Downstream boundary conditions are important for all hydraulic models, especially unsteady flow models. Downstream boundary conditions can often be a source of model error, as well as model instability. More often than not, the true stage for a given flow at the downstream end of our models is not known. Because of this we often use either Normal Depth (Manning's equation), or a rating curve computed from a steady flow model. The normal depth boundary condition requires the user to enter a single energy slope, which is then in turn used in Manning's equation to compute the downstream stage for any flow occurring. Occasionally this forced slope, or even a single valued rating curve can end up with stages that are not correct for the given flow at a given point on the flood hydrograph. In general, the best solution is to make sure that the downstream boundary condition is downstream from any of the locations in which stages are being used to compute damages or loss of life, such that the error in the water surface elevation at the boundary condition does not affect the area of interest.
Additionally, if a boundary condition is ill posed, this can be a source of model instability (i.e. rating curves with not enough points, or the user entered stages are too low for a given flow rate; and normal depth boundaries where the user has entered to steep of a slope for the energy gradeline). In other words, the downstream boundary condition may be causing abrupt drops or rises in the computed water surface near the location of the boundary condition. An example of what can happen when using a Normal Depth boundary condition, and entering too steep of an energy slope is shown in the figure below. In this case, the steep energy slope caused the program to compute lower stages than appropriate for a given flow, which in turn caused the model to over steepen the flood wave at the downstream end of the model.
