A Boussinesq-type model is derived which is accurate to O(kh)4 and which retains the
full representation of the fluid kinematics in nonlinear surface boundary condition
terms, by not assuming weak nonlinearity. The model is derived for a horizontal bottom,
and is based explicitly on a fourth-order polynomial representation of the vertical
dependence of the velocity potential. In order to achieve a (4,4) Padé representation of
the dispersion relationship, a new dependent variable is defined as a weighted average
of the velocity potential at two distinct water depths. The representation of internal
kinematics is greatly improved over existing O(kh)2 approximations, especially in the
intermediate to deep water range. The model equations are first examined for their
ability to represent weakly nonlinear wave evolution in intermediate depth. Using a
Stokes-like expansion in powers of wave amplitude over water depth, we examine
the bound second harmonics in a random sea as well as nonlinear dispersion and
stability effects in the nonlinear Schrödinger equation for a narrow-banded sea state.
We then examine numerical properties of solitary wave solutions in shallow water,
and compare model performance to the full solution of Tanaka (1986) as well as the
level 1, 2 and 3 solutions of Shields & Webster (1988).