  The problem of choice of boundary conditions are discussed for the case of
numerical integration of the shallow water equations on a substantially
irregular relief. In modeling of unsteady surface water flows has a dynamic
boundary partitioning liquid and dry bottom. The situation is complicated by
the emergence of sub- and supercritical flow regimes for the problems of
seasonal floodplain flooding, flash floods, tsunami landfalls. Analysis of the
use of various methods of setting conditions for the physical quantities of
liquid when the settlement of the boundary shows the advantages of using the
waterfall type conditions in the presence of strong inhomogeneities landforms.
When there is a waterfall on the border of the computational domain and
heterogeneity of the relief in the vicinity of the boundary portion may occur,
which is formed by the region of critical flow with the formation of a
hydraulic jump, which greatly weakens the effect of the waterfall on the flow
pattern upstream.
