Data Report

Data Report
Author:
Publisher:
Total Pages:
Release: 1911
Genre:
ISBN:

Funding was provided by the Office of Naval Research under Grant Nos. N00014-89-J-1058 and N00014-90-J-1046.

Data Report: Stress Measurements in the Bottom Boundary Layer with BASS Tripods STRESS II 1990-1991

Data Report: Stress Measurements in the Bottom Boundary Layer with BASS Tripods STRESS II 1990-1991
Author:
Publisher:
Total Pages: 44
Release: 1993
Genre:
ISBN:

Two Benthic Acoustic Stress Sensor (BASS) equipped tripods were deployed in the Sediment TRan5port Events on Shelves and Slopes (STRESS) experiment on the California Shelf acquiring data from January to March 1991. They measured velocity profiles in the bottom boundary layer over the lowest 5 meters. Transmissometers, thermistors, and a pressure sensor on each tripod provided suspended sediment concentration, stratification, and wave spectral information, as well. Benthic boundary layer, Sediment transport, Boundary layer stress.

Data Report

Data Report
Author: Thomas F. Gross
Publisher:
Total Pages: 456
Release: 1993
Genre: Boundary layer
ISBN:

High Frequency Bottom Stress Variability and Its Prediction in the Code Region

High Frequency Bottom Stress Variability and Its Prediction in the Code Region
Author: William Denny Grant
Publisher:
Total Pages: 834
Release: 1983
Genre: Boundary layer
ISBN:

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.

Near Bottom Velocity Profile Measurements Using the Field Prototype of the BASS Rake Wave Bottom Boundary Layer Sensor

Near Bottom Velocity Profile Measurements Using the Field Prototype of the BASS Rake Wave Bottom Boundary Layer Sensor
Author:
Publisher:
Total Pages: 16
Release: 2000
Genre:
ISBN:

The BASS Rake is an acoustic travel time current meter designed to make spatially and temporally dense velocity profile measurements in the continental shelf wave bottom boundary layer. The thinness of the layer is responsible for high levels of bottom shear stress which are important contributors to the sediment entrainment process and which enhance turbulent dissipation of flow energy. The BASS Rake is a modification of BASS, the Benthic Acoustic Stress Sensor, using a new geometry to image flow in the WBBL. A laboratory prototype has previously demonstrated the features and near bed capabilities of the new design. The field prototype described here was constructed with standard BASS components to evaluate the measurement technique and the performance of the support frame in the near shore zone. The field prototype measures the horizontal velocity vector at ten heights from the bottom up to 30 cm above the bottom. The results of tow tank calibration and cosine response measurements are presented. After calibration the field prototype was deployed in 3 m of water immediately outside the surf zone of a local beach. Profiles were recorded continuously at 1 Hz for approximately 3.5 weeks in December of 1996. This period includes both calm and storm conditions. Selected data from this deployment are presented.