Bioretention Cell Monitoring, Omaha Sewer Maintenance Facility

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Stormwater management methodologies are shifting from the traditional storm sewer to a more green-infrastructure approach that stresses the importance of capturing, retaining and treating stormwater. The implementation of green infrastructure uses various Best Management Practices (BMPs) to mitigate the effects of stormwater on flooding and water quality. A bioretention cell is a BMP that captures and treats stormwater, by promoting evapotranspiration and infiltration, thereby reducing the quantity of stormwater. Bioretention cells have been used for a number of years in the United States, and factors such as soil type, vegetation, evaporation rate and cell design can affect performance. Site-specific scientific data is needed to evaluate and validate green-infrastructure BMPs.

Green infrastructure (GI) is gaining consideration as an effective form of stormwater management, especially in communities like Omaha, Nebraska that are faced with combined sewer overflow issues.  The U.S. Environmental Protection Agency (USEPA) and USGS are assessing soil characteristics and monitoring GI performance in several locations nationwide to support GI design guidelines that account for factors that are unique to a region. 

One of these locations is in Omaha, Nebraska, where a cooperative monitoring project between the City of Omaha, USEPA, and USGS has been established for a bioretention cell.  Monitoring has focused on an assessment of the water balance in the cell, and a characterization of soil moisture around the cell.  This monitoring design was made possible because strong communication was maintained among the partner agencies, the design team, and the contractors before and during construction.  The implementation of monitoring has not been without challenges, and the lessons learned contribute to ongoing improvement in the form and function of GI.  Preliminary monitoring results from this study are being used to develop a practical understanding of good monitoring practices and local hydrologic sinks and sources, such as insights related to infiltration characteristics in the clay loam soils.  This information will ultimately be leveraged toward effective stormwater management. 

Sanitary and stormwater sewers in Omaha use a combined pipe system called a combined sewer overflow (CSO). When rainfall is greater than approximately 0.1 inches, the stormwater runoff exceeds the capacity of the combined sewer system causing raw sewage to be discharged into Papillion Creek. To reduce the impact of runoff during precipitation events, we are working with the USEPA and the City of Omaha on green infrastructure that uses various Best Management Practices (BMPs) to mitigate the effects of stormwater on flooding and water quality.

A bioretention cell is a BMP that captures and treats stormwater, by promoting evapotranspiration and infiltration, thereby reducing the quantity of stormwater. For this study, both a bioretention cell and permeable pavement were constructed near the Omaha Sewer Maintenance Facility (OSMF) to catch and retain stormwater from the adjacent building and parking lot.  The OSMF is located in the Big Papillion Creek watershed, which is part of the Papillion Creek Basin. The BMPs used in this study are unique because of the interaction between the permeable pavement adjacent to bioretention cell as well as monitoring equipment being used.  

To evaluate the performance of the BMPs, we are monitoring the water balance of the stormwater entering the bioretention cell.

The water balance is measured using

  • precipitation,
  • surface inflow,
  • outflow through the infiltration trench,
  • outflow through the overflow,
  • evapotranspiration, and
  • the change in stored runoff.   

The remaining component of the water balance, infiltration, is being estimated as the residual of the water balance.  In addition, the spatial characteristics of soil moisture are being monitored by an array of soil moisture probes nested at various depths and locations throughout the bioretention garden. 

Additional details about this study, and other related studies in Omaha are available from our website: Monitoring the Effectiveness of Bioretention Cells