Inlet flooding with the HEC-22 Third Edition headloss method, but not with HEC-22 Second Edition hea


Applies To 
Product(s):Bentley StormCAD
Version(s):10.00.xx.xx, 08.11.04.54 +
Area: Calculations
Original Author:Terry Foster, Bentley Technical Support Group

Defect Description

I'm getting flooding at an inlet, but when I run as a culvert, no flooding.  Why? In my StormCAD model, a single lateral conduit has an incredible exit loss at a manhole connection.

Explanation

  1. This is a rare, known limitation with our current implementation of the HEC-22 3rd edition structure loss method. This can occur in a specific condition where the downstream node is using the 3rd edition structure loss method and is considered "plunging", but the plunging depth is very small (difference between the upstream pipe's outgoing invert and the structure's HGL). The velocity used in the exit loss calculation is based on the area created by this difference along with the respective flow. So, you can see that with a very small plunging depth, the velocity used is very high, leading to a high exit loss.
  2. To illustrate further, in the case with the manhole, you can set the "bolted" option for this manhole to "false". This causes the HGL to be effectively "capped" at the rim, allowing you to see more clearly the small plunging depth mentioned above (the manhole in question is on the right side)

  3. The conduit/manhole situation being very close to non-plunging makes the model sensitive.  For example when you lower the invert of the node by enough of a distance so that the upstream pipe's stop invert is above the node's HGL, it is enough to cause a non-plunging condition. 

Workaround

Our developers have implemented an enhancement to check for this condition and handle it more appropriately.  You can get this enhancement by downloading the Select Series 4 cumulative patch or installing the latest version of the software. Directions for how to do that can be found here:

Downloading OpenFlows | Hydraulics and Hydrology Software

If you are unable to apply the Select Series 4 cumulative patch or install the latest version of the software, there are a number of potential workarounds.  You could: 1) Set the structure loss method to HEC-22 2nd edition for that node; 2)  Lower the invert to prevent the plunging condition  3)  Calculate the desired structure loss externally and input that as an "absolute" loss for the node (or estimate a K for the standard method).

See Also

What are the differences between the HEC-22 3rd edition and the HEC-22 2nd edition?

Flat hydraulic grade line over nodes for the HEC-22 3rd edition energy method