Evaluation of environmental conditions in a Norwegian fjord based on field measurements

Abstract

As part of the project “Ferry-free E39”, a 5 km long floating bridge is planned to be built in Bjørnafjorden to replace the existing ferry connection. The demanding nature of the fjord and the sensitivity of the bridge to wind and wave excitations make the stability of the structure under harsh environmental conditions important. This thesis focuses on evaluating the environmental conditions in the fjord based on wind measurements in the area, by studying the effect of mean wind speed, mean wind direction and atmospheric stability on the various wind characteristics in the fjord. A total of 289 wind events were identified from 2015 to 2020 by selecting a threshold for the mean wind speed of 6 m/s. These wind events were used to study the wind characteristics, including angle of attack, turbulence intensity, wind spectra and coherence. The wind spectrum and coherence model from N400 were fitted to the wind measurements, and the fitted parameters were compared to the suggested values in the code. Overall results indicate that the complex topography of the fjord surrounding the wind masts significantly affects the wind characteristics. The main directions for the wind flow in the fjord were found to be from the north-west, south-west and south-east, which all correspond to wind flow over water with long fetch. As the mean wind speed increases, the suggested values in N400 represents the wind characteristics better compared to lower wind speeds. For wind flow over land, it was observed that N400 tends to underestimate the turbulence intensity and spectral parameters, while for wind flow over water it tends to overestimate these same values. For the coherence, N400 consistently underestimates the coherence in the horizontal plane and overestimates the coherence in the vertical plane. The distribution of the fitted parameters with atmospheric stability was difficult to interpret, but it was concluded that the stability only affected the distribution of the spectral parameter, and not the coherence parameter.