Development in Maryland and Virginia has exploded in recent years and we have witnessed the conversion of rural areas to urban. This conversion has created increased impervious cover, such as roads, sidewalks and roofs. This in turn has increased the amount of storm water runoff. Problems created by runoff include increased pollutants and temperatures in waterways, and increased flooding.
A current practice used to minimize the impact of storm water runoff is bioretention. Rain gardens are a form of bioretention that can be implemented on an individual level and have a significant impact on water quality locally and regionally. A rain garden collects water runoff and stores it, permitting it to be filtered and slowly absorbed by the soil.
This is a small contribution that anyone can make to help improve the water quality in our region. A rain garden should be placed so that impervious surfaces drain into the depressed area. Impervious surfaces can include a patio, driveway, roof (downspouts), a large lawn or any other surface that water does not penetrate well.
Components of a Rain Garden
A grass buffer strip around the garden will slow the velocity of the runoff and filters particulates from the runoff. A mulch/organic layer provides for the decomposition of organic material and helps in the removal of metals from the runoff.
Shredded hardbark allows the maximum surface area for binding and resists floatation and washout. The plants will use the runoff for moisture and nutrient requirements, and they should be selected for their ability to cycle and assimilate nutrients, pollutants and metals. Most authorities encourage the use of native plants in rain gardens because they’re preferred by wildlife.
A soil layer is where the plant roots will collect the moisture and nutrients for their growth. The soil layer will also absorb heavy metals, hydrocarbons and other pollutants.
A sand bed beneath the soil layer further slows runoff. The sand helps prevent anaerobic conditions in the planting soil. A ponding area or depression of the garden will provide the storage needed for the runoff. The surface area must be level for maximum infiltration.
A berm that is at least 6” of soil that works like a dam to pond the runoff.
Lots more Information
The best all-in-in resource for information about rain gardens is here, the website of the Low Impact Development Center. Thanks to them for that!