LEED credit 6.1, Storm Water management: Rate and Quantity, specifically requires that if existing imperviousness is greater than 50% a storm water management plan is to be implemented that results in a 25% decrease in the rate and quantity of stormwater runoff. Several calculation methods are utilized in standard engineering design to quantify these values. The LEED credit 6.1 work sheet uses imperviousness as a simplified method for quantifying storm water rate and quantities. This is a uniform and simple method to assess if the credit requirement is met, but since all design variables are not taken into account with this method the results are not necessarily consistent with engineering design calculations. Typically, the Rational or Santa Barbara Unit Hydrograph (SBUH) methods are used to calculate storm water runoff rates. Both of these methods are dependent upon several variables: Pervious/impervious land areas, time of concentration (Tc), runoff curve numbers (CN), and the design storm. Each of these variables has an impact on the calculation results and is required for an accurate determination of storm water rate. The SBUH method is typically used for storm water runoff quantity calculations. It determines a runoff hydrograph for an urbanized area, which can then be used to calculate the runoff volume. This method has been approved by the City of Portland, Oregon, Bureau of Environmental Services for determining runoff when doing water quality and quantity calculations. Most other jurisdictions in our region have adopted this calculation method, as well. As mentioned above, the variables involved have an impact on the calculation results and an engineered assessment allows for a more accurate determination of storm water runoff rate and quantities than the simplified LEED imperviousness method. Based on the above, we request the council accept storm water calculations provided by a licensed engineer as an alternate to the net imperviousness calculation methodology currently required when submitting for this credit. The intent of credit 6.1 would be met, but allows for professional judgment in assessment and calculations for a more accurate representation of the storm water characteristics of a development site.
Any defensible calculation methodology which demonstrates a 25% reduction in rate and quantity of stormwater runoff for the 1.5 year, 24 hour storm on a site with greater than 50% existing imperviousness is acceptable for achievement of SSc6.1. To achieve this credit, the LEED Reference Guide requires that declaration and demonstration of this reduction be provided, along with determination of existing site imperviousness. The methods you propose, as with all other methods, are only acceptable if they meet these standards.