I have a simple catchment area that I've routed to an inlet and then to an outfall. The 10 year storm peak flow going into the inlet is 3.91 cfs and my flow in the link to the outfall is 10.55 cfs. I do not understand why it is increasing going into the pipe. I've attached the pdf results here because for some reason it won't let me attach.spf file.
From what I noticed, it looks like the pipe is just picking up velocity. If you drop the pipe down to 0.50% you do get 3.9cfs.
I'm wondering if the slope of the pipe is increasing my flow by such a large amount. I'm thinking it has to be some other reason that I am missing. I've attached a simple example here. The flow goes from 2.2 entering the inlet to 7.7 cfs when the slope of the pipe is 5.5%. I've reduced the pipe slope down to 1.5% and it still amost doubles the peak flow once it enters the pipe. This doesn't make sense to me and it's causing my pipe to surcharge.
Please help if you can. Thank you.
Is it because of the Link routing method? I think when I change it to hydrodynamic in the project options it fixes the problem. What does that link routing method change?