Performance Monitoring of A High Flow Bioretention System and Permeable Paver-High Flow Bioretention System in North Carolina

North Carolina State University (NCSU) conducted a third-party analysis of the sediment and nutrient removal performance and hydrologic mitigation of a stand-alone Filterra Bioretention System and a Filterra BioPave System, which is a Filterra Bioretention System coupled with permeable interlocking concrete pavers (PICP). The project was installed in Fayetteville, North Carolina in 2012 with the purpose to obtain performance data that supports approval by the North Carolina Department of Environmental and Natural Resources (NCDENR). This monitoring was performed in accordance with Preliminary Evaluation Period (PEP) guidelines described in the 2007 NCDENR Stormwater BMP Manual and the Quality Assurance Project Plan (NC State 2013) previously submitted to NCDENR. In addition, further data analyses utilizing the state of Washington’s Technology Assessment Protocol - Ecology (WADOE, 2011) criteria was performed.The NCDENR total suspended sediment (TSS) design criterion is 85% removal. The stand-alone Filterra system reduced TSS and suspended sediment concentration (SSC) by median values of 95% and 96%, respectively, over a 23 month study period. Load reduction for the system on a storm-by-storm basis demonstrated median values of 67% and 82%, respectively, for TSS and SSC. Storms for which bypass did not occur showed median load reductions of 96% for both TSS and SSC. Nitrogen and phosphorus nutrient data analysis will be presented as well.The Filterra system was sized according to the 1-inch water quality design storm for volume-based best management practices in North Carolina. When plotting the observed rainfall intensity vs. site peak outflow against the theoretical peak flows from the Rational equation’s pre- and post-development conditions, the Filterra device nearly mimics the pre-development site peak. Results show the Filterra system reduced median peak flow by 58% for storms monitored in the study (0.03 to nearly 5 inches in depth). Statistically, bypass did not significantly occur below 1 inch of rainfall depth and below 1 inch per hour intensity.