Rethinking Bioretention Design - Maximize in-Situ Soils, Enhance Function, & Reduce Costs.

Bioretention system design, from its beginning, has relied upon bioretention soil mixes (BSM) to perform a multitude of tasks that include regulating flow to the underdrain, filtering stormwater for a variety of parameters, and providing an environment for plants to grow in.   This has resulted in BSMs with a wide variety of materials and quantities.  For most bioretention systems, the BSM extends across the entire bottom of the bioretention system and to a typical depth of two feet or greater, with layers of aggregate below to facilitate flow to the underdrain.  This approach presents multiple concerns to consider in the overall performance of a bioretention system, including the limiting of deep-rooting plants with layers of aggregate and/or geotextiles, variability in infiltration rates with a limited ability to modify, and significant costs associated with material and labor inputs.  The City of Omaha has developed bioretention system design details that aim to address these concerns that include a limited footprint of the BSM and underdrain, installation of a valve on the underdrain, and amending in-situ soils as needed.  This presentation will discuss the evolution of bioretention systems in Omaha, NE, provide details of multiple project experiences, and a description of current and future bioretention monitoring efforts.