Impervious surface is defined as a constructed hard surface that either prevents or retards the entry of water into the soil and causes water to run off the surface in greater quantities and at an increased rate of flow than prior to development. Examples include but are not limited to, rooftops, sidewalks, patios, decks, driveways, parking lots, storage areas and concrete, asphalt, or gravel roads, or tightly compacted soils. Surface area of a lot physically separated by a public or private road right-of-way or access shall not be included for the purposes of the impervious surface calculation.
When looking at reducing impervious surface, the location and size are important. Removing impervious surface from sensitive areas such as the bluff, bluff impact zone, and shore impact zone is extremely important in order to help reduce stormwater runoff. Impervious surface causes stormwater runoff to concentrate and increase in velocity which as a result increases the potential for erosion issues and negatively impacts water quality.
The image above shows how surface water will flow off of impervious surfaces.