Evidence for the influence of form drag on bottom boundary layer flow

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Abstract
An experiment in 199 m of water on the Oregon shelf produced continuous current speed profiles down to the sediment‐water interface. These profiles show that the velocity structure above the viscous sublayer is consistent with that expected when form drag influences the boundary layer flow. They show two logarithmic‐profile regions, each yielding a different stress estimate. The stress calculated from the upper one reflects the influence of form drag and is more than 4 times the bed stress determined frm the shear in the viscous sublayer. When form drag is significant, the application of logarithmic profile or Reynolds stress techniques to measurements more than a few tens of centimeters from the bed may yield bed stress estimates inappropriate for use in near‐bed sediment transport or entrainment calculations. Large roughness‐length or drag‐coefficient values do not prove that a viscous sublayer does not exist.