The Effectiveness of Adaptive Beach Protection Methods Under Wind

Home , Beach nourishment, Cobble (geology), Grain size, Tetrapod (structure)

Journal of Marine Science and Engineering

Article The Eﬀectiveness of Adaptive Beach Protection Methods under Wind Application

Kyu-Tae Shim 1 , Kyu-Han Kim 1,* and Jun-Ho Park 2

1 Department of Civil Engineering and Catholic Kwandong University, Gangneung 25601, Korea; [email protected] 2 Coast & Ocean Technology Research Institute, Seoul 08584, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-33-643-3436

Received: 23 September 2019; Accepted: 28 October 2019; Published: 30 October 2019

Abstract: A physical model test was carried out to evaluate a measure of reducing sediment transport in a condition of erosive wave incidence. The erosion trend was analyzed in a beach profile consisting of 0.1 mm sand, and a scenario in which 1 mm and 5 mm materials were applied to the erosion section was conducted. The effects of beach nourishment profiles with different sand diameters were verified by comparing the results when the submerged breakwater was installed. In addition, because high waves are usually accompanied with strong wind, to determine the wind effect, morphological change was examined under waves only and the coexistence of waves and wind together. The experimental results showed that sediment transport around the shoreline decreased in a condition of nourishment with 1 mm grains, and the total amount of morphological change was similar to the case in which the submerged breakwater was installed. The results illustrated that a change in wind velocity increased the wave energy density, as well as the range of morphological change.

Keywords: beach proﬁle change; beach nourishment; wind velocity; wave deformation; submerged breakwater

1. Introduction The coastline is an important location for protecting the land from high waves, as well as for providing a recreation function for leisure and other amenities. It also maintains the ecological environment by degrading marine pollutants [1,2]. The loss of sand has occurred continuously at beaches for various reasons, such as the frequency increase of high waves, occurrence of abnormal climate caused by global warming, and imbalances in the sediment budget produced by artificial structures. Accordingly, the issue of serious beach erosion is one that confronts the whole world [3,4]. Issues like sediment transport and erosion have been discussed by many researchers, but the existence of many coasts in the world is still threatened by rises in sea levels and beach erosion, which are causing a reduction and inundation of the total land area [5]. To reduce the occurrence of these situations, many types of countermeasures are being applied; such measures can be comprehensively classified into structural and non-structural measures. Structural measures, which involve methods for effectively controlling sediment transport caused by waves and currents in a coastal region, can be classified into the hard-type method, soft-type method, and hybrid method applying both approaches simultaneously. In the hard-type method, a hard structure is installed in a nearshore area, such as a submerged breakwater, jetty, groin, or headland, thereby inducing the blockage or reduction of waves and currents in the target area and protecting the surrounding coast at the same time. However, this approach involves an enormous expense, a long construction period, and the need to carry out a detailed examination in advance to prevent against secondary damage that may occur after installation [6–11]. It also has a disadvantage in that it cannot respond actively to changes in the natural

J. Mar. Sci. Eng. 2019, 7, 385; doi:10.3390/jmse7110385 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 385 2 of 17 environment, such as sea level rise or external forces. The soft-type method includes artificial reefs, beach nourishment, and vegetation schemes. However, these methods have disadvantages including issues related to lack of permanence, maintenance requirements, and the fact that the period after construction is limited. These phenomena are determined by the surrounding coast’s environmental conditions. In addition, extremely limited vegetation species can be applied, and vegetation methods cannot be adapted in a region with large range of tidal variation. However, the soft-type method has advantages in that it exhibits a short construction period, low cost, and easy maintenance compared with the hard-type method. It is also eco-friendly, and there is a low probability of secondary damage occurring. Moreover, it is easier to actively respond to changes in the coastal environment with the soft-type method [12,13]. Recently, the soft-type application of nourishment has been carried out more extensively than the hard-type method has. In the case of normal sand nourishment, the aim is to maintain the shape of the target coast, but beach reshaping could occur repeatedly, so nourishment with a larger grain size is applied more frequently. Nourishment has advantages, including a long preservation period and easy maintenance. According to a previous experiment [14] and in regions where gravel nourishment has been applied, a coast with a certain width and area can be maintained [15–17]. Waves and currents can be considered the most important external force to be understood in the investigation regarding the mechanism of sediment transport or the reasons for erosion and the establishment of beach erosion countermeasures. Waves are classified into erosive waves and accretive waves by analyzing wave observation data and compiling and applying the statistics of the frequency of occurrence at the target coast; various situations can be predicted through simulations. The empirical formulas suggested by many researchers are currently being used and applied to the analysis of morphological changes. As mentioned above, increasing research results have recently been published based on the soft-type method. As can be seen from the observation data, nourishment with gravel has the effect of reducing the sediment transport [5,16–18]. In the work of [13], the researchers examined topographical variation by applying a drainage system with sand nourishment and a submerged breakwater in a physical model test. Moreover, in the literature [14,19,20], sediment transport was reviewed according to variations in particle sizes when nourishment was applied. These research initiatives were conducted under circumstances when only wave action was considered. However, as can be seen from the observational data, high waves are accompanied by strong winds. In particular, the approach of a typhoon can be considered as a good example, and cases of significant morphological change occurring at coasts affected by typhoons must be considered. Therefore, it is necessary to analyze the phenomenon of erosion according to the incident wave and review morphological change by accounting for the wind factor to carry out research that properly assesses the mechanism of sediment transport; the findings of such research can be used for establishing a mitigation plan. Nevertheless, wind-based research is limited to the measurement of waves overtopping discharge or aeolian transport [21], and most studies—especially model tests—have been carried out on the morphological change with waves. In the field, strong wind velocity is observed when a high wave occurs, so the results of an experiment carried out in conditions where waves and wind coexist will be highly similar to the real-world phenomenon. The goal of this research was to assess the influence of wave action with wind variation on morphological change by reproducing the occurrence of a high wave run-up and large-scale sediment transport in the foreshore in circumstances where a high wave hit the coast. A further goal was comparing the erosion and accretion trend between conditions with and without wind. In addition, the nourishment method based on the results of the literature [13,14,19], representing a typical soft-type method, was applied to examine appropriate measures to reduce a large degree of morphological change when it occurs on a natural coast. A submerged breakwater [13,22–26], which is one among various hard-type methods, was installed, and the trend of sediment transport was compared and analyzed to verify its effectiveness. J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 3 of 17 J. Mar. Sci. Eng. 2019, 7, 385 3 of 17 2. Movable Hydraulic Model Experiment 2. Movable Hydraulic Model Experiment 2.1. Experimental Setup 2.1. ExperimentalIn this study Setup, a two-dimensional physical model test was conducted to evaluate beach deformationIn this study, according a two-dimensional to external force physical changes model and test wasbeach conducted erosion tocountermeasures. evaluate beach deformation The wave accordingflume was to40 external m long, force1.5 m changes high, and and 1.0 beach m wide, erosion and countermeasures.a wind tunnel was The installed wave at flume the top was of 40 the m long,wave 1.5operation m high, system and 1.0 to m reproduce wide, and aa windsituatio tunneln involving was installed the coexistence at the top ofof thea maximum wave operation wind systemvelocity toof reproduce20 m/s and a a situation maximum involving wave height the coexistence of 0.5 m. When of a maximuma wave is created wind velocity in a closed of 20 water m/s andtank, a seiche maximum or resonance wave height phenomena of 0.5 m. may When be agene waverated is created owing into athe closed grouping water tank,effect seicheof wave or resonancereflection, standing phenomena waves, may or be low-frequency generated owing wave tos the [27,28]. grouping Porous eff ectwave-abs of waveorbing reflection, materials standing were waves,installed or on low-frequency both ends of the waves water [27 tank,28]. in Porous this experiment wave-absorbing to prevent materials such phenomena, were installed and external on both endsfactors of that the watercould tankoccur in in this the experiment wave tank to were prevent controlled such phenomena, using the wave-absorbing and external factors system. that In could the occurwave influme, the wave a wave tank gauge were is controlled attached on using the the wave wave-absorbing generating panel. system. If the In wave the wave is increased flume, a wavemore gaugethan the is target attached wave on signal, the wave the generating wave generating panel. pa If thenel wavereduces is increasedthe wave height more than and theperiod target to fit wave the signal,signal during the wave the generating test. As shown panel in reduces Figure the 1, it wave was heightintended and to period evaluate to fitmorphological the signal during changes the test.according As shown to different in Figure grain1, it was sizes intended by setting to evaluate the nourishment morphological section changes on accordingan eroded to area. di fferent The grainmeasurement sizes by settingof beach the profile nourishment evolution section was conducted on an eroded at 0.5 area. h intervals The measurement for the first of 0.5 beach h and profile 1 h evolutionintervals until was a conducted total of 4.5 at h. 0.5 h intervals for the first 0.5 h and 1 h intervals until a total of 4.5 h.

