Dear All,

I have some questions regarding SS C4 Rainwater Management: Natural Land Cover conditions, option 2.

According to LEED v4 BD+C web-based reference guide:

  • The recommended method to calculate land use runoff volume is the “Small Storm Hydrology Method”, which takes into account the rainfall depth, area of land use and Runoff Coefficient (Rv);
  • As an alternative for detailed/sophisticated calculations, LEED web-based guide suggests that Runoff Coefficient (Rv) can be calculated according with following equation: Rv= 0.05+0.009(I), where I = percent impervious area.

The questions are the following:

  1. When calculating runoff volume associated with the natural land cover conditions is it acceptable to simply consider 0% of impervious area (Rv = 0.05) without further justification (since this is a wort-case scenario)?
  2. When calculating runoff volume associated with the postdeveloped conditions:
    1. can we assume that rainwater management structures (e.g., retention basins) are completely dry prior to the rainfall event? If not, how shall this calculation be performed?
    2. How shall we classify a green roof: impervious (Rv = 0,95), pervious (Rv = 0.05) or any intermediate Rv value?

Thanks for your attention. I'm looking forward to your reply.

Best regards,