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Considerations for HEC-RAS 2D Flow Hydrograph Boundary Conditions

HEC-RAS version 5.0.3 includes several boundary condition types for 2D flow areas. Flow and stage hydrographs are typically used to input flows to the 2D area. The rating curve and normal depth boundary conditions are available where flow is leaving the 2D area. There is also a precipitation boundary condition, which can be used to apply time series of rainfall excesses to the 2D elements. This article focuses on the flow hydrograph boundary condition.

A flow hydrograph boundary condition is created by: (1) drawing a boundary condition line along the edge of the 2D area, (2) entering an input flow hydrograph, and (3) entering an input energy slope. The energy slope and cross section from the underlying terrain are used to calculate the normal depth. Flow is then distributed between 2D cells adjacent to the boundary condition line (whose terrain elevations are less than the normal depth water surface) using a conveyance weighting approach.

It is important to consider the location of the 2D area boundary and boundary condition line, as these can significantly affect the distribution of flow between 2D cells. Figure 1 below depicts a scenario in which the 2D area boundary and boundary condition line pass through a low spot in the terrain (corresponding to an excavation site). In the second scenario, the 2D area boundary and boundary condition line have been moved such that they no longer pass through the low spot.

Figure 1. Terrain and 2D Area Boundary for Scenario 1

Figure 2. Terrain and 2D Area Boundary for Scenario 2

Figures 3 and 4 below depict the depth-times-velocity results for each scenario. For Scenario 1, the normal depth calculations and conveyance weighting concentrate the flow in the low spot. For Scenario 2, the flows are distributed more broadly across the full length of the boundary condition line.

Figure 3. Depth-times-velocity Results for Scenario 1 

Figure 4. Depth-times-velocity Results for Scenario 2    

Several considerations should be taken into account when setting up boundary condition flow hydrographs:

  1. The boundary condition line should be generally perpendicular to the direction of flow.
  2. If possible, locate the boundary along natural high ground or elevated features that function as weirs, such as drainage divides or elevated roads.
  3. Select the energy slope with care, as this also affects normal depth calculations.
  4. Validate the computed normal depth with depths from the model which was used to develop the hydrograph.
  5. Utilize the depth-times-velocity results in RAS Mapper to help visualize how the flow is being distributed. Profile lines can also be utilized to view depths and flows.

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