The user has the option of fixing the stagnation point locations or allowing the program to solve for them within user defined limits. In general, it is better to let the program solve for the stagnation points, because it provides the best flow distribution and computed water surfaces. Also, allowing the stagnation points to migrate can be important when evaluating several different flow profiles in the same model. Conversely though, if the range in which the stagnation points are allowed to migrate is very large, the program may have difficulties in converging to a solution. Whenever this occurs, the user should either reduce the range over which the stagnation points can migrate or fix their location.
Within HEC-RAS, stagnation points are allowed to migrate between any bridge openings and/or culvert groups. However, if the user defines a conveyance area opening, the stagnation point between this type of opening and any other must be a fixed location. Also, conveyance area openings are limited to the left and right ends of the cross section.