Geomorphological and Meteorological Control of Estuarine Processes: a Three-Dimensional Modeling Analysis
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Geomorphological and Meteorological Control of Estuarine Processes: A Three-Dimensional Modeling Analysis Quamrul Ahsan, P.E., M.ASCE1; Alan F. Blumberg, M.ASCE2; Andrew J. Thuman, P.E., M.ASCE3; and Thomas W. Gallagher, P.E., M.ASCE4 Abstract: The proper timing, duration, and direction of wind events interacting with the geometry of an estuarine system can control the intensity of stratification. A three-dimensional, time-dependent hydrodynamic model was used to examine this process. Intense mixing is closely tied with wind-generated internal velocity shear. A south wind generates up-estuary directed surface currents, which eventually leads to downwelling movements of water. This downwelling process in the upper bay region accelerates the bottom current in a down-estuary direction. A vertical instability occurs, especially in the upper bay region, due to the generation of shear across the pycnocline, causing mixing sufficient to destratify the entire water column. On the other hand, strong stratification occurs when a north wind advects fresher upper bay surface water into the lower bay. A downwelling movement of water is produced, which in turn drives bottom saline water in the up-estuary direction. DOI: 10.1061/͑ASCE͒0733-9429͑2005͒131:4͑259͒ CE Database subject headings: Three-dimensional models; Hydrodynamics; Estuaries; Wind; Geometry; Stratification. Introduction the tidal excursion to the distance between major bathymetric and shoreline features. They concluded that in estuaries where the Stratification in an estuarine environment is of significant impor- typical spacing of topographic features is less than the tidal ex- tance to phytoplankton populations, nutrient recycling, and dis- cursion, tidal dispersion may contribute significantly to the mix- solved oxygen distributions since they are strongly controlled by ing processes. In Chesapeake Bay, Blumberg and Goodrich the vertical structure of the water column. In a fully stratified ͑1990͒ have found that wind-driven internal shear is a more ef- estuarine system, such as Pensacola Bay, Fla., the presence of a fective mechanism of inducing destratification than turbulence strong pycnocline forms a barrier to the downward transport of generated at the surface. oxygen; this event is a dominant influence in causing hypoxia in The advective effects of wind forcing have been examined in the bottom layer. The amount of stratification also controls the many estuarine systems. Observations of local wind forcing in region’s physical characteristics, such as temperature, salinity, estuaries, especially in Chesapeake Bay, have shown that wind- and velocity structure. induced currents are typically larger than density-driven gravita- Astronomical tides have long been considered the major tional currents and can be of the same order as the tidal currents, source of mixing energy for the breakdown of stratification. especially during wind events ͑Goodrich 1985͒. Weisberg ͑1976͒ Stratification and destratification as a result of neap-spring tidal has found that wind-induced velocity fluctuations at a time scale mixing have been reported by Ruzecki and Evans ͑1985͒ in the between the steady-state gravitational circulation and tidal oscil- York River estuary. Geyer and Signell ͑1992͒ have argued that the lations are of equal or greater importance to the circulation phys- effectiveness of tidal dispersion depends on the relative scale of ics. Large variations in the salinity distributions associated with wind-driven velocity fluctuations were also observed in Chesa- 1PhD, Sr. Project Manager, HydroQual, Inc., 1200 McArthur Blvd., peake Bay by Wang ͑1979͒. Wang ͑1979͒ clearly demonstrated Mahwah, NJ 07430 ͑corresponding author͒. E-mail: qahsan@ that wind forcing has a twofold effect on the water column, pro- hydroqual.com 2 ducing wind-induced mixing and causing advective currents. PhD, George Meade Bond Professor of Ocean Engineering, Dept. of In general, wind-induced mixing tends to raise the potential Civil, Environmental and Ocean Engineering, Stevens Institute of energy of a water column by redistributing the salinity gradient Technology, Castle Point on Hudson, Hoboken, NJ 07030 and Consultant, HydroQual, Inc., 1200 McArthur Blvd, Mahwah, NJ 07430. over depth, thereby altering the horizontal pressure gradient. It is E-mail: [email protected] this gradient which essentially generates the gravitational flow. 3Associate, HydroQual, Inc., 1200 McArthur Blvd., Mahwah, NJ Several studies have demonstrated that a depth-dependent re- 07430. E-mail: [email protected] sponse to local wind forcing is a very effective mechanism in 4Principal Engineer, HydroQual, Inc., 1200 McArthur Blvd., maintaining stratification