Figure 1. Experimental setup condition with wind–wave flume. Figure 1. Experimental setup condition with wind–wave flume. For the determination of a wave signal, erosive waves were selected after applying various empiricalFor the formulas. determination Preliminary of a tests wave were signal, performed erosive to waves judge whetherwere selected the waves after were applying proper various for the sedimentempirical transportformulas. test. Preliminary It was confirmed tests were that perfor the sandmed movement to judge whether reached the a quasi-equilibrium waves were proper state for at the endsediment of the transport test (4.5 h).test. Various It was empiricalconfirmed formulas that the sand were movement suggested byreached the authors a quasi-equilibrium of [29–33] for thestate standard at the end regarding of the test the (4.5 erosive h). Various and accretional empirical beaches, formulas and were the suggested experiment by was the carried authors out of after[29– evaluating33] for the thestandard existence regarding of erosion the basederosive on and the accr empiricaletional formulas beaches, suggested and the experiment by the works was of carried [31,32], whichout after are evaluating summarized the asexistence follows: of erosion based on the empirical formulas suggested by the works of [31] and [32], which are summarized as follows: F = H /(Vt T). (1) 0 0 × F0 = H0 / (Vt × T). (1) Here, F0 is the Dean number, H0 is the deep water wave, Vt is the fall velocity, and T is the wave period. WhenHere, F 0 is< 1,the accretion Dean number, occurs, andH0 is when the deep F0 > water1, erosion wave, occurs Vt is [ 31the]. fall velocity, and T is the wave period.In FormulaWhen F0 (2),< 1, accretion occurs, and when F0 > 1, erosion occurs [31]. 0.27 0.67 In Formula (2), C = H0/L0 (tanβ) (d50/L0)− , (2) × × where C is the non-dimensional coeCffi =cient H0/L that0× (tan determinesβ)0.27×(d50/L erosion0)-0.67, or accretion, H0 is the deep water(2) wave height, L0 is the deep water wavelength, tanβ is the beach slope, and d50 is the median grain size. where C is the non-dimensional coefficient that determines erosion or accretion, H0 is the deep water In this formula, accretion occurs when C < 4, accretion and erosion occur repeatedly in the section of wave height, L0 is the deep water wavelength, tanβ is the beach slope, and d50 is the median grain 4 C 8, and erosion occurs when C > 9 for the model experimental conditions [32]. In addition to size.≤ In≤ this formula, accretion occurs when C < 4, accretion and erosion occur repeatedly in the the above formula, the morphological change characteristics are evaluated using the mobility number section of 4 ≤ C ≤ 8, and erosion occurs when C > 9 for the model experimental conditions [32]. In (3) and Shields parameter (4) [34], which indicate the non-dimensional bottom friction stress: addition to the above formula, the morphological change characteristics are evaluated using the mobility number (3) and Shields parameterψ = (A (4)ω) 2[34],/(s which1)gd, indicate the non-dimensional bottom (3) friction stress: −

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θ = τˆ/ρ(s 1)gd = 0.5fwψ. (4) − Here, ψ is mobility number, A is the water particle semi-excursion, ω is the radian frequency, τˆ is the peak bed shear stress under waves, d is the grain diameter, and fw is the wave friction factor. At this time, the wave applied to the formulas was a wave with a high approach frequency selected by analyzing the long-term wave monitoring data [35], and three wave periods caused erosion, as presented in Table1. However, the preliminary experiment was conducted to prevent a situation in which most sediment transport occurred at the beginning of wave generation or the equilibrium state was reached before the experiment was ﬁnished. As a result, it was conﬁrmed that the quasi-equilibrium state was reached according to continuous occurrence of sediment transport in the condition of Case B, so Case B was selected as the experimental wave.

Table 1. Test Conditions.

Nourished Water Depth Hs Ts Wind Interval Case Area Slope C F Ψ θ (m) (m) (sec) (m/s) (h) 0 (d50, mm) 16.6 8.43 122.4 0.48 Case A 0.9 0.08 1.13 0 Type 1: 0.1 0.5 1/10 3.5, 072 12.2 0.11 3 Type 2: 1.0 ~ (foreshore) 1.2 0.31 2.5 0.05 6 Type 3: 5.0 4.5 1/30 17.9 8.44 122.8 0.45 Case B 0.9 0.1 1.41 8 Type 4: 0.1 (oﬀshore) 3.8, 0.72 12.3 0.10 1.3 0.31 2.5 0.05 19.0 8.40 121.7 0.42 Case C 0.9 0.12 1.70 4.1, 0.72 12.2 0.09 1.4 0.31 2.4 0.04

The wind was generated in parallel with the water surface within a range to avoid affecting the formation of a wind wave as much as possible. According to an experiment carried out previously, windblown sand may be generated on the coast when the wind velocity is 10 m/s or higher [21]. In this experiment, wind velocities of 0, 3, 6, and 8 m/s were applied to verify the effect of wind velocity change at the upper part of still water level and intensity change, such as the occurrence position of a surf zone and wave runup on sediment transport in the condition of offshore water depth of 0.9 m. A 1:10 sea-bottom slope for the section of 0–2.7 m and 1:30 sea-bottom slope for the section of 2.7–8.0 m were configured among the total reproduction range of 8 m for the profile to create erosive conditions, and the beach was reproduced with well-sorted 0.1 mm fine sand. At this time, preventing sand from being blown by wind at the section the wave did not reach owing to wave runup was included in the design. Nourishment was applied to the 110 cm section around the shoreline in the erosion range reproduced through the preliminary experiment. Median sand grain sizes of 0.1 mm and 1 mm and 5 mm crushed gravel were used as nourishment materials (Figure2). The measured fall velocities of materials were 0.0084, 0.098, and 0.23 m/s, respectively. In addition, the submerged breakwater, which did not interrupt the landscape, was also installed away from the coast and reduced high waves in front of the coastal area. It was applied along with 0.1 mm sand in the experiment. Many studies have shown that the morphological change was directly affected depending on the detached distance, crest width, crest depth, porosity, and shape of the submerged breakwater [22–25]. In this study, 2.6 m for detached distance from the shoreline, 0.5 m for crest width, 0.02 m for crest depth, and 1:2 for the front and rear breakwater slopes were selected, and Tetrapod (T.T.P.)—the generally used wave dissipation block—was placed in the regular manner with two layers. J. Mar. Sci. Eng. 2019, 7, 385 5 of 17 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 5 of 17

