  This paper presents the governing equations of a tidally-averaged,
width-averaged, rectangular estuary in completely nondimensionalized forms.
Subsequently, we discover that the dynamics of an estuary is entirely
controlled by only two variables: (i) the Estuarine Froude number, and (ii) a
nondimensional number related to the Estuarine Aspect ratio and the Tidal
Froude number. Motivated by this new observation, the problem of estuary
classification is re-investigated. Our analysis shows that the two control
variables are capable of completely determining the stratification at the
estuary mouth, and therefore can specify the estuary type. The theoretical
estuary classification scheme proposed in this paper is validated against real
estuarine data collected from existing literature. Our classification scheme on
comparison with the state-of-the-art theory shows significant improvement.
