This study presents an open-source Python library for simulating two-dimensional incompressible Kelvin-Helmholtz (KH) instabilities in stratified shear flows. The solver employs a fractional-step projection method with spectral Poisson solution via fast sine transform (FST). While diffusion and pressure terms achieve second-order spatial accuracy through central differencing and spectral methods respectively, advection employs first-order upwind differencing that provides numerical stability for sharp gradients characteristic of KH billows. Implementation leverages NumPy for array operations, SciPy for spectral methods, and Numba just-in-time (JIT) compilation, balancing computational performance with code transparency and accessibility. Four canonical test cases systematically explore parameter space spanning Reynolds numbers 1,000-5,000 and Richardson numbers 0.1-0.3: classical shear layer, double shear configuration, rotating flow, and forced turbulence. Statistical analysis employing Shannon entropy, complexity indices, and nonparametric tests reveals non-monotonic relationships between flow parameters and mixing efficiency, with double shear layers achieving 4.3x higher mixing rates than forced turbulence despite lower Reynolds numbers. The developed complexity metrics combining entropy, gradient variability, and higher-order moments provide quantitative benchmarks for turbulence model validation complementing conventional statistical measures. While the two-dimensional framework excludes spanwise secondary instabilities, it isolates primary mixing mechanisms relevant for understanding atmospheric and oceanic transport processes. The solver executes efficiently on standard desktop hardware, with the most demanding 384x192 grid simulation completing in approximately 31 minutes for 30 seconds physical time, enabling systematic parameter studies on modest computational resources. Results demonstrate that mixing efficiency depends fundamentally on instability generation pathways rather than intensity measures alone, challenging existing Richardson number-based parameterizations and suggesting refinements for subgrid-scale representation in climate models.

The Korteweg–de Vries (KdV) equation is a foundational model in geophysical fluid dynamics (GFD), governing the propagation of long internal and surface gravity waves in stratified and shallow ocean environments where the interplay between nonlinear steepening and frequency-dependent dispersion gives rise to solitons. Although the analytical tractability of the KdV equation through inverse scattering is well established, systematic numerical exploration of multi-soliton interactions remains valuable for benchmarking solvers, probing conservation properties under varied oceanic initial conditions, and building intuition for more complex ocean wave phenomena. This article presents sangkuriang, an open-source Python library that solves the KdV equation using Fourier pseudo-spectral spatial discretization and adaptive eighth-order Runge–Kutta time integration. The implementation leverages just-in-time (JIT) compilation to achieve research-grade computational efficiency on standard hardware, making it readily accessible for coastal and ocean engineering applications, including idealized modeling of internal solitary waves on continental shelves, rapid parameter studies for solitary wave propagation in stratified basins, and pedagogical investigations of nonlinear dispersive wave dynamics. The solver is validated through four progressively complex idealized scenarios motivated by oceanic wave dynamics: isolated soliton propagation, symmetric interactions, overtaking collisions, and three-body interactions. High-fidelity conservation of mass, momentum, and energy is demonstrated, with relative errors remaining below $\mathcal{O}\left({10}^{-4}\right)$ across all test cases. Measured soliton velocities align with theoretical predictions within 5%, confirming the capture of the amplitude-dependent dispersion characteristic of oceanic solitary waves. Complementary diagnostics, including spectral entropy and recurrence quantification analysis (RQA), verify that the numerical solutions preserve the regular phase-space structure characteristic of integrable Hamiltonian systems. These results establish sangkuriang as a robust, lightweight platform for reproducible numerical investigation of idealized nonlinear dispersive wave dynamics relevant to coastal and ocean engineering applications.

Turbidity serves as a crucial indicator of coastal water health and productivity. Twenty years of remote sensing data (2003–2022) from the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) satellite were used to analyze the spatial and temporal variations in turbidity, as measured by total suspended matter (TSM), in the Berau Coastal Shelf (BCS), East Kalimantan, Indonesia. The BCS encompasses the estuary of the Berau River and is an integral part of the Coral Triangle, renowned for its rich marine and coastal habitats, including coral reefs, mangroves, and seagrasses. The aim of this research is to comprehend the seasonal and interannual patterns of turbidity and their associations with met-ocean parameters, such as wind, rainfall, and climate variations like the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). The research findings indicate that the seasonal spatial pattern of turbidity is strongly influenced by monsoon winds, while its temporal patterns are closely related to river discharge and rainfall. The ENSO and IOD climate cycles exert an influence on the interannual turbidity variations, with turbidity values decreasing during La Niña and negative IOD events and conversely increasing during El Niño and positive IOD events. Furthermore, the elevated turbidity during negative IOD and La Niña coincides with rising temperatures, potentially acting as a compound stressor on marine habitats. These findings significantly enhance our understanding of turbidity dynamics in the BCS, thereby supporting the management of marine and coastal ecosystems in the face of changing climatic and environmental conditions.

