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bioretention Tag

What I’ve Learned About Bioretention: Part 2

Read Part 1 of this post here. 2. Bigger is not better Like most everything we design, when big things fail, they fail in a big way. Bioretention works best in applications where the drainage area to an individual cell is less than about a quarter acre. Larger bioretention areas are more likely to fail due to erosion because of larger flows, creation of low spots due to variations in the surface, and clogging of the surface layer. This can be avoided by dividing the area into multiple cells with smaller drainage areas. [caption id="attachment_6401" align="aligncenter" width="770"] Larger basin divided into three cells[/caption]   The largest bioretention area I know of is an example of this. Designed by others, it collects runoff from several acres at a highway rest stop. The measures designed to evenly distribute incoming flow have been overwhelmed by large flow rates, the engineered soils mix layer does not drain quickly enough, and...

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What I’ve Learned About Bioretention: Part 1

Bioretention areas (also sometimes called rain gardens) are a very useful tool in the toolbox of stormwater treatment BMPs. They provide a high level of pollutant removal and runoff volume reduction, and, if properly designed, require little maintenance. Bioretention was developed to mimic the hydrology of a natural forest. It consists of a shallow (typically 4-8 inches) basin with 3 inches of mulch and a variety of selected plants on the surface that can accommodate periodic flooding and drought. Under the mulch is a layer of engineered soil mix (typically 12-36 inches). The basin collects surface runoff and filters out nitrogen and phosphorus (nutrients for plants, but pollutants in stormwater) as water passes through the engineered soil mix. Water is absorbed by surrounding soils or is collected by an underdrain. The plants convert the nitrogen and phosphorus collected in the engineered soil mix into woody plant matter and leaves. If designed and...

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Putting the LID on Stormwater Runoff

The following post is provided by guest blogger Thomas Powers, P.E., LEED AP, CFM, CPESC, A Project Manager with Wight & Company in Chicago Illinois and former colleague of The Inlet's Carolyn Howard. [caption id="attachment_688" align="alignleft" width="234" caption="Thomas Powers, PE, LEED AP, CFM, CPESC"][/caption] How many gallons of rain do you think falls each year on just one acre of land in Norfolk, Va.? Ten thousand? One hundred thousand? Would you believe more than one million? Unfortunately, most of that water isn’t absorbed by the land and instead becomes stormwater runoff, carrying debris, chemicals, sediment or other pollutants. The U.S. Environmental Protection Agency (EPA) ranks urban runoff and storm-sewer discharges as the fourth most prevalent source of impairment of our lakes, streams and rivers. The current best practices in stormwater management is called low impact development (LID), which refers to a comprehensive land planning and engineering design approach that emphasizes conservation and the use of...

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Cisterns and Stormwater

[caption id="attachment_206" align="alignleft" width="200" caption="Rainwater collection system at Northside Family Learning Center, Richmond, VA"][/caption] Cisterns make sense, why not use stormwater as resource and reduce the demand on potable water supply systems. I have successfully designed cistern systems, I encourage the use of cisterns, and I look forward to the standardized methodology to calculate benefits in the proposed regulations http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html . However, I feel that there are some misconceptions about cisterns that need to be addressed: If the cistern is used only for irrigation, then the cistern does nothing for reducing stormwater impacts in the 6 or 7 months out of the year when there is no irrigation. Cisterns used solely for irrigation need to overflow to another measure, such as bioretention.  Is there a stormwater benefit for a measure that works for only half of the year  Rainfall is unpredictable. In the last ten years in Richmond, the annual rainfall varied by...

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