drifter1. Nonetheless, indicated thatthe the survey average result flow was velocity not completely of all trials reliablewas 0.4 because ms−1, and the each drifter value returned is listed without going in Table 1. Nonetheless,beyond the the surf survey zone, result and the was result not wascompletely not suitable reliable for because this research’s the drifter analysis. Table1 returned withoutshows going the dailybeyond average the surf wave zone, data and (wave the result heights was (Hs), not wavesuitable periods for this (Ts), re- and direction) search’s analysis.provided Table by 1 Haeundae’sshows the daily buoy average wave data wave from data KHOA (wave [19 heights]. The wave(Hs), datawave from the third periods (Ts), and fourthdirection) trials provided were not by recorded Haeundae’s as they buoy were wave too highdata andfrom unsafe. KHOA [19]. The wave data from the third and fourth trials were not recorded as they were too high and unsafe.

Table 1. Wave conditions of each GPS drifter survey trial.

Trial and Date Hs (m) Ts (s) Direction (˚) Avg. Flow Velocity (ms−1) 1st, 17 Jun 2019 0.73 8.51 134 0.60 2nd, 18 June 2019 0.53 8.13 147 0.43 3rd, 24 July 2019 N/A (>1.0m) N/A N/A N/A

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Table 1. Wave conditions of each GPS drifter survey trial.

◦ −1 Trial and Date Hs (m) Ts (s) Direction ( ) Avg. Flow Velocity (ms ) Atmosphere 2021, 12, x FOR PEER REVIEW 8 of 12 1st, 17 Jun 2019 0.73 8.51 134 0.60 2nd, 18 June 2019 0.53 8.13 147 0.43 3rd, 24 July 2019 N/A (>1.0 m) N/A N/A N/A 4th, 25 July 2019 N/A (>1.0 m) N/A N/A N/A 4th, 25 July 2019 N/A (>1.0m) N/A N/A N/A 5th, 26 July 2019 0.46 7.32 171 0.2 6th, 5 September 2019 0.515th, 26 July 2019 6.91 0.46 7.32 150 171 0.2 0.23 6th, 5 September 2019 0.51 6.91 150 0.23

3.2. Rip Rip Current Current Investigation Using Drone and Dye As describeddescribed above,above, the the survey survey method method using using a GPS a GPS drifter drifter requires requires the drifter the drifter to move to movein the in wave the wave direction direction by the by mass the mass transport transport of the of wave; the wave; however, however, it cannot it cannot go beyond go be- yondthe surf the zone surf duezone to due the breakingto the breaking of the waves,of the waves, which iswhich a critical is a limitationcritical limitation to the method. to the method.Along with Along this limitation,with this factorslimitation, including factors the including inflow of the long-period inflow of high long-period waves and high the wavesmarine and environment the marine (e.g., environment typhoon) were(e.g., thoughttyphoon) to were have thought an enormous to have impact, an enormous thereby impact,resulting thereby in survey resulting difficulties in survey and difficulties limitations and in derivinglimitations reliable in deriving results. reliable In addition, results. Inconducting addition, aconducting survey using a survey GPS drifters using GPS was riskydrifters at timeswas risky of rip at currenttimes of occurrence rip current with oc- currencehigh waves with because high waves the divers because or the divers observers or the had observers to go into had the to watergo into by the swimming water by swimmingand install theand buoy.install Several the buoy. years Several ago, ripyears current ago, rip surveys current were surveys conducted were inconducted the United in theStates United [20] andStates Japan [20] [ 14and] using Japan GPS; [14] however,using GPS; the however, output reliability the output was reliability not considered was not to consideredbe sufficiently to be high sufficiently because the high results because from the the results movement from of the the movement buoys or drifters of the buoys were not or driftersaccurate. were Therefore, not accurate. this research Therefore, aimed this to research obtain the aimed results to obtain by conducting the results investigations by conduct- ingusing investigations both drones using and dyes both (Figure drones8 )and to improve dyes (Figure the accuracy 8) to improve of the the rip currentaccuracy survey of the ripand current the quality survey of theand derived the qual results.ity of the derived results.

Figure 8. Drone and Sea Marker (Dye).

