What slope percentage indicates rapid surface drainage with cover?

A slope percentage greater than 14% indicates rapid surface drainage with cover due to the steepness of the terrain. Steeper slopes promote quicker runoff of water because gravity drives water to move downward more rapidly, reducing the time water remains on the surface. This quick drainage is particularly pronounced when there is vegetation or ground cover present, as the plants can slow down the water momentarily but do not significantly impede its overall movement off the slope.

In this context, slopes categorically less than 14% would either retain some water longer or allow for different drainage characteristics. For instance, slopes between 0-5% tend to have much slower drainage, while those between 5-14% can vary in drainage speed but generally still hold more moisture longer than what is seen in steeper slopes. While a specific percentage like 9% falls within the slower drainage range, it does not reach the rapid drainage outcome that is characteristic of slopes exceeding 14%. Thus, slopes greater than 14% are well recognized for their capacity to facilitate rapid surface drainage.