Time scales of circulation and mixing processes of San Francisco Bay waters

Abstract

Conceptual models for tidal period and low-frequency variations in sea level, currents, and mlxmg processes in the northern and southern reaches of San Francisco Bay describe the contrasting characteristics and dissimilar processes and rates in these embayments: The northern reach is a partially mixed estuary whereas the southern reach (South Bay) is a tidally oscillating lagoon with density-driven exchanges with the northern reach.The mixed semidiurnal tides are mixtures of progressive and standing waves. The relatively simple oscillations in South Bay are nearly standing waves, with energy propagating down the channels and dispersing into the broad shoal areas. The tides of the northern reach have the general properties of a progressive wave but are altered at the constriction of the embayments and gradually change in an upstream direction to a mixture of progressive and standing waves. The spring and neap variations of the tides are pronounced and cause fortnightly varying tidal currents that affect mixing and salinity stratification in the water column.Wind stress on the water surface, freshwater inflow, and tidal currents interacting with the complex bay configuration are the major local forcing mechanisms creating low-frequency variations in sea level and currents. These local forcing mechanisms drive the residual flows which, with tidal diffusion, control the water-replacement rates in the estuary. In the northern reach, the longitudinal density gradient drives an estuarine circulation in the channels, and the spatial variation in tidal amplitude creates a tidally-driven residual circulation. In contrast, South Bay exhibits a balance between wind-driven circulation and tidallydriven residual circulation for most of the year. During winter, however, there can be sufficient density variations to drive multilayer (2 to 3) flows in the channel of South Bay.Mixing models (that include both diffusive and dispersive processes) are based on time scales associated with salt variations at the boundaries and those associated with the local forcing mechanisms, while the spatial scales of variations are dependent upon the configuration of the embayments. In the northern reach, where the estuarine circulation is strong, the salt flux is carried by the mean advection of the mean salt field. Where large salinitygradients are present, the tidal correlation part of the salt flux is of the same order as the advective part. Our knowledge of mixing and exchange rates in South Bay is poor. As this embayment is nearly isohaline, the salt flux is dominated entirely by the mean advection of the mean salt field. During and after peaks in river discharge, water mixing becomes more dynamic, with a strong density-driven current creating a net exchange of both water mass and salt. These exchanges are stronger during neap tides.Residence times of the water masses vary seasonally and differ between reaches. In the northern reach, residence times are on the order of days for high winter river discharge and of months for summer per...

Hydrodynamic response in a microtidal and shallow bay under energetic wind and seiche episodes

“…This fundamental mode was persistent during summer consistent with wavelet analysis presented in Fig.5b. The mean seiche excursion length, defined here as the distance travelled by a body of water between low and high water slack, could be estimated from the RMS current speed times the half tidal period (Waiters et al 1985). For first mode (S1-*) events was around 650m in A2 and 500m in A1 (in 30 minutes).…”

Submarine Groundwater Discharge-Derived Nutrient Loads to San Francisco Bay: Implications to Future Ecosystem Changes

“…South Bay receives very little freshwater from tributaries and has a water residence time of several months during summer and approximately 30 days during winter (Walters et al 1985). Circulation in South Bay is set by the interplay of tidal forcing modified by windforcing and intermittent density forcing due to freshwater from Central Bay, wastewater discharge, and small streams .…”

“…Water in Central Bay is primarily composed of coastal Pacific water, which is influenced to various degrees by Delta water contribution. Density gradient drives estuarine circulation, and water residence time is on the order of days during high river discharge periods and up to months during the dry season (Walters et al 1985). The surrounding coastline is comprised of sandstone and shale along a narrow shoreline, and the Central Bay seafloor is comprised mostly of sand and silty sand (Conomos and Peterson 1977;Chin et al 2010).…”

Sediment dynamics in the lowerMekongRiver: Transition from tidal river to estuary

“…Estuarine regime shifts driven by seasonal river discharge variability have long been observed [e.g., Hansen and Rattray, 1966] in estuaries and deltas of all sizes. For example, the Columbia [Hughes and Rattray, 1980], San Francisco Bay [Walters et al, 1985], Tamar [Bale et al, 1985], and Merrimack [Ralston et al, 2010] estuaries transition from highly stratified to moderately stratified conditions depending on seasonal river discharge.…”