Journal of Marine Science and Engineering Article Analysis of Longshore Drift Patterns on the Littoral System of Nusa Dua Beach in Bali, Indonesia Anthony Harlly Sasono Putro and Jung Lyul Lee * Graduate School of Water Resources, Sungkyunkwan University, Suwon-si 16319, Korea; [email protected] * Correspondence: [email protected] Received: 26 August 2020; Accepted: 24 September 2020; Published: 26 September 2020 Abstract: Bali is one of the most popular tourist areas in Indonesia. With its coastline stretching 633 km, Bali has many beautiful beaches that have become world tourism attractions, thus making tourism the economic engine of Bali. The biggest priority of major tourism sites is maintaining sufficient beach width. However, based on a survey from Balai Wilayah Sungai (BWS) Bali Penida, Bali has suffered from a series of erosions along 215 km of its coastline, including Nusa Dua, in 2015. The location of the study area for this report is a 2.8 km stretch of coastline at Nusa Dua Beach. The erosion problem at Nusa Dua Beach was assessed by analyzing the longshore drift patterns. Simulations are required to assess this erosion problem, combined with the erosion rate and the simulated equilibrium shoreline for each sublittoral cell. To estimate the erosion rate, this study employed profile monitoring data of the beach obtained from 2003 to 2016. This advanced study was based on the mass conservation principle as a governing equation used to predict longshore drifts between sublittoral cells. The satellite image for every sublittoral cell was also used to check the equilibrium condition and estimate the predominant wave direction as the shoreline orientation. Nusa Dua Beach was found to suffer from the change of wave direction and the consequent generation of littoral drift after the reclamation project of Serangan Island located north of the Benoa strait. The correlation between the transportation of longshore sediments and the predominant wave direction indicates the effect of longshore drift in the system. The groin system also created a unique longshore transport pattern in the coastal area. The results obtained in this study can help manage the longshore drift system of Nusa Dua Beach and can be used to predict the beach area subject to erosion and deposition after every beach conservation project. Hence, a strategic plan for managing the shore target lines can be formulated. Keywords: beach erosion; equilibrium shoreline; longshore drift; predominant wave direction; shoreline orientation 1. Introduction Beaches play an essential role in the life of an ecosystem. They participate in water purification, nutrient renewal, coastal defense, recreational activities, etc. Previous studies have stated that the use of beach does not integrate well with the importance of maintaining sustainable resources. These studies considered beach use as a cause of environmental degradation and imbalance in the sedimentary process [1–3]. Fast-growing tourism, coastal development, and global climate change can worsen the degradation impact [4–6]. To better understand their socioeconomic and engineering impacts, these erosion problems need to be assessed via the physical processes of the coastal zone [7]. Some indicators, such as shoreline positions, beach morphology, coral reef condition, hydro-oceanography condition, and coastal structures, can be used to assess erosion problems, and the results of this assessment should be considered for the development of coastal management. J. Mar. Sci. Eng. 2020, 8, 749; doi:10.3390/jmse8100749 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 749 2 of 19 Some cases of beach erosion are due to imbalances in sediment transport, which are caused by the longshore sediment transport. Previous studies have analyzed the impact of longshore sediment transport on the conditions of coastal areas [8–12]. Bergillos et al. [10], analyzed the relationship between the morphology of the conserved beach and longshore sediment transport. These conditions can be affected by some factors, such as the change in bathymetry condition and the predominant wave condition [8]. The changes in sediment volume can also be analyzed by using the longshore sediment transport gradient [8]. Furthermore, Andrade et al. [1], concluded that via the change in beach width and shoreline position, the gradients of longshore drift play an essential role in erosion processes. The main aim of this study is to assess the erosion problems at Nusa Dua Beach of Bali Indonesia, based on the data from the beach monitoring survey. The assessment of the longshore drift process and equilibrium shoreline are the primary tools used for checking the erosion problems. The results of this research will serve as guidance for planning and developing the next strategic approach in coastal management. 