Pipe drainage Sizing
1. calculate area A in acres to be drained, by surface type. 1 acre is 43560 sf.
2. determine rainfall intensity rate I in inches per hour
3. determine runoff coefficient Cr for the surface type. It's 0.05 to 0.3 for cultivated lawns
4. determine how much slope you have available for drain pipe
5. calculate flow in cu.ft/sec = Cr x I x A. For 0.5 acre, 2"/hr rainfall and 0.2 Cr, it's 0.5x2x0.2 = 0.2 cu.ft/sec.
6. using flow rate and drain slope, determine the pipe size using a Manning's pipe flow Chart
(from Army Technical Manual TM 5-814-1, Aug. 1966)
"Manning's pipe flow Chart" - Google Search
[I couldn't get this document to display correctly on my computer.]
If you post your values I'll try to use my hard copy of this chart to tell your pipe size.
"Rainfall rate is generally described as light, moderate or heavy. Light rainfall is considered less than 0.10 inches of rain per hour. Moderate rainfall measures 0.10 to 0.30 inches of rain per hour. Heavy rainfall is more than 0.30 inches of rain per hour."