How are stormwater charges calculated

Stormwater charges are typically calculated based on the impervious area of a property. Impervious areas are surfaces that do not allow water to penetrate, such as roofs, driveways, parking lots, and sidewalks. The rationale behind this calculation is that these surfaces contribute to increased stormwater runoff.

Here is a simplified explanation of how stormwater charges are calculated:

1. Determining Impervious Area: The first step is to measure or estimate the total impervious area on a property. This is usually done by using aerial imagery, property surveys, or other assessment methods. The impervious area is typically measured in square feet or square meters.

2. Assigning Equivalent Residential Unit (ERU): To simplify billing and ensure fairness, stormwater charges are often standardized based on an Equivalent Residential Unit (ERU). An ERU represents the average impervious area of a typical residential property.

3. Calculation: The impervious area is divided by the ERU to determine the number of ERUs associated with the property. For example, if the property has an impervious area equivalent to 2 ERUs, the stormwater charge would be calculated as 2 times the rate associated with one ERU.

4. Applying Rates: Once the number of ERUs is determined, it is multiplied by the stormwater charge rate set by the local stormwater utility or municipality. The stormwater charge rate is typically expressed in dollars per ERU per billing cycle (e.g., monthly, quarterly, or annually).

5. Additional Factors: In some cases, there may be additional factors considered in the calculation. This could include discounts or credits for implementing stormwater management practices on the property that reduce runoff or fees based on the property's classification (e.g., residential, commercial, industrial).

It's important to note that stormwater charge calculation methods can vary between different municipalities or regions. Local stormwater management authorities or utilities establish the specific rates, formulas, and considerations used for billing. It's recommended to consult the local stormwater utility or municipality for precise details on how stormwater charges are calculated in a particular area.