For some quick estimates of the expected behavior of the current we make a bold approximation. We assume that the current has a simple shape, a well defined “box.” In the present axisymmetric case the box is a cylinder of radius rN (t) and height hN (t). The main objective is to determine the behavior of these two variables, which means that we need two equations. The first is provided by the obvious volume continuity requirement

2 r2N (t)hN (t) = V . (7.1)

The second equation must involve some dynamic considerations. In general, simplified forms for the velocity and reduced gravity fields are postulated, and integrals of the buoyancy, inertial, and viscous effects are obtained for the volume under consideration. Then some governing momentum integral balance is applied and an equation for uN is derived. The integration of this equation provides rN (t). The method is illustrated and discussed below for the axisymmetric current.