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The Elmwood Park Diversion, just south of Elmwood Park Drive and west of 60th Street, was completed in the spring of 2012 as part of the CSO Program’s Aksarben Village Neighborhood Sewer Separation Project. The Elmwood Park Diversion Plan pairs the cleaning of water using green infrastructure with new pipe systems. Seven slotted-weir structures were constructed within the ravine to reduce stormwater velocity and lower erosion potential. The weir structures have vertical drops that vary from one to three feet, with three bioretention gardens located just above. At the base of the ravine, a dry detention basin collects flows into an outlet structure that connects with a bypass pipe and directs flows into Elmwood Creek. The diversion is estimated to have saved $550,000 on the project overall, which includes the costs associated with building the diversion.
The diversion pipe, built on the southeast side of the park, leads into a series of grade controlled slotted weirs that allow water to pass after reaching a certain depth. A weir is a concrete wall that is engineered to slow down large surges of water behind the wall. In between the weirs are bioretention gardens. These gardens are used to absorb some of the stormwater runoff. They contain different grasses and plants that will naturally clean and absorb excess water. During larger wet weather storm excess, any remaining water will flow downstream into a wet detention pond just north of the bioretention gardens and the weirs.
The grasses planted in this project are native to the area. The grasses, 1 to 3 feet tall, include Blue Grama, Sideoats Grama, Little Bluestem and Buffalo Grass. Wildflowers were also used in the seeding mixture and in the bioretention gardens. Over the next few years, the gardens will need weeding, watering and fertilizing. However, eventually they will require very little maintenance as they will naturally thrive.
All the plants in the project are deep-rooted, and each year the roots of the plants die. This leaves behind vertical straws of roots that carry water deep into the soil. By absorbing more water into the ground, there is less overflow to manage above ground.
The project followed the Omaha Parks and Recreation Department's tree mitigation standards, planting at least three times as many trees as were removed in the area. Fifty (50) native trees were added to Elmwood Park including Northern Catalpa, Swamp White Oak, Hackberry and Kentucky Coffeetree. All of the trees are well-adapted to the area.
The Elmwood Park Diversion Project utilizes natural ways of water flow while also implementing new systems to help control water during wet weather. The weirs help to manage the water while the gardens retain moisture. The green infrastructure not only decreases the amount of water above ground, but it also cleans water naturally and reduces the overall cost of the CSO Program.