(a) (b)

Figure 2. Beach proﬁles.profiles. (a) SandSand (d(d5050:: 0.1mm), 0.1 mm), (b (b) )Sand Sand (d (d5050:0.1: 0.1 mm), mm), Gravel Gravel (d (d50:150 :mm), 1 mm), (c) (Sandc) Sand (d(d50:0.1: 0.1 mm), mm), Gravel Gravel (d (d50:550 mm),: 5 mm), (d) (Sandd) Sand (d50:0.1 (d50 mm),: 0.1 mm), Submerged Submerged BW BW

2.2.2.2. Calibration This study aimed aimed to to evaluate evaluate the the effect effect of of wind wind on on wave wave propagation propagation and and wave wave runup, runup, and and the the distributiondistribution of vertical vertical wind wind velocity velocity on on the the sh shorelineoreline was was measured measured before before reproducing reproducing the the beach beach profileprofile to control the phenomena according according to to an an increase increase in in wind wind velocity velocity (Figure (Figure 3).3). A Adigital digital anemometeranemometer was used used for for measuring measuring the the wind wind velocity, velocity, and and it itwas was measured measured at at15 15positions positions at 5 at cm 5 cm intervalsintervals basedbased onon the the shoreline. shoreline. The The experimental experimental results results showed showed that that the the wind wind velocity velocity at the at bottom the ofbottom the shoreline of the shoreline decreased decreased slightly slightly and the and wind the velocity wind velocity at the topat the of top the of shoreline the shoreline increased. increased. A wind velocityA wind deviationvelocity deviation within a within range ofa range0.2 mof/s ± occurred 0.2 m/s occurred based on based the shoreline, on the shoreline, but it was but judged it was that ± thisjudged would that notthis awouldffect the not execution affect the ofexecution the experiment of the experiment significantly. significantly. Next, measurement Next, measurement was taken inwas conditions taken in conditions where only where the wateronly the level water was level reproduced was reproduced to evaluate to evaluate the variation the variation of wave of wave energy densityenergy density according according to the windto the velocitywind velocity change change when when a wave a wave is propagated. is propagated. The The experimental experimental result revealedresult revealed that a similarthat a valuesimilar was value obtained was basedobtained on thebased value on ofthe 0 mvalue/s when of it0 wasm/s accompaniedwhen it was by 3accompanied m/s wind velocity, by 3 m/s and wind short-term velocity, components and short-te increasedrm components as the windincreased velocity as the changed wind velocity to 6–8 m /s. Itchanged is judged to 6–8 that m/s. such It a is trend judged resulted that such from a thetren formationd resulted of from an increased the format surfion andof an swash increased zone surf owing toand the swash continuous zone owing effect to of the wind, continuous although effect the inertialof wind, force although increased the inertial as the waveforce andincreased kinetic as energy the onwave the and surface kinetic of theenergy water on increasedthe surface in of proportion the water increased to the intensity in proportion of wind, to creating the intensity a breaking of wind, wave increating the form a breaking of a plunging wave in wave the form and of dispersing a plunging energy. wave and In dispersing this experiment, energy. eight In this capacitive experiment, wave gaugeseight capacitive were installed wave gauges and the were wave installed deformation and the characteristics wave deformation were characteristics measured to were assess measured changes in waveto assess height changes when in beach wave deformation height when occurred beach deformation and waves occurred were propagated and waves according were propagated to sediment transport.according to This sediment study sought transport. to evaluate This study the sought wave height to evaluate change the according wave height to the change shoreline according regression to the shoreline regression and occurrence of wave runup on P1 and P2, and wave deformation and occurrence of wave runup on P1 and P2, and wave deformation according to morphological according to morphological change in front of the shoreline was measured on P3–P6. P7 and P8 were change in front of the shoreline was measured on P3–P6. P7 and P8 were installed in the open ocean installed in the open ocean to verify wave actions at a position where they were relatively deep in the to verify wave actions at a position where they were relatively deep in the water. It was judged water. It was judged that there would be no significant difference in the result between the condition that there would be no significant difference in the result between the condition of the coexistence of of the coexistence of wave and wind and a change in the wind velocity and the propagation condition wave and wind and a change in the wind velocity and the propagation condition of only wave if the of only wave if the breaking wave was excluded, so the experiment was conducted based on the breaking wave was excluded, so the experiment was conducted based on the conditions evaluated conditions evaluated above. It is thought that various uncertainties may exist in relation to such above. It is thought that various uncertainties may exist in relation to such phenomena, because the phenomena, because the spatial distribution for wind to reach the coast in the water tank is different spatialfrom that distribution of the real for ocean. wind However, to reach theconsidering coast in the the water observation tank is result, different indicating from that that of high the real waves ocean. However,are accompanied considering by strong the observationwind and sediment result, indicatingtransport occurring that high on waves the coast are accompanied simultaneously, by strongit is

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J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 6 of 17 wind and sediment transport occurring on the coast simultaneously, it is thought that if the topography, wave,thought and that wind if the are topography, reproduced aroundwave, and the shoreline,wind are reproduced a qualitative around result could the shoreline, be obtained a qualitative even if the scaleresult e couldﬀect issue be obtained occurs. even if the scale effect issue occurs.

