| Product(s): | StormCAD, CivilStorm, SewerGEMS, OpenRoads Designer |
| Version(s): | 10.03.04.53 and earlier |
| Area: | Calculations |
Background
Engineers often use standards such as HY-8 for HDS-5 based culvert calculations, and compare its results to the culvert (headwall) results in OpenFlows StormCAD, CivilStorm or SewerGEMS, or the drainage calculations in OpenRoads Designer (which use the OpenFlows technology). Primarily the results being compared are the headwater elevation and overtopping.
The headwall culvert calculations in these Bentley products are also based on the HDS-5 standard methodology which is similar to HY-8 and therefore results should be close. However, there are a few things you may need to do to ensure the results match.
How to Match HY-8
To ensure your culvert headwater results are accurate and match HY-8, the maximum headwater should be set and the user should view the right headwater result field.
It is also advisable to upgrade to the latest version (24.00.03 for OpenFlows and 2025 for OpenRoads) to ensure you can access relatively new features such as the dedicated "Elevation (Headwater)" result field.
Set the Maximum Headwater Elevation
First, ensure that the maximum headwater elevation is set high enough, to prevent the calculated headwater from being unexpectedly limited.
If you expect that the headwater elevation may extend above the headwall element's ground elevation (the "Elevation (Ground)" in the headwall hydraulic properties), select "User Defined" for the "Headwater Range Type" in the properties of the conduit that represents the culvert. Then, enter the desired maximum headwater elevation in the field "Elevation (User Defined Maximum Headwater)".
By default, the headwater range will be limited by the ground elevation, which can cause results to not appear to match HY-8 if the headwater needs to extend above the ground elevation.
View the Correct Headwater Result
After computing hydraulics, you can find that culvert's headwater elevation in the result field "Elevation (Headwater)" in the properties of the conduit representing the culvert. This shows the water surface elevation on the upstream side of the culvert headwall assuming zero velocity (i.e., equal to the energy grade line), in keeping with HDS-5 / HY-8 methodology:
Note:
- You may need to upgrade to OpenRoads version 2025 or OpenFlows version 24.00.03 to see this conduit result field. For earlier versions, look at the Hydraulic Grade result in the headwall element.
- "Depth (Headwater)" is the depth of water at the inlet end of the culvert, measured from the upstream invert of the barrel. A velocity of zero is assumed upstream of the culvert.
- "Depth (Tailwater)" is the depth of water at the downstream end of the culvert, measured from the invert of the downstream channel or outlet (endwall) structure. A velocity of zero is assumed downstream of the culvert.
- You can also find the Hydraulic Grade Line result in the headwall properties.
- If you do not specify that the culvert has an overtopping weir (see further below), the headwater at the headwall node (Hydraulic Grade Line In and Out result) will be limited to ground elevation (and cannot rise above), similar to unbolted manholes and catchbasins (even if you have specified a higher maximum headwater range in the culvert's conduit properties). The correct "Elevation (Headwater)" will still be shown in the conduit properties.
Specify Overtopping Roadway if Present
If your culvert passes under a road and you need to account for the possibility of roadway overtopping, ensure that you have the option selected and configured in the properties of the conduit representing the culvert. HY-8 has equivalent functionality for this.
Note:
- A broad crested weir is assumed.
- In HY-8, a dropdown is used to select the roadway type (paved or gravel), but in OpenFlows/OpenRoads, the weir coefficient is entered directly. To match HY-8's "paved" or "gravel" options, you may need to define the table with the "use Weir C-Depth table" option in the headwall properties, based on figure 3.11 in the HDS-5 Manual (third edition).
- When the culvert Elevation (Headwater) result exceeds the headwall node's rim elevation, the user will see new warning: "The culvert has a computed headwater elevation that exceeds the top of its headwall structure."
- The result field "Flow (Roadway Overtopping)" shows the flow overtopping the culvert's roadway at the current time step.
- If you do not specify that the culvert has an overtopping weir, the headwater at the headwall node (Hydraulic Grade Line In and Out result) will be limited to ground elevation (and cannot rise above), similar to unbolted manholes and catchbasins (even if you have specified a higher maximum headwater range in the culvert's conduit properties). The correct "Elevation (Headwater)" will still be shown in the conduit properties.
Use the Most Recent Version
Some of these features were introduced in recent versions, so upgrading may be needed (version 2025 of OpenRoads or 24.00.03.XX of OpenFlows). More recent versions also have the following minor improvements:
- The depth calculation increment for the headwall rating curve has been made smaller, for finer accuracy in results.
- Hydraulic profiles of culverts have been improved, to make it easier to understand visually.
If you need to use an older version, you may need to raise the headwall's ground elevation in cases where the headwater is above the ground, and ensure that you look at the Hydraulic Grade Line result field in the headwall properties, to see the headwater elevation.
A note on Culvert Control Type - in some situations this may not match HY-8 - for example you may see "Outlet Control" when it should be "Inlet Control" (or vice-versa), or it shows as "N/A". This is planned to be fixed for the next versions released after March 2026. (reference # 2011934)
Reference #987818