Bioretention cells or basins are engineered WATER QUALITY TREATMENT systems often required in new development to curtail runoff and reduce pollution. Typically, bioretention basins are designed and constructed in order to meet specific water quality criteria, such as total suspended solids (TSS), nitrogen and phosphorus removal from stormwater runoff. Bioretention basins must be sized and designed in accordance with standard specifications. Construction materials typically include perforated underdrain pipes placed in washed stone, filter fabric, engineered soil, hardwood mulch and vegetation. Details
Backyard rain gardens are considered landscaping projects and can vary from small and simple to elaborate. Rain gardens are shallow depressions designed to intercept, treat, and infiltrate stormwater before it runs off into neighbors' yards, storm drains or waterways. Rain gardens can use native soil, but usually will require amendments to the soil. Plants used in rain gardens are native to the region and help retain pollutants that could otherwise harm nearby waterways. The philosophy with rain gardens is that even small rain gardens can make a difference for our water quality, as long as they are properly placed and erosion control measures are taken during construction and in the design for flow into and overflow out of the finished garden.
There are many guides online. Chapel Hill Stormwater Management favorites are listed below. Construction of rain gardens is basically the same in all areas of the country. However, keep in mind that you are in the Central Piedmont region of North Carolina and may have to adapt certain directions to soil and plants found here. Some helpful tips:
1) Choose native plants tolerant to BOTH flooding and drought that grow well in North Carolina. This will reduce the need to water and fertilize. The purpose of a rain garden is to remove nutrients from runoff before it reaches streams or storm drains, not to add nutrients!
2) Remove heavy clay and replace with a well-draining mix (50% screened sand, 30% topsoil, 20% compost mix is recommended).
3) Plant so that root balls are in the soil, not just in the top layer of mulch.
4) Sizing a backyard rain garden can be confusing. A general rule of thumb for our type of soil is to make the 3" deep rain garden (or series of rain gardens) 25%-30% of the size of the area draining into it. If it is 6" deep, you can usually make it about 15%-20% of the drainage area. Smaller rain gardens will still work, but may not have the capacity to hold runoff from a heavy storm, and will overflow.