Morpho-dynamic Induced Rip Currents in Klayar Beach, Pacitan, East Java, Indonesia

Klayar Beach (KB), Pacitan, East Java, Indonesia is a rugged coast with a narrow sand beach, a relatively small embayment with rocky headlands, and a very exotic view as a coastal tourism destination. Despite its attractiveness, KB poses a hazard due to rip currents. From 2009 to 2022, KB visited more than 3 million people, and more than 30 people were reported dead due to drowning in rip currents. This paper presents the result of fieldwork to investigate morpho-dynamic driven rip current hazard of the study area. This study carried out bathymetric and topographic mapping and sediment sampling. The last 10 years’ wave data was used for predicting hydrodynamic parameters. The study shows a good correlation between a variety of incoming significant wave heights and tide fluctuation vs. rip current velocity. The existence of headland plays a significant role in rip current formation in the study area. The headland influences either shadow or deflected rip currents formation for the incoming wave angle. The study also indicates that the type of rip current in the KB is structural boundary controlled, and the most important thing to be alerted for promoting beach safety in this area is that this rip current is predicted permanently to occur.


Introduction
Klayar Beach (KB) is located in the southern part of Pacitan Regency, the southern coastal area of East Java Province, Indonesia (Fig. 1).As a coastal tourism destination for both domestic and foreign visitors, KB provides attractive views with mountainous and cliffed coast, wave cut terrace and sea stacks, and embayment beach with beautiful rocky headlands and spectacular incoming breaking waves.This feature is common to be found along the southern coast of Java.The last 10 years record of visitors to KB shows a significant increase (Fig. 2).Increasing tourism in KB is supposed to be a local economic driver for the local community as well as for coastal communities along the southern coast of Java.Coastal waters, particularly in the nearshore is an active zone that is very attractive to the tourist.1298 (2024) 012035 IOP Publishing doi:10.1088/1755-1315/1298/1/012035 2 However, despite their attractiveness coastal water can be very dangerous to their visitors due to rip currents which are really inhospitable to them [1,2].Rip currents are among the most deadly coastal hazards for the swimmer in the coastal water [3,4], particularly in a swell-dominated environment (as it is possessed by the southern Java coastal area), because the swell can result in more deadly rip current hazard than wind waves [5,3].

Figure 1. Location map of the study area
Beach accidents caused by rip currents have been a major global hazard worldwide, but there is a lack of attention to rip currents compared to the other hazards [6], since rip currents never destroy buildings or infrastructures and never cause environmental damage and economic loss.In fact, however, it has been reported that an estimated 100 people drown in rip currents each year in the U.S., 21 people in Australia, and 53 people in Costa Rica [7]; and 71 % of all recorded incidents were due to rip current in the UK, and in many other countries (Brazil, Colombia, Israel, and New Zealand) have reported that rip current are major hazards [8].[9] reported that more than 200 people had drowned in rip currents and 30 people were dead every year in Korea.Surfzone fatalities in China have been reported reached 660 from 2009 to 2019, even more than the 628 casualties of maritime disasters during the same period [10].In Indonesia, fatalities due to rip currents on the Southern Coast of Java have been reported as many as 28 people dead each year in the last 5 years, only on the southern coast of Java [11].Beach accident due to drowning in rip current in KB has been recorded as high as 30 people's death since 2009 (Fig. 2).
A rip current is defined as water masses that move contrary in direction to the incident wave which extend outside the surf zone normal to the beach [1,2,5,6,12,13,14], narrow, strong and concentrated seaward-directed flows [5,6,8,14], associated with wave action in the breaker zone [12,14,15], its characteristics are depending on the incoming wave height variability [5,6], and variability in the offshore bottom topography [12,15,16], it is much shallower than its wide and its lifetime is correspond to its development time [17].This present study examines the existence of rip current and headlandinduced morpho-dynamic processes in generating a rip current, and the implication for beach safety campaigns in KB, as well as on the southern coast of Java.

Material and Methods
We carried out fieldwork to investigate the occurrence of rip currents in KB.We conducted shore topographic and seafloor bathymetric mapping, shore and sea bottom surface sediment samplings, and secondary waves data collecting.Before the field works, satellite imageries of the KB were collected to select locations where the rip current existed.

