From the 2D User manual:
The Energy Slope is used to compute Normal Depth from the given flow rate and the cross section data (underlying terrain data) along the Boundary Condition Line for each computational time step. A flow distribution in the cross section is then computed (based on the normal depth water surface and the conveyance in the cross section) and this flow distribution is used to appropriately distribute the flow to the cells along the boundary condition line that are wet. At any given time step, only a portion of the boundary condition line may be wet, thus only the cells in which the water surface elevation is higher than their outer boundary face terrain will receive water. However, if the computed Normal Depth water surface is higher than all the boundary face elevation data along the Boundary Condition Line, then all the cells will receive water based on a conveyance weighting approach.
My experience is that if you enter a slope similar to the bed slope things generally work fine.