Changes

Bioretention practices have been shown to reduce runoff volume through both means of evapotranspiration and infiltration. The primary body of research is separated into bioretention practices either with underdrains and those without (therefore, relying solely on full infiltration into underlying soils). Volumetric performance improves when:   

Bioretention practices have been shown to reduce runoff volume through both means of evapotranspiration and infiltration. The primary body of research is separated into bioretention practices either with underdrains and those without (therefore, relying solely on full infiltration into underlying soils). Volumetric performance improves when:   

* Native soils have high infiltration capacity and facility is designed for full infiltration, without an underdrain;

* Native soils have high infiltration capacity and facility is designed for full infiltration, without an underdrain;

* Size of the impervious drainage area relative to the facility permeable footprint area is kept within recommended range of 5:1 (HSG C and D soils) to 20:1 (HSG A and B soils).

* Size of the impervious drainage area relative to the facility permeable footprint area (i.e., I:P area ratio) is kept within recommended range of 5:1 (HSG C and D soils) to 20:1 (HSG A and B soils).

* Perforated pipe or outlet connection is elevated above the bottom of the practice in the underdrain cross-section;  

* Perforated pipe or outlet connection is elevated above the bottom of the practice in the underdrain cross-section;  

* A flow restrictor (e.g., orifice, valve) is installed on the underdrain or storm sewer outlet pipe.

* A flow restrictor (e.g., orifice, valve) is installed on the underdrain or storm sewer outlet pipe.