Site Selection
The KB is located in the southern part of Pacitan Regency, East Java Province, Indonesia with latitudes and longitudes boundaries as follows: 8 o 13'23"S; 110 o 56'46"E to 8 o 13'28"S; 110 o 56'43"E and 8 o 13'30"S; 110 o 56'54"E to 8 o 13'33"S; 110 o 56'30"E (Fig. 1).Coastal morphology of the study area shows highly exposed coast with wave dominated erosional coast which is characterized by the presence of sea stacks, wave-cut terraces, narrow sandy beach, natural structural headlands, and embayed beach.This is typical of the secondary coast which is dominated by wave actions (Fig. 1 and 3).This coastal feature is common in the southern coastal area of Java, Indonesia, and it is almost always crowded by beach visitors throughout, especially during the holiday season.This study used Google Earth™ map to investigate the existence of rip current in KB.Utilizing satellite imagery from Google Earth™ for rip currents identification has been done by previous researchers worldwide [18,19,20,21,22,23]). KB was selected as the study area based on the result of the visual analyses of the eight satellite imageries of the Google Earth™ maps (Fig. 3).Eight satellite 1298 (2024) 012035 IOP Publishing doi:10.1088/1755-1315/1298/1/0120354 imageries from 2009 to 2022 show that there are rip currents persistently occur, particularly at the lefthand (east) embayment close to the headland.

Environmental Dataset
Shore zone and seabed topographic mappings were carried out using a single beam echosounder for bathymetrics (Fig. 4), and using Total Station for shore zone topographic.We collected the last 10 years' winds and waves data from secondary published data released by The Agency of Meteorological Climatological and Geophysical of The Republic of Indonesia (BMKG).We processed it to find significant wave height (Hs) and its peak periods (Tp) and to predict breaking wave heights (Tabel 1).Significant wave height Hs (m) Source: Meteorology Climatology and Geophysical Agency

