Abstract
Detention Basins (EDBs), an established stormwater Best Management Practice (BMP), are used to treat polluted stormwater runoff from urban areas and highways. Treatment occurs primarily through the settling of particles in the EDB. Typically, the design volume is based on a 2-year storm event and is large enough to capture about 85 percent of the annual runoff. Effluent is discharged through an outlet orifice typically sized to drain the entire basin in 24 to 48 hours. Because a high percentage of storm events are smaller than the typical design storm, the 2-year storm, the storm events pass through the EDB quickly because of the relatively large outlet orifice and receive little treatment. This project expands on previous investigations of strategies to increase hydraulic detention time and improve treatment in these devices. The strategy of interest is dividing an EDB with a partition wall, thus creating primary and secondary cells in series. The primary cell, equipped with a correspondingly small outlet orifice, captures and holds the frequent, smaller storm events. Large storm events overflow into the secondary cell and are treated there. The anticipated outcome is a longer average detention time and better total suspended solids (TSS) removal. An ExcelTM Visual Basic for Applications (VBA) computer model was created to simulate EDBs with and without a partition wall. The software can model irregularly-shaped basins and internal cells with unequal volumes. Hourly rainfall is converted to runoff using the Rational Method and routed through hypothetical EDBs. Detention times for each storm event are calculated from the time difference between the centroids of the influent and effluent hydrographs. In this project, 50 years of historic hourly rainfall data from Orange County, California were passed through a one-cell (unpartitioned) EDB and three partitioned EDBs with primary cells sized for 75, 50, and 25 percent of the design volume. Model results showed that event mean hydraulic detention times were increased in the partitioned EDBs by 64 to 94 percent compared to the one-cell control EDB. The longest detention time was achieved in the EDB with the smallest primary cell. Partitioning also increased the volume-weighted mean hydraulic detention times by 36 to 60 percent, but the longest detention time in this case was achieved in the EDB with the largest primary cell. Compared to the one-cell EDB, the total volume of water that overflowed the EDB (i.e. exceeded the design volume) over the 50-year hydrologic record decreased slightly for the basin with a 75-percent design volume primary cell, but increased for the other two cases with smaller primary cells. Direct modeling of TSS was not attempted in this project. When the calculated detention times are applied to TSS removal curves from the literature, though, event mean TSS removal was estimated to increase by up to 6 percentage points due to adding an internal partition.