Prior to European settlement, Iowa’s native prairies and savannas maintained soils with high organic matter and lots of pore space. This allowed the landscape to absorb rainfall, while shedding little runoff. Most rainfall infiltrated into the soil, where it recharged groundwater flow. Clear flows of groundwater fed and maintained Iowa’s river and lakes.
Iowa’s soil resources have been significantly altered by tillage-based agricultural practices and land development in urban areas. Consequently, less rainfall infiltrates into the landscape and more surface runoff occurs today.
Urban landscapes are dominated by impervious surfaces such as roadways, parking lots, and rooftops. Urban green spaces often feature turf grass with short roots over compacted soils. Such impervious and compacted surfaces decrease the amount of stormwater that can infiltrate into the soil, resulting in higher volumes of stormwater runoff. Stormwater carries pollutants into storm drains, which discharge directly into local streams and lakes. Urban runoff causes flashy flows that erode stream corridors and compound local flooding problems.
Watershed Concerns: Quantity and Quality
Runoff Quantity- Flooding
Iowa has six major river basins. River basins are comprised of smaller watersheds- expanse of land drained by smaller streams, rivers, lakes or wetlands. The majority of land within these watersheds is used fir agricultural purposes. In general, land devoted to row crop agriculture sheds runoff after 1.25 inches or rain. Urban areas located downstream of large watersheds in Iowa are heavily affected by this agricultural runoff.
In 2008, more that 90% of the 3.6 million acres in the Cedar River Watershed above Cedar Rapids was comprised of rural land, primarily used for planting of corn and soybeans. Consequently, the runoff from these agricultural fields was the primary contributor to the record flood flows that devastated Waverly, Cedar Falls, and Cedar Rapids. It is in the best interested of cities to partner with agriculture to restore the landscape’s ability to absorb more rainfall and shed less runoff.
In smaller watersheds, stormwater runoff has a significant impact on the flash flooding of local streams in areas of urban development. Fourmile Creek Watershed in central Iowa measures almost 77,000 acres, and 36 percent of this watershed is urban land. Urban runoff in this area contributes to downstream flooding.
Runoff from both urban and agricultural areas has a direct effect on water quality. Most water quality efforts have been directed at the agricultural sector in the past. Now, urban runoff is being recognized as a significant contributor to water quality degradation, especially in smaller watersheds. Major pollutants in stormwater include sediment from construction sites and eroding urban streams, nutrients from excess fertilizer applied to lawns, bacteria from pet wastes, oil, grease, heavy metals from vehicles and chloride from road salt applications. In some communities, overflows of combined sanitary and storm sewer flows discharge pollutants into receiving streams.
Unified sizing criteria
Traditional stormwater management focuses on large storm events, to reduce impacts of large flooding events. While this is important, these structures do not address water quality concerns or smaller storm events causing excessive streambank erosion in urban areas.
When planning and designing for post construction stormwater management, considerations 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:
Water Quality Volume (WQv) treats runoff from the 1.25 inch or less rainfall, 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 bank full flows and helps minimize downstream channel erosion.
Overbank Flood Protection (Qp) provides peak discharge control for 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 practices can also be effective techniques. Most communities adequately mange the Qp and Qf. In order to improve Iowa’s water quality and provide additional protection form flooding, further management of the WQv and CPv is needed.