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HomeMy WebLinkAboutControlling Stormwater RunoffControlling Stormwater Runoff Water needs a place to go! When it rains or snow melts, water needs a place to go. If the soil conditions are right, some water will soak into the ground, replenishing our groundwater supplies. But water that flows over hard surfaces like driveways, streets, and parking lots can create problems. First, this water (called stormwater runoff) can pick up contaminants—things like oil, fertilizer, pesticides, and dirt—and move, untreated, through storm sewers to local waterbodies. And, if storm sewers can’t handle the water or property is not properly drained, flooding can also occur. Saturated green space can also lead to flooding; for more information, see page 2. Urban impacts and ecological issues Design-related issues: Both initial urban development and redevelopment of land can impact an area’s flood potential. Initial development disturbs natural flow patterns to make way for roads, homes, and utilities. During redevelopment, if proper stormwater management and grading aren’t considered, water may be directed towards buildings. Redevelopment can also create a need for increased storm sewer capacity. Climate change issues: Our changing climate has created a higher frequency and intensity of flood events that is anticipated to increase. Rising temperatures can also result in faster snow melt, creating more spring flooding. The result is oversaturated green spaces, high water tables, and flooding of local waterbodies. The increased water can also challenge the capacity of local storm sewer systems. Environmental issues: Both improper site design and climate change can mean more water flowing to and from sites. But, groundwater levels in the area are also rising— translating to historic high water levels at many landlocked basins. Degraded soils from development and pollution also limit the ability to infiltrate and treat water on site. These factors can create additional runoff, resulting in environmental and flooding consequences. A typical water cycle involves vegetated spaces infiltrating and treating water, replenishing our groundwater, and runoff from hardscape to storm sewers or waterbodies. For more information on controlling stormwater runoff, contact the Engineering Department at 952-826-0371. Controlling Stormwater Runoff Long ago, the area that is now the City of Edina changed from open green space and wetlands to developed land— disturbing natural drainage patterns and the ability to move, collect, and treat stormwater. That change set the stage for water management challenges today. While limiting impervious areas during redevelopment may mitigate some flooding, the frequent and intense storms caused by climate change make the situation more complex. Not all green space is made to infiltrate Even without considering the change in rainfall frequency and amount, other environmental factors can limit the land’s ability to infiltrate stormwater. Some soil types, like clay, are just difficult to infiltrate, wet or dry. Rising groundwater levels also play a part. According to recent groundwater modeling, recharge to groundwater has increased approximately 37% in the western Twin Cities metropolitan area over the last 5 years. This increase is apparent at locations throughout the metro—translating to historic water levels and flooding at many landlocked basins. The high groundwater limits infiltration from basins that historically would have successfully managed the stormwater runoff. Pollutants and landscape Increased water quantity is one of the challenges we face as we move from natural to developed landscapes. But, water quality issues related to stormwater runoff create additional health and safety concerns. While we often think of pollutants associated with impervious pavement or industrial areas, it is important to remember that chemicals used in green spaces (like nitrogen and phosphorous) can also harm our waterbodies and local wildlife. Oversaturated green space can act like hardscape With heavy, intense rain events and frequent back-to-back storms, green space can sometimes result in just as much runoff as impervious pavement. Water falling on oversaturated lawns and open spaces simply cannot infiltrate the soil and, instead, creates runoff and flooding. While green infrastructure improvements such as rain gardens and green spaces are effective for smaller or more temporary water storage needs, they can’t mitigate flooding during heavy, intense, and frequent storms.Saturated soils create flooded yards and have potential to add to stormwater runoff. Use of fertilizers should be minimized to avoid adding pollutants to our waterbodies. Native landscapes are great low-maintenance alternatives that limit the need for fertilizers. To find out if your soils can properly Infiltrate stormwater, use soil and infiltration tests, like the one shown above (https://stormwater.pca.state. mn.us/index.php/Determining_soil_infiltration_rates) For more information on controlling stormwater runoff, contact the Engineering Department at 952-826-0371.