Author(s): N. Jarry; F. Gouaud; V; Rey
Linked Author(s):
Keywords: Waves; Refraction; Diffraction; Sea level; Caustic
Abstract: In the perspective of climate change and sea level rise especially in the South East Asia area, many emerged obstacles such as inlet and islands could become submerged shoal. That would lead certain coasts to be affected by waves while they were previously protected. And whereas wave propagation above a shoal or any submerged structures has been a subject that has passionate oceanographers for decades and has been studied through several experimentations, the difference of free surface monitoring around emerged compared to submerged structure has rarely been studied in a careful way. That’s why this paper presents an experimental investigation of the waves’free surface behavior propagating around or above a shoal at different sea levels. As it is evident that the waves’crests do cross behind an emerged structure, the crests’behavior is completely different when the shoal is lightly or even deeply submerged. Experimental results coupled with a study of the mild-slope equation show that contrary to set ideas, since the shoal is submerged enough and the wave breaking doesn’t affect the wave propagation anymore, diffraction effect prevents waves crossing in modifying waves’characteristics. Indeed, diffraction phenomenon tends to scatter and distribute the energy along the crests from one part, and increases wave length and wave phase speed on the other part, where amplification due to the refraction is high. This competition between refraction and diffraction leads to a free surface organization where the crests, that have braked and bended passing the shoal, straighten up, dissociate and dephase compared with the rest of the crest that hasn’t been affected by the shoal. The dephasing is symbolized by two lines of low amplitude that we name “isophases” lines, starting above the shoal and moving apart due to diffraction, and whose opening angle seems to depend on the shoal and incident wave characteristics. Once the water depth is high enough above the shoal to limit refraction effects, the dephasing doesn’t occur anymore, and refraction and diffraction plays together along the same wave crest.
Year: 2014