Figure 3. Wave spectrum with wind velocity variation before beach profile installation. Figure 3. Wave spectrum with wind velocity variation before beach profile installation. 3. Experimental Results 3. Experimental Results 3.1. Morphological Change 3.1. MorphologicalIn this study, Change the experimental result for Case B is summarized, showing moderate findings amongIn allthis the study, test cases.the experimental In addition, result the conditions for Case ofB windis summarized, velocities ofshowing 0 and 8 moderate m/s, where findings a clear diamongfference all occurredthe test cases. according In addition, to wind the change, conditions are illustrated. of wind Thevelocities results of of 0 the and morphological 8 m/s, where changea clear afterdifference 0.5 h, occurred 1.5 h, and according 4.5 h measured to wind at thechange, center ar ofe illustrated. the water tankThe areresults shown of the in Figures morphological4 and5. Forchange the wholeafter 0.5 beach h, 1.5 profile h, and consisting 4.5 h measured of 0.1 mm at sandthe center in the noof the wind water (U = tank0 m/ s)are condition, shown in the Figures erosion 4 occurredand 5. For around the whole the shoreline beach profile in proportion consisting to theof 0. duration1 mm sand of the in incidentthe no wind wave, (U and = 0 eroded m/s) condition, sand was transportedthe erosion occurred to the seaside. around For the the shoreline result after in 4.5propor h andtion when to the a quasi-equilibrium duration of the incident state was wave, reached, and theeroded shoreline sand was retreated transported approximately to the seaside. 30 cm from For the initialresult topographicalafter 4.5 h and conditionswhen a quasi-equilibrium and loss of sand occurredstate was in reached, a wide area the fromshoreline the foreshore retreated to approx the backshore.imately Meanwhile,30 cm from transported the initial sandtopographical formed a sandbarconditions at aand position loss of 2 msand away occurred from the in shorelinea wide area of the from final the profile. foreshore If the to erosion the backshore. section was Meanwhile, replaced withtransported 1 mm sand, sand anformed extremely a sandbar small at morphological a position 2 m change away from occurred the shoreline in the foreshore of the final area. profile. Eroded If sandthe erosion near the section shoreline was replaced was transported with 1 mm landward, sand, an formingextremely a small dune, morphological and drift sand change in the foreshore occurred belowin the theforeshore water surfacearea. Eroded was transported sand near tothe the shoreline offshore bywas 4 m,transported forming a landward, bar. The shoreline forming retreated a dune, byand approximately drift sand in 5the cm foreshore at the end below of the the wave water duration. surface Especially, was transported a significantly to the offshore lower amount by 4 m, of morphologicalforming a bar. changeThe shoreline occurred retreated in comparison by approx withimately the 0.1 5 mm cmsand at the profile. end of The the 0.1 wave mm sandduration. was transportedEspecially, a by significantly the wave runup lower and amount rundown of morphological after the sea bottom change was occurred suspended in comparison instantaneously with the by the0.1 mm breaking sand wave.profile. However, The 0.1 themm gravel sand was with transp d50 1 mmorted was by suspendedthe wave runu instantaneouslyp and rundown when after a wave the brokesea bottom near thewas shoreline, suspended while instantaneously sediment transport by the decreased,breaking wave. as this However, is less aff theected gravel by uprush with d and50 1 downrushmm was suspended owing to theinstantaneously fast fall in velocity. when However,a wave brok wavee near runup the shoreline, was accelerated while whensediment it returned transport to thedecreased, seaside, as and this a is strong less affected undertow by uprush owing and to rundown downrush aff owingected theto the deposit fast fall (0.1 in mm)velocity. located However, at the boundarywave runup of thewas nourishment accelerated section,when it creating returned scouring to the overseaside, a wide and range. a strong When undertow the grain owing size was to replacedrundown with affected 5 mm the gravel, deposit a dune(0.1 mm) with located a large at amount the boundary of gravel of wasthe nourishment created at the section, beginning creating part ofscouring the wave over action; a wide its range. magnitude When increasedthe grain size and was it was replaced transported with 5 tomm the gravel, rear side a dune astime with passed,a large creatingamount of a steepgravel front was slope.created Unlike at the 0.1beginning and 1 mm part sand, of the 5 wave mm gravel action; was its magnitude transported increased gradually and to theit was leeside transported by impact, to the without rear side being as time suspended, passed, creating although a thesteep wave front applied slope. Unlike direct impact0.1 and to1 mm the particles,sand, 5 mm and gravel overlapped was transported particles were gradually deposited to in th thee leeside nearshore by impact, area owing without to friction being and suspended, accretion (cohesion),although the rather wave than applied being direct transported impact to to the the pa leesiderticles, continuously. and overlapped Large-scale particles accretionwere deposited occurred in fromthe nearshore the shoreline area toowing the land to friction side after and the accretion last 4.5 (cohesion), h, and the shorelinerather than retreated being transported by approximately to the 5leeside cm from continuously. the coast. In Large-scale the foreshore accretion section occurred under sea from level, the the shoreline wave rundown to the land was side accelerated after the bylast a steep4.5 h, and slope, the creating shoreline undertow retreated owing by approximately to a strong runup 5 cm from phenomenon, the coast. In and the sand foreshore transported section byunder the sea level, the wave rundown was accelerated by a steep slope, creating undertow owing to a strong runup phenomenon, and sand transported by the strong undertow was deposited at a position of 300–600 cm, forming a sandbar. When comparing the above phenomena with the submerged

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 7 of 17

breakwater installation condition, gradual sediment transport occurred near the shoreline because of waves, and the shoreline retreated by 10 cm at the end of the wave generation. A sand dune was formed in the backshore, and erosion occurred in the foreshore according to the formation of a surf zone. Transported sand formed a thick sandbar in the range of 200–300 cm (x direction), and the overall range of morphological change decreased significantly in comparison with the condition of 0.1 mm. A large amount of sediment transport occurred from the beginning part of the experiment under the 0.1 mm profile condition in circumstances where there was an accompanying 8 m/s of wind shown as Figure 5, and sand was transported over a wide area, exceeding the nourishment section at the end of the wave duration. The erosion range was mostly from the backshore to the position of 200 cm, and the shoreline retreated by 45 cm. Transported sand formed an approximately 5 cm high thick sandbar at the position of 200 cm as the x distance. The result of the experiment with a 1 mm material size revealed that the range of morphological change occurred at the beginning of the wave action owing to the effect of wave energy by the wind effect, which increased in proportion to the duration of the wave, and a sand dune forming on the backshore. The shoreline retreated by 10 cm, and less sediment transport occurred from the shoreline to the position of 200 cm in the foreshore owing to J. Mar. Sci. Eng. 2019, 7, 385 7 of 17 the effect of replaced particles. Large-scale scouring occurred as a result of the effect of a strong undertow after 200 cm, and scoured sand was transported to the seaside, forming a sandbar. The strongresult of undertow the experiment was deposited with 5 atmm a position gravel showed of 300–600 that cm, a large-scale forming a sandbar.gravel dune When was comparing formed from the abovethe part phenomena adjacent to with the shoreline the submerged to the breakwaterpoint of the installationnourishment condition, section (x: gradual 50 cm), sediment and the transportdune size occurredincreased near in proportion the shoreline to because the wave of waves,duration, and while the shoreline the front retreated slope bybecame 10 cm steep. at the endWhen of thethe waveexperiment generation. ended, A an sand approximately dune was formed 10 cm high in the gr backshore,avel dune was and formed, erosion and occurred the shoreline in the foreshore retreated accordingby 10 cm. Scouring to the formation occurred of over a surf a wide zone. range Transported owing to sand a strong formed downrush a thick sandbarin the foreshore, in the range and up of 200–300to 9 cm of cm scour (x direction), depth was and formed the overall at the range position of morphological of 155 cm. In addition, change decreased the loss of signiﬁcantly sand occurred in comparisonup to the position with the of 300 condition cm (X distance), of 0.1 mm. and the transported sand accumulated at the seaside. When

(a) (b)

Figure 4. Beach profile evolution with wind velocity; U: 0 m/s : (a) d50, 0.1mm, (b) d50, 1 mm, (c) d50, Figure 4. Beach proﬁle evolution with wind velocity; U: 0 m/s : (a) d50, 0.1 mm, (b) d50, 1 mm, (c) d50, 5mm, (d) d50, 0.1mm + Sub. B.W. 5 mm, (d) d50, 0.1 mm + Sub. B.W.