Coastal Morphodynamic
Oceanographically, the area along the coast of the southern part of the Indonesian Archipelago especially the southern coast of Java faces directly the unlimited fetch of the Indian Ocean, consequently this area is characterized by swell swell-dominated environment with high to extremely high incoming wave height.Tectonically, Java Island is a part of the island arcs that has been being uplifted since at least ~2 ma [24], as a result of the subduction between the subducting Indian-Australian plate below overriding the Eurasian plate (Fig. 6).Because of this uplift, the southern coast of Java has high topography with hilly to mountainous morphology.Combined oceanographical and geological processes lead to the high energy of highly exposed coasts with wave-dominated erosional coasts.
Therefore, the southern coast of Java exhibits spectacular views characterized by high incoming waves height, cliffed coasts, small to large embayments with rugged rocky headlands, and beautiful sand beaches.
Figure 5. Tectonic setting of the Indonesian archipelago [24] As a system, beach morpho-dynamic consists of elements of morphology, waves and flow, sediment transport and bed change, and as a process refers to interactions between morphology and hydrodynamic including wave, tide, and wave-driven currents involving sediment movement [25,26,27]).Interactions between beach morphology and hydrodynamic processes take place when waves reach the surf zone.As waves propagate from deeper to shallower water, the friction of the seabed starts to decrease wave celerity and deform wave characteristics, which are wave height becomes higher and wavelength becomes shorter, and eventually wave breaks when breaker index (b) reaches 0.78.Breaking waves have been believed as a driving mechanism of rip current formation [2,5,12,14].Waves and tides are predominantly factors influencing coastal morphology and processes [25], and rip current is one of the phenomenons created by the interactions between waves and coastal morphology [8,14,25,28], and often modulated by tides [2,29].[30] discovered that rip currents vary significantly in occurrence, spacing, and associated nearshore topography, depending on the predominant and the origin of wavebeach conditions.
Since morpho-dynamic processes can generate coastal hazards, one of those is rip current, therefore, in Australia, the ABSAMP (Australian Beach Safety and Management Program) used beach morphodynamics to develop a beach hazard rating system [31].Previously, [30] combined sediment characteristics with breaking wave parameters to categorize beach types.He used the non-dimensional settling velocity () of [32] as follows: where  is the non-dimensional settling velocity, Hb is the height of wave break, T wave period, and Ws settling velocity of the sediment.
Based on the , [33] described beach types into three categories, as the followings: (1) reflective beach when  < 1; (2) intermediate beach with 1 <  < 6; and (3) dissipative beach if  > 6.In order to investigate coastal morpho-dynamic processes, usually researchers do not use only a single criterion with a single parameter, but they often use many parameters and combine more than one criterion [30,33,34,35].[33] developed a method for describing the circulation pattern in the surf zone of an embayment beach with a dimensionless scaling factor (').He used ' for predicting the influence of headland to the circulation pattern, which is obtained from: where Sl is the length of the embayed beach which is a sum of both lengths of the headland and beach (Fig. 7).
This method is now widely used to classify embayed beaches [35].The surf zone circulation pattern can be described by the range of the '; normal beach circulation (' > 19); transitional circulation (8 < ' < 19); and cellular circulation (' < 8) [30].Recently and more specifically, [35] developed a new and low-cost method for classifying swell-dominated embayed beaches utilizing morphometric parameters.They used this classification to determine morphological processes that help in effectively assessing the vulnerability of general beach embayments.To classify swell-dominated embayed beaches, [35] developed embayment morphometric parameter (e) and utilized the dimensionless scaling factor (') of [30], and headland orientation (H1 to H6).The embayment morphometric parameter is defined as: where a is an indentation of the embayment from the headland point to the back-beach limit, and Ae is the embayment area.We followed the works of [30,33,35] to classify the beach type of the study area and to investigate the morpho-dynamic process and its influence in generating rip currents in KB.We summarized embayment geometry and its components, hydrodynamic characteristics and its interaction with the seafloor topography, and beach and surf zone sediments of the study area (Fig. 7), and used them to determine embayment morphometric parameter, dimensionless fall velocity, and headlands orientation (Table 2).Those parameters are then employed to investigate the dominant process that is playing an important role in generating rip currents in KB.According to the dimensions of the embayment parameter components (Table 2), KB has two inward-oriented headlands (Fig. 7) implying that shadow rip is more susceptible to occur than deflected one.The dimensionless falling velocity () ranges between 4-6, indicating that the study area is categorized as an intermediate beach, which rip currents are susceptible to occur.The Dimensionless embayment scaling factor (') of the KB falls to the range of 4.0 to 6.6 suggesting that the existence of the structures or headlands is very significant in influencing morphodynamic processes in the study area with cellular circulation pattern.The Embayment morphometric parameter (e) of the study area is 0.72, indicating that the embayment of the KB consists of a relatively lower distance between headlands and greater coastal indentation., width between headlands (Ro), the total length of embayment that is a sum of headlands and beach length (Sl), and orientation of headland direction to the beach (hw and he: west and east headland, respectively); dominant incident waves directions (W1 to W5), because of the valleyed seafloor, all direction of incident waves are refracted and eventually the prevailing wave direction entering to the nearshore zone tends relatively constant all the times (WI); east headland plays an important role in shadowing shadowed zone by blocking incident waves.

