Sandbar Long-Term Evolution Analysis on Xbeach for Cabinda Breakwater

2. Group of Hydrology and Water Resources, Faculty of Engineering, Agostinho Neto University, Angola

Abstract: When understanding the dynamics of erosion-deposition patterns and sediment transport in the area of a coastal structure, it is crucial to evaluate the prediction of changes in coastal dynamics and performance of coastal structures caused by a specific structure, and in this study, an evaluation on the Cabinda Breakwater was performed. In coastal structures' nearshore, there is a huge variation in hydro-morpho dynamics responses, as they are complex functions with plenty of parameters, including the materials making up the beach and structures, wave dynamics, and sediment, the slope of the coast and angle of approach, and the structural design. In this research, the sediment transport in the area of a breakwater and the erosion-deposition patterns are located in the province of Cabinda, in Angola. Data used for this study were attained from the Portuguese Meteorological Department. Every two months starting from January 2011 to November 2013, a monitorization of Seabed elevations was proceeded for the investigation of the quantification of seabed changes in the erosion-deposition patterns of the cohesive sediment due to the existence of breakwater. Additionally, there was a performance of numerical modeling for the understanding of the breakwater impacts on the nearshore hydrodynamics and investigation of the dynamics of fine sediment transport around the breakwater structure. Based on the XBEACH model, a coupled two-dimensional hydrodynamics-sediment transport model, with the inclusion of a flexible meshing approach proceeded in this study. According to the analysis of the results the breakwater diminished both wave height, peak period, and direction behind the structure and the current speed by an average of 4 m, 15 sec, 90, and 0.40 m/s respectively. The breakwater does not allow the settlement of trapped suspended sediment in a sheltered area that is approximately 2.5 m in height nearshore from 1 year after its construction. There was an alignment of the numerical results and the field measurements, where concentrated accumulations of sediment in the landward area behind the breakwater were depicted. Notably, during the Northeast (NE) season there were sediment accumulations concentrated along the main segments of the breakwater structure. During the Southwest (SW) season, near the first main segment concentration of the breakwater was recorded. The evaluation illustration concluded that differences in structure design, seasonal hydrodynamic conditions, sediment type, and sediment supply from the Congo River were significantly influenced by depositional patterns. Breakwaters are hardly regarded as cohesive shores; thus, this study aims to provide new, significant benefits for scientists, coastal management authorities, and engineers, regarding seasonal dynamic changes negatively impacted by a breakwater and its performance on a cohesive coast.