Hi,
I have three questions here that hope to get clarification.
I have a surface water network model consists of open channels and culverts built on SewerGEM using DynamicWave solver and Modified Rational Method.
Question 1:
When I compare the hand calculation on Peak Flow, the model seem to be underpredict. Taking Catchment H-41 as example. It has an pervious area of 15.593ha and runoff coefficient of 0.35 at Tc of 7.185. Based on the IDF curve, the 100yr rainfall intensity at this Tc should be 325.6318 mm/hr. from manual calculation, the peak runoff should be 4.93 m3/s, however the model only predict around 3.1m3/s peak flow (see attached graph “H-41 runoff”). Is this correct?
Question 2:
I have a branch channel at cross section CS-230 with a side weir at the branch sewer. However the downstream channel surcharged but the surface flow doesn’t seem to be spilling over the weir at the side branch channel. See profile CS-230 where it shows the surcharge HGL is much higher than the spill link. Is this correct?
Question 3:
The surface water channel consists of numerious open cascaded drains. I have model them as box with steps using manhole transition nodes. Is this the right way to model cascaded drain in SewerGEM? See network at node MH-29, M2-15, M1-9991, MH40.
I have attached a copy of the model for your comments.files.zip
Regards,
Boon
Hello Boon,
For Question 1, this is related to how the Modified Rational Method works. The Modified Rational Method creates a hydrograph based on the time of concentration as the storm duration that you enter. The storm duration here is the key. Based on graphs of the hydrographs for the catchments, it appears the storm duration is about 22.5 minutes. This will lead to a lower intensity than you might see just using the plain Rational Method. You can find more information at the following link: Understanding the Modified Rational Method. So this appears to be working as designed. If you strictly need to use peak flow and the Rational Method, the StormCAD solver can do that, but a lot of what you are modeling (pumps, storage, etc.) will not really work in the StormCAD solver.
For Question 2, from what I can tell, the control structure is limiting flow in the downstream channel CH-441. This could cause the hydraulic grade at CS-230 to be higher. Basically, the flow is building up at the start control structure, causing the hydraulic grade to be higher. The placement of the control structure is important here. Since it is a start control structure on the downstream channel, this will cause the HGL at the cross section to be higher, but your profile Profile-6 does show limiting hydraulic grade in the channel, which is accurate.
For Question 3, I will defer to the engineering judgment of others here to the proper why to model cascaded drains.
Scott
Hi Boon, I would suggest posting a separate thread regarding the separate topic of "cascading drains".
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
HI Scott,
Re Question 2:, when i look at the flow graph for the downstream conduit at node CH-224, the surcharge flow from the continuation pipe doesn't seem to spill over the side branch. is this correct?
ok Jesse, thanks.
I computed the model with and without the control structure in place on channel CH-226. With the control structure in place, the flow results without the control structure were lower in CH-226 and higher in CH-227 than when there was no control structure. This tells me that the extra flow is being directed into the other channel when there is a control structure.
If you are seeing different results, please send a screenshot of what you are seeing, both with and without the control structure.