Bio Swale calcs in Phoenix AZ

Not to shoot holes in your question, but these are things you need to figure out in order to design your swale:

1. It’s a 100 year flood, not a 100 year storm (that was Point Break)

2. By “retention” I believe you mean “detention”. Retention areas hold onto water. Detention detains a given amount of water for a given period. I spent a fair amount of time in detention in high school, so I’m familiar with the distinction. Thank god it wasn’t “retention” otherwise I’d still be there. (that’s a handy way to remember the difference)

3. Bio-swales neither “retain” nor “detain”. They move water from point A to point B. They are a “green/sustainable/environmentally sound” (choose your buzz-word) alternative to traditional, below ground, storm water management (drains/pipes). It better be 1% grade, minimum, otherwise you have a retention/detention pond.

4. You will be calculating how much water your swale can hold in cubic feet not square feet. And you will be determining how fast that water will be moving in Cubic Feet Per Second. Slope, course, soil-type and vegetation will all influence how fast the water moves. It may move the water to a detention/retention area but that’s another design issue you’ll have to address. 

5. A 100 year flood event is not based on a specific rainfall amount but rather a rise in water level. Especially in AZ an area can get little/no rainfall yet experience a 100 year flood. I.E. it rains in the mountains, multiple drainages converge and send a torrent of water into town. (flash flood – the nature of arroyos). You need to figure out how much water your swale will need to accommodate.

It’s pretty involved stuff… I’m guessing (I hope) you’ll need an engineer to do the calculations/sign off on it. You’re job should be to make it designerly/ an amenity and get the calculations as close as you can. You make it a work of art, the engineer makes your work of art, work. But get it close so that you represent our profession and respect theirs… Here are a few sites to get a better idea of what’s involved.

http://www.brighthub.com/engineering/civil/articles/52905.aspx

http://extension.ucdavis.edu/unit/center_for_water_and_land_use/soil_based.asp

http://pepi.ucdavis.edu/mapinfo/mapserv/biosize.html