Friction Loss for gravity fed rainwater downspouts/pipes?

  • Thread starter saxman2u
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  • #1
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I am building a rainwater collection system and want to know how much higher the tops of my PVC downspouts have to be compared to where the water exits into my rain tank. Or, how tall my rain tank can be if my downspouts are 10 feet tall. Let's say that the rain tank and home foundation are level. The importance of this is that the higher my rain tank is, the more water I can store if I am limited in width. I understand the concepts of friction loss, ft of head, length of pipe, and elevation change as they apply to sprinkler systems or water services/ water towers that are under pressure by a pump or gravity, incur elevation changes, etc... These pipes are always full of water. What I am not sure how to calculate is for gravity fed systems that have multiple downspouts that are not full of water all the time?? The horizontal pipes will be full of water as the water makes its way over from the downspouts and then moves up to the rain tank. The downspouts will not always be full of water, since it depends on how hard it is raining and how high the rain tank is.....

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.
 

Answers and Replies

  • #2
tnich
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If it backs up too much, then the water will backup into the gutters which will lead to a real mess of things.
I think you have a couple of choices here: 1) you can over-design your system so that it will never back up, or 2) you can build an overflow capability into the system so that when the water backs up to a specific height, the excess water drains off into the storm sewer or someplace down-hill. If you have 100 GPM flow from your roof, your tank will fill up quickly anyway, so you will need an overflow built into the tank, too.
 
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  • #4
tnich
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I think you have a couple of choices here: 1) you can over-design your system so that it will never back up, or 2) you build an overflow capability into the system so that when the water backs up to a specific height, the excess water drains off into the storm sewer or someplace down-hill. If you have 100 GPM flow from your roof, your tank will fill up quickly anyway, so you will need an overflow built into the tank, too.
Also, rise over run of 1/80 is a little worrisome. It would only take a little debris in the pipe to create pools of water in your 80' pipe, which would make a great breeding ground for mosquitos.
 

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