Green Roof Design

By now I’m sure everyone (engineers and regular folks alike) have seen or heard about green roofs.

(from treehugger.com green roof post – give it a click to read)

As an exercise in “let me prove to myself what I already know” I have spent some time trying to understand what the effect of a green roof is on a drainage network.

Over the past year I’ve seen the number of green roofs actually installed on projects increase significantly. Sure we were discussing them five years ago, but now we’re starting to put them in and they are seen as more of an asset now.

But with this increase in popularity I’ve had an increase in interest from both clients and other consultants asking me, “If we install a green roof, how much will it reduce the attenuation?”. To which my stock response was usually, “they make a big difference in the smaller return period events but will not provide much benefit in the larger design events we have to design around (i.e. 100 year return period with 30% adjustment for climate change)”.

With Micro Drainage’s release of Win Des 11.4 last year they even supplied a mechanism for adjusting the runoff from a roof area to include for green roofs. So I thought I’d sit down and try and understand what Micro Drainage have done to accommodate green roofs into their Simulation software.

Essentially, what the software does is take the drainage area of the roof and cut it into small bits that it spreads out over time. Micro Drainage refer to this as a Time Area Diagram and it is effectively a longer time of entry.

The equation for the percentage of roof area looks like this:

And a graph of the curve looks like this:

Generally the water that lands on a roof will enter the drainage system within five minutes of touching down. As the graph illustrates, a green roof will only discharge around 18% of its area in the first five minutes. With the rest spread out over time. This means that the peak discharge from the roof will not coincide with the usual peak flows from a hard standing site. Simples.

Micro Drainage caveat their whole help file with a little blurb about where the data came from. Apparently research at the University of Sheffield where they modeled different rain events through different roof systems and plotted the best fit line you see above. It goes on to make outrageous claims about us being the engineers and them just designing the software and that we should always satisfy ourselves with our own designs. This goes without saying. The fact that Micro Drainage have added a function to their software does not mean it is covered by any enforcable standard.

So the input in Simulation looks like this:

The help files say that 5% of rainfall that falls onto the green roof will stay there. And it gives further guidance that you can expect 1mm (winter) to 3mm (summer) of evapotranspiration per day off the roof. Once you confirm the K value you think appropriate it then cuts your roof area up over time for this drainage area.

I have gone on to model a theoretical network over a range of different scenarios. I won’t post all that today though as this one has gone on too long already!

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