As part of the kickoff of Tulsa Partner's Green Building Resource Library on September 24th, we will be installing a raingarden at the TPi office! There's a good bit of prep work that goes into building a raingarden, so I thought I would use the blog as repository of everything we can't get into the short on-site workshop we'll conduct that evening.
I went by today and took few photos of the office. The downspouts are already disconnected, so that is one thing we won't have to do. There is one downspout on the southeast corner of the building that discharges to a sunny, grassy spot. Its visible from the street and building entrance, which is great from an educational perspective. Its also in a nice sunny spot, which gives us a good range of plants to work with. See the downspout on the right side of the roof overhang.
I then walked off the area of the roof handled by this downspout. Its about 20' across and 25' long, or 500 sf. I'll do a field check of infiltration later, but just as a starting point we could assume 6" of infiltration/24 hours, resulting in a 6" deep garden. For a garden that would handle the first 1" of runoff, that would require 500/6 = 83 sf of garden. The space available is 80sf, max. So, let's say we get more infiltration, say 8"/24hours. That would make our garden 500/8 = 62.5 sf. Probably more likely to fit the space, but we'll check the infiltration to be sure the soil can handle it.
What if we don't have the infiltration needed for this size roof? Just size the garden depth for the infiltration (depth = infiltration in 24 hours but 12" maximum). A smaller garden than your runoff volume requires just means you'll be capturing less than the first 1", and the first 0.5" is typically the most important, so that gives you a good range for the available space.
Here's another look at our site.
