Implicit SewerGEMS error

Hello Videet,

Please make sure that you have specified the value of "Duration (Modified Rational) in the Global storm event, which can be accessed through Components>Global Storm Events. 

Once you set the value of duration of storm, which should be greater than or equal to time of concentration, you should no longer receive the mentioned error message. 

You can read more about modified rational method and how it works, using the link mentioned below. 

Understanding the Modified Rational Method 

Hello, Sushma

The value of "Duration (Modified Rational) in the Global storm event was already set to the desired one.

-> "Duration (Modified Rational) in the Global storm event is set to 7min.

-> Minimum Tc in Calculation option is set to 5 min. and

-> The value of Time of concentration in Catchment flextable is also set to 7min.

Still, the error persists. If needed I can share the model with you.

Thank You!

Hello Videet,

The model is working for any other value than 7 minutes, if you set the Duration (Modified Rational) in the Global storm event to say 8 minutes and also the time of concentration for catchments to the same value 8 minutes, the model runs fine, just issue with 7 minutes. 

We are investing further, will keep you updated. 

Ok, Sushma. The error is something different and unrecognizable.

I have also checked the model with a different value (Tc=6.9 min.) and it is running fine.

Thank You!

Hi Videet,

As Sushma mentioned, we will look into improving this for a future version. However, I wanted to point out that setting the critical storm duration equal to the catchment Tc may not be the best method. You might be thinking that this will yield results compatible with the "straight" Rational Method (Q=CIA for peak flow, used in StormCAD's GVF-Rational solver for example). This would indeed achieve the same peak flow that you would normally see from the straight Rational Method (intensity based on Tc), but since the *Modified* Rational Method needs to construct a hydrograph to route in an unsteady simulation (as opposed to a steady state), the low storm duration will yield a relatively small total runoff volume.

To understand this better, set up another scenario with a higher storm duration set in the global storm events dialog and compare the catchment runoff hydrograph in the same graph as the 6.9/7.1 minute scenario. You will see that with a higher storm duration, the peak flow will be less, but the duration of the hydrograph will be greater.

There exists a "critical" storm duration that yields a maximum *volume*, which tends to be an important factor in an unsteady simulation. For more details on this, please read the article that Sushma linked to previously: Understanding the Modified Rational Method 

Hello Videet,

I have filed a defect for this, the reference number is #756293, which shall be considered for future release of the product. I have created service request for you, so when this is resolved in the future version of SewerGEMS, you will be notified about the same. 

Thank you Sushma!

Thank you Sushma!