as well as leading to destratification. Mahwah, NJ 07430. E-mail: [email protected] Among these studies, work in Narragansett Bay by Weisberg Note. Discussion open until September 1, 2005. Separate discussions ͑1976͒ and in Chesapeake Bay by Wang ͑1979͒, Grano and Prit- must be submitted for individual papers. To extend the closing date by chard ͑1982͒, Goodrich ͑1985͒, Goodrich et al. ͑1987͒, and Blum- one month, a written request must be filed with the ASCE Managing ͑ ͒ Editor. The manuscript for this paper was submitted for review and pos- berg and Goodrich 1990 , and in Mobile Bay, by Noble et al. ͑ ͒ ͑ ͒ sible publication on January 5, 2004; approved on July 30, 2004. This 1996 and Schroeder et al. 1990 are notable. They suggest that paper is part of the Journal of Hydraulic Engineering, Vol. 131, No. 4, a down-estuary wind, which is frictionally coupled with the sur- April 1, 2005. ©ASCE, ISSN 0733-9429/2005/4-259–272/$25.00. face currents, tends to drive relatively fresh surface water in the JOURNAL OF HYDRAULIC ENGINEERING © ASCE / APRIL 2005 / 259 Fig. 1. Geographic and bathymetric features of Pensacola Bay system. Field observation stations are marked; these data were used for model calibration purposes seaward direction, and subsequently a compensating more-saline water level measurements. Favorable comparison of the model- bottom water movement can occur in the up-estuary direction. computed salinity distribution against the observed data provides This kind of response to the wind is more pronounced both in a firm basis for believing that the model-computed circulation is space and time if the coastline of the estuary changes direction an accurate representation of the bay circulation. near its confluence with the ocean, for example, the Gulf Breeze Peninsula in Pensacola Bay ͑Fig. 1͒. On the other hand, destrati- fication may occur due to an up-estuary wind. Goodrich ͑1985͒ Physical Characteristics of Pensacola Bay suggested that this depth-dependent response to wind is an effec- tive mechanism for generating vertical shear which, in turn, pro- duces significant vertical mixing to destratify the entire water The general morphological features of the Pensacola Bay system ͑ ͒ ͑ column. Fig. 1 are well documented USDI, FWPCA 1970; Wolfe et al. ͒ A three-dimensional hydrodynamic model was used to simu- 1988; NOAA 1993 . The present hydrodynamic study focuses on late the physics of the bay in response to various forcing mecha- the processes governing stratification and destratification in nisms, including tides, fresh water inflow, and atmospheric and Pensacola Bay and their connection in controlling water quality meteorological conditions. The modeling analysis was directed at processes such as the vertical dissolved oxygen distribution and examining both the mechanism of mixing and the temporal and phytoplankton growth. These physical processes are dependent on spatial distribution of salinity in the bay. Model calibration was the geomorphology, freshwater discharge, tidal energy dissipa- performed through comparison of the model results against ob- tion, and atmospheric and meteorological conditions in the sys- served data. The observed data include salinity, temperature, and tem. 260 / JOURNAL OF HYDRAULIC ENGINEERING © ASCE / APRIL 2005 The morphology tends to divide the entire study area in to two in the Blackwater Bay and East Bay areas, where modeling re- major estuarine systems; the Pensacola-Escambia Bay and East sults are of lesser interest. This variable grid spacing allows for Bay-Blackwater Bay systems. Unlike most estuarine systems, this the design of computationally efficient and time-effective model- estuary is physically separated from the coastal water ͑Gulf of ing system. Mexico͒ by a strip of land mass, the Gulf Breeze Peninsula ͑Fig. The modeling framework is configured such that it can accom- 1͒. The system however is dynamically connected to the Gulf of modate time dependent river flows, and water level, temperature, Mexico through the Pensacola Inlet. The Gulf Breeze Peninsula, and salinity along its open coastal boundaries and wind stress on which provides a physical barrier between these two systems, the water surface. River flows along with their temperature and estuary. The degree of stratification and destratification in the bay, salinity are used for Escambia, Blackwater, Yellow and East Bay accentuated by the oppositely directed near surface and near bot- Rivers, and Mulatto Bayou and Bayou Texar ͑see Fig. 1 for geo- tom currents, is substantially affected by the presence of the Gulf graphic locations of these rivers͒. Freshwater flows are obtained Breeze Peninsula. A detailed analysis of this process is provided from USGS maintained gages. The Pensacola Bay Bridge area is in this paper. where we have placed our open boundary. Through the specifica- The