An understanding of estuarine circulation patterns, and the forces controlling them, is essential for a broader understanding of the ecology of Chesapeake Bay and of environmental problems affecting the Bay. The estuary is, first and foremost, a giant mixing bowl where salt water from the ocean meets fresh water draining off the land surface. The basic two-layer estuarine circulation pattern is driven by the differences in density between salt water and fresh water, as we will see. The excerpt from Kennish reviews the major types of estuaries on the basis of their circulation patterns and the extent of mixing. One important factor distinguishing between different types of estuaries is the relative importance of diffusion and advection - where diffusion refers primarily to mixing processes that tend to smear out what would otherwise be fairly sharp distinctions between waters of differing salt content, and advection refers primarily to processes that cause net transport of a particular parcel of water - i.e. up-Bay transport of salty bottom water or down-Bay transport of relatively fresh river water. If diffusion is more important than advection, than salt gradients from top to bottom of the water column or from one side of the estaury to the other are less significant. If advection is more important, then gradients are fairly sharp and discrete parcels of saline and fresh water are more easily differentiated from one another.
In order to really understand how the estuary works, however, you also need to have a good grasp of what waves are and how they behave, how wind can alter the circulation, what controls tides and tidal currents (as it turns out, the tides can be viewed as very long waves driven by the moon's gravitational pull rather than by the wind), and the role of the Coriolis effect. For this reason one of the readings in this week's set of assignments is a fairly extensive set of excerpts from Gross's oceanography text, which goes into some detail on progressive waves and standing waves (seiches), tides and tidal currents, storm surge, the Ekman spiral, and related topics. How all of these relate to Chesapeake Bay will be explained in class.
Another reading, by Schubel and Pritchard, discusses the basic characteristics of freshwater flow entering Chesapeake Bay, tides and tidal currents, spatial patterns of temperature, salinity, and density in the Bay and their seasonal variations, the classical pattern of two-layer circulation in the main Bay, and the major types of circulation patterns in tributaries to Chesapeake Bay. Of particular importance is the discussion of variations on the classical two-layer circulation pattern: there are six major types of estuarine circulation that alternate at different time scales in the main Bay. The classical two-layer pattern referred to earlier actually occurs less than half of the time, and the other patterns are driven by meteorological forcing. Here's where the effects of waves and of phenomena like the Ekman spiral have to be taken into account. Also, keep in mind that all of these patterns describe the net nontidal circulation - in other words, these are the patterns that we get when we average over one or two full tidal cycles. In fact, both the upper and lower layers are always sloshing back and forth as the tide goes in and out; you have to average out the effects of the changing tidal currents in order to see the larger patterns.
With an expanding knowledge base from detailed field data collection efforts, we can now characterize some aspects of the physical circulation that add greater complexity to the system. These are briefly discussed in the article from Science (July 1998), as well as the first two Web pages listed below, in the context of a major interdisciplinary research project now underway in the University of Maryland's Center for Environmental Science.
The last reading considers the effect of a really big flood on estuarine circulation by looking at a case study of the biggest flood on record - Tropical Storm Agnes. Catastrophic events like this occur rarely but may have a major impact on the system when they do occur, so we need to take a close look at how they influence circulation.
Some other aspects of estuarine circulation that aren't covered here will be addressed in the next packet of readings on estuarine sediment. As we will see, sediment transport in the Bay is affected not only by the net nontidal circulation pattern but also by short-term variations in the direction and velocity of tidal currents, particularly in bottom waters. You'll hear more on that subject later.
1. Readings in Adobe Acrobat (pdf) format: