| Product(s): | CulvertMaster |
| Version(s): | CONNECT Edition (v 10.03.00.03) |
| Area: |
The Outlet Control Headwater Elevation is calculated higher in CulvertMaster as compared to HDS-5 Third Edition.
This difference in Outlet Control Headwater Elevation is usually seen for steep culverts. As per HDS-5 methodology, the Outlet Control is calculated as follows;
Outlet Control Headwater = Invert Elevation + Depth
However, CulvertMaster also considers the velocity head in the above calculation as is does an energy balance to get the headwater elevation immediately upstream of the culvert entrance. This is an extra step CulvertMaster considers above HDS-5 methodology, so the difference you see may be due to velocity head (V^2/2g).
In open channel flow, when you have a transition to a steep channel (supercritical flow), the flow depth = critical depth (dc) at the upstream end of the steep channel.
So, (Energy grade just inside the culvert) = (Flow depth + channel elevation + velocity head) = dc + z + v2/(2g)
Here, the energy loss due to the culvert entrance is K* v2/(2g).
Therefore, the HGL at the headwater location = dc + z + v2/(2g) + [K* v2/(2g)].
We assume that the velocity upstream of the culvert is 0, so EGL=HGL at this location.
This can lead to differences in results when considering such specific cases. The addition of the velocity head in CulvertMaster is expected and can be considered a difference in interpretation of the underlying open channel hydraulics theory.