2. Characterization of Study Area Bali is one of the most popular tourist sites in Indonesia. In 2017, tourism constituted almost 23% of the gross regional domestic product of Bali [13]. With a coastline stretching 633 km, beaches remain the most popular tourist destination in Bali. The southern part of Bali has a flourishing tourism industry and some of its beaches, such as Nusa Dua, Sanur, Kuta, and Tanah Lot, are particularly popular tourist destinations. Additionally, the beaches are of significant value to the Balinese Hindu population. However, according to the 2015 survey data obtained by the Indonesian Ministry of Public Works and Housing, erosion in Bali has affected 215 km of the coastline. Coastal erosion has recently become a global environmental problem. Most coastlines suffer from erosion owing to the sediment deficit caused by human disturbances such as damming, land reclamation, urbanization, and coastal area development [14–19]. Over 70% of the world’s beaches suffer from erosion [20]. One of the goals of integrated coastal zone management's (ICZM) is to improve beach quality. The European Commission [21] has noted coastal erosion as an issue that must be addressed in ICZM strategies. Coastal erosion may generate several negative impacts on social-economical aspect such as the potential loss of life, damage of property and infrastructure, loss of tourism revenue, and degradation of the ecosystem. The study area for this work is located on Nusa Dua Beach, from groin G12 to the north part of groin GA2, with a total shoreline of approximately 2.8 km (Figure1). Located in the southern part of Bali Island, in front of Benoa Strait, Nusa Dua has been one of the most famous Bali beach resorts for several years. Up to the eastern part of the north coast of Tanjung Benoa, the beach is made of carbonaceous sand and foraminiferan components. The beach slope is slightly steep and steadily levels as it nears the sea. Based on the data obtained after a beach nourishment project, the grain size (D50) at Nusa Dua Beach is 0.606 mm. The beach erosion problem at Nusa Dua was caused by the interruption of the northward littoral drift by a natural tombolo behind the Nusa Kecil and Nusa Besar islands that occurred several decades [22]. The other reason for the erosion problem is a reduction in the reef flat owing to coral mining. The lower reef flat allows waves with high energy to be propagated to the beach. The decreased supply of sand creates an imbalance in the sediment budget. Furthermore, the general stress from the disturbance on the beach area induces localized erosion. From 2001 to 2003, the Indonesian Ministry of Public Works in collaboration with Japan International Cooperation Agency completed the Bali Beach Conservation Project Package II at Nusa Dua by implementing civil works such as beach nourishment, and constructing coastal structures. The coastal conservation and protection efforts essentially propped the tourism at Nusa Dua Beach. The project focused on maintaining the beach from a functional perspective and developing a plan for its utilization as a tourist area and its landscaping while protecting and conserving the beach environment. Furthermore, the project facilitated the construction of several groins to stabilize the J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 3 of 20 shoreline and protect it from longshore drift. Hence, the coastline has the same phase potential line. These artificial headlands divided Nusa Dua Beach into several sublittoral cells. To optimize its effectiveness, the beach nourishment project needs to be adjusted with the coastline morphology, J.sediment Mar. Sci. Eng. grain2020 size,, 8, 749 and input volume [11]. 3 of 19 The wave direction is predominantly south and south east. This oblique wave is reflected by the shoreline generating a northerly longshore current. Its motion is one of the external forces responsible shorelinefor either erosion and protect or sedimentation it from longshore in the area. drift. The Hence, tidal thecharacteristic coastline hasin the the Nusa same Dua phase area potential is mixed line.semidiurnal. These artificial Based on headlands the tidal elevation divided Nusadesign Dua for Beachthe beach into conservation several sublittoral project, cells. the highest To optimize water itslevel eff ectiveness,obtained was the 2.6 beach m higher nourishment than the projectlowest water needs level to be (LWL). adjusted Hence, with the mean coastline sea morphology,level was 1.3 sedimentm above the grain LWL. size, and input volume [11]. Figure 1.1. Study area location—Nusalocation—Nusa Dua Beach. TheFrom wave 2003 direction to 2016, a is beach predominantly profile survey south was and conducted south east. in Thissome oblique sections wave of Nusa is reflected Dua Beach by the by shorelinethe Indonesian generating Ministry a northerly of Public longshore Works current.and Housing. Its motion This is activity one of theprovided external useful forces information responsible fornecessary either erosionfor understanding or sedimentation the beach in the erosion area. Thephenomena. tidal characteristic After the beach in the conservation Nusa Dua area project is mixed was semidiurnal.completed, the Based change on thein shoreline tidal elevation was evident design in for a thetime beach series conservation of satellite images project, from the highestGoogle waterEarth level(Figure obtained 2).