Morpho-hydrodynamic processes impacted by the 2022-extreme La Niña event and high river discharge conditions in the southern coastal area of West Java, Indonesia

Coastal areas play a major role in human life and activities, including economic, sociocultural, agri-aquaculture, tourism, and other ecological benefits. These various potencies support the growth of coastal areas, developing more rapidly than areas far from the coast. Most of the big cities in the world lie in coastal areas such as New York City, USA; Perth, Australia; Tokyo, Japan; Shanghai, China; Seoul, Korea; and Jakarta, Indonesia. Approximately 40% of the world's population lives within 100 km of the coast (Sedac 2008). Unfortunately, the coastal areas are highly dynamic and vulnerable to coastal hazards, making it a challenge to manage their sustainability. For several decades, researchers have been focusing on coastal dynamics related to the effects of climate change (Hirons & Klingaman 2016; Barnard et al. 2017; Rossi & Soares 2017).

Climate change is a long-term threat that is expected to affect the severe environmental degradation in coastal areas by altering the ocean temperature and circulation patterns (Darby et al. 2016). The La Niña event is characterized by anomalous sea surface temperatures (SSTs) in the Eastern Tropical Pacific Ocean, which became cooler than usual. This anomaly increased the air pressure off the western coast of South America, where wind-driven rain blows toward the Western Tropical Pacific Ocean, causing high precipitation, flooding, and erosion in Indonesia (Encyclopedia 2024). The extreme La Niña event in 2022 was the triple-dip La Niñas for three consecutive years from 2020 to 2023, changing the seasonal and annual climate patterns in several equatorial regions (Shi et al. 2023). The La Niña event can be observed by visualizing the oceanographic parameters (SST, sea level pressure, wind velocity, and precipitation). Three ranges of the La Niña event are normal, moderate, and extreme based on the Oceanic Niño Index (ONI), in which the lower index (below −1.0) indicates the extreme La Niña. In this study, the 2022 La Niña event is visualized to figure out its phenomenon and its effects on the Tropical Pacific regions, particularly in West Java, Indonesia, during boreal winter (December 2021–February 2022) and boreal autumn (September–November 2022).

The La Niña event is strongly related to coastal hazard, in which the wave-induced currents erode the coastal areas due to the extreme difference of SST. Odériz (2020) found that the El Niño-Southern Oscillation (ENSO) impacted the direction and magnitude of the waves. The coastal areas of Indonesia are categorized as high response to the La Niña event (level 2), and vulnerable to erosion and accretion. However, the studies of morpho-hydrodynamics due to the influence of ENSO and high river discharges in coastal areas near river mouths and port jetties have rarely been studied in previous literature studies. This study aims to fill a gap in understanding the substantial impact of the La Niña event on the possibility of sedimentation in the port basin surrounding the river mouth, particularly in the southern coastal area of Indonesia at the Bojong Salawe Beach, Pangandaran.

High precipitation is the major effect of the La Niña event, extremely causing erosion in the catchment areas and upstream riverbanks (Darby et al. 2013). The sediment-laden high river discharges flow toward the river mouth, inducing sedimentations at the estuary. In addition, wind velocity-generated high waves produce strong energy to erode the coastal areas. Erosion and accretion are the main problems of the morpho-hydrodynamic changes that commonly occur in coastal areas. Shoreline changes due to erosion and accretion can be investigated using spatial methods by comparing the satellite images with a certain range of years (Hastuti et al. 2022). For instance, the digital shoreline analysis system (DSAS) program was usually used in the study of shoreline changes. DSAS is used to calculate the rate-of-change statistics from a time series of vector shoreline positions, providing an automated method for establishing measurement locations, performing rate calculations, and also includes a beta model for forecasting shoreline position (USGS 2021). A previous study using DSAS was utilized to observe the shoreline changes in Pangandaran in two decades between 1994 and 2014 (Harahap et al. 2022). The results show that the shoreline changes in Pangandaran attained the highest erosion at a rate of −4.7 m/year, while the highest accretion rate was 40.1 m/year. In addition, numerical models are also widely used in shoreline change research due to their effectiveness and advanced capabilities. Mike21 developed by the Danish Hydraulic Institute (DHI) for coast and marine modeling is also used to observe the morpho-hydrodynamic processes due to its accuracy and highly close to real conditions (www.mikepoweredbydhi.com). Kulkarni (2013) used the Mike21 numerical model to analyze coastal erosion in Baydara Bay, Russia. The model results demonstrate that the combined wave and current produced realistic simulations, which predict a bed level change to the order of 0.2 m. In this study, a numerical model using Mike21 is utilized to observe and calculate the coastal morpho-hydrodynamics during the 2022 extreme La Niña event.

