Open AccessBottom friction and bed forms on the southern flank of Georges Bank
Author(s) -
Werner Sandra R.,
Beardsley Robert C.,
Williams Albert J.
Publication year - 2003
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2000jc000692
Subject(s) - geology , drag coefficient , flank , benthic zone , drag , ocean bottom , mean flow , oceanography , seismology , geodesy , geomorphology , meteorology , mechanics , physics , sociology , anthropology , turbulence
We present observational estimates of bottom stress, bottom roughness, and quadratic drag coefficient for the southern flank of Georges Bank, a shallow region dominated by strong semidiurnal tides. Our estimates are based on near‐bottom velocity measurements from two Benthic Acoustic Stress Sensor (BASS) tripod deployments at a depth of 76 m taken during winter (February–April 1995) and summer (July–August 1995). The dominant tidal constituent is the M 2 , which provides 76–89% of the total kinetic energy. Typical bottom friction velocities are 1.1–1.2 cm s −1 in the time mean, with standard deviations of 0.4–0.5 cm s −1 . A representative drag coefficient is c D = 3.0 ± 0.1 × 10 −3 at 1.2 m above the bottom. Our drag coefficient estimates at this and other elevations correspond to the apparent bottom roughness range z 0 = 0.05–0.09 cm. Bottom photographs (taken February–June) show that from February until April the sea floor was covered by sand ripples 1–2 cm in height. These ripples were approximately aligned with the local isobath; their flanks were perpendicular to the major axis of the tidal ellipse. Ripples exhibited stationary behavior; that is, they did not migrate with the flow. Intermittent modification of the ripple pattern happened during times of strong wave‐current interaction, but along‐isobath ripples formed again after each wave event. Observations of bed forms affected by surface waves coincided with periods of wave‐enhanced bottom stress predicted by the Grant and Madsen [1979] combined wave‐current interaction model. Model results and bottom photographs indicate that sediments were kept in suspension only under extreme conditions, here the February 1995 storm. Sand ripples gradually eroded during late March and April until the sea floor was almost flat. Coinciding with slow ripple erosion, bioturbation levels increased. Our estimates of bottom stress do not exhibit seasonal changes, so that seasonal variation of bottom shear stress cannot explain the transition from a rippled to a nearly flat bottom. We conclude that biogenic modification of bottom sediment enhanced the critical shear stress for initiation of sediment movement, thereby prohibiting ripple formation in spring and summer.