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error when analyzing by SewerGEMS

Hi, I am analyzing a rainwater drainage network. I have some problems and need support.
When changing a conduit, the results of network analysis results change. I think a small area will be affected when just adjusting a conduit. I checked and could not find the cause.
In addition, the "TC Data collection" value according to the software seems smaller than normal.
Is the data I have given wrong?Existing Network.rar

  • Hello Phi,

    I checked your model and following are my observations;

    1. If you are using the GVF-Rational Solver since your catchments are set to Rational Method then you cannot have more than one outfall. I ran the "Validate" command and found this error;

    "Gravity sub-network has more than one outfall or wet-well.."

    To resolve this you can either do some modeling adjustments or use our dynamic solvers such as Implicit or Explicit solver. See below article for more details on this;

    How to resolve the user notification: Gravity sub-network has more than one outfall or wet-well

    2. When I changed the solver to Implicit Solver this was the error message;

    "There are 1 isolated nodes. Be sure all nodes are connected to a pond or outfall by a link (Boundary Element boundary condition type is not counted)..."

    This is due to manhole BS-MH106 having both upstream and downstream conduits are inactive. If they are supposed to be inactive in the current scenario, make this manhole also inactive. 

    3. Many conduits have a less than 3 m. Hence after validation this user notification is generated;

    "Length (1.0 m) is less than (3.0 m). Check if the length value is correct."

    This is because the minimum computation length required for analysis. Kindly ensure these are correct. You can combine a couple of pipes to avoid unnecessarily small length pipes.

    4. Many conduits have slope as "zero" or "adverse". This user notification can be seen;

    "Link slope is nearly zero."

    Check the upstream and downstream manhole invert levels to see if they are entered properly. For adverse slope condition, check if the downstream invert is higher than upstream invert.

    In addition, the "TC Data collection" value according to the software seems smaller than normal.

    As far as the Tc calculation is considered, I checked a few of the catchments; and most of the overland flow lengths entered are above 300 ft or approximately 92 m. This is the limit for this formula as specified in NRCS guidelines. Also it uses an iterative method to compute time of concentration. You can try using some other method for computation of time of concentration.

    Hope this helps.


    Regards,

    Yashodhan Joshi

  • Thanks for the reply

    I am analyzing a existing network. It is not well designed. As you can see: Link slope is zero with main channels; the pipelines connect to each other (When analyzing I disconnected them).

    1. Network has more than one outfall and pipelines connect to each other. I can not use the GVF-Rational Solver. I have used Implicit solver and catchments are set to Rational Method. Should I use another analysis menthod?

    2. Manhole BS-MH106 will be inactive in the current scenario (I was not careful).

    3.  "Many conduits have a less than 3 m" These are pressure pipelines in front of the pump. I am using vertical shaft pump so these pipes are actually not available. I have set up it following way "is virtual: false; length: 1 m). Setting like this is wrong? How should I set up?

    4. As I said above.

    5. About Tc Data collection: I can not use the Kinametic Wave method to my catchments.  "You can try using some other method for computation of time of concentration". I have set Rational Method for my catchments. According to you, which the best method should I use for mine?

    What is the main reason for the problem "When changing a conduit, the results of network analysis results change. I think a small area will be affected when just adjusting a conduit" ? How should I set up this existing network?

  • Hello,

    1. If you have diversions and loops, the Implicit or Explicit solver tend to be the better options. For the runoff method, I would use what you have data for. Other runoff methods are available, such as the SCS unit hydrograph method. What you use will be up to your engineering judgment. Here is some further information on using the Modified Rational Method.

    3. Using a virtual pipe when connecting to a pump may work well. You can find more information on that here.

    5. This is largely up to your engineering judgment. There is some information on the different Tc calculation methods in the Help documentation. However, you can also include a user-defined Tc if you know what value you want to use.

    Changing the time of concentration will impact the results, since it will have an impact on the runoff entering the subsurface network. Adjusting the conduit may impact results like headloss and velocity. Depending on what changes are being made, other results could be impacted. This is due, in part, to the continuity error reported with the Implicit solver. If adjusting some parameter in the system creates a large change in the continuity error, this could impact some of the results you are seeing. Generally speaking, you would want to try to get as low of a continuity error as is reasonable.

    Regards,

    Scott

  • Thanks for your helpful comments.

    I can understand that my choices are not wrong. I think I can use the Rational method and the Kinematic Wave method to my catchments. I am analyzing for an industrial zone. The area of the catchments that are subdivided <20 ha. They also have similar surfaces.

    According to you, should I use Modified Rational for my project. when I adjust from Rational Method to  Modified Rational, continuity error reduce from 20% to 10%.However, report an error: "22000 "Base" "Manhole" "155" "No-M 211" 0.000 "Flooding is occurring at this element during this time step. Hydraulic Results". Why was the element flooded at 0.000  before the analysis began.

    "This is due, in part, to the continuity error reported with the Implicit solver. If adjusting some parameter in the system creates a large change in the continuity error, this could impact some of the results" I will adjust the network to reduce continuity error.

    Regards,

  • I think I can use the Rational method and the Kinematic Wave method to my catchments.

    The Rational method is a steady state/peak flow method, and only applicable to the GVF-Rational solver. If you attempt to use it with the Implicit or Explicit solver (by first setting the solver to rational, selecting the rational method for catchments, and then switching to the implicit or explicit solver), the program will assume that you wanted to use the Modified Rational Method, which utilizes the critical storm duration that you enter in the Global Storm Events window, or in the Rainfall Runoff alternative.

    According to you, should I use Modified Rational for my project. 

    The method to use is up to your judgement and local standards. Although the Modified Rational Method is typically used for smaller areas, you'll want to ensure you have a clear understanding of the assumptions and how the hydrograph is calculated based on the critical storm duration that you have entered. See the article that Scott previously sent for more information.

    when I adjust from Rational Method to  Modified Rational, continuity error reduce from 20% to 10%.However, report an error: "22000 "Base" "Manhole" "155" "No-M 211" 0.000 "Flooding is occurring at this element during this time step. Hydraulic Results". Why was the element flooded at 0.000  before the analysis began.

    As explained above, if the Rational Method was selected and you ran the Implicit or Explicit solver. it should automatically interpret it as the modified rational method. So, the results should not change. Did you change anything else other than the Runoff Method field in the catchment properties? What do you have the storm duration set to?

    Regarding the flooding notification - this could be due to initial instability when the dry pipes start filling with water - try looking at some profiles and graphs.

    "This is due, in part, to the continuity error reported with the Implicit solver. If adjusting some parameter in the system creates a large change in the continuity error, this could impact some of the results" I will adjust the network to reduce continuity error.

    Yes, the overall stability of the model will impact the results, and it is a good idea to use the tools available to locate data entry issues (which can lead to unstable results) and reduce mass balance error using the calculation options.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.