  One of the most intriguing facets of Saturn's rings are the sharp edges of
gaps in the rings where the surface density abruptly drops to zero. This is
despite of the fact that the range over which a moon transfers angular momentum
onto the ring material is much larger. Recent UVIS-scans of the edges of the
Encke and Keeler gap show that this drop occurs over a range approximately
equal to the rings' thickness. Borderies et al. (1982, 1989) show that this
striking feature is likely related to the local reversal of the usually outward
directed viscous transport of angular momentum in strongly perturbed regions.
In this article we revise the Borderies et al. (1989) model using a granular
flow model to define the shear and bulk viscosities and incorporate the angular
momentum flux reversal effect into the axisymmetric diffusion model we
developed for gaps in dense planetary rings (Gr\"atz et al. 2018). Finally, we
apply our model to the Encke and Keeler division in order to estimate the shear
and bulk viscosities in the vicinity of both gaps.
