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My question is, does the Hazen-Williams equation still apply for the vertical downspouts as well? If there is a light rain, the water should flow through all the pipes with ease, but if it is raining cats and dogs, how far will the water "back up" into the vertical portion of the downspouts? If it backs up too much, then the water will backup into the gutters which will lead to a real mess of things. Do I use a different formula for the down spouts? Or, do I not include the downspouts in my calculations and my feet of head is how far the water will "back up" into all of the downspouts? That is, only include horizontal/linear sections of pipe?

My example would be, if it's raining at a 100 GPM rate, and two 4" SCH 40 PVC downspouts are taking on 20 GPM and 80 GPM of water each and are 20 feet apart, and then transfer the water another 80 ft down a 4" line, will a water tank that is 9 feet tall accept all of the 100 GPM. This leaves me with a 1 ft or 12 in differential for the rainwater system. There are 5 segments to calculate, A - downspout 1, B - downspout 2, C- 20 ft piece of pipe between both downspouts, D - 80 ft piece of pipe to rain tank, and E - then vertical portion up into the rain tank. Or, what is the tallest tank possible for this scenario to maximize my storage since the tank can't be higher than 10 ft.

What is PSI loss and feet of head for segments A, B, and E or do all of the vertical sections pipe just cancel/equalize each other out since the tank and house are at the same elevation? Segments C and D added together equal almost 0.5 ft of head, if the vertical sections cancel each other, does that mean the rain tank can be 9.5 ft high?

Thanks for your help.