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# HGL & Surcharged Flow Rate for outfall pipe!

## HGL & Surcharged Flow Rate for outfall pipe!

(OP)
Hi there!
First of all thank you all for your valuable time and help. I'm almost new to this engineering field and I've learned a lot from this forum.
So, I'm sorry if my questions are long and silly!

We have a project in south Texas that is a 0.7 AC small land with a proposed hotel in it. I've attached our storm sewer design in photo #1. We asked to make pipes 6" from underground detention to the outfall, we did and next time they came with another comment asking:
"Calculate the hydraulic grade lines and give the actual flow rates for the 100-year event. What is the surcharged flow rate through the 6-inch outfall pipe from "8" to "Out"? (See attached #1)
My questions are:
1. In a conduit system with variable pipe sizes (not consecutive, ex. 8" --> 10" --> 36" ---> 6" --> outfall) how would be the HGL? I modeled this system in SSA (my very first time) and HGL was eventually dropping at the upstream nodes (see table in photo #2). We've been told that HGL will never drop at upstream in a conduit system, is it a rule?
2. If i'm right, the surcharging happens when the pipe capacity is smaller than what it should convey, right? Is it OK to have it in a system? In our design this pipe supposed to act as a restrictor, then what is the Surcharged flow rate? Is it the difference between peak flow and the capacity?
3. Any one can help how to model a similar system in StormCAD or SSA? I have difficulty when a small pipe connecting to a bigger size pipe but not a junction.
4. I'm attaching my drainage table (photo #2), is the actual rate the same as 100-yr partial flow rate? or what? I'm some how confused with "Design" , "Peak" and "Actual" flow rates definition. That could be because of my english, too!

Thank you so much in advance!

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