  We investigate the numerical artifact known as a carbuncle, in the solution
of the shallow water equations. We propose a new Riemann solver that is based
on a local measure of the entropy residual and aims to avoid carbuncles while
maintaining high accuracy. We propose a new challenging test problem for
shallow water codes, consisting of a steady circular hydraulic jump that can be
physically unstable. We show that numerical methods are prone to either
suppress the instability completely or form carbuncles. We test existing cures
for the carbuncle. In our experiments, only the proposed method is able to
avoid unphysical carbuncles without suppressing the physical instability.
