The most fair and equitable basis for calculating stormwater charges is based upon the amount of a property's impervious area, which can be measured and has a direct and accepted relationship to the estimated amount of water that leaves a property as stormwater runoff. A property's impervious area is the most significant factor affecting both stormwater quality and quantity because stormwater cannot be absorbed by these surfaces and must be managed through some sort of stormwater system.
A unit of impervious surface area on an average single-family, residential property, or "equivalent residential unit" (ERU), is the quantity used for assessing the new stormwater charges. The size of one ERU was determined by averaging the impervious surface areas existing on all single-family properties around the city. The value of one ERU in the City of Johnston is set at 4,000 square feet of impervious area. Thus, the stormwater utility fee system is based on the total impervious surface area divided by 4,000 square feet; properties with greater than 4,000 square feet (1 ERU) of impervious surface are charged for more than one ERU.
For example: A property with 8,000 square feet of the total impervious surface would be billed for 2 ERUs. In all cases there are caps on the maximum number of ERUs that can be billed to the property, the cap for single-family residential properties is 4 ERUs and the cap for commercial, industrial, nonprofit, and all other properties is 40 ERUs.