Manning's roughness coefficient (n):

For smooth-wall interior corrugated plastic, n=0.012

For 3, 4, 5 and 6 inch single-wall corrugated plastic, n=0.015
For 8 inch single-wall corrugated plastic, n=0.016
For 10 inch single-wall corrugated plastic, n=0.017
For 12 and 15 inch single-wall corrugated plastic, n=0.018
For 18 and 24 inch single-wall corrugated plastic, n=0.020

For clay or concrete drain tile, n=0.013

Source: Corrugated Polyethylene Pipe Association (2000) and ASABE EP260.5 FEB2015 (R2019)

Area drained (Ad) calculation

Ad= Drain spacing × (Total lateral length + Total main length)

Rate of water movement through the drainage pipe network.

Choose a drainage coefficient that is greater than or equal to drainage intensity for best performance.

To learn about drainage intensity, watch this video