Gray vs. Green

Stormwater Treatment

Urban Green Infrastructure

Cities are often constructed from materials which are waterproof, often called gray infrastructure. To offset the negative consequences of large areas of gray infrastructure, we use urban conservation practices to control rainfall runoff and flooding.

Rural Green Infrastructure

Rural conservation practices in Iowa are usually added to private land. Conservation practices allow farmers to protect their soil and water.

Financial Assistance

Unified Sizing Criteria

Gray vs. Green Infrastructure

Rainfall Event Classifications

Most communities in Iowa adequately manage the Qp and Qf level storms. In order to improve Iowa’s water quality and provide additional protection from flooding, further management of WQv and CPv classes is needed.

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 and smaller rainfall events, to the less frequent flooding events. Design specifications for all storm events can be found in the ISWMM posted on the Iowa DNR website.

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.


Long-term inspection and maintenance is critical to ensure proper performance and aesthetics of GI practices. Project plans should include short and long-term maintenance schedules and property owners should be educated about typical design features to perform typical maintenance tasks. 

BMP Loans

Green Infrastructure (GI) practices, such as bioretention cells, are engineered to capture, infiltrate, cleanse, and detain the rain. The practices are also considered “Best Management Practices” which have available funding for cost-share.


One example of a treatment train might include biocells in a subdivision that drain to a bioswale system, which then 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.


Staff Profiles







Staff Profiles


Employment Opportunities

No job positions are available at this time. We appreciate your interest in Iowa’s Polk County SWCD.

Polk Soil and Water Conservation District

1513 North Ankeny Blvd., Suite 3

Ankeny, IA, 50023

(515) 964 1883 ext. 3

Monday – Friday

7:30 am – 4:00 pm

Polk Soil and Water Conservation District is an Equal Opportunity Employer and Provider.

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