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When using rational loading (StormCAD or the GVF Rational solver in CivilStorm or SewerGEMS), rational peak flow decreases when moving downstream.How can I prevent the attenuation effect of rational flows in StormCAD?Product: StormCADVersion: 08.XX.XX.XXArea: Analysis/Computation, Problem ID#: 52648, PROBLEMSOLVING
This is expected behavior with StormCAD, which mimics the effect of attenuation.This may seem counter-intuitive at first, with questions like "Where did the rest of the flow go?" coming to mind. In reality, the rest of the flow was not lost, but an attempt to balance peak flows is not valid. Picture standing at the top of a hill with a bucket of water. If you empty the entire bucket into the gutter in one second, then the peak rate of discharge at the top of the hill is one bucket per second. Racing to the bottom of the hill, you can observe the flow and see that the peak flow is much less than one bucket per second. However, the flow lasts longer than one second. There was no water lost, but the peak was lower.This is explained further in the help documentation under Contents > Theory > Special Considerations > Flow Balance. it is also discussed under Contents > Theory > Hydrologic Principles > Rational Loading > System Time / Controlling Time / Duration.As seen in the above help topic, if you do not wish to have this flow attenuation taken into account, you should specify Known or Additional flows at the catch basins. The "known" and "additional" flows are not considered "rational" flows, so they will remain constant, since the intensity is not adjusted as the travel time increases (as is done with rational flows).If this situation occurs in a Carrier Pipe (pipe with no subcatchment connected to the upstream node), you can also use the "Ignore Travel Time in Carrier pipes? " Calculation option (available in the latest version under Analysis > Calculation options). For more on this including illustrations, see the section in the Help called "Carrier Pipes".