The southern coastal areas of West Java, Indonesia, are potentially impacted by climate change due to high waves and strong currents from the adjacent Indian Ocean. Their length of coastlines is approximately 428 km, covering five regencies (i.e., Sukabumi, Cianjur, Garut, Tasikmalaya, and Pangandaran) (Windupranata et al. 2020). The existence of a large estuary and its economic activities (port, shrimp ponds, and tourism) highly increase the coastal morpho-hydrodynamics with a unique sediment distribution pattern in this area. Bojong Salawe Beach, Pangandaran, is the area focused for this study due to the large estuary in which several tributaries and four rivers flow, potentially causing high erosion and sedimentation. This study aims to assess the impacts of the 2022 La Nina extreme on the Bojong Salawe Beach and to observe the different effects during boreal winter and boreal autumn. The level of erosion and accretion caused by the sediment-laden river discharges, inducing a high risk of flooding around the river mouth and port jetty, is also observed in this study. Furthermore, the shoreline changes of the study area are investigated to provide input for the development of coastal areas.

In this study, the description of the study site is depicted in Section 2. The methods and supporting data are explained in Section 3. The analysis of results and discussion are presented in Section 4. The paper concludes in Section 5.

The descriptive analysis method was used to visualize several oceanographic parameters such as SST, sea level pressure, wind velocity, total precipitation, and significant wave, presenting the La Niña event in 2022. These oceanographic data were obtained from the reanalysis dataset ERA-5 Copernicus Climate Change Service. During boreal winter, SST ranged between 28.7 and 29.5 °C in the study area. The dominant wind direction blew from west to east with velocity ranging from 0.11 to 10.22 m/s. The SST during boreal autumn 2022 ranged between 25.8 and 29.0 °C. The wind direction dominantly blew from southeast to northeast with a mean velocity of 5.2 m/s. This low wind speed increased the precipitation levels by 19.23 mm in November 2022. The observed significant waves were recorded to be 1.09–2.62 m.

Other parameters used in the simulation included river discharges, tides, and sediment data. The maximum high river discharge taken from the Balai Besar Wilayah Sungai (BBWS) Citanduy was 174.54 m³/s, triggered by intense precipitation due to the 2022 La Niña event. The Pangandaran monitoring station recorded the tidal data with a maximum of 1.07 m and a minimum of −0.97 m. The sediment data properties taken from the previous study (Nirwana et al. 2021) were used in the simulation approximately of 80% silt and 20% sand.

In this study, the boundary outlines consist of polylines with 771 vertices. The land boundary data are manually digitized from Google Earth, covering the shoreline length of 3.78 km. Meanwhile, the length of the open-sea boundary used in the model is 4.17 km. The elevation data from the digitized topography and bathymetry represent the x, y, z scatter data and water depth values. Both sea–land boundary and elevation data are used to generate the flexible triangular mesh. The computational grid specifies the domain area, and the mesh resolution determines the simulation accuracy. Some scenarios are conducted to observe the patterns of the current and wave with consideration of river discharges on the rate of non-cohesive sediment transport. The models are simulated within a month from 1 November 2022 00:00:00 a.m. to 30 November 2022 23:00:00 p.m. due to high rainfall intensity that increases the river discharges (see Figure 7). The simulation consists of 707 steps with a time step of 3,600 s. Furthermore, the models are divided into three parts: hydrodynamics, spectral waves, and sediment transport, which are integrated in Mike 21/3 coupled models. The mesh model involves three open-sea boundary conditions and two river boundary conditions to input the time series data of water level conditions and river discharges, respectively. The model type used in this study is wave and current-induced sediment transport.

