Hello,
I'm simulating a drainage network in SewerGEMS. In my network, when I calculate the time of concentration, it comes as 30 min. So I created my design storm for 30 min event since runoff will be maximum when the duration of rainfall is equal to the time of concentration.
Since I wanted to show the same in modelling perspective, I used 20 min, 30 min and 1 hr event and created three scenarios. Then I used Critical storm analysis option and selected Maximum HGL option.
But in the results, I always got the highest duration event as the critical event.(20 min,30 min and 1 hr set , critical event is 1 hr. And 20 min,30 min ,1 hr and 2 hr set, critical event is 2 hr event). Have I missed anything here? Shouldn't it be the rainfall event with time of concentration is the critical R/F event?
Thanks and Regards
Nishadi
Nishadi, are you referring to the "Duration (modified rational)" used with the Modified Rational Method? (set in the Global Storm Events dialog) If so, the longer the duration, the lower the peak flow, but the longer the storm will last. See more here: Understanding the Modified Rational Method
It could be that with the longer duration storms, even though the peak flow is less, the total volume might be more, and causing storage to overflow?
Have you tried computing three separate scenarios with the different durations, then examined the results? This will enable you to confirm that the peak flow is indeed less with the longer durations (so you know the storms were configurated correctly), and will also enable you to look at profiles for example to see if flooding is occurring.
If this does not help, please provide a copy of the model and reply with steps to reproduce: Sharing Hydraulic Model Files on the OpenFlows Forum
Also, ensure that you are using the latest version of SewerGEMS (currently 10.03.04.53, check under File > Help > About). See: Downloading OpenFlows Software
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Hello Jesse,
Thanks for the Reply. I was referring duration as design storm duration. I uploaded model. There I was expected 1 hr event to be the critical event as the Tc for that model is 62 min (I have included the calculation sheet too)
BR
Hi Nishadi,
Thanks for providing a copy of the model.
If you look at the runoff hydrograph for a catchment and choose all of the scenarios in the Graph Series Options (see: Viewing graph data for multiple scenarios), you can see that the longer duration storms have a higher peak flow, which is why they are marked as the critical storm (since the higher peak flow causes a higher peak hydraulic grade).
If you then look at the corresponding storm events, you can see that the volume of rainfall is higher for the higher durations (which is why the peak flow is higher for the runoff hydrographs). Meaning, even though the peak depth is the same in the rainfall curves, the higher duration storms have more volume under the incremental hyetograph curves. It might help to visualize them in cumulative form.
Answer Verified By: Nishadi
Thanks for the answer Jesse