Numerical investigations on seasonal variation of waves in the Cat Ba – Ha Long coastal area (Vietnam) in 2021

The Delft3D model, equipped with a high-resolution grid, was employed to simulate wave conditions in the Cat Ba-Ha Long coastal area by combining hydrodynamic and wave modules. The model was calibrated and validated using the measurement data at Hon Dau station, and demonstrated a good match between the data and simulation results (NSE = 0.55–0.69). In 2021, the simulation results revealed that wind waves are the primary type of waves in the Cat Ba-Ha Long coastal area. The lowest monthly-averaged wave heights were found in the January-February period (about 0.4 m), followed by a gradual increase, reaching its peak in the May-June period (1.18 m), and subsequent fluctuations throughout the rest. Wave heights ranging from 0.2 to 0.6 m were frequently observed in the majority of months, including January, February, March, April, July, November, and December. The wave heights ranging 0.6–0.8 m are primarily recorded during the months of August and October. The occurrence of high-altitude waves of 1.0–2.0 m in height was predominantly seen during the months of May and June. The areas with elevated wave heights (>1 m) are primarily located in the southern part of Cat Ba and the southeast of Ha Long, accounting for around 30 % of the region. The areas characterized by gentle undulations are primarily concentrated in Ha Long Bay, where wave heights range from 0.2 to 0.6 m, accounting for 70 % of the region. The variations in wave height primarily result from the southern region of Cat Ba and the southeastern area of Ha Long Bay, which are more exposed and less influenced by the topography, hence facilitating the development of wind waves. Simultaneously, the Ha Long area is encircled by small islands, characterized by shallow depths, with waves that are diffused in direction and exhibit modest heights. In this area, the dominant wave directions are south-southeast (32.9 %) and east (29.7 %).