In situ coastal observations of wave homogeneity and coherence

Abstract

A better understanding of wave homogeneity, i.e. the spatial variations of the wave characteristics, and wave coherence in coastal areas and fjords is essential for the design and analysis of sea-crossing infrastructures, such as floating bridge concepts. The wave conditions in fjords that are exposed to the open sea are complex and often characterized by a mixed swell–wind sea state. This study investigates the spatial coherence and homogeneity of ocean waves using two years of unique buoy observations from Sulafjorden – a fjord partly exposed to the open sea. We analyze both long term statistics and four selected cases with different sea states. The most exposed locations are dominated by long waves (swell), while the energy of the wind sea is comparable to the swell energy in the more sheltered locations. Despite the study area being relatively small (ca. 2 km ×1 km), the differences in wave conditions are significant because the complex fjord geometry blocks the incoming offshore waves, and changes in fetch and wind conditions affects the local wave growth. For swell waves we measured an along-crest spatial coherence (ca. 0.6) over a 1–2 km distance. The coherence between consecutive crests for swell was weaker (up to 0.3–0.4) for distances between 0.6 km and 1.3 km (up to about 5 wavelengths). Wind sea (both along crest and between crests) showed no coherence over these distances.