Fluorescent dyes dyes are are usually usually used used in in offshore offshore rescue rescue missions missions and and areare harmless harmless to the to humanthe human body; body; hence, hence, they they were were used used for for the the dy dyee tracking tracking investigation investigation in in this this research. research. Based on the the numerical numerical simulation simulation results, su suspectedspected areas of rip current current occurrences occurrences were selected as the drone-shooting section, which is the rip current hot spot. After installing a mount that can be attached or detached to a drone, some dye was released at the desired spot. At this time,time, twotwo dronesdrones (i.e., (i.e., dye dye releasing releasing and and image image shooting shooting drones) drones) were were operated oper- atedsimultaneously simultaneously for the for investigation. the investigation. Phantom Phantom 4 Pro 4 dronePro drone manufactured manufactured by DJI, by capableDJI, ca- pableof up toof 25up min to 25 of flightmin of time flight and time a receiver and a distancereceiver ofdistance 5 km, wasof 5 used km, forwas the used survey. for the survey.The aerial image of the fluorescein tracer dye in a rip current was taken via a drone, as shownThe aerial in Figure image9, and of the the fluorescein current velocity tracer was dye calculated. in a rip current After was the videotaken footagevia a drone, was ascorrected shown forin Figure distortions, 9, and images the current of this velocity drone footagewas calculated. were captured After the at 1-min video intervals footage was and correctedwere color-corrected for distortions, according images to of the this dye drone concentration footage were to identify captured the at rip 1-min current intervals flows. and were color-corrected according to the dye concentration to identify the rip current flows. During correction, red, green, and blue (RGB) color channels were used for adjustment [21]. The white balance levels were preset by changing the white and grey areas using only blue channels to be able to clearly detect the fluorescein dyes, which changes

Atmosphere 2021, 12, 719 9 of 12

During correction, red, green, and blue (RGB) color channels were used for adjustment [21]. Atmosphere 2021, 12, x FOR PEER REVIEW 9 of 12 Atmosphere 2021, 12, x FOR PEERThe REVIEW white balance levels were preset by changing the white and grey areas9using of 12 only blue channels to be able to clearly detect the ﬂuorescein dyes, which changes the lighting conditions of the image [21]. During this process, the ﬂuorescein dye was recolored for the lighting conditions of the image [21]. During this process, the fluorescein dye was re- analysis. The dye survey was performed on 14 October 2019, at 13:45 for 5 min (Table2, coloredthe lighting for analysis. conditions The of dye the survey image was[21]. perfDuriormedng this on process, 14 October the fluorescein 2019, at 13:45 dye for was 5 min re- Figure(Tablecolored 10 2,). for Figure analysis. 10). The dye survey was performed on 14 October 2019, at 13:45 for 5 min (Table 2, Figure 10).

FigureFigure 9. 9.Color-correction Color-correction process of of fluorescein ﬂuorescein tracer tracer dye. dye. Figure 9. Color-correction process of fluorescein tracer dye.

Figure 10. Dye tracking at Haeundae Beach. FigureFigure 10. 10.Dye Dye trackingtracking at at Haeundae Haeundae Beach. Beach. The wave observation information from KHOA [19] was referred for the wave char- acteristicsThe wave at the observation time of the information dye tracking from observation. KHOA [19] The was maxi referredmum forwave the heightwave char- was 1.62acteristics m, the at maximum the time ofwave the perioddye tracking was 10 observation.s, and the wave The directionmaximum was wave 135°. height The flowwas 1.62 m, the maximum wave period was 10 s, and the wave direction was 135°. The flow

Atmosphere 2021, 12, x FOR PEER REVIEW 10 of 12

velocity analyzed through the images, as shown in Figure 10, showed a maximum instantaneous velocity of 0.25 ms−1 and an average velocity of 0.17 ms−1.

Table 2. Velocity from dye tracking at Haeundae Beach.

Colored Dots Avg. Velocity (ms−1) Max. Instantaneous Velocity (ms−1) Red → Yellow 0.08 Yellow → Blue 0.25 → Atmosphere 2021, 12, 719 Blue Purple 0.17 0.17 10 of 12 Purple → Orange 0.17 Orange → Black 0.17

4. Results and DiscussionsTable 2. Velocity from dye tracking at Haeundae Beach.

ColoredFigure Dots 11 shows the results Avg.of the Velocity numeri (mscal− 1simulations) using Max. the Instantaneous real-time wave Velocity (ms−1) configurations from 13:45 of 14 October 2019. Based on the data from the wave buoy at Red → Yellow 0.08 Haeundae Beach, the significant wave height and its period at the time of the survey were Yellow → Blue 0.25 Blue1.22→ Purplem and 8 s, respectively [21]. Through0.17 the numerical simulations, the average 0.17velocity Purpleof →theOrange rip currents in the zone designated as its hot spot was found to be 0.21 ms0.17−1; this Orangewas→ measuredBlack to be 0.17 ms−1 when observed using fluorescent dye. 0.17 Meanwhile, the incident wave height of the numerical simulation was used as input data with the averageThe wavevalue observation of the significant information wave from height KHOA measured [19] was at referred 2-h intervals. for the wave charac- Therefore, it isteristics somewhat at the difficult time of theto quantitatively dye tracking observation. compare the The result maximum of the wavenumerical height was 1.62 m, simulation withthe the maximum velocity waveat the moment period was of observation. 10 s, and the Nevertheless, wave direction it was was confirmed 135◦. The flow velocity that the observedanalyzed values through and the the numerical images, simulation as shown in results Figure are 10 qualitatively, showed a maximum consistent instantaneous not only in ordervelocity but al ofso 0.25 in a ms flow−1 andpattern. an average velocity of 0.17 ms−1. The rip current trends at other regions in Korea where rip currents frequently occur 4. Results and Discussions would be similar if various numerical methods can be complemented using the observation data. The resultFigure of the 11 orthometric shows the corre resultsction of theof the numerical dye observation simulations images using obtained the real-time wave using the droneconfigurations demonstrated from a higher 13:45 accuracy of 14 October than the 2019. GPS Based drifter on results the data because from they the wave buoy at were measuredHaeundae without Beach,any limitations the significant that prevent wave height going and beyond its period the surf at the zone, time unlike of the survey were that in the case1.22 of mbuoys and 8or s, drifters. respectively However, [21]. Through this method the numerical cannot perform simulations, observations the average velocity −1 at multiple placesof the simultaneously; rip currents in hence, the zone combining designated the asresults its hot of spota numerical was found simulation to be 0.21 ms ; this −1 as well as a dyewas observation measured tocan be enable 0.17 ms fast andwhen accurate observed prediction using fluorescent of rip currents. dye.

