For unsteady flow simulations, hydraulic structures such as bridges and culverts and storage area connections are converted into families of rating curves that describe the structure as a function of tailwater, flow and headwater. Then, internally, while the unsteady flow analysis is running, the HEC-RAS computational engine will refer to the computed rating curves to determine what the headwater elevation at the structure is. This speeds up the unsteady flow simulation by not having to iterate the computations at the structure.
The user can set several parameters that can be used in defining the rating curves, but generally only the maximum headwater elevation needs to be defined. This headwater elevation needs to be high enough to account for the maximum computed water surface elevation expected at the upstream side of the structure.
Users set these parameters in HEC-RAS by selecting the “HTab Parameters” (Hydraulic Table) button from the Geometric Data Editor, Bridge Culvert Data Editor or Connection Data Editor. In GeoHECRAS, these same parameters are accessed from the Analysis ribbon menu.
Follow these steps to set these parameters in GeoHECRAS:
- From the Analysis ribbon menu, select Rating Curves – Hydraulic Parameters and then Hydraulic Parameters – Bridges & Culverts command.
- The Hydraulic Parameters – Bridges & Culverts dialog box will be displayed.
- Enter the maximum elevation that you expect the water surface to reach on the upstream side of the bridge in the Maximum headwater elevation data entry field. Click on the […] Select Headwater Elevation button to pick the elevation from the graphical plot.
- The other parameters defined in the dialog box can usually retain their default values.
- Click the [OK] button to apply the defined changes and close the dialog box.
When the unsteady flow analysis runs, the HEC-RAS computation engine will construct a family of headwater elevation vs flow vs tailwater elevation rating curves. To speed up this rating curve computation, enter the maximum flow (discharge) expected at the structure and the software will limit the computations to that maximum value. This can reduce the time required for computing the rating curves.