  In this paper I develop a nonlinear theory of tightly-wound (highly twisted)
eccentric waves in astrophysical discs, based on the averaged Lagrangian method
of Whitham. Viscous dissipation is included in the theory by use of a
pseudo-Lagrangian. This work is an extension of the theory developed by Lee \&
Goodman to 3D discs, with the addition of viscosity. I confirm that linear
tightly-wound eccentric waves are overstable and are excited by the presence of
a shear viscosity and show this persists for weakly nonlinear waves. I find the
waves are damped by shear viscosity when the wave become sufficiently
nonlinear, a result previously found in particulate discs. Additionally I
compare the results of this model to recent simulations of eccentric waves
propagating in the inner regions of black hole discs and show that an ingoing
eccentric wave can be strongly damped near the marginally stable orbit,
resulting in a nearly circular disc with a strong azimuthal variation in the
disc density.