Wave parameters were considered as wave stress radiation to investigate the influence of waves on hydrodynamic processes in the study area. The predicted tide parameters were used as time series in the boundary conditions (northwest, southwest, and southeast). To obtain a stable model, the Courant–Friedrichs–Lewy (CFL) adjustment number of 0.6 was chosen, in which the minimum and maximum internal time steps were set at 0.001 and 30 s, respectively. The Coriolis force was included in the simulation due to its important influence to the wind and current circulation in the study area. The eddy viscosity of 0.28 (Smagorinsky formulation) and Manning's number of 32 m^(1/3)/s were chosen in the model. These values are important to determine the effects of the wave-induced currents and bed level changes. The sediment properties such as porosity of 0.4 and grain coefficient of 1.1 are set as default, while the grain size of 0.0625 mm is based on observation data by Nirwana et al. (2021) in the Bojong Salawe Beach.

The hydrodynamic conditions simulated by Mike21 greatly determine the results and accuracy of other generated models such as the spectral waves and sediment transport. The model simulates water level variations and flows over time in response to the force exerted. Using the continuity equation and two horizontal momentum equations for the x- and y-components, the model determines the solution of the incompressible Reynolds averaged Navier–Stokes equations subject to the assumptions of Boussinesq and hydrostatic pressure. The continuity equation integrated over depth can be written as follows.

Understanding SST and wind circulation can lead to a better comprehension of the La Niña event. The anomaly is characterized by a cooling surface-ocean off South America's tropical west coast. The cooling of SST conditions develops when the equatorial trade winds intensify and cause the upwelling, essentially decreasing the temperature of the Pacific Ocean due to the colder water from the deep sea rising to the surface. SST is a significant indication of climate change, directly related to the wind magnitude and direction due to the temperature differences (Emery 2015). Colder SST increases the atmospheric pressure, moving the air to a lower pressure, which has a warmer SST, and vice versa.

Rainfall intensity is influenced by several factors such as topography, humidity, temperature, air pressure, wind speed, and magnitude. Wind-driven high rainfall is composed of two main components (u and v), representing zonal and meridional flows. Zonal and meridional flows are described as air movement parallel to the longitude and latitude, respectively (Segura et al. 2020). The positive zonal values indicate wind blowing from west to east, and vice versa. Meanwhile, positive values for meridional indicate wind blowing from south to north (Australian Monsoon) and negative meridional values represent wind blowing from north to south (Asian Monsoon).

The monthly precipitation and wind components data in 2022 are visualized in Figure 7. In boreal winter, the highest precipitation of 15.53 mm was obtained in December 2021. The magnitude of westerly winds increased in January with the weakening Australian monsoon, showing a decrease in precipitation to 6.43 mm. It is suspected that the warmer SST in the study area affects the decrease of precipitation. The SST based on observed data was 28.96 °C in December 2021, while it escalated to 29.09 °C in January 2022, reducing the amount of precipitation. This was further strengthened by the increase of precipitation in February due to the SST of 28.83 °C, in which the meridional wind direction shifted to the Asian monsoon of −0.4 m/s to increase the precipitation of 8 mm.

On the other hand, the precipitation in boreal autumn peaked in November at 19.23 mm, which corresponds to the change in direction from easterly to westerly winds. Overall, the change in zonal wind directions was more significant than those of the meridional wind directions in 2022. This can be understood as the influence of La Niña on the direction of wind and rainfall fluctuations that occur in the study area.

