Abstract
The paper investigates the local topographic effect on the mean and integral flow characteristics recorded by sonic anemometers mounted on tall masts near the shoreline of three different Norwegian fjords. Two years of measurements are analysed, using data from 25 three-dimensional sonic anemometers mounted at heights from 12 m to 95 m above the ground. The goal is to explore the potential and challenges of using wind measurements from the masts located on the shores of the fjords in the design of planned bridge crossings. Therefore, the study explores the deviations of the mean and turbulent flow characteristics from the traditional case of flat and homogeneous terrain. Only records with mean wind speeds of 12 m s−1 and above at all elevations above the ground are considered due to their relevance in buffeting response, which led to the identification of a limited number of sectors representative of strong wind conditions. Mean incidence angles with absolute values above 6° and low mean wind shear are measured in several of the selected sectors. This highlights the major influence of the local terrain and vegetation around the masts on the wind conditions at the mast locations. Nevertheless, non-dimensional variance and covariance estimates of the velocity components are found to be consistent with values previously measured from bridge decks crossing narrow fjords. The paper explores also an alternative approach to compute the friction velocity, the estimation of which is challenging in a fjord-like topography. This first part of the paper focuses on integral flow characteristics, a second follow-on part will investigate in details which eddy wave-numbers are most affected by the local terrain, based on the analysis of the spectra of the velocity fluctuations.
Highlights
The topography of large parts of the Norwegian west coast is characterized by fjords, i.e. long, deep inlets of the sea, typically surrounded by steep mountainsides
The wind measurements in Sulafjorden, Halsafjorden and Julsundet are strongly affected by the local topography, the similarities of the values of the turbulence intensities among the three locations as well as with previous studies confirm the relevance of the measurement data for the design of the planned fjord crossings
The paper explores a subset of a larger data set aiming to identify the wind conditions relevant for bridge design at three potential fjordcrossing sites, in complex coastal terrain, in western Norway
Summary
The topography of large parts of the Norwegian west coast is characterized by fjords, i.e. long, deep inlets of the sea, typically surrounded by steep mountainsides. The flow field over the fjord, in the vicinity of steep mountain slopes, will be strongly affected by topographic effects, such as channelling (Jackson and Steyn, 1994), downslope wind storms (Sandvik and Harstveit, 2005), and extreme gusts (Grønås and Sandvik, 1999), e.g caused by turbulent eddies, either generated locally due to flow over or along complex terrain (Agústsson and O lafsson, 2004), or aloft in steep and possibly overturning gravity waves (Guarino et al, 2016) These fine-scale flow features challenge the identification of the flow characteristics used to model the wind load on slender structures such as long-span bridges.
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