Historically, stormwater utilities (SWUs) in Virginia and around the country have primarily utilized the impervious surface rate methodology to develop the billing unit by which the SWUs fees are charged. In its simplest form, the SWU uses either a sample of several hundred single-family residential (SFR) parcels or an entire SFR parcel planimetric dataset, to establish its “billing unit” based on the average impervious cover found on those parcels. From there, multiples of that billing unit, often referred to as the Equivalent Residential Unit (ERU), may be used to assess SWU fees for the other parcel types found in the community. Fractions of units may be applied to parcels with townhouses or condominiums, multiple units may be applied to non-residential properties, such as commercial parcels.

In recent years, additional, feasible possibilities for calculating SWU billing units in Virginia have emerged. First, data collection capability and data quality continue to improve. Data components that once took significant time to construct, such as impervious surface GIS layers, are now much more easily constructed from planimetric data associated with photogrammetry and pictometry that now includes more precision and detail than was feasible previously. In addition, with the introduction of updated stormwater management regulations in 2014, Virginia adopted the Runoff Reduction methodology for calculating stormwater quality and quantity impacts from new and re-development projects. Emphasizing biological nutrient uptake and infiltration practices, while still recognizing traditional stormwater management BMPs (i.e., ponds, etc.), the Virginia Runoff Reduction (VRR) spreadsheet tool quantifies water quality volume and specific nutrient and sediment runoff rates based on site inputs, as well as calculating the impacts of a specified BMP, or menu of BMPs, top reduce those impacts on a proposed site.

Using these catalytic factors, this presentation explores the potential for the development of an Equivalent Hydrological Area (EHA) assessment, to establish runoff characteristics of an entire parcel – rather than simply the impervious cover – for purposes of SWU rate creation and parcel billing. The author will present the basic concepts of the EHA methodology and the potential to use the VRR calculations to establish such a methodology, while describing some of the potential advantages and disadvantages of such a system.