Help me determine drain pipe diameter

Old 09-02-21, 12:08 PM
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Help me determine drain pipe diameter

Currently four 3" pvc drainage pipes (for three downspouts plus one walkway drain grate) end together halfway down a hill causing severe erosion. I want to extend the pipes to the bottom of the hill. For esthetics and to save work my plan is to connect the pipes and have only 1 drainage pipe from the current drainage pipe ends to the bottom of the hill.

I'd like to use a corrugated pipe but am not certain what diameter pipe will handle the flow. I've tried using online calculators and every one shows 3" is fine but that doesn't seem sufficient. I'd appreciate assistance on determining whether 3" corrugated pipe is adequate.

The roof in that area is 1,100 square feet and the walkway is 50' long. My community averages 50" of rain annually but we just got 8" in 6 hours so I'm wondering if the historical calculators are accurate. The hill slope is approx. 10 degrees. Any input is appreciated.
Old 09-02-21, 08:14 PM
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You want to extend four 3" PVC drain lines.
I'd use a 4" PVC line. I'm not big on corrugated as it has a tendency to clog.

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