High frequency bottom stress variability and its prediction in the CODE region

Abstract

High quality bottom boundary layer measurements obtained in the CODE region off Northern California are described. Bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions and during a winter storm are analyzed to obtain both velocity profiles and mean bottom stress and bottom roughness estimates. The spring/summer measurements were taken in June, 1981 during CODE-1 at C3 (90 m) by Grant and Williams, WHOI; the winter storm data was taken in November 1980 prior to CODE-1 at the R2 (80 m) site by Cacchione and Drake, USGS. The mean near-bottom (< 2m) velocity profiles are logarithmic (R2 > 0.993) much of the time for everyday flows; deviations are primarily due to kinematical effects induced by unsteadiness from internal waves. Stress profiles show the logarithmic layer corresponds to a constant stress layer as expected for the inertial region of a boundary layer. Stress estimates made from dissipation and profile techniques agree at the 95 percent confidence level. Typical z0 values estimated from measurements greater than 30 cm above the bottom have magnitudes of approximately 1 cm; an order of magnitude larger than the physical bottom roughness. Corresponding u* values have typical magnitudes of 0.5-1.0 cm/sec; more than twice as large as expected from a usual drag law prediction (corresponding to over a factor of four in mean stress). These values are demonstrated to be consistent with those expected for combined wave and current flows predicted theoretically by Grant and Madsen (1979) and Smith (1977). The u* values estimated from the CODE-1 data and predicted by the Grant and Madsen (1979) model typically agree within 10-15 percent. Similar results are demonstrated for the winter storm conditions during which large sediment transport occurs. (Typical z0 values are 4-6 cm; typical u* values are 3-6 cm/sec). The waves influencing the mid-shelf bottom stress estimates are 14-20 second swell associated with Southern and Western Pacific storms. These waves are present over most of the year. The results clearly demonstrate that waves must be taken into account in predicting bottom stress over the Northern California Shelf.