I've been tasked w/ routing analysis of 40+ acre urban watershed...

[Spartan5]

and associated storm sewer using the Rational Method. The purpose of the analysis is to identify any deficiencies in the sizing of the storm sewer. The system consists of about 25 subcatchments/inlets and is quite dendritic in nature.

I understand the Rational Method for smaller, simpler systems, but I am having trouble wrapping my mind around how tc and Q are routed through the system. I've got lots of nodes and lots of pipe.

I've tried modeling the system in UDSEWER, but it is choking on a couple of the larger subcatchments at outer reaches of the system. It's throwing a "backwater solver" error at me.

Intuitively, it seems like the Rational Method is not the appropriate approach to this situation, but it's been what I've been asked to do.

I'm seeking any and all guidance on this matter and appreciate any help. Either in formulating (and justifying) a better way to attack this... or in attacking it with the Rational Method. My H&H is a bit rusty and this is a tall order for something to start warming up that though process again.

 

[Spartan5]

For what it's worth... I've built the model in SWMM5 but I've been told I need to do a Rational analysis of the system. What's the most comparable thing I can do with SWMM5 that will approximate a rational approach?

 

[froude]

Spartan5,

For 40 acres, the Rational Method may be okay for your analysis.

StormCAD will conduct the Rational analysis of your system. However, I don't know if they still offer trial software or not.

I believe that Hydraflow (now incorporated with AutoCAD packages) will also analyze your system.

Otherwise, you can also calculate the HGL of the system by hand. Your Tc's will consist of any flow in the drainage areas plus your pipe flow times. The drainage design manual for the City of Fort Worth, TX has a spreadsheet example of how to analyze a SD network using Rational.

Good luck!

 

[jgailla]

Also FDOT has a good manual with worked examples.
You can develop a spreadsheet from it. The spreadsheet probably only takes about 10 to 20 hours to develop.

http://www.dot.state.fl.us/rddesign/dr/Manualsandhandbooks.shtm

 

[francesca]

If this is an urban storm sewer then the majority of your time of concentration is open channel flow within the storm sewer. Rational method seems appropriate for the small drainage areas to the inlets. You will need a dynamic system that can find the peak flow; it will be some permutation of a storm long enough to have most of the storm sewer network draining to the outlet, but short enough to be high intensity.

I just don't believe that you can't do the Rational Method with SWMM.

 

[beej67]

Rational analysis of this is easy if you use what's called the "modified rational method," which is also what programs such as StormCad use. Basically, refigure your basin at each node in your network and generate a new rational flow based on the watershed characteristics to that node. Note that the TC will change node by node based on the travel time along the pipe, and so will the intensities from your IDF curve. The end result is not a set of flows that the pipe system will ever experience from one given storm, but rather a worst case scenario of the selected return period.

Make sense?

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