Slide 46 of 52
Notes:
Similar to sand filters, an off-line design is recommended for most bioretention applications. Larger storms are likely to cause erosion problems at the inflow points, disrupt the mulch layer, and otherwise negatively affect the plant material. For situations where it is not possible to separate the WQv from the larger storms an on-line design may be utilized. For these applications, it is imperative that adequate precautions are taken to protect the inlet, mulch layer, and plant material (e.g., stone stabilization or synthetic erosion protection materials). On-line designs should only be considered for very small drainage areas.
The basic flow regulation design objective is to capture and divert the WQv to the bioretention area and "bypass" the larger storms to the downstream storm drainage system, detention pond or receiving water. In some cases, utilizing bioretention structures for treating the WQv throughout a site or subcatchment may also provide significant runoff attenuation to effectively manage smaller �quantity control� storms as well, and therefore, the need for downstream detention facilities may be eliminated.
On-line design applications should be limited to a maximum drainage area of 0.5 acres. The designs should incorporate adequate overflow measures to accommodate larger flows. A drop inlet storm drainage structure, with the throat opening held 6" above the top of the mulch layer is one technique for handling overflow.