D.G. Aubrey | Geology Reports

USGS1996

The role of thermal stratification in tidal exchange at the mouth of San Diego Bay

We have examined, from an observational viewpoint, the role of thermal stratification in the tidal exchange process at the mouth of San Diego Bay. In this region, we found that both horizontal and vertical exchange processes appear to be active. The vertical exchange in this case was apparently due to the temperature difference between the'bay water and ocean water. We found that the structure of the outflow and the nature of the tidal exchange process both appear to be influenced by thermal stratification. The tidal outflow was found to lift-off tan the bottom during the initial and later stages of the ebb flow when barotropic forcing was weak. During the peak ebb flow, the mouth section was flooded, and the outflow extended to the bottom. As the ebb flow weakened, a period of two-way exchange occurred, with the surface layer flowing seaward, and the deep layer flowing into the bay. The structure of the tidal-residual flow and the residual transport of a measured tracer were strongly influenced by this vertical exchange. Exchange appeared to occur laterally as well, in a manner consistent with the tidal-pumping mechanism described by Stommel and Farmer [1952]. Tidal cycle variations in shear and stratification were characterized by strong vertical shear and breakdown of stratification during the ebb, and weak vertical shear and build-up of stratification on the flood. Evaluation of multiple tidal-cycles from time-series records of flow and temperature indicated that the vertical variations of the flow and stratification observed during the cross-sectional measurements are a general phenomenon during the summer. Together, these observations suggest that thermal stratification can play an important role in regulating the tidal exchange of low-inflow estuaries.

California↗

USGS1982

Holocene sedimentation in the shallow nearshore zone off Nauset Inlet, Cape Cod, Massachusetts

Present conditions and sedimentary evolution of the shallow offshore region near Nauset Inlet on Cape Cod, Massachusetts were clarified using high-resolution seismic-reflection profiles, sidescan-sonar records, surface grab samples and current meter measurements. The study area contains three provinces: (1) a nearshore province (shallower than 18 m) with a relatively steep slope (0.6°) and a cover of medium sand; (2) a northern offshore province covered with coarse sand, gravel, and boulders, interpreted to be glacial drift; and (3) a southern offshore province with a gentle seaward-dipping slope (0.3°) and a surface sediment of coarse sand. The glacial drift exposed in the northern offshore province can be traced southward under the coarse sand province. The overlying fill is comprised of either outwash sediment derived from the Pleistocene South Channel ice lobe to the east or Holocene-aged marine sediments eroded from seacliffs to the north. Latest Holocene sediment appears to be limited to the zone shoreward of 18 m where the medium sand occurs. Near-bottom mean flows (measured over two winter months in 10 m water depth) average 6 cm sec −1 to the south. Mean flows exceeded 20 cm sec −1 approx. 23% of the time. Ninety percent of the flows exceeding 20 cm sec −1 were directed to the south, reflecting the dominant atmospheric forcing during these winter months. Waves had an average variance of 650 cm 2 with variance exceeding 5000 cm 2 , 3% of the time, indicating moderate wave activity. Present processes are actively reshaping the nearshore province, which is characterized by many east to northeast-trending shore-oblique channels that do not extend seaward of the 18-m contour. Coarse sand in the floors of these channels suggests they may be erosional features, and the presence of megaripples oriented perpendicular to the channel axes indicates active transport in these channels. Megaripple orientation and the current and wave regime of the study area support a rip-current origin for these channels.

Massachussetts↗