Modeling Bioretention Hydrology with DRAINMOD

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by Robert A. Brown, Graduate Research Assistant; Dept. of Biological and Agricultural Engineering, North Carolina State University, Campus Box 7625, Raleigh, NC 27695-7625,,
William F. Hunt, Associate Professor and Extension Specialist; Dept. of Biological and Agricultural Engineering, N.C. State University, Campus Box 7625, Raleigh, NC 27695-7625,,
R. Wayne Skaggs, William Neal Reynolds Professor and Distinguished University Professor, Dept. of Biological and Agricultural Engineering, N.C. State University, Campus Box 7625, Raleigh, NC 27695-7625,,

Document Type: Proceeding Paper

Part of: Low Impact Development 2010: Redefining Water in the City

Abstract: It has been documented that bioretention cells are an effective low impact development (LID) stormwater practice to remove nitrogen, phosphorus, and other pollutants at high rates, as well as to restore a site's pre-developed hydrology. However, hydrologic performance has varied greatly in past field studies because of the impact of underlying soils, physiographic regions, drainage configuration, and media depth. Development of a long-term hydrologic model that generates an annual water balance is needed to more accurately describe hydrologic performance to predict pollutant loads and to determine whether a site meets LID hydrology criteria. The only models available are either unable to run continuous simulations or do not accurately model underdrain flow for typical designs of bioretention cells. DRAINMOD, a widely accepted long-term agricultural drainage model, is being used to predict bioretention hydrology. The concepts of water movement in bioretention cells are very similar to agricultural fields with drainage pipes, so many bioretention design specifications correspond directly to DRAINMOD inputs. As a result, calibration of DRAINMOD to model bioretention hydrology is currently underway. Detailed hydrologic data have been specifically collected from two field sites over the past 24 months to calibrate the model. Additional hydrology data are available from at least 10 other bioretention cells across NC to validate the model. DRAINMOD is also unique from other bioretention models in that it accounts for evapotranspiration (ET), and the controlling factor in DRAINMOD is the drainage configuration which employs the Richards Equation. The model can be used on an hour-by-hour basis for long periods of climatological records (e.g. 50 years). Hydrologic outputs from the model include: volume of runoff, overflow, drainage, exfiltration, and ET. These outputs will allow users to examine the water balance to discern a most suitable design based on varying design configurations.

Subject Headings: Drainage | Hydrology | Hydrologic models | Retention basins | Model accuracy | Data processing | Sustainable development | Hydraulic design | North Carolina

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