Checking the soil infiltration rate on site“Bioretention cell” is the formal name for what we informally call a rain garden. Bioretention is the actual process in which stormwater is retained in a treatment area where its contaminants and sediments can be filtered out. Generally, the treatment area consists of an area of large stone or riprap where the inflow pipe or swale directs the stormwater. This area, known as a forebay, serves to slow the velocity and evenly distribute the flow of the water to help prevent erosion. The stormwater then filters into the ponding area, which is composed of a soil mix of approximately 80% sand, 20% organic material and native soil. The sand in the soil encourages rapid infiltration into the ponding area, and the organic material and clay in the native soil increases the soils water storage capacity and creates surface area that encourages the growth of microorganisms. Soil microorganisms aid in the breakdown of organic material, therefore making nutrients readily available to plants. Plant roots and organic materials in the soil help to further filter the water and hold it in place until it is able to infiltrate into the underlying soil or is evapotranspired through the plants.
In order to understand the impact that bioretention cells potentially have, we should understand what they help to prevent. When stormwater is channelized and piped into storm sewers or large-scale treatment plants, it results in an overall reduction of groundwater recharge. This leads to the long-term lowering of groundwater tables and the loss of stream flows during dry weather months. Also, when water isn’t allowed to slowly filter into the ground, it loses the chance to have the sediments and pollutants it may have picked up filtered out by soil microorganisms. Furthermore, when we plan new developments without providing areas for bioretention, the chance of “flash” flooding becomes greater.
Although the New Norris House site may seem like a small area, the benefits of treating our stormwater on site are actually quite significant. The roof area will receive over 31,000 gallons of rainwater on an average year—that is about 520 bathtubs of water that we will treat in the bioretention beds. And, hopefully, through our educational tours and community outreach, the impact of our rain gardens will become even greater.
I never thought stormwater management is that important. Homeowners should learn this, especially if they don't want to experience nasty floods. If bioretention cells could be expanded for use at a bigger scale, then the same principle could be used to prevent flooding in low-lying areas.
ReplyDelete80% sand? More than I would have guessed. Thanks for sharing this post.
ReplyDelete-Jon @ inlet filter