A large amount of sediment transport occurred from the beginning part of the experiment under the 0.1 mm profile condition in circumstances where there was an accompanying 8 m/s of wind shown as Figure5, and sand was transported over a wide area, exceeding the nourishment section at the end of the wave duration. The erosion range was mostly from the backshore to the position of 200 cm, and the shoreline retreated by 45 cm. Transported sand formed an approximately 5 cm high thick sandbar at the position of 200 cm as the x distance. The result of the experiment with a 1 mm material size revealed that the range of morphological change occurred at the beginning of the wave action owing to the effect of wave energy by the wind effect, which increased in proportion to the duration of the wave, and a sand dune forming on the backshore. The shoreline retreated by 10 cm, and less sediment transport occurred from the shoreline to the position of 200 cm in the foreshore owing to the effect of replaced particles. Large-scale scouring occurred as a result of the effect of a strong undertow after 200 cm, and scoured sand was transported to the seaside, forming a sandbar. The result of the experiment with 5 mm gravel showed that a large-scale gravel dune was formed from the part adjacent to the shoreline to the point of the nourishment section (x: 50 cm), and the dune size increased in proportion to the wave duration, while the front slope became steep. When the experiment ended, an approximately 10 cm high gravel dune was formed, and the shoreline retreated by 10 cm. Scouring occurred over a wide range owing to a strong downrush in the foreshore, and up to 9 cm of scour depth was formed at the position of 155 cm. In addition, the loss of sand occurred up to the position of 300 cm J. Mar. Sci. Eng. 2019, 7, 385 8 of 17

(X distance), and the transported sand accumulated at the seaside. When a submerged breakwater was installed on a sandy beach, the range of initial morphological change was not large, but sediment transport occurred based on the foreshore section at the end of the test. The range of morphological change in the foreshore area was larger, although sediment transport also occurred at the upper part of the shoreline owing to the eﬀect of wind. This happened because the wave energy, diminished by the submerged breakwater, was being developed because of an increase of the x-direction component by the wind; then, the wave energy broke in front of the shoreline, and the intensity of the wave runupJ. Mar. Sci. decreased Eng. 2019, as7, x a FOR result PEER of REVIEW dispersed wave energy. Transported sand was deposited to the seaside,8 of 17 and some of it was transported into the submerged breakwater.

(a) (b)

Figure 5. Beach profile evolution with wind velocity; U: 8 m/s : (a) d50, 0.1mm, (b) d50, 1 mm, (c) d50, Figure 5. Beach proﬁle evolution with wind velocity; U: 8 m/s : (a) d50, 0.1 mm, (b) d50, 1 mm, (c) d50, 5mm, (d) d50, 0.1mm + Sub. B.W. 5 mm, (d) d50, 0.1 mm + Sub. B.W.

3.2.a submerged Wave Deformation breakwater was installed on a sandy beach, the range of initial morphological change was Annot assessmentlarge, but sediment was conducted transp onort wave occurred variation based that on took the placeforeshore when section sediment at the transport end of occurred the test. accordingThe range toof themorphological profile condition change and in changesthe foreshor in winde area velocity. was larger, Wave although height datasediment were collectedtransport fromalso occurred eight positions, at the upper and the part results of the of shoreline P1–P6, located owing on to the the foreshore, effect of wind. were summarized.This happened The because wave heightthe wave was energy, measured diminished with the by same the time submerged intervals breakwater, during the test,was andbeing the developed results (Figures because6 and of 7an) wereincrease obtained of the at x-direction the first 15 mincomponent of each timeby the step. wind; The then, measurements the wave wereenergy taken broke at 20 in Hz front sampling of the rates,shoreline, and aand significant the intensity wave of height the wave value runup was presented decreased using as a 16,384 result samplingof dispersed data. wave In addition, energy. theTransported final morphological sand was deposited change results to the were seaside, displayed and some together of it to was understand transported the waveinto the height submerged change morebreakwater. easily.

3.2. Wave Deformation An assessment was conducted on wave variation that took place when sediment transport occurred according to the profile condition and changes in wind velocity. Wave height data were collected from eight positions, and the results of P1–P6, located on the foreshore, were summarized. The wave height was measured with the same time intervals during the test, and the results (Figures 6 and 7) were obtained at the first 15 minutes of each time step. The measurements were taken at 20 Hz sampling rates, and a significant wave height value was presented using 16,384 sampling data. In addition, the final morphological change results were displayed together to understand the wave height change more easily. As illustrated in the upper panel of Figure 6, the experimental result of the profile consisting of 0.1 mm sand showed that waves approaching from the offshore to the nearshore decreased gradually in proportion to the water depth decrease. The change in wave height at the position located at the seaside (x: 235) was extremely small, and wave fluctuation increased to the landside with the time duration. At the position of 200 cm, a sandbar was visible and breaking waves occurred in the section

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 9 of 17 J.J. Mar.Mar. Sci.Sci. Eng.Eng. 20192019,, 77,, 385x FOR PEER REVIEW 99 ofof 1717

(a) (b) (a) (b)

(Figurec) d50, 5mm,6. Wave (d) deformationd50, 0.1mm + distributionSub. B.W. with wind velocity; U: 0 m/s : (a) d50, 0.1mm, (b) d50, 1mm, Figure 6. Wave deformation distribution with wind velocity; U: 0 m/s : (a) d50, 0.1 mm, (b) d50, 1 mm, (c) d50, 5mm, (d) d50, 0.1mm + Sub. B.W. (c) d50, 5 mm, (d) d50, 0.1 mm + Sub. B.W.

(a) (b) (a) (b)

(c) (d) (c) (d) FigureFigure 7.7. WaveWave deformationdeformation distribution distribution with with wind wind velocity; velocity; U: U: 8 8 m m/s/s : (:a ()a d) 50d50, 0.1, 0.1mm, mm, ( b(b)) d d5050,, 11mm, mm, ((Figurecc)) dd5050, 55mm,7. mm, Wave ( (dd) ) deformationd d5050, ,0.1mm 0.1 mm + distribution+Sub.Sub. B.W. B.W. with wind velocity; U: 8 m/s : (a) d50, 0.1mm, (b) d50, 1mm, (c) d50, 5mm, (d) d50, 0.1mm + Sub. B.W. As illustrated in the upper panel of Figure6, the experimental result of the proﬁle consisting of 0.1 mm sand showed that waves approaching from the oﬀshore to the nearshore decreased gradually

