Green Infrastructure (GI), unlike gray infrastructure, which is designed for the single-purpose of moving water through a sewer system, is designed to treat rainwater at the source through enhanced infiltration. According to the EPA, these practices can be a cost-effective and resilient approach to management wet weather impacts.
Through a combination of vegetation, soils, engineered structures, and other elements, GI practices attempt to restore the natural function of the landscape and soil. Increased infiltration reduces the amount of stormwater runoff that is carried into the storm sewer.
There are many benefits ton using green infrastructure in combination with traditional gray infrastructure. GI practices can save developers, cities, and homeowners money in the long-term. These practices also ensure less polluted drainage and stormwater runoff enter Iowa’s waterways. Other benefits include fewer localized flooding events and a decrease in stream bank erosion.
GI practices, such as bioretention cells, are engineered to capture, infiltrate, and detain the rain. Other Rainscaping practices include soil quality restoration, native landscaping, rain gardens, permeable pavers, green roofs, extended retention basins, and wetlands.
The Unified Sizing Criteria
When planning and designing for post construction stormwater management, consideration should be given to both water quality and water quantity (flood control). The Unified Sizing Criteria in the Iowa Stormwater Management Manual (ISWMM) provides a comprehensive approach to managing stormwater, from the more frequent, smaller rainfall events to the less frequent flooding events (see inside of this brochure for more information on each design criteria).
Water Quality Volume (WQv) treats runoff from the 1.25 inch or less rain, which is the most frequent rainfall
in Iowa. Managing this size of event helps reduce the most pollution.
Channel Protection Volume (CPv) manages the 1-year, 24-hour duration event. Managing this size of storm reduces bankfull flows and helps minimize downstream channel erosion.
Overbank Flood Protection (Qp) provides peak discharge control of the 5-year, 24-hour duration event. Managing this size of storm prevents downstream capacity issues and minimizes localized overbank flooding.
Extreme Flood Protection (Qf) manages the 100-year, 24-hour duration event. Managing this size of storm minimizes extreme flooding downstream. Flood management typically occurs through detention controls and/or floodplain management. Constructed wetlands and other GI practices can also be effective techniques.
Stormwater Treatment Train
Site conditions and treatment goals dictate which green infrastructure practices are implemented in series since each method targets different rainfall events. Implementing a train has the potential to greatly reduce the quantity of discharge and pollutants leaving a site.
One example of a treatment train might include biocells in a subdivision that drain to a bioswale system, which subsequently discharges to a retention pond or wetland. In an ultra urban area, where space is at a premium, a treatment train could consist of a green roof, permeable pavers, and plant filter boxes that eventually discharge to an underground detention system for larger rainfall events.