MIDUSS MODEL

[jvoisin]

Hi,

I have a green roof which I would like to use flow control roof drains on to control post development flows. First off, how would you go about modelling a green roof? I know the roof is 100% impervious beneath the green roof, but I don't think I should use this since some water will be absorbed into the plantings.

Anyone have some input on green roofs and SWM.

Thanks

 

[pinkpig]

interesting question. have you thought of SWMM? the newest SWMM5 has LID capability and I am sure green roof can be modeled.

 

[gbam]

I've never designed a green roof nor attempted to model one hydrologically; very interesting. Intuitively thinking, shouldn't there be volumetric reduction in precipitation due to the vegetation, maybe a percentage. If that is ignored and you are looking to store water on the roof standard practice would prevail, wouldn't it?

 

[cvg]

vegetation does not reduce the amount of precipitation. it does reduce the amount and rate of runoff. For conservative drainage design, you would want to use a high runoff coefficient and size your scuppers accordingly. For LID and structural design of the roof, you would want to use a low runoff coefficient and assume very little runoff. If you plan to store water than you should also consider back to back storms which could fully saturate the subgrade and fill up all storage so that your runoff coefficient is high.

 

[pinkpig]

green roof may be useful to reduce runoff for small events, like 1 or 2-yr storm, but for larger ones,its effect diminishes quickly once it is saturated. There are cases that I saw engineers overstated greenroof's effectiveness.

 

[gbam]

correct cvg, I was referring to direct runoff in inches.

 

[beej67]

Step one is check with your regulatory agency to see if they'll give you any credit whatsoever, and if so, how they would like you to present it. Many regulators want you to use green roofs, but give you no hydrologic credit for using them.

If it were me, I would not take any credit for reduced runoff volume, since your large event storms are likely to saturate the green elements. I would, however, treat the green roof as a reservoir with a flow control if I thought it was necessary to do so. How I did that would vary based on the green element design.

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