  The evolution of piecewise constant distributions of a conserved quantity
related to the frozen-in canonical vorticity in effectively two-dimensional
incompressible ideal EMHD flows is analytically investigated by the Hamiltonian
method. The study includes the case of axisymmetric flows with zero azimuthal
velocity component and also the case of flows with the helical symmetry of
vortex lines. For sufficiently large size of such a patch of the conserved
quantity, a local approximation in the dynamics of the patch boundary is
suggested, based on the possibility to represent the total energy as the sum of
area and boundary terms. Only the boundary energy produces deformation of the
shape with time. Stationary moving configurations are described.
