Vegetated areas

Patricia, on item 1 I agree with your calcuations. On item 2, it gets more complicated since all green roofs are different—I think you could use some input from a civil engineer on that.

Hi, two questions that came to my mind when reading the above comment: 1 -when calculating the treated run-off, do you include the water volume in the calculation that infiltrates into the ground/evaporates and also the volume that leaves the area? I mean, in case of a vegetated area where runoff coefficient is 0.1, do you get the credit without any additional BMP, saying that the 90% is treated with 100% efficiency, equaling a 90% average; or do you only get the credit if the actual runoff (10% of total rainfall) flows through a BMP of 80% efficiency before leaving the site? 2 - What is the difference between the removal efficiency of the flat vegetated area and i.e. a grass swale? Grass swale should capture water and infiltrate it into the ground - however, average efficiency is only 60% according to EPA. Can water infiltrating into the ground really be considered to remove 100% TSS?

1.The curve number used to generate a runoff volume already takes into account the volume that infiltrates. A higher CN produces a higher runoff volume. 2. Your question regarding removal efficiencies is a long-standing argument with any regulatory agency. 85% TSS is typically what I see/use with any type of vegetated BMP. Definitely not 100% with any type of open-channel flow.

Thanks Michael, So you say that in question 1 that you do have to treat the volume of runoff, even if the infiltration is around 90%?

If you are infiltrating 90% of the sites runoff, through a BMP with 80% TSS removal, you are good.