J. Mar. Sci. Eng. 2019, 7, 385 10 of 17 in proportion to the water depth decrease. The change in wave height at the position located at the seaside (x: 235) was extremely small, and wave fluctuation increased to the landside with the time duration. At the position of 200 cm, a sandbar was visible and breaking waves occurred in the section within 150 cm. Changes in the water depth occurred at the section of x: 90–190 cm because of large-scale sediment transport. Thus, the wave height generated increased up to 120% on P1 when it was compared between initial and 4.5 h data. Changes in wave height are affected by changes at the sea bottom where the measurement position is, but the wave height at a certain position is more affected by the topography in front of the measurement area rather than the topography at this certain position. In addition, the wave height is the value at the beginning of each test interval and the topography result is the value at the end of the tests, so the time difference should be considered when analyzing the correlation between the wave height and topography result. For the profile consisting of 1 mm sand particles, a constant wave height appeared at the position located closest to the seaside as time passed. The result showed that a change in the water depth did not significantly affect the wave deformation, and the wave deformation increased from the position of 230 cm, where morphological change occurred in a significant manner. The wave height change also decreased from the position of 150 cm as the section where sediment transport decreased, and the wave height changed by a small amount in the water level near the shoreline owing to the formation of breaking, reflected, and multiple waves. The wave runup appeared dominantly at the innermost position (x: 85 cm), and the wave height increased up to 51% on P2 according to an increase in water depth at the section of 150–220 cm, where a large amount of sediment transport occurred. As illustrated in the lower panel of Figure6, in the condition in which 5 mm crushed gravel was applied, the wave propagated while maintaining a constant phase according to changes in water depth until the position of 160 cm. A breaking wave occurred at the position of x: 150 cm, where the water depth changed rapidly. A significant change in wave height occurred at the front of a rubble dune up to 38% on P2 in proportion to the (P2, P3) dune formation and change in water depth. However, waves did not reach the position of P1 directly, and changes in the water level according to changes in the internal water level were measured. In the condition in which the submerged breakwater was installed, a constant wave height appeared up to the position of 150 cm owing to sediment transport, as the wave reduced by the submerged breakwater was less affected by the topography when it propagated to the coast. However, changes in wave height increased near the shoreline up to 108% on P1 as a result of the direct influence of morphological change. In the circumstance in which there was an accompanying 8 m/s wind, a significant change in water depth occurred at the 0.1 mm sand profile owing to sediment transport. As illustrated in the upper panel of Figure7, no change in wave height occurred at the position of P6 located on the o ffshore side, but wave deformation occurred up to 48% on P1 as the water depth decreased at the position of 210 cm, where the sandbar was formed, and the wave energy increased as the wind force increased in proportion to changes in water depth at the section within 180 cm where the wave broke or right before the wave broke. In the condition with 1 mm sand, the variation of wave height occurred up to 26% on P2 at the section of 100–180 cm, where the water depth changed rapidly according to the morphological change. Relatively small wave variation of about 10.5% occurred at the position of x: 85 cm, which was affected by wave runup as time passed. The change in water depth was not significant under the 1 mm profile condition, because the range of morphological change was small. The range of wave height change also decreased accordingly. As illustrated in the lower panel of Figure7, for the profile where the crushed gravel with 5 mm was applied, changes in wave height occurred up to 35% on P2 owing to the effect of water depth change according to the occurrence of morphological change. The wave height increased in the section rear side of x: 200 cm, where the change of water depth occurred on a relatively smaller scale as sediment transport was in progress, and, in particular, the wave height change was the greatest at the position of x: 115 cm in front of the shoreline. The water surface displacement value at x: 85 cm located in the riprap dune increased as a consequence of the reduction of the water level in the foreshore and the transfer of wave energy by J. Mar. Sci. Eng. 2019, 7, 385 11 of 17 wind, and such a result was an outcome of a change in the water level inside the riprap layer. In the case of the profile where the submerged breakwater was installed, wave deformation occurred as a result of a change in water depth and the effect of wind in proportion to the time duration of the wave action; this deformation was the largest at the position of 115 cm up to 48% on P1. However, a constant water depth change was measured at the position of x: 85 cm that was affected by wave runup.

4. Data Analysis and Discussion

4.1. Morphological Change In this section, the objective is verifying how much a change in wind velocity influenced each profile condition using the sediment transport experiment results. For the profile featuring 0.1 mm sand, as shown in Figure8, a large degree of morphological change occurred in the foreshore and backshore, regardless of the presence of wind, and accretion started to occur after the erosion section (x: 195 cm). However, when wind accompanied the wave, seawater reached the end of the backshore owing to the development of wave runup, and the erosion area increased. Eroded sand was transported to the offshore side, forming a thick sandbar. For the 0.1 mm profile, a similar erosion trend appeared, regardless of the coexistence of wind, and the erosion range increased under the condition of 8 m/s, where the external force increased. Wind increased the range of the swash zone as a result of wave breaking-point change and accelerated the wave propagated to the coast after wave breaking. At this time, asymmetric wave skewness formed, and a morphological change occurred from the foreshore to the backshore [36,37]. According to the result of previous research [38], more sediment transport occurs when the skewness of flow in the shape of asymmetric saw teeth propagated to the coast is accelerated, and the same phenomenon appeared in this experiment. Another reason for the increase in the sediment transport range due to the action of wind was that the boundary layer of a wave propagated in the direction of the coast after breaking is accelerated by a half cycle, and this takes less time than the deceleration time by the loss of energy, such as friction; furthermore, the shear strength also increased in proportion to an increase in the vertical components of the flow velocity [39–41]. When the nourishment section was replaced with 1 mm sand, the overall sediment transport trend decreased, but the erosion in the land direction in the shoreline increased as a result of a strong wave runup phenomenon caused by the wind. Moreover, a sand dune with a steep front slope was formed in the backshore. In addition, the undertow caused by strong wave rundown created strong scouring on the ocean floor; as the wave runup phenomenon became stronger, the downrush phenomenon also increased. Scouring occurred over a wide area under circumstances where the wind coexisted with the wave. In the case in which the nourishment section was replaced with 5 mm crushed gravel, a large-scale gravel dune formed in the backshore area and strong scouring occurred in the foreshore. When the wind was present in this case, the size of the dune increased and the front breakwater slope also increased, as shown in Figure9. In addition, the slope from the dune crest to the foreshore where scouring occurred became steep when there was an accompanying wind. However, if there was no wind, only 5 mm gravels in the foreshore were partially transported to the rear, forming a dune. The remaining gravels stayed in place so that a constant slope was maintained at the section of x: 105–155 cm, and rapid scouring occurred from the boundary region (x: 168 cm) where material and sand encountered one another. However, with accompanying wind, gravels that were present previously were transported to the rear of the shoreline. The bottom was eroded in proportion to the amount of sediment transported, so that the range of morphological change was different. In particular, when the undertow was accelerated by the effect of wind, sediment transport occurred in more areas. It is known that topography in a stair-like structure formed at a position of 150–200 cm occurs in natural coasts as a result of the approach of typhoons or high waves with a steep wave slope [42]. Therefore, a morphological change similar to the situation at the time of a storm occurred at the front of the 5 mm nourishment section. Under circumstances where the submerged breakwater was installed, the energy of the wave developed from the offshore side was reduced primarily by the submerged J. Mar. Sci. Eng. 2019, 7, 385 12 of 17

breakwater.J. Mar. Sci. Eng.The 2019, morphological 7, x FOR PEER REVIEW change in the shoreline was significantly reduced in comparison 12 with of 17 theJ. Mar. profile Sci. Eng. condition 2019, 7, x FOR consisting PEER REVIEW only of fine sand. However, as the wind became stronger, an increase 12 of 17 in the magnitude of the wave breaker and wave runup phenomenon expanded the erosion area.

