The secret to most Rainscaping practices, including the Rain Garden is a good quality soil. To manage the water quality event in Iowa, work to infiltrate rainfall runoff generated by the 1.25" rain from the targeted "watershed," which could be your rooftop or driveway. A 1.25" rain equates to .78 gallons of water per square foot of drainage area.
In determining how big of a rain garden to build use the "rule of thumb" of 7 - 20% of the impervious surface area (ie. roof) based on infiltration rates of soil.
To learn more please refer to the Iowa Rain Garden Manual; however, here are a few musts for a successful rain garden:
- Soil infiltration rates should be greater than .50" per hour, any less and the water will pond for too long.
- They should be located at least 10 feet from building foundations.
- The bottom of the rain garden should be level from side to side and front to back - no slope.
- There should be at least 6" of ponding depth, allowing the runoff time to infiltrate into the ground.
- Plant plants with deep roots to help maintain soil quality and good infiltration / percolation rate.