The bottom boundary layer of the bay stem plains environment of lower Chesapeake bay

Field observations of bottom boundary layer processes in the bay stem plains environment of lower Chesapeake Bay (depth 11–12 m) were made using a bottom mounted tripod supporting electromagnetic current meters and a pressure sensor. The Prandtl-von Karman equation, the inertial dissipation method, and the Grant and Madsen wave-current interaction model were applied to the field data to estimate time-varying bed stress and to evaluate the likely contributions of waves and currents to total shear stress and to skin friction. Biogenic bed micromorphology dominated bed roughness. A background signal of long period, low amplitude swell was present for most of the time. Although these oscillations were too weak to agitate the bottom sediments, interactions of the swell with tidal currents in the presence of a biogenically-roughened bottom, caused the hydraulic roughness height, z' 0 and drag coefficient, C D, ‘sensed’ by the mean current to be significantly increased. In addition, internal waves with frequencies in the neighbourhood of the pycnoclinal Brunt-Vaisala frequency (period range approximately 60–100 s) appeared intermittently and caused near-bottom velocity fluctuations with amplitudes of over 10 cm s −1.