I just received this question about downstream boundaries in HEC-RAS, and thought it would make a nice post here on RASModel.com.
“…regarding the boundary condition, if I use critical depth instead of normal depth will it be unacceptable? When choosing the normal depth I need to enter a slope, can i use just one slope for the whole stream?”
Either critical or normal depth will work fine IF they are far enough downstream from your area of interest. Normal depth is usually better, because it is typically a better representation of actual stream conditions. Critical depth is only accurate when used at a significant grade break, a drop structure, a waterfall, etc., where flow passes through critical depth. Critical depth does not happen in natural streams very often, and usually the water depth is nowhere near critical depth. Using critical depth for a typical section of natural river is wrong. That’s why I tend to always use normal depth. However, using critical depth as a boundary condition is convenient (it’s easy to use, right?!?!), so it does have some merit.
For normal depth, the slope you enter is not the slope of your modeled reach. It represents the slope of the reach downstream of your downstream-most cross section. You can often times approximate this slope by using a topographic map and locating where topo lines cross the stream, then measure the streamwise distance between them. But remember this should be done DOWNSTREAM of your downstream cross section. Normal depth as a boundary condition in RAS is applied to one cross section only-the downstream cross section, and using it means you are assuming that the reach just downstream of your downstream cross section is flowing under uniform (normal) conditions. This too, is almost never the case in a natural stream due to the constant variation of cross section size and shape. However, it is much closer to the true solution than critical depth is.
The bottom line is both methods (in fact ALL methods) for assigning a downstream boundary have some inaccuracies. That is why it is important to maintain distance between your downstream boundary and the area of your model that you are interested in. One of the great things about the solution scheme in HEC-RAS is that it is “self-healing”. In other words, if there is an error at a specific location (due to a bad boundary condition, improper n value, poor survey data, that error will diminish the further away from that location you move. At some point upstream, the effect of that error will no longer be “felt”. How far downstream you need to place the downstream boundary is variable, but you can test this by trying different downstream boundary assumptoins (critical depth and normal depth, or simply normal depth with a bunch of different normal slope assumptions) and seeing how far upstream you need to be to not see an effect from different downstream boundary assumptions. This post demonstrates this technique: http://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.html