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This chapter discusses definitions and classification of estuaries. It presents both the classical and more flexible definitions of estuaries. Then it discusses separate classifications of estuaries based on water balance, geomorphology, water column stratification, and the stratification–circulation diagram – Hansen–Rattray approach and the Ekman–Kelvin numbers parameter space.
The most widely accepted definition of an estuary was proposed by Cameron and Pritchard (1963). According to their definition, an estuary is (a) a semienclosed and coastal body of water, (b) with free communication to the ocean, and (c) within which ocean water is diluted by freshwater derived from land. Freshwater entering a semienclosed basin establishes longitudinal density gradients that result in long-term surface outflow and net inflow underneath. In classical estuaries, freshwater input is the main driver of the long-term (order of months) circulation through the addition of buoyancy. The above definition of an estuary applies to temperate (classical) estuaries but is irrelevant for arid, tropical and subtropical basins. Arid basins and those forced intermittently by freshwater exhibit hydrodynamics that are consistent with those of classical estuaries and yet have little or no freshwater influence. The loss of freshwater through evaporation is the primary forcing agent in some arid systems, and causes the development of longitudinal density gradients, in analogy to temperate estuaries. Most of this book deals with temperate estuaries, but low-inflow estuaries are discussed in detail in Chapter 9.
Estuaries are of high socioeconomic importance with twenty-two of the thirty-two largest cities in the world located on river estuaries. Estuaries bring together fluxes of fresh and saline water, as well as fluvial and marine sediments, and contain high biological diversity. Increasingly sophisticated field observation technology and numerical modeling have produced significant advances in our understanding of the physical properties of estuaries over the last decade. This book introduces a classification for estuaries before presenting the basic physics and hydrodynamics of estuarine circulation and the various factors that modify it in time and space. It then covers special topics at the forefront of research such as turbulence, fronts in estuaries and continental shelves, low inflow estuaries, and implications of estuarine transport for water quality. Written by leading authorities on estuarine and lagoon hydrodynamics, this volume provides a concise foundation for academic researchers, advanced students and coastal resource managers.
This book resulted from the lectures of a PanAmerican Advanced Studies Institute (PASI) funded by the United States National Science Foundation and the Department of Energy. The topic of the PASI was “Contemporary Issues in Estuarine Physics, Transport and Water Quality” and was held from July 31 to August 13, 2007 at the Unidad Académica Puerto Morelos of the Mexican National University (UNAM). One of the requirements was that the PASI had to involve lecturers and students from the Americas, with most from the United States. The institute was restricted to advanced graduate students and postdoctoral participants. Because of the requirements, this book includes authors who work in the United States but tries to be comprehensive in including aspects of estuarine systems in different parts of the world. The book, however, reflects regional experiences of the authors and obviously does not include exhaustive illustrations throughout the world. Nonetheless, it is expected to motivate studies, in diverse regions, that address problems outlined herein.
This book should be appropriate for advanced undergraduate or graduate courses on estuarine and lagoon hydrodynamics. It should also serve as a reference for the professional or environmental manager in this field. The sequence of chapters is designed in such a way that the topic is introduced in terms of estuaries classification (Chapter 1). This is followed by the basic hydrodynamics that drive the typically conceived estuarine circulation consisting of fresher water moving near the surface toward the ocean and saltier water moving below in opposite direction (Chapter 2).