  In this paper, a unified theory of internal bores and gravity currents is
presented within the framework of the one-dimensional two-layer shallow-water
equations.
  The equations represent four basic physical laws: the theory is developed on
the basis of these laws. Though the first three of the four basic laws are
apparent, the forth basic law has been uncertain. This paper shows first that
this forth basic law can be deduced from the law which is called in this paper
the conservation law of circulation.
  It is then demonstrated that, within the framework of the equations, an
internal bore is represented by a shock satisfying the shock conditions that
follow from the four basic laws. A gravity current can also be treated within
the framework of the equations if the front conditions, i.e. the boundary
conditions to be imposed at the front of the current, are known. Basically, the
front conditions for a gravity current also follow from the four basic laws.
When the gravity current is advancing along a no-slip boundary, however, it is
necessary to take into account the influence of the thin boundary layer formed
on the boundary; this paper describes how this influence can be evaluated.
  It is verified that the theory can satisfactorily explain the behaviour of
internal bores advancing into two stationary layers of fluid. The theory also
provides a formula for the rate of advance of a gravity current along a no-slip
lower boundary; this formula proves to be consistent with some empirical
formulae. In addition, some well-known theoretical formulae on gravity currents
turn out to be obtainable on the basis of the theory.
