Stormwater Mitigation

In many Midwest cities like Cincinnati, combined sewer overflows (or CSOs) affect the health of the region’s waterways. During flooding or times of heavy precipitation, rainwater mixes with sanitary waste in combined sewer pipes and is discharged directly into rivers and streams without treatment. This makes the water unsafe for people to be around or use it for recreation purposes.

If left untreated or improperly addressed, vacant lots throughout the city can contribute more rainwater runoff, adding to an already overburdened sewer system. However, if designed correctly, vacant lots can serve as a network of low-impact, natural solutions that address the rainwater issue by reducing rainwater runoff volume and possibly improving water quality. At some urban sites with extremely compacted soils, it will be beneficial to allocate additional resources to aerating the soil sufficiently to encourage water filtration.

Vegetation can be used as a sustainable, low-cost solution to alleviate excessive runoff and improve water quality. The following section further discusses cost-effective ways to utilize vacant lots as solutions to the city’s rainwater runoff problems.

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Site Strategies for Managing Rainwater


Rainwater Runoff Coefficient

RunoffRunoff coefficient is a unitless value that relates the amount of rainwater runoff to the amount of rainwater received. Impervious, paved areas have a larger value than well-draining, vegetated areas. The diagram below characterizes the runoff coefficient of each surface condition.

Reducing impervious surface area in urban areas can decrease rainwater runoff to an already over-burdened sewer system and prevent flooding of natural stream systems. On vacant lots, removing impervious surfaces, including existing built and hardscape elements (e.g., driveway, parking area, patio) is the first step toward increasing on-site infiltration.

The diagram to the right shows the runoff coefficient for common development typologies. Dense urban development has a larger value than single-family residential neighborhoods and parks. Thus, focusing on reducing impervious surfaces in urban environments would have the greatest impact on managing rainwater runoff.