status report

[Eveline Brouwer]

Hello,

I'm a beginner in using SWMM. I have simulated some examples but now I have a few questions about the results.

1)Why you can't specify a depth of an outfall node?
2)How is it possible that outflow is bigger than inflow(runoff is bigger than precipitation)?
3)Status report-->routing time step summary: what does mean...
-"percent in steady state": I thought that it would be 100% in the steady flow routing, but the program did give someting else.
-"average iterations per step"
4)How can I calculate the continuity error by hand, and what does it exactlly mean? Is it possible there is a continuity error when outflow is the same as inflow?
5)What does runon mean?
6)What does "time-step critical element" mean in the status report?
7)The depth in the outfall node is all the time the same as the maximum depth only in dynamic wave routing method not. How come?

So, that's it for the time being. I hope someone can help me, because I need the information for my final report on school, because there nobody can help me. Thanks!

Eveline

[hawstom]

Sorry I can't help more.

2) I think outflow can be bigger than inflow only if there is computaional error. Maybe your time step is too long.

4) Continuity error, to me, indicates lost water. I dont' know how to calculate it by hand (that's what the billion bean-counting accountants inside SWMM are for). Continuity errors could mean make your model last longer to catch the hydrograph tail.

6) Time-step critical element is the element of the system that hd the most problems with the time step. If you can't accept the modelling of that element, and you don't want to reduce the time step, SWMM gives you this information so you can maybe try a modelling trick like making a pipe longer or something to make that element less sensitive to time step.

[groutanker]

Hi,

isn't it so that swmm reduces the timestep by iteration untill the calculation is stable? Look at the "routing time step summary" in the status report, there is mentiones minimum time etc.

So decreasing your timestep would only improve calculation time and only if the average timestep as indicated in the status report is small anyway.

I tried reducing my time step because I had a system continuity error of around 15%. Nothing helped. What did "help" was increasing the total calculation time. I went from 2 hours calculation time to 4 hours, and my error reduced to just 3.5%. Not that this is satisfying, as it only means that errors resolve each other...