Effect of a submerged plate on the near-bed dynamics underincoming waves in deep water conditions
Résumé
It is well known that wave induced bottom oscillations become more and more
negligible when the water depth exceeds half the wavelength of the surface
gravity wave. However, it was experimentally demonstrated for regular waves
that the bottom pressure oscillations at both first and second wave harmonic
frequencies could be significant even for incoming waves propagating in deep
water condition in the presence of a submerged plate.
For a water depth h of about the wavelength of the wave, measurements
under the plate (depth immersion of top of plate h/6, length h/2) have shown
bottom pressure variations at the wave frequency, up to thirty times larger
than the pressure expected in the absence of the plate. In this paper, not
only regular but also irregular wave are studied together with wave
following current conditions. This behavior is numerically verified by use
of a classical linear theory of waves. The wave bottom effect is explained
through the role of evanescent modes and horizontally oscillating water
column under the plate which still exist whatever the water depth. Such a
model, which allows the calculation of the velocity fields, has shown that
not only the bottom pressure but also the near bed fluid velocity is
enhanced. Two maxima are observed on both sides of the location of the
plate, at a distance of the plate increasing with the water depth. The
possible impact of such near bed dynamics is then discussed for field
conditions thanks to a scaling based on a Froude similarity. It is
demonstrated that these structures may have a significant impact at the sea
bed even in very deep water conditions, possibly enhanced in the presence of
current.