Deferent engine Results in SewerGEMS

Hi Mohamad,

Results are expected to be different between the two engine versions if you are using features that would be impacted by the things that we enhanced with the "Bentley" engine version. Please see the section at the bottom of the following Wiki article:

EPA SWMM solver versions used by SewerGEMS and CivilStorm

Note this part:

If the compatibility mode is set to “EPA (5.1.012)”, these enhancements are turned off:

• On Grade Inlet Calculations - All on grade inlets will assumed to be full capture.
• Node Headlosses - The EPA version of the SWMM engine assumes zero headloss across nodes.
• Pond Outlet Structures - In EPA mode, composite outlet structures associated with ponds are loaded as a single outlet rating curve assuming free outfall.
• Tractive Stress Calculations
• Hydrogen Sulfide Analysis
• Calculation Options - The following options were added by Bentley to improve the overall stability of simulations under certain conditions, and are not used in runs in EPA mode: “Inlet Transition Depth” and “Use Bentley Transition Equation?”

So, differences in results can be expected if you have catalog inlets on grade for example. Or since you are referring to a pond, the difference could be due to the third bullet point.

Hello Jesse,

As you mentioned the difference is there, and I do NOT think due to the third bullet, I do not have outlet structure active, and I do not have on grade inlet, the model shows some are in sag (which full capture) and others side entry gully. where I refer to pond I made outfall only (no pond structure).

that why I do not expect that difference, and if would be a difference it should not be like 2 m3/s.

Regards,

Mohamad  

Hi Mohamad, I see you uploaded a copy of a model so I took a look. Most of your catchbasins (1593 out of 1732) have the inlet location set to on-grade. You can filter or query on the "inlet location" field to see this. As mentioned earlier, the "EPA" version of the SWMM solver does not support HEC-22 inlet capture calculations and assumes full capture, whereas the "Bentley enhanced" SWMM solver does support on-grade capture calculations, so results will be different due to the difference in efficiency.

For illustrative purposes if I global edit all catchbasins to be in-sag and re-compute (I set up two scenarios with different calculation options where the solver version was changed), the results are the same.

Note that it appears you are using a slightly older version of SewerGEMS: 10.02.01.04. For upgrading, see: Downloading OpenFlows Software

Thank you Jesse.

Please correct me if I am wrong; you are saying EPA considering the On Grade gully is full capture (100%), while Bentley Enhanced calculates with considering the efficiency that we used in the input? if so; you mean that when we are using EPA there is no difference between "In Sag" and "On Grade" and the EPA deals with both the same; am I correct?

On another hand, if the EPA considering the full capture for "On Grade" how are the gutters show and convey flow?

Regards,

Mohamad

Mohamad,

The EPA-SWMM software does not have inlet elements or inlet capture calculations. So, if a catchment is directed to a "node" in EPA-SWMM, all of the flow from the catchment enters the system. Meaning, 100% of the catchment flow is "captured" - this is why we assume full capture in this case if you are using the EPA-SWMM compatibility engine calculation option (to match what you'd get in EPA-SWMM). You can see this for example if you export the "office project" sample model (which has on-grade inlets) to SWMM5 format and import into EPA-SWMM 5.1. 

SewerGEMS is able to perform HEC-22 inlet capture calculations and inject the resulting flow into the system (to be solved by the SWMM engine) based on the calculated capture efficiency. Since EPA-SWMM itself cannot do that, SewerGEMS ignores those calculations when setting the engine to EPA-SWMM, so that the results are comparable to EPA-SWMM. I have added some additional clarification to the related Wiki article.

So - unless you have a need to produce results that are compatible with EPA-SWMM itself, then since your model uses on-grade catalog inlets, you should use the "SewerGEMS" engine version.