Wind-Generated Waves in Fjords and Coastal Areas

Abstract

Accurate predictions of coastal and fjord wave conditions are vital for several sectors such as fish farming, fisheries, as well as coastal and maritime infrastructure. In complex coastlines such as the Norwegian coast with thousands of islands, islets, and narrow fjords, accurate wave prediction is challenging. In addition to the necessity of using a state-of-the-art nearshore wave model with high spatial resolution, there is a need for high quality and resolution of forcing fields such as wind, surface currents and bathymetry. Moreover, in such areas, satellite remote sensing techniques are not reliable due to the proximity to the land. The observations are thus mainly limited to point measurements, e.g., wave buoys. In recent years, the Norwegian Public Roads Administration has conducted one of the largest measurement campaigns for wind, wave, and current conditions along the Norwegian coast. The campaign aims to obtain the essential data to construct a ferry-free E39 highway route. It provides observations for advanced met-ocean studies in coastal areas and fjords that previously have not been possible. Using these unique measurements together with a high-resolution wave simulation system based on a spectral wave model, this thesis advances our knowledge about coastal and fjord waves.

Papers I-III investigate wind-generated gravity waves in a complex coastal system with narrow fjords, partly exposed to some of the most energetic offshore waves at the western coast of Norway in the Norwegian Sea. In Paper I, we investigate the importance of wind forcing on coastal and fjord wave conditions. The results indicate that a high-resolution wind forcing is essential to obtain a realistic wind field in complex fjord topography. The best model performance is found at the exposed to open sea locations using high-resolution wind forcing. Local phenomena such as lee effects and wind channelling significantly affect the wave estimates. During extreme cases, simulations without wind forcing are unable to predict the wave height accurately in any of the fjord locations. Paper II focuses on the performance of three different deep-water source term formulations in narrow fjords, known as the Komen approach (based on a pressure-pulse white-capping), the saturation-based white-capping approach, and the observation-based scheme (ST6). The results pinpoint that the fetch geometry has a distinct effect on the model's accuracy at inner fjord locations. The saturation-based white-capping approach performs most accurate in fjords with mixed swell-wind sea conditions. In narrow fetch geometries without swell, all source term packages overestimate the wave energy, with ST6 showing the highest sensitivity to fetch geometry and local wind variations. At inner locations and during strong wind conditions, the results illustrate that the white-capping in ST6 is relatively weak compared to its strong wind input. In Paper III, we quantify the impact of surface currents on wave conditions in fjords and coastal areas. The results highlight the significant role of ocean surface currents on wave modulation at inner fjords. In such areas, the incorporation of ocean forcing considerably improves the wave height estimates. Wave-current interaction is also found to have an effect on wave characteristics such as the relative frequency, spectral bandwidth, and directional spreading in narrow fjords.