Dam break in rectangular channels with different upstream-downstream widths

•The Stoker dam-break problem is revisited for different upstream/downstream widths.•A suitable augmented shallow water system is analytically investigated.•Across the contact wave the total discharge and the specific energy are conserved.•Different types of solutions are investigated in the (width ratio/depth ratio) plane.•An original nonlinear path in DOT methods improves the reproduction of contact wave.