TABLE OF CONTENTS What is special about alluvial estuaries and what makes them different from nonalluvial and man-made estuaries? This chapter presents a classification of estuaries and related types of salt intrusion. Subsequently it focuses on the characteristics, peculiarities, and the resulting behavior of alluvial estuaries.
The shape of an alluvial estuary is similar all over the world. The width reduces in upstream direction as an exponential function. In coastal plain estuaries, there is no significant bottom slope, but in estuaries with strong relief, the depth may decrease exponentially. As a result in both types of alluvial estuaries, the cross-sectional area varies exponentially, and so does the flood volume, also called tidal prism (the volume of water that enters the estuary on the tide). Morphological equilibrium and minimum stream power lie at the cause of this typical shape. There is a similarity between the exponential reduction of the width of an estuary (in upstream direction) and the exponential increase of the drainage area of a catchment (in downstream direction) described in Rodriguez-Iturbe and Rinaldo (2001).
The constant depth and exponentially varying width correspond with the shape of an ideal estuary as described by Pillsbury (1939) and Langbein (1963), a topography that can be observed in coastal plain estuaries all over the world. The shape of an alluvial estuary is characterized by the ratio of the depth of flow ( h) to the convergence length ( b), the length scale of the exponential function. Although there is both empirical evidence (examples...