Coastal hydrodynamics are mainly influenced by the climate, changing the patterns of tidal, wind, and waves (de Lalouvière et al. 2020). Together with bathymetry, those parameters also determine the behavior of coastal currents. However, the existence of an estuary increases the complexity of coastal hydrodynamics during extreme climate change. The intense precipitation due to La Niña in the boreal autumn of November 2022 triggered the high river discharges to affect the interaction between river flow and shallow-water tides in the estuary (Sampurno et al. 2022). The currents intrude into the estuary during flood tide and spread during ebb tide generating the higher magnitude of currents to seaward. Tidal currents in shallow-water areas are formed by the fluctuations of water level in the ocean. The interactions of tides and river discharges cause several problems in the Bojong Salawe Beach and estuary, which has shallow-water characteristics. The combined effect of river and tide currents leads to sedimentation in the estuary, disrupting vessel movement and the estuarial ecosystem.

The current conditions are observed separately into three sections, namely, the mouth of estuary, updrift area, and downdrift area. The simulated results show that the river discharges generated currents of 0.32 m/s, flowing into the estuary, and decreasing in speed with an average of 0.21 m/s. However, the interactions between the intrusion of tides, high river discharge conditions, and smaller channels in the mouth of estuary generated an ebb-tidal jet of 0.36 m/s. The averaged updrift currents of 0.13 m/s along the coastline interacted with the ebb-tidal jet, deflecting the currents northward and causing the circulated currents surrounding the mouth estuary. At the downdrift areas, shallow-water tides produced currents with an average of 0.084 m/s, flowing southward and circulating in the port basin.

In addition, the wind-induced currents impact the morpho-hydrodynamics in the study area. The direct effect of wind generates the sea waves from the ocean toward the coast, resulting in longshore and cross-shore currents. In the Bojong Salawe Beach, the wind dominantly blows from the southeast (see Figure 6(b)), propagating the longshore currents along the coast, particularly in updrift areas. The currents caused the littoral drift, which induced the coastal sediment transport processes. From the current conditions, the sediment transport patterns could be estimated where the sediment is distributed and dispersed.

However, tidal currents and wave-generated longshore currents also caused shoreline erosion. Point t1 experienced severe erosion, which is considered to continuously increase (Figure 13(a)). Accretion is also investigated in point t5 (Figure 13(e)), which received the sediment supply from the updrift (Figure 13(f), point t6) and estuary (point t4). The updrift area (point t6) was seen to remain stable because the position is quite far from the influence of sediment-laden river discharges.

The port jetty is also considered to influence the sedimentary processes. The port jetty blocked the sediment supply to points t1 and t2 (Figures 13(a) and 13(b)), inducing erosion processes. From the analysis, shoreline retreat could occur in the downdrift areas (points t1 and t2), while accretion threatened the updrift areas (points t3, t5, and t6) (Figures 13(c), 13(e), and 13(f)), particularly the surrounding estuary.

The analysis was established with 134 and 77 transects in the updrift and downdrift areas, respectively, with a 25 m distance between transects. Transect IDs are started from the mouth of the estuary to the end of the generated transects along the shorelines (see Figure 14(a) and 14(b)). In the updrift areas, the shoreline positions of 2022 (yellow line) are examined to retreat, particularly on the right side of the estuary's mouth. High river discharges during La Niña 2022 are expected to erode the channel bank, while wave-driven longshore currents from the southeast transported the coastal sediments to downdrift areas, settling surrounding the port. The port jetty construction proved to block sediment supply from the estuary, causing sedimentation in the port basin.

In general, river discharges are an important indicator of shoreline changes in the Bojong Salawe Beach. The lower precipitation levels decrease the river discharges causing the accretion at the mouth of the estuary, as in 2013 (green line), 2015 (orange line), 2017 (red line), and 2019 (blue line). The data show that the shoreline positions near the mouth of the estuary both updrift and downdrift are highly dynamic, while the areas far from the influence of river discharges are observed to be insignificantly dynamic.

