Fees can be based on a variety of standards, but the Commissioners approved the recommendation of the City’s consulting engineer to base the fee on the amount of impervious surface on a developed residential property, which is a common standard. An index called the “equivalent residential unit” or ERU is used to compare the stormwater runoff generated by different size properties with different amounts of impervious surface. (Impervious surfaces are those disturbed or hardened surface areas that prevent or limit the natural entry of water into the soil and include rooftops, sidewalks, walkways, patios, driveways, parking lots, detached garages/sheds, compacted gravel/soil surfaces, awnings or other coverings.) After the engineer’s analysis of sample properties in Belle Meade, the ERU was determined to be 12,200 square feet of impervious area. As approved by the Commissioners, the monthly stormwater user fee rate per single ERU is $7.47. Based on the engineer’s analysis of lot sizes in Belle Meade, residential properties were divided into three categories according to square footage. The data on lot square footage was obtained from the Metro Nashville/Davidson County tax rolls. The table below indicates the stormwater user fee for each category of residential lot sizes: