Comparing the Hydrologic Performance of a Bioretention Cell with Predevelopment Values

Abstract

Bioretention cells have been found to improve the hydrologic and water quality performance of urban impervious areas. This study recorded continuous hydrologic data from a bioretention cell in Silver Spring, Maryland, over a period of 2 years. Two evaluation methods were used to assess bioretention performance, curve number (CN) volumetric analysis, and flow-duration flow regime analysis. CN-derived Woods B and C land uses were used to analyze cell volumetric performance, and data from a nearby forested stream were used to evaluate the cell flow regime. A CN of 75 was fit to the cell outflow with relation to rainfall depth. A larger cell, from 4.5 to 8.3% of the drainage area, was required to match the CN of woods land use. Flow duration comparisons between cell outflow and forested streamflow data suggested that bioretention may not match the predevelopment hydrologic regime despite performing similarly volumetrically. The overall natural hydrologic regime, not only peak flows and flow volume, of an area should be used both as a design factor as well as a performance metric for bioretention cells and other low-impact development (LID) facilities.