The multiple opening features in HEC-RAS allow users to model complex bridge and/or culvert crossings within a one dimensional flow framework. HEC-RAS has the ability to model three types of openings: Bridges; Culvert Groups (a group of culverts is considered to be a single opening); and Conveyance Areas (an area where water will flow as open channel flow, other than a bridge or culvert opening). Up to seven openings can be defined at any one river crossing. The HEC-RAS multiple opening methodology is limited to subcritical flow profiles. The program can also be run in mixed flow regime mode, but only a subcritical profile will be calculated in the area of the multiple opening. An example of a multiple opening is shown in the figure below.
As shown in the figure below, the example river crossing has been defined as three openings, labeled as #1, #2, and #3. Opening #1 represents a Conveyance Area, opening #2 is a Bridge opening, and opening #3 is a Culvert Group.
The approach used in HEC-RAS is to evaluate each opening as a separate entity. An iterative solution is applied, in which an initial flow distribution between openings is assumed. The water surface profile and energy gradient are calculated through each opening. The computed upstream energies for each opening are compared to see if they are within a specified tolerance (the difference between the opening with the highest energy and the opening with the lowest energy must be less than the tolerance). If the difference in energies is not less than the tolerance, the program makes a new estimate of the flow distribution through the openings and repeats the process. This iterative technique continues until either a solution that is within the tolerance is achieved, or a predefined maximum number of iterations is reached (the default maximum is 30). The distribution of flow requires the establishment of flow boundaries both upstream and downstream of the openings. The flow boundaries represent the point at which flow separates between openings. These flow boundaries are referred to as "Stagnation Points" (the term "stagnation points" will be used from this point on when referring to the flow separation boundaries). A plan view of a multiple opening is shown in the figure below.