Type of the KB Rip Currents and Its Implication for Promoting Beach Safety
Classical taxonomy of any type of rip current comprises the rip feeder(s) which is initially part of longshore current that is parallel to the coastline and becomes a feeder when its direction turns to the open sea and perpendicular to the coastline; a rip neck is the longest part of rip current with seaward direction flow and normal to the shoreline, and a rip head that formed outside of breaker zone.Even though they vary significantly in occurrence, spacing, and associated nearshore topography, rip currents do not occur on all-type beaches [32].Rip currents could not occur on the reflective type beach swash oscillations perpendicular to the shore are dominant in this environment, and do not occur on fully dissipative beaches.Rip currents are a typical feature and essential element of the intermediate type of beach on the transition state from the intermediate to dissipative beach type.Based on the morphometric analyses, we found that all embayment parameter components of the study area suggest that rip currents are susceptible in the KB.Coastal scientists and coastal engineers have identified and described rip currents to classify rip currents [32,8,36,6] and [14].[32,8] classified rip currents into four types based on hydrodynamic and topographic controls.Those are accretionary and erosional rip currents and topographic and mega rips.Based on the generating mechanisms of rip currents, [36] described four categories of rip currents: (1) waves-structures interactions, (2) waves-waves interactions, (3) shear instabilities of strong alongshore currents, (4) hydrodynamic and morphologic instabilities.In addition, they showed that the rip current type that most frequently occurs is hydrodynamically generated by uneven seafloor topographic-induced wave refraction.To increase awareness and promote a better public understanding, [6] suggested using descriptive terminology.[6] classifies rip current types based on the description of environmental conditions including seafloor morphology and waves and sea conditions.He also describes the appearance and persistence of each rip current type, and its relative threats and proposes ways to avoid drowning in each rip current type.[6] proposed five categories of rip current type: (1) rips occur between bars, (2) cusped-shore type rips, (3) structurally generated rips, (4) flash rips, and (5) large-scale or mega rips.[14] considered interactions between wave breaking and morphology and tides influence induced flow behaviors to describe and classify the type of rip current.This type is categorized by the generation processes.[14] divide rip currents generating forces into four major mechanisms: (1) hydrodynamic-generated rip currents, (2) bathymetric-induced rip currents, (3) boundary-influenced rip currents, and (4) other generated-mechanism rip currents.
Referring to the above-mentioned four rip currents classifications, KB rip currents are categorized as topographic rips of [8]; classified as waves-structures interactions rip by [36]; identified as structurallycontrolled rips of [6], and termed as boundary-controlled rips of [14].Even though all classifications applied for KB rip currents are semantically different, they refer to the same driving mechanism, they mention the same processes, and they show the same appearances.Characteristics of sediment may be important as an indication to determine whether positive or negative feedback of the wave action in the study area.[37] also considered the availability of coarse sediment as an indicator of a wave-dominated coast, not only based on the absolute parameter of wave height.Sediment from KB consists of predominantly well to very well-sorted coarse sand [11] indicating that the study area is categorized as a wave-dominated coast [37].In addition, sediment is only found at the shore zone, and during low water elevation, fringing reefs and bedrock are exposed.This is interpreted that KB is categorized as erosional rather than accretional beach.Therefore, according to [32], KB rip currents could have been classified as erosional rips.However, since the morphology of the embayment including the size of the embayment is relatively small, its geometry as shown by the sea bottom valley and direction of the east headland tends to refract and block the incident waves, respectively, then eventually, any direction of incident waves almost always perpendicular to the shoreline (Fig. 7) and are blocked by the east headland.Therefore, the above-mentioned classification of rip types for the study area is applicable.More specifically the existence of east headland which is playing an important role in blocking the incident waves that lead the down waves area as a shadow zone that is very susceptible for shadow rips to take place.
KB, especially at the east headland looks calm as protected from incident waves and breaking waves, also looks very attractive since very close to the cliffed headland.Despite that attractiveness, structural (headland) boundary shadow rip current in KB persistently occurs and could be very fast and dangerous to the bathers.Concerning the condition, it is suggested to formulate mitigation measures to lower the risk due to this deadly hazard.The first thing that can be done is to stake a red flag at the most dangerous location on this beach and make an alert announcement board there.These measures are necessary not only in the KB but also along the southern coast of Java Island

Conclusion
We have carried out fieldwork and analyzed the morphodynamics of the embayment beach concerning rip formation in the study area.Tectonically and oceanographically, the study area is categorized as a highly exposed swell-dominated coast that exhibits spectacular views characterized by high incoming wave height, cliffed coasts, small to large embayments with rugged rocky headlands, and beautiful sand beaches.Despite the attractiveness, the KB poses a deadly hazard due to rip current.The morphometric embayment parameters of the KB indicate that this area is susceptible to the rip current, this is consistence with the evidence from the satellite imageries.According to this, it is suggested to formulate mitigation measures to reduce the risk due to the deadly hazard.

Figure 4 .
Figure 4.The result of the topographic and bathymetric mapping of the KB

Figure 6 .
Figure 6.Morphometric and morpho-dynamic parameters of KB: area of embayment (Ae), the indentation of the embayed beach (a), width between headlands (Ro), the total length of embayment that is a sum of headlands and beach length (Sl), and orientation of headland direction to the beach (hw and

Table 2 .
Beach embayment parameter and its componentsEmbayment/beach parameters of the Klayar Beach