Q. What Is an Impervious Surface?

A. An impervious surface is a surface covering or pavement of a developed parcel of land that prevents the land’s natural ability to absorb and infiltrate rainfall. Impervious surfaces include, but are not limited to: rooftops, walkways, patios, driveways, parking lots, storage areas, impervious concrete and asphalt, and any other continuous watertight pavement or covering.

Q. Does Pervious Paving Count as Impervious Surface?

A. Pervious paving, including pavers with permeable openings and seams, is not considered impervious if it is underlain with pervious soil or pervious storage material, such as a gravel layer that is sized to hold the volume of stormwater runoff specified in Provision C.3.d of the MRP (80 percent of the average annual runoff). Guidance for calculating this amount of runoff is provided in Section 5.1 of the San Mateo
Countywide Water Pollution Prevention Program’s (Countywide Program) C.3 Technical Guidance.