Widening Channel Does Not Lower Max HGL

Hello, 

I am using SewerGEMS (version 10.03.01.08) utilizing the Implicit Solver to model a series of open channels. There is one section of channel that is overtopping. To get rid of the overtopping issue I decided that widening the channel would be best, but doing so has not lowered the HGL. I significantly increased my bottom width and even made the top of banks higher and the Max HGL budged by only 0.03 ft. 

It appears to me that drastically increasing conveyance of channel is having negligible effect on the Max HGL, which seems off to me as I would expect my Max HGL to lower if I increase channel conveyance. 

Does SewerGEMS base max HGL for a particular cross section from its station elevation table? 

If yes, does that mean there is possibly a different issue with my model if drastically changing my cross section station elevation has negligible effect the Max HGL? 

- CM

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  • Hello Cecily,

    There is one section of channel that is overtopping.

    Do you see a user notification stating that the cross-section is overtopping. When using the implicit solver, if the HGL exceeds the ground elevation, the program will extend the elevation at the element "vertically", something like a wall going up from the either side of the cross section. Thus when the HGL gets to a level where overtopping would occur, the HGL will increase above the ground elevation. See the below article which explains more on this;

    What Happens When the Water Level Exceeds the Top Elevation of an Open Channel?

    To get rid of the overtopping issue I decided that widening the channel would be best, but doing so has not lowered the HGL.

    Even after increasing the channel width, do you see any overtopping user notification? Or is there still overtopping occuring in the channel?

    To troubleshoot this, you can try using a smaller calculation time-step and observe the HGL fluctuation through time to see the impact of channel width on this. It is possible that the cross-section might not be hydraulically adequate for the flows in the channel. We would have to take a look at your model to ascertain if this is the case. See the below article on sharing hydraulic model files over the forum;

    Sharing Hydraulic Model Files on the OpenFlows | Hydraulics & Hydrology Forum


    Regards,

    Yashodhan Joshi

    Bentley Technical Support

Reply
  • Hello Cecily,

    There is one section of channel that is overtopping.

    Do you see a user notification stating that the cross-section is overtopping. When using the implicit solver, if the HGL exceeds the ground elevation, the program will extend the elevation at the element "vertically", something like a wall going up from the either side of the cross section. Thus when the HGL gets to a level where overtopping would occur, the HGL will increase above the ground elevation. See the below article which explains more on this;

    What Happens When the Water Level Exceeds the Top Elevation of an Open Channel?

    To get rid of the overtopping issue I decided that widening the channel would be best, but doing so has not lowered the HGL.

    Even after increasing the channel width, do you see any overtopping user notification? Or is there still overtopping occuring in the channel?

    To troubleshoot this, you can try using a smaller calculation time-step and observe the HGL fluctuation through time to see the impact of channel width on this. It is possible that the cross-section might not be hydraulically adequate for the flows in the channel. We would have to take a look at your model to ascertain if this is the case. See the below article on sharing hydraulic model files over the forum;

    Sharing Hydraulic Model Files on the OpenFlows | Hydraulics & Hydrology Forum


    Regards,

    Yashodhan Joshi

    Bentley Technical Support

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