# Friction Loss for gravity fed rainwater downspouts/pipes?

• saxman2u
In summary, the conversation discusses the calculation of the height of PVC downspouts and rainwater tanks for a rainwater collection system. The importance of this is to maximize water storage while considering factors such as friction loss, ft of head, length of pipe, and elevation change. The question also arises whether the Hazen-Williams equation applies for vertical downspouts and if there is a different formula to use. Another consideration is the possibility of overflow and the need for an overflow system built into the tank. The conversation also mentions the concern of debris in the pipes creating pools of water and becoming a breeding ground for mosquitos.
saxman2u
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?

saxman2u said:
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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Can you please provide a diagram?

tnich said:
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.

## 1. What is friction loss and how does it affect gravity fed rainwater downspouts/pipes?

Friction loss is the loss of energy that occurs when water flows through a pipe due to the surface of the pipe and the fluid rubbing against each other. This can result in a decrease in flow rate and pressure within the downspout or pipe.

## 2. How do I calculate friction loss for gravity fed rainwater downspouts/pipes?

Friction loss can be calculated using the Hazen-Williams equation, which takes into account the diameter, length, and roughness of the pipe, as well as the flow rate and velocity of the water. Alternatively, there are online calculators and tables that can help determine friction loss for specific pipe sizes and materials.

## 3. What are some common causes of friction loss in gravity fed rainwater downspouts/pipes?

Some common causes of friction loss include obstructions or bends in the pipe, rough interior surfaces, and changes in pipe diameter. Additionally, the velocity of the water can also impact friction loss, with higher velocities resulting in greater frictional resistance.

## 4. How can I minimize friction loss in gravity fed rainwater downspouts/pipes?

To minimize friction loss, it is important to use smooth, properly sized pipes with minimal bends and obstructions. Regular maintenance and cleaning of the downspouts and pipes can also help reduce friction loss. Additionally, reducing the velocity of the water by increasing the diameter of the pipe can also help decrease frictional resistance.

## 5. What are the consequences of high levels of friction loss in gravity fed rainwater downspouts/pipes?

If friction loss is too high, it can result in a decrease in flow rate and pressure, which can cause problems such as clogging, backflow, and overflow. This can lead to water damage to the surrounding area, as well as potential damage to the downspouts and pipes themselves. In extreme cases, high levels of friction loss can even cause the downspouts or pipes to burst.

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