(a) (b) (a) (b)

Figure 8. Comparison50 between50 beach profile50 evolution and wave distribution after 4.5 h. (a) d50, Figure0.1mm, 8. (b)Comparison d , 1mm, (c) between d , 5mm, beach (d) d proﬁle, 0.1mm evolution + Sub. B.W. and wave distribution after 4.5 h. (a) d50, 0.1mm, (b) d50, 1mm, (c) d50, 5mm, (d) d50, 0.1mm + Sub. B.W. 0.1 mm, (b) d50, 1 mm, (c) d50, 5 mm, (d) d50, 0.1 mm + Sub. B.W.

(a) (b) (a) (b) FigureFigure 9.9. Comparison between sediment transporttransport results and beach profileprofile types.types. (a) Wind velocityvelocity 00m/s,Figure m/s, ( b9.)) WindComparison Wind velocity between 88m/s. m/s. sediment transport results and beach profile types. (a) Wind velocity 0m/s, (b) Wind velocity 8m/s. TheThe comparison resultresult ofof thethe profileprofile formform according toto the same external force condition showed that,that, The whenwhen comparison onlyonly thethe wavewave result waswas of the applied,applied, profile thethe form smallestsmallest according rangerange to oftheof morphologicalmorphological same external force changechange condition occurredoccurred showed whenwhen thethethat, submergedsubmerged when only breakwaterbreakwater the wave was was applied, installed the and smallest the condition range of with morphological a 1 mm grain change size wasoccurred employed. when However,However,the submerged thethe smallestsmallest breakwater morphologicalmorphological was installed changechange and underunder the condition thethe 11 mm with condition a 1 mm occurred grain size from was the employed. backshore totoHowever, thethe sectionsection the wherewheresmallest nourishmentnourishment morphological waswas change appliedapplied under (x:(x: 0–170 the cm).1 mm When condition there occurred was accompanying from the backshore wind in suchsuchto the circumstances, circumstances,section where a nourishment smalla small degree degree was of morphological appliedof morphological (x: 0–170 change cm). change occurred When thereoccurred when was the accompanying submergedwhen the submerged breakwater wind in wasbreakwatersuch installed circumstances, was in combinationinstalled a small in withcombinationdegree the 1of mm morphological with grain the size 1 profile.mm change grain This occurred size profile profile. waswhen advantageousThis the profilesubmerged was for controllingadvantageousbreakwater sedimentwas for installedcontrolling transport in sedime nearcombination thent transport shoreline. with near the the1 mm shoreline. grain size profile. This profile was advantageous for controlling sediment transport near the shoreline. 4.2. Wave Deformation 4.2. Wave Deformation In the experiment, the surf zone moved toward the offshore and the swash zone broadened whenIn the the wind experiment, velocity increased.the surf zone The heightmoved of toward wave runupthe offshore increased and in the the swash coast because zone broadened the wind when the wind velocity increased. The height of wave runup increased in the coast because the wind

J. Mar. Sci. Eng. 2019, 7, 385 13 of 17

4.2. Wave Deformation

J. Mar. Sci.In theEng. experiment, 2019, 7, x FOR thePEER surf REVIEW zone moved toward the offshore and the swash zone broadened 13when of 17 the wind velocity increased. The height of wave runup increased in the coast because the wind energy energyand wave and motionwave motion applied applied in parallel in parallel to the water to the surface water wassurface stronger was stronger after the after surf zone.the surf As zone. shown As in shownthe morphological in the morphological change experiment change experiment in Figure 10 ,in the Figure wave characteristics10, the wave werecharacteristics also different were because also differentdifferent because erosion different and accretion erosion trends and accretion appeared tr accordingends appeared to the according profile condition to the profile change. condition For the change.profile consistingFor the profile of 0.1 consisting mm sand, of a similar0.1 mm value sand, appeared a similar at value the position appeared of P6at the (x: 235position cm), regardlessof P6 (x: 235of windcm), velocity,regardless as shownof wind in velocity, the wave as height show measurementn in the wave result. height When measurement compared withresult. the When wave comparedsignal set beforewith the installing wave signal the profile, set before the formula installing and the measurement profile, the formul result ata and the positionmeasurement of P6 showed result atalmost the position the same of P6 result, showed but almost after the the beach same profile result, was but installed,after the beach the long-term profile was components installed, the increased long- termas a components result of the increased effects of waveas a result reflection, of the breaking,effects of wave and standing reflection, waves. breaking, A breaking and standing wave waves. did not Aoccur breaking in such wave a phenomenon, did not occur and in such the change a phenomen in energyon, and density the duechange to the in energy effect of density wind velocity due to the was effectsmall of in wind the area velocity with a was suffi cientsmall water in the level area for with wave a propagation;sufficient water however, level thefor resultwave measuredpropagation; near however,the shoreline the result (x: 115 measured cm) showed near that the theshoreline short-term (x: 115 components cm) showed decreased that the asshort-term a result of components the effect of decreasedshoaling andas a breaking result of waves, the effect while of the shoaling low-frequency and breaking area increased. waves, while The comparison the low-frequency result of area wind increased.velocity change The comparison showed that result stronger of wind wave velocity deformation change occurred showed near that the stronger shoreline, wave and thedeformation short-term occurredcomponents near increased the shoreline, in particular. and the Inshort-term the condition components where 1 increased mm sand wasin particular. deployed, In the the wave condition height whereat the 1 position mm sand of was P6 wasdeployed, constant the regardlesswave height of at the the wind position velocity of P6 change, was constant and the regardless overall energyof the winddensity velocity at the change, position and of the P2 nearoverall the energy shoreline density increased at the inposition comparison of P2 near with the the shoreline condition increased in which inonly comparison the wave with was the applied. condition Here, inwe which can only understand the wave that was the applied. low-frequency Here, we wave can understand is created by that the theoverlapping low-frequency of the wave short is wave created or shortby the wave overlapping and long of wave the short within wave the or range short of wave the surf and zone long [wave43]. within the range of the surf zone [43].

(a) (b)

Figure 10. Wave spectrum analysis on beach profile variation. (a) d50, 0.1mm, (b) d50, 1.0 mm, (c) d50, Figure 10. Wave spectrum analysis on beach proﬁle variation. (a) d50, 0.1 mm, (b) d50, 1.0 mm, (c) d50, 5.0 mm, (d) d50, 0.1mm + Sub. B.W. 5.0 mm, (d) d50, 0.1 mm + Sub. B.W.

TheThe spectral spectral distribution distribution analysis analysis result result when when the the 5 5mm mm grain grain size size was was applied applied showed showed that that constantconstant energy energy density density was was formed formed at at the the position position of of P6 P6 regardless regardless of of the the wind wind velocity velocity change. change. RegardingRegarding the the wave characteristicscharacteristics inin the the shoreline, shoreline, a valuea value of 1.0of 1.0 Hz Hz or higheror higher showed showed a similar a similar trend, trend, but the long-term components increased significantly in the area of 0.2 Hz or less. If the submerged breakwater was installed, a change in the energy density according to wind velocity change at the position of P6, which corresponded to the outer side of the foreshore, was very small when the wave reduced by the submerged breakwater was propagated to the coast. However, the short- and long-term components increased at the P2 position. An increase in energy density

J. Mar. Sci. Eng. 2019, 7, 385 14 of 17 but the long-term components increased signiﬁcantly in the area of 0.2 Hz or less. If the submerged breakwater was installed, a change in the energy density according to wind velocity change at the positionJ. Mar. Sci. ofEng. P6, 2019 which, 7, x FOR corresponded PEER REVIEW to the outer side of the foreshore, was very small when the 14 wave of 17 reduced by the submerged breakwater was propagated to the coast. However, the short- and long-term componentsindicated the increased growth of at theenergy P2 position. applied Anto the increase coast, in and energy significant density sediment indicated transport the growth occurred of energy in appliedproportion to the to coast,the growth and signiﬁcant of energy. sediment transport occurred in proportion to the growth of energy.

