I am writing regarding some results using the HEC 22 loss method. We are working on a sewer model using the explicit calculation engine and HEC 22 loss method by request of our client, so this cannot be modified. We calibrate the model using field data basing on flow, depth and velocity. We have found cases in which depth results show an unexpected jump due to HEC22 losses. See the following screenshot:
We wanted to ask you about these jumps because it does not seem to conform to real behaviour, and there is nothing similar on any of our meters.
In short, why do these jumps appear? Is there a real explanation?
What sort of continuity error are you getting when you compute the model? You can try lowering the Routing Time Step in the calculation options properties to see if that helps. Try a value of 1 second and see if the results look better. You can also find some additional things you can try at this link.
If that doesn't help, it would be useful to see a copy of the model to have a better understanding of what is happening in the system. This link has the steps on how to send the model files to us.
Already have 1 second Routing Time Step calculation. These are the model options and summary results:
I have sent you the model.
Apologies for the late reply but I couldn’t work in this project for a while. I have sent you a new model with one scenario, where you can see some graphs with problems like the ones explained in the first message. In this case there are bigger jumps:
This issue may be because the conduit has a hydraulically steep slope. This link has some information on this. This forum post has some additional information as well.
I don't think this issue can be explained with that information. The problem is solved when HEC-22 loss is removed from the manhole. In the model I have sent, if absolute loss (0.00) is set in manhole 8612160:
As you can see in the images, there are no "jumps" when manhole loss is removed. So the problem is in HEC22 loss calculation.
I am asking for input from our development team on this. I will let you know when I have some information for you.
Based on calculations for the HEC-22 Headloss Method (Third Edition), this appears to be working as expected. The depth at the downstream end of the conduit is calculated to have a higher depth than what is seen at the upstream end of the next conduit. This was confirmed using the equations from the HEC-22 manual.
I will note that if you were to upgrade to the latest version of SewerGEMS, there is a feature that allows you to change the solver compatibility so that it will run the SWMM solver with none of Bentley's upgrades, like the addition of the HEC-22 Headloss Method. This would be similar to the results you see when you use the Absolute headloss method. This link includes other features included in the latest version of SewerGEMS.
The steps to upgrade to the latest version can be found here.