A Nonlocal Formulation of Rotational Water Waves
  The classical equations of irrotational water waves have recently been reformulated as a system of two equations, one of which is an explicit non-local equation for the wave height and for the velocity potential evaluated on the free surface. Here, in the two dimensional case, (a) we generalise the relevant formulation to the case of constant vorticity, as well as to the case where the free surface is described by a multi-valued function; (b) in the case of travelling waves we derive an upper bound for the free surface; (c) in the case of constant vorticity we construct a sequence of nearly Hamiltonian systems which provide an approximation in the asymptotic limit of certain physical small parameters. In particular, the explicit dependence of the vorticity on the coefficients of the KdV equation is clarified. Also, in the irrotational case we extend the formalism to n>2 dimensions and analyse rigorously the linear limit of these equations. 
Anthony C.L Ashton, A.S. Fokas