Wind-generated waves are among the primary environmental factors in the design and functionality of coastal structures. In addition to exerting determining forces on the structural design of coastal facilities, their irregular and asymmetric characteristics can mobilize sediment on the seabed, leading to adverse effects such as sedimentation and seabed erosion (scour), which in turn can reduce the functionality of coastal infrastructure. In this study, numerical modeling was conducted to determine offshore and nearshore extreme wave conditions in order to provide a basis for the design and analysis of planned or existing coastal and marine structures in İzmit Bay. To reflect more recent conditions, wind climate analyses were performed using 25 years of hourly wind data from 1999 to 2023. Based on these anlyses, the 100-year return period extreme wind speed was determined to be 31.0 m/s from the north-northeast direction using the Gumbel distribution. These data, together with waves approaching from the west through the Sea of Marmara, were used to calculate design parameters such as the significant offshore wave height (𝐻𝑠 = 4.43 m) and peak wave period (𝑇 𝑝 = 6.7 s) that may occur in İzmit Bay. The results obtained show good agreement with detailed

Wind-generated waves are among the primary environmental factors in the design and functionality of coastal structures. In addition to exerting determining forces on the structural design of coastal facilities, their irregular and asymmetric characteristics can mobilize sediment on the seabed, leading to adverse effects such as sedimentation and seabed erosion (scour), which in turn can reduce the functionality of coastal infrastructure. In this study, numerical modeling was conducted to determine offshore and nearshore extreme wave conditions in order to provide a basis for the design and analysis of planned or existing coastal and marine structures in İzmit Bay. To reflect more recent conditions, wind climate analyses were performed using 25 years of hourly wind data from 1999 to 2023. Based on these analyses, the 100-year return period extreme wind speed was determined to be 31.0 m/s from the north-northeast direction using the Gumbel distribution. These data, together with waves approaching from the west through the Sea of Marmara, were used to calculate design parameters such as the significant offshore wave height (H_s=4.43 m) and peak wave period (T_p=6.7 s) that may occur in İzmit Bay. The results obtained show good agreement with detailed analysis results from applied projects. Wave transformation modeling was conducted using a Gulf model supported by digital bathymetry data, considering nine different directions to determine nearshore wave characteristics. As expected, waves from the west were found to be the most effective within the bay. In contrast, waves from the east were observed to have lower energy due to their limited fetch distance. It is concluded that the spatial distribution findings presented in the study will provide a useful foundation for practitioners involved in the design and analysis of coastal structures in İzmit Bay.