Figure 11. RipFigure current 11. generation Rip current comparisons generation ofcomparisons numerical simulation of numeri resultcal simulation and dye trackingresult and result dye (Numerical tracking re- simulation sult (Numerical simulation October 14, 2019, 13:45, Dye tracking: 14 October 2019, 13:45–50). October 14, 2019, 13:45, Dye tracking: 14 October 2019, 13:45–50).

5. Conclusions Meanwhile, the incident wave height of the numerical simulation was used as input data with the average value of the significant wave height measured at 2-h intervals. Therefore, it is somewhat difficult to quantitatively compare the result of the numerical simulation with the velocity at the moment of observation. Nevertheless, it was confirmed that the observed values and the numerical simulation results are qualitatively consistent not only in order but also in a flow pattern. The rip current trends at other regions in Korea where rip currents frequently occur would be similar if various numerical methods can be complemented using the observation data. The result of the orthometric correction of the dye observation images obtained using the drone demonstrated a higher accuracy than the GPS drifter results because they were measured without any limitations that prevent going beyond the surf zone, unlike that in the case of buoys or drifters. However, this method cannot perform observations at Atmosphere 2021, 12, 719 11 of 12

multiple places simultaneously; hence, combining the results of a numerical simulation as well as a dye observation can enable fast and accurate prediction of rip currents.

5. Conclusions Wave-induced currents are recognized as significant mechanisms for sediment trans- portation, which is tied to erosion and deposition on beaches. Rip currents that occur in wave-induced currents can be a significant cause of deaths in coastal regions. Therefore, it is crucial to detect the formulation of rip currents by accurately measuring the location, size, and direction of rip currents to prevent casualties. In general, a tracking observation using GPS drifters is used to identify the characteristics of rip currents. Likewise, the Coastal Improvement Project Design Guidebook [8] also proposes the observation of rip currents using a GPS observation methodology. However, many researchers have faced difficulties with this method due to the limited safety and effectiveness. This study specifically investigated rip currents at Haeundae Beach, where rip current occurrences are frequent. Haeundae Beach is open wide to the south, where in summer, south wind is dominant, and the waves hit the beach directly. If the coastline is curved, the waves pushed to the coast cannot gather power and scatter in one place, making it difficult to generate a rip current. Still, in the case of Haeundae, the coastline has a gentle shape close to a straight line, which increases the risk of the rip current generation. Besides, because the slope of the coast is gentle, the wave breaks; that is, the wave breaking zone is wide, so that much of the breaking wave energy accumulates, resulting in perfect conditions for rip current generation. In general, when detecting the generation of rip currents, GPS drifters or buoys are used; however, various problems were discovered during the observation that resulted in low accuracy. As an alternative, a dye tracking methodology employing a drone was used in this study to measure and determine the location and movement paths of rip currents at a hot spot in Haeundae Beach. Wave-induced current numerical simulations with shallow water equations that incorporate radiation stress were performed to determine the characteristics of the rip currents. The results obtained from the numerical simulations were then compared with the result of the dye tracking method, and it was found that the observed result showed relatively sufficient reliability for rip current generation at the hot spot with a maximum instantaneous velocity difference of only 0.04 ms−1. In the future, if a method that allows the simultaneous observation of rip currents at multiple hot spots in front of the beach can be devised, it is expected that a more useful and practical rip current investigation system can be constructed. In this study, the origin and size of rip currents in front of the beach were investigated, confirming the effectiveness of the dye tracking method using drones. In future studies, it is necessary to install a fixed current meter at the points where rip current occurrences have been identified through this investigation and observe the changes in the velocity of rip currents over time using the Euler method. In such a future study, it is also necessary to grasp more quantitative characteristics of the rip currents in front of Haeundae Beach by performing continuous and simultaneous observations on both the wave height and the current velocity of rip currents.

Author Contributions: Conceptualization, K.-H.K.; Formal analysis, K.-H.K.; Investigation, H.D.K.; Methodology, H.D.K.; Supervision, K.-H.K.; Validation, H.D.K. and K.-H.K.; Writing—original draft, H.D.K.; Writing—review & editing, K.-H.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Atmosphere 2021, 12, 719 12 of 12

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