Modeling Bioretention Hydrology with DRAINMOD

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 sites 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 currently available either are 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 adapted to simulate 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 for 24 months to calibrate the model. Additional hydrology data are available from 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 one of the controlling factors in DRAINMOD is the drainage configuration. The model is capable of being 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.