Three statistical method indicators, namely, Net Shoreline Movement (NSM), End Point Rate (EPR), and Linear Regression Rate (LRR), are also used to calculate shoreline change rates. Tables 1 and 2 present the calculation results of those indicators in the updrift and downdrift areas, respectively. The NSM values describe the distance between the shoreline in 2022 (yellow line) and 2013 (green line) on each transect. The results show that 73.13% of total transects experience accretion with a maximum distance of 29.96 m in the transect ID 102. Despite the erosion being insignificant, the negative values were high in the channel bank, particularly in the transect ID 2 with the maximum negative value of −92.04 m (see Table 1). Compared to the NSM values of updrift areas of −20.27 m, the erosion events in downdrift areas were significant with an average of −14.65 m. However, the higher values with an average of 23.4 m presented the accretion in the downdrift areas (see Table 2). From the analysis, it can be noted that the shoreline of updrift areas has advanced seaward. Nevertheless, because of the narrow range of percentages between erosion (57.14%) and accretion (42.86%) in the downdrift areas, it can be concluded that balanced shoreline movements are still occurring in this area.

EPR presents the rates of shoreline movements by dividing the distance of each transect by the time between 2013 and 2022. The EPR results show that the dominant activities in the updrift areas were accretions of 1.2 m/year (73.13%). The erosion processes were not dominant but have high rates of −2.25 m/year (26.87%), which is concentrated in the channel bank due to high river discharges during 2022. However, the erosion processes were significant in the downdrift areas of −1.63 m/year covering 57.14% of total transects. The accretion rates of 2.6 m/year were indicated surrounding the port.

Shoreline change rates are also computed using LRR, which includes trend or accuracy of change in all shoreline data. The LRR of updrift areas demonstrates that the highest erosion rate occurred with a value of −9.67 m/year. The average rate was −1.52 m/year covering 44.78% of all transects. The downdrift areas were indicated with the highest accretion rate of 9.61 m/year with an average of 2.03 m/year covering 45.45% of transects.

La Niña potentially threatens the coastal ecosystem of Bojong Salawe Beach due to wind-driven high rainfall. The anomaly of this climate change significantly affects the coastal morpho-hydrodynamics, triggering shoreline changes. The existence of an estuary with several rivers flowing in was exposed to high sedimentation due to riverbank and riverbed erosion. This study confirms that the 2022 extreme La Niña boreal autumn leads to lower the temperature in the Pacific Ocean, blowing wind-driven high rainfall to Indonesia. The high wind velocity blew the rain clouds from the southeast and gradually decreased reaching the southern coast of Java with an average of 3.45 m/s. The wind-driven rainfall plays a major role in intensifying the precipitation on Bojong Salawe Beach, which peaked in November 2022. The high precipitation levels caused high river discharges with a maximum of 180.44 m³/s flowing to the estuary.

Mike21 model results are highly capable of describing the influence of river discharges on coastal hydrodynamics. High river discharges caused the riverbank and riverbed erosion, which can be seen from changes in the bed level of the estuary. High sediment-laden river flows substantially increased the bed level surrounding the mouth of estuary. Moreover, the influence of significant high waves of 0.66 m from the southeast penetrated the estuary, which considerably accelerated the accretion processes. The high wave impacts could be reduced by the port jetty in the Bojong Salawe Beach, which effectively decreased the wave height. The simulated results indicate that the existence of a port jetty effectively decreased the significant waves from 0.88 to 0.27 m, reducing the wave energy to erode the coast. Shoreline changes analysis using DSAS confirmed that Bojong Salawe Beach was indicated to experience high erosion events, particularly adjacent to the mouth of the estuary. The analysis concluded that river discharges play a major role in inducing morpho-hydrodynamics by supplying high sediment-laden discharges from catchment areas, riverbank, and riverbed erosion.

We want to express our gratitude to the Rector of Siliwangi University, the Dean of the Faculty of Engineering, and the Head of the Department of Civil Engineering for their essential support and contribution to finishing this research.

FRA and AR contributed to conceptualization, methodology, writing the original draft, and reviewing and editing the writing. FA contributed to software analyses, data curation, amd visualization. AR contributed to supervision. All authors have read and agreed to the published version of the manuscript.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.