SewerGEMS_Max Out Flow

Hello Mohammad,

I checked all the catchments for your outfall OF-PP.2 (approximately 256) and compared the runoff's for both of them for all times, however the runoff for the scenario (with normal CN values) is higher at each time step for every catchment as compared to the runoff for the scenario (with reduced CN values).

Also total flow out is higher in the scenario of normal CN values (189.03 L/s) as compared to the scenario of reduced CN values (4.25 L/s).

Try adjusting the calculation options as given in the below article to see if you get better results. You can try reducing the calculation time-step to get some conservative results. Currently the continuity percentage is 0.7%; you can try bringing this value up by tweaking the constraints and see if it helps.

Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver

Another observation I had was that smaller catchments with same CN values in the reduced CN scenario had higher runoff than in the case of normal CN scenario. This could be due to the peak flow occurring later in the former case. This could lead to flow attenuation and result in higher peak values downstream. See the article below for details;

Conduit flow increases as you move downstream, with no additional inflow

Let me know if this helps. 

Hi Yashodhan,

I have looked to your articles, however; the only difference between the two scenarios is that I reduced the CN value and the expectation from the model to achieve lower flow without changing the timing steps since nothing changed in the physical elements. also for the fair comparison, we cannot use a different time step for the scenario and the child scenario.

I have changed the Time Steps to 0.1 min but I am experiencing more spikes in the graph, and the changes are not big in the scenario with reduced CN; I was expecting the reduction of CN will change the total outflow more!

why the Ghaphs shows negative outflows; would you elaborate about that.?

In the Hydraulic Reviewer; Manhole (MH PP.2/8) what couse flow deviation? and why the amount is high?

Other Manholes show Devation (node) (%) while the flow deviation is zero for those particular manholes?

Regards,

Mohamad.

The problem is that there is some initial instability (which is typical), which happens to be where the peak flow occurs. So, although the change you see is not logical, it is because the slight change in input can sometimes result in a relatively large or unpredictable change in results during times of instability. 

You can use the information in the article that Yashodhan pointed to, to help improve stability. As an example, if I reduce the calculation timestep (and output increment) from 0.083 hr to 0.025 hr and change Start Type to Warm Start (with the default 15 minute warm up time), the initial instability is reduced, and the peak flow at the outfall is less in the scenario with the smaller CNs.

Either way though, as Yashodhan mentioned, the total volume decreases in the scenario with the smaller CNs (even as seen in the Calculation Summary).

Regarding the hydraulic reviewer - click the help button to see definitions of the fields and how they are calculated. With my example adjustments

Hello Jesse,

why the Ghaph shows negative flows at the outflow? and what that means?

what is the way to fix the flow deviation at the manhole when the flow deviation is accountable and high (the continuity error is less than 0.5%)?

Thank you,

Hello Mohamad,

The negative flows may be related to the hydraulic grade in the system. Since you are defining the elevation at the outfall, if the elevation is higher than the hydraulic grade further upstream, this may result in flow moving upstream into the network.

If the pipe network is sloped downward and you do not expect that the outfall could ever experience reverse flow, then it could be due to instability. See my previous comments regarding troubleshooting this, and how it can be common but often short-lived, during the initial hour or so.

Mohamad Azzam said:

what is the way to fix the flow deviation at the manhole when the flow deviation is accountable and high (the continuity error is less than 0.5%)?

Even if the overall continuity error is low, you may still have a few particular locations that experience relative high continuity error. Consider the overall volume involved, zoom to the element in question, look for data entry issues or identify challenging situations and adjust the input or calculation options to address the situation. Again, see the "troubleshooting" article for more specific tips.

If the pipe network is sloped downward and you do not expect that the outfall could ever experience reverse flow, then it could be due to instability. See my previous comments regarding troubleshooting this, and how it can be common but often short-lived, during the initial hour or so.

Mohamad Azzam said:

what is the way to fix the flow deviation at the manhole when the flow deviation is accountable and high (the continuity error is less than 0.5%)?

Even if the overall continuity error is low, you may still have a few particular locations that experience relative high continuity error. Consider the overall volume involved, zoom to the element in question, look for data entry issues or identify challenging situations and adjust the input or calculation options to address the situation. Again, see the "troubleshooting" article for more specific tips.