In multi-hazard environments, women’s social reproductive labor often constitutes a foundation of community survival, yet remains undertheorized in disaster scholarship. This study contributes to an active scholarly conversation by examining Daya Annisa, a women-led grassroots organization in Bantul Regency, Yogyakarta, Indonesia, a region under continuous geological stress from the Sunda Megathrust, the Opak Fault, and Mount Merapi. Drawing on in-depth interviews and focus group discussions analyzed through Social Reproduction Theory (SRT), with a Strengths, Weaknesses, Opportunities, and Threats (SWOT) framework reinterpreted as an analytical lens on the structural conditions of reproductive labor, the analysis traces four interlinked practices: preparedness embedded in arisan and pengajian gatherings, community-based vulnerability mapping, trust-based crisis response, and informal post-disaster livelihoods. The paper argues that resilience in such settings is best understood not as a passive capacity to absorb shocks, but as the active, gendered, and largely uncompensated labor through which communities are materially sustained when formal systems are stretched. Three policy shifts follow: long-term flexible funding calibrated to continuous reproductive preparedness; institutional integration of community-generated vulnerability data with appropriate privacy and inclusion safeguards; and inclusion of grassroots women’s organizations as autonomous decision-making actors in disaster governance.

Hydraulic experiments in a two-dimensional physical laboratory were conducted to evaluate the performance of pile breakwaters in reducing wave energy. The piles on the breakwater were arranged in a staggered pattern in two rows. A total of seventy-two simulation scenarios were run based on variations in the incoming wave height and period, and the spacing between the piles, using a 1:10 model scale. The data from the hydraulic tests were then processed using a spectral method that could separate the energy spectra of reflected and transmitted waves. These changes result in reflected and transmitted waves. The laboratory test data were used to estimate the values of the transmission coefficient and reflection coefficient. Both coefficients were then used to validate semi-empirical formulas for the two coefficients. The semi-empirical formulas for the two coefficients were developed based on a model for estimating the spatial porosity of pile breakwaters. The porosity estimation model takes into account the dimensions of the piles and the dimensional components of the breakwater, including the arrangement, number of rows, and spacing between the piles. The validation results of the semi-empirical formulas with the laboratory test data showed a coefficient of determination of 0.917 and a root mean square of 0.077. The staggered arrangement improves the effectiveness of the breakwater in reducing the transmission wave height. The developed semi-empirical model can be used to design the pile dimensions to achieve optimal reflected and transmitted wave heights for coastal protection. Keywords: pile breakwater, coastal protection, hydraulic experiment, physical wave simulation, wave transmission new formula

Balikpapan Estuary, a semi-enclosed estuarine system in East Kalimantan, is influenced by tidal variability and freshwater input from surrounding rivers, which shape the vertical distribution of salinity and density. This study examines the spatial and temporal characteristics of salinity-driven stratification based on in-situ observations conducted at 13 stations along the estuary during a spring tide using a CTD profiler and a water quality profiler. Stratification was quantified using the Brunt–Väisälä frequency and exhibits clear variability across tidal phases. During low tide, relatively higher water column stability was confined to a thin surface layer (approximately 2–3 m) from the bay mouth (0 km) to upstream regions (25 km), corresponding to fresher surface waters and shallow density gradients. During high tide, vertically extended stable layers were observed. Along the lower to middle estuary (0–20 km), elevated stability occurred at depths of about 3–4 m, whereas in the upper estuary (20–25 km), stable layers were identified between 3 and 13 m depth, coinciding with the presence of more saline water at depth. Temporal observations at a fixed station show that stratification varied within the tidal cycle, with stronger vertical density gradients during ebb conditions and weaker gradients during flood conditions. Salinity ranged from 31.64 to 32.52 PSU, and density varied from 1018.73 to 1019.59 kg/m3, indicating moderate but temporally variable stratification. These observations demonstrate that semidiurnal tidal variability is closely associated with changes in the vertical structure of salinity and density in the Balikpapan Estuary.