4.3.4.3. Literature ComparisonComparison AsAs mentionedmentioned in in the the previous previous section, section, studies studie usings using submerged submerged breakwater breakwater and nourishmentand nourishment have beenhave carriedbeen carried out by manyout by researchers many researchers and are currentlyand are beingcurrently conducted being conducted under various under conditions. various Inconditions. order to demonstrate In order to demonstrate the objectivity the of theobjectivity experimental of the resultsexperimental described results so far, described we are to so compare far, we oneare to of thecompare results one of severalof the results recent of studies several as recent an example studies in as this an section. example In in the this work section. of [44 In], thethe beachwork nourishmentof [44], the beach method nourishment was examined method through was examined a physical through model test. a physical Figure1 modelshows test. the results Figure for 1 shows sand nourishmentthe results for of sand d50 0.16 nourishment mm and gravel of d50 nourishment 0.16 mm and of dgravel50 5 mm, nourishment which are similarof d50 5 to mm, the conditionswhich are performedsimilar to the in thisconditions experiment. performed The resultsin this ofexperiment. [44] cannot The be results quantitatively of [44] cannot compared be quantitatively because the experimentalcompared because conditions the experimental such as indent conditions waves, beachsuch as proﬁle, indent and waves, range beach of beach profile, nourishment and range are of dibeachﬀerent nourishment from those are of thisdifferent paper. from However, those of the this experimental paper. However, results the for aexperimental similar particle results size for can a besimilar compared particle qualitatively. size can be compared Figure 11a qualitatively. shows that the Figure nourishment 11 (L) shows section that isthe made nourishment of sand and section the erosionis maderange of sand due and to the wave erosion activity range gradually due to increases.wave activity Figure gradually 11b shows increases. that erosion Figure occurs11 (R) shows at the beginningthat erosion of occurs the experiment, at the beginning but large-scale of the experi accretionment, occurs but large-scale on the rear accretion side of the occurs shore on line the after rear a certainside of periodthe shore of time. line Theafter results a certain of [ 44period] show of a ti similarme. The tendency results toof those[44] show of this a experiment.similar tendency If wind to isthose considered, of this experiment. the erosion If trend wind is is expected considered, to increase the erosion further. trend In is addition, expected there to increase are many further. related In documents,addition, there but are they many were related omitted documents, because the but comparison they were withomitted the because research the results comparison analyzed with in this the paperresearch lacks results cohesion. analyzed in this paper lacks cohesion.

(a) (b)

Figure 11. Experiment result of literature review; (a) d50: 0.16mm, (b) d50: 5mm. Figure 11. Experiment result of literature review; (a) d50: 0.16 mm, (b) d50: 5 mm.

5.5. ConclusionsConclusion TheThe trendtrend ofof morphologicalmorphological changechange waswas verifiedverified byby applyingapplying nourishment,nourishment, whichwhich isis oneone ofof thethe typicaltypical soft-typesoft-type methods,methods, toto aa coastcoast wherewhere erosionerosion occurred,occurred, andand thethe characteristicscharacteristics ofof eacheach profileprofile werewere analyzedanalyzed by by comparing comparing the the findings findings with with the resultsthe results of the of submerged the submerged breakwater breakwater method, method, which iswhich applied is applied in the field in the more field frequently more frequently as a hard-type as a ha structure.rd-type structure. In addition, In addition, the effect the of windeffect intensity of wind changeintensity on change morphological on morphological change when change an erosive when wave an approachederosive wave the approached coast was evaluated the coast at was the sameevaluated time. at In the circumstances same time. whereIn circumstances only the wave where existed, only the erosionwave existed, increased the undererosion the increased 0.1 mm sandunder profile the 0.1 in mm proportion sand profile to the in wave proportion generation to the time. wave In particular, generation large-scale time. In particular, loss of sand large-scale occurred overloss of a widesand rangeoccurred from over the a position wide range of x: from 200 cmthewhere position waves of x: broke 200 cm to where the backshore waves broke area, andto the a backshore area, and a sandbar was formed up to the position of x: 200–400 cm to the seaside. When d50: 1 mm sand was applied to the eroded section based on the shoreline, the sediment transport decreased significantly. This is because the sediment transport caused by wave action was restricted by a fast fall in velocity, and wave runup and rundown did not affect the sediment transport significantly. With regard to the 5 mm material, no suspension or traction due to waves occurred, and when the wave was applied to the material directly, the material was transported to the rear by

J. Mar. Sci. Eng. 2019, 7, 385 15 of 17

sandbar was formed up to the position of x: 200–400 cm to the seaside. When d50: 1 mm sand was applied to the eroded section based on the shoreline, the sediment transport decreased significantly. This is because the sediment transport caused by wave action was restricted by a fast fall in velocity, and wave runup and rundown did not affect the sediment transport significantly. With regard to the 5 mm material, no suspension or traction due to waves occurred, and when the wave was applied to the material directly, the material was transported to the rear by a momentary impact, but the material accumulated from the position within x: 100 cm, forming a steep rubble dune. When the submerged breakwater was installed, sediment transport decreased because of the effect of wave energy being decreased in comparison with the 0.1 mm profile, and the total amount of sediment transport was similar to the condition with 1 mm material application. When there was an accompanying wind velocity of 8 m/s, the erosion trend in most profiles was similar to the situation in the case of 0 m/s, but a difference was revealed in that the erosion range was expanded. In the case of the 0.1 mm profile condition, the erosion occurred in most of the backshore area owing to strong wave runup, and in the case of the 1 and 5 mm profile conditions, the undertow developed by the strong rundown phenomenon created large-scale scouring near the boundary between the nourishment section and existing sandy ground. Although a submerged breakwater was installed, the erosion range also increased owing to the effect of increased wave energy caused by wind. The profile with 1 mm and submerged breakwater installation showed the smallest range of sediment transport. On the basis of the wave data collected from the positions of P2 and P6, a clear wave change according to the change in wind velocity did not appear at P6, where no breaking wave or wave deformation occurred in the experiment. However, complicated flows, such as breaking waves, reflection, and standing waves, occurred at P2 on the shoreline, and the overall energy density increased in conditions where wind was applied, although the overall energy density could vary according to the morphological conditions. To summarize the above experimental results, if nourishment is selected for the purposes of reducing erosion, a 1 mm particle size is advantageous for sediment transport, but an increase in the grain size, such as with the 5 mm condition, does not produce greater reductions in sediment transport. In addition, an increase in the magnitude of wind velocity produces a greater range in morphological change. It is deemed appropriate that situations where waves and wind coexist should be considered when establishing measures to reduce erosion in the future.

Author Contributions: K.-T.S., writing—original draft preparation; K.-H.K., review and editing; J.-H.P., formal analysis. Funding: This research received no external funding. Acknowledgments: The authors thank Hyun-Dong Kim for support in the data comparison. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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