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On Grade Inlet with SWMM

Ref: https://communities.bentley.com/products/hydraulics___hydrology/f/haestad-hydraulics-and-hydrology-forum/193866/deferent-engine-results-in-sewergems

Hello Jesse,

In reference to the above case; and as I understood from you, the "Bentley SWMM" does suite the modelling gully on grade with Dynamic Wave routing rather than the EPASWMM engine, hence "the EPASWMM solver does not support HEC-22 inlet capture calculations and assumes full capture".

1- With "Bentley SWMM" When a catchbasin (on grade) floods and the hydraulic grade rises above the rim elevation, does overflow passes into the downstream gutter?

2- In EPASWMM, the catchbasin (on grade) will capture 100% of the connected surface flow, and the flow in the connected gutter will have a value only once the catchbasin floods and the hydraulic grade rises above the rim elevation, while in "Bentley SWMM" percentage of connected surface flow will enter the catchbasin and the rest go to the gutter, is that correct?

Accordingly; the difference in the flow results and flood location will occur, correct me if I am wrong or I misunderstand the fact?

As described in the link: https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/26172/modeling-catch-basin-overflow-into-gutter

Modeling catch basin overflow into the gutter; The Kinematic Routing Methods should be used when the model has catchbasin (on grade) and that routing method is preferable rather than the dynamic wave, is it correct?

would you please answer my above queries? I am trying to fix the understanding of the case.

Now; if the above understanding is correct; the best is to use Bentley SWMM engine and not to use the EPASWMM as you advised in the first link (I am fine with that), however; is there any workaround to use the EPASWMM engine for a model has "on grade inlets" and get results matching with "Bentley SWMM results"? or that cannot be done since the EPASWMM engine itself does not support the HEC-22 inlet capture calculations and assumes full capture? and that kind of network configuration and getting trusted output should only be done by using the Bentley SWMM engine?

I tried to model the network with the same configuration "on grade" by using  InfoSWMM which has exactly the same engine "EPASWMM" shipped in Bentley SewerGeMS and match the results when I run the model in SewerGEMS "EPASWMM", is it possible to work around and to match the results in Bentley SewerGEMS "EPASWMM" with "Bentley SWMM", if I can do that; then I move the model to InfoSWMM the results will be matched because is has exactly "EPASWMM engine".

Would you please clarify all the above my queries? I am working in this case for a while to figure out the best suitable engine for modeling the networks which will have hundreds of gullies (on grade) connected to gutters and get results close to the reality. for the other type of models (without Gullies nor Gutters) where the catchments are connected directly to the manholes, the difference is very minimal when running the model with Bentley SWMM and EPASWMM engine, am I correct and can clarify that?

I believe this is really interesting and very helpful topic to be documented and investigated and your support and your massive feedback are appreciated.

Regards,

Mohamad Azzam

  • Hello Mohamad,

    I will try to answer your queries serially,

    1- With "Bentley SWMM" When a catchbasin (on grade) floods and the hydraulic grade rises above the rim elevation, does overflow passes into the downstream gutter?

    Yes. As Jesse explained in the earlier post, the Bentley SWMM engine is a modified version of the EPA-SWMM engine. We have modified it to include inlet capture calculations which will result in part of the incoming flow to be captured and sent downstream (through the subsurface pipe system). The balance flow will flood the inlet and make the hydraulic grade rise above the rim elevation. In such a case, if there is a gutter connected the balance flow will be diverted to it.

    E.g. If the incoming flow is 30 L/s and as per inlet capture calculations the capture efficiency of the inlet is say 70% then only 21 L/s (0.7*30) of flow will be captured by the inlet and passed downstream through the conduit connected. The balance 9 L/s will be the overflow which is passed to the gutter if connected to the inlet.

    2- In EPASWMM, the catchbasin (on grade) will capture 100% of the connected surface flow, and the flow in the connected gutter will have a value only once the catchbasin floods and the hydraulic grade rises above the rim elevation, while in "Bentley SWMM" percentage of connected surface flow will enter the catchbasin and the rest go to the gutter, is that correct?

    Yes. In the EPA-SWMM engine if you use the solver compatibility as SWMM (EPA 5.1.014) then the capture at the inlets would be 100% because the EPA method does not account for inlet capture calculations, so 100% of the "catchment" flow will be captured by the inlet. The overflow in these cases can be due to contribution from upstream conduits which caused flooding at the inlet and then the overflow is transferred to the gutters connected.

    In reality SewerGEMS ignores the inlet capture calculations which are normally performed in this case so that the results match up with the EPA-SWMM solver.

    Accordingly; the difference in the flow results and flood location will occur, correct me if I am wrong or I misunderstand the fact?

    Yes. For a better understanding I have put the comparison in a table;

    SWMM (Bentley 5.1.014) SWMM (EPA 5.1.014)
    First perform inlet capture calculations to determine inlet capture efficiency

    Inlet capture calculations ignored by SewerGEMS

    Catchment flow captured based on capture efficiency calculated

    100% catchment flow captured

    Overflow passed on to downstream gutter

    Overflow passed on to downstream gutter

    Now; if the above understanding is correct; the best is to use Bentley SWMM engine and not to use the EPASWMM as you advised in the first link (I am fine with that), however; is there any workaround to use the EPASWMM engine for a model has "on grade inlets" and get results matching with "Bentley SWMM results"? or that cannot be done since the EPASWMM engine itself does not support the HEC-22 inlet capture calculations and assumes full capture? and that kind of network configuration and getting trusted output should only be done by using the Bentley SWMM engine?

    Not sure if this is possible. When you run the model with the solver compatibility set to SWMM (EPA 5.1.014), you would get a user notification;

    "All Inlet / capture data was excluded for Catch Basins.  All Catch Basins are assumed to be full capture.  To perform these calculations set the "Solver Compatibility" in the calc options to "SWMM (BENTLEY...)".

    This indicates that regardless of the inlet condition specified, all catch-basins (inlets) are assumed to be full capture. So to get the capture efficiency calculations you have to change the solver compatibility to SWMM (Bentley 5.1.014).

    I tried to model the network with the same configuration "on grade" by using  InfoSWMM which has exactly the same engine "EPASWMM" shipped in Bentley SewerGeMS and match the results when I run the model in SewerGEMS "EPASWMM", is it possible to work around and to match the results in Bentley SewerGEMS "EPASWMM" with "Bentley SWMM", if I can do that; then I move the model to InfoSWMM the results will be matched because is has exactly "EPASWMM engine".

    If InfoSWMM has only the EPA-SWMM method available, then you would have to use the SWMM (EPA 5.1.04) method in solver compatibility. This will give you results which match the EPA-SWMM method. But this will have the assumptions of 100% inlet capture and the capture efficiency calculations would be ignored as stated earlier.

    If you do want to match the EPA-SWMM results with the Bentley SWMM method, you would have to see the capture efficiency and actual captured flow for all your inlets. Then you would have to set the catchment in such a way that the runoff from the catchment matches the captured flow (from using the Bentley SWMM method). This way the 100% flow will be captured which would be equal to the captured flow (from the Bentley SWMM method).

    However, modifying the catchment could be tedious in such cases and you could use the inflow (wet) collection to specify the flows entering the catchment.

    Let me know if this helps. In his response, Jesse has also shared a few articles which you can find useful in this context. Please go through them for a better understanding. 


    Regards,

    Yashodhan Joshi