What is the runoff coefficient?
Almost the only runoff coefficient that has ever been measured on properly built porous pavements is zero: There is no runoff, because the surface permeability is so high. But surface runoff coefficient does not take into account the limited capacity of the pavement’s base reservoir: In a long, intense storm, the base could become saturated and overflow, either across the surface or through a perforated drainage pipe if one is provided. At that point, the pavement would in effect be generating runoff. So it would be prudent to use some positive number—not zero—for the runoff coefficient. An example would be to set the runoff coefficient equal to that of the local jurisdiction’s “predevelopment” condition, which might be forest, meadow, or grass. To assign a coefficient larger than predevelopment would be arbitrary. A predevelopment grass surface generates some runoff during large storms, so it provides a valid analogy for porous pavement hydrology. Q: How much credit should be given fo
