Near bottom measurement of wave environment in a tidal current

Abstract

Passenger ferries running from Woods Hole to Martha's Vineyard in Massachusetts enter Vineyard Sound at the Woods Hole Harbor entrance and turn from south to east where summer southwesterly winds often set up a short chop. Occasional stronger blows create a swell running across or opposed to the strong tidal current Northeasters are common from October to May and set up a chop and swell at the Martha's Vineyard end of the passage as well as at the turning point off Woods Hole. Directional wave measurements were made for the Woods Hole and Martha's Vineyard Steamship Authority in June and July 2004 to characterize the wave climate for design of a new fast ferry that will serve this route. Heavy boat traffic makes surface buoys at the critical turning region impractical but shallow depths permit a directional wave spectrum to be computed from near bottom measurements of vector velocity and pressure. However the strong tidal currents add a new element to the computational task of projecting near bottom wave velocities back to the surface. In extreme cases, the wave period observed at the bottom may be very long when the waves propagate into the current yet the attenuation relation for waves near the bottom must use the wave period relative to the near surface moving water. This back tracking of current presents a problem that has not been well addressed by existing wave models. In the Vineyard Sound case, shallow depths and unstratified conditions permit a computation of the surface current from the near bottom measurements for this reconstruction.