Changes

A [[bioretention]] cell with footprint of 30 x 10 m is planned to received runoff from adjacent roadways and parking facilities. The design includes 600 mm depth of filter medium, 75 mm wood based mulch, and ponding of 300 mm. Two underdrain pipes will be embedded at the base of the storage reservoir. These will connect together and then have an upturn within a manhole at the downstream end to prevent discharge until the head of water reaches the top of the storage reservoir within the cell. The lab test for the filter medium state that it has a hydraulic conductivity of 80 mm/hr. The downstream pipe in the manhole can convey 0.002 m³/s on a 1% slope.:  

A [[bioretention]] cell with footprint of 30 x 10 m is planned to received runoff from adjacent roadways and parking facilities. The design includes 600 mm depth of filter medium, 75 mm wood based mulch, and ponding of 300 mm. Two underdrain pipes will be embedded at the base of the storage reservoir. These will connect together and then have an upturn within a manhole at the downstream end to prevent discharge until the head of water reaches the top of the storage reservoir within the cell. The lab test for the filter medium state that it has a hydraulic conductivity of 80 mm/hr. The downstream pipe in the manhole can convey 0.002 m³/s on a 1% slope.:  

<math>Q_{max, pipe}=\frac{80 mm/hr\times 300\ m^{2}\times \left (\frac{0.975\ m}{0.6\ m}  \right )\times 2}{3.6\times 10^{6}}=0.0022\ m^{3}/s</math>

<math>Q_{max, media}=\frac{80 mm/hr\times 300\ m^{2}\times \left (\frac{0.975\ m}{0.6\ m}  \right )\times 2}{3.6\times 10^{6}}=0.0022\ m^{3}/s</math>

As the maximum flow rate exceeds the downstream maximum permissible flow, the design must be amended. In order of preference, some options include:

As the maximum flow rate exceeds the downstream maximum permissible flow, the design must be amended. In order of preference, some options include:

#Reformulating the bioretention filter media in collaboration with the soils provider, to reduce the hydraulic conductivity,  

#Reformulating the bioretention filter media in collaboration with the soils provider, to reduce the hydraulic conductivity,  

#Reducing the depth of the filter media bed in agreement with a Landscape Architect, and increasing pretreatment (as filter bed < 600 mmm will provide less treatment),   

#Reducing the depth of the filter media bed in agreement with a Landscape Architect, and increasing pretreatment (as filter bed < 600 mmm will provide less treatment),   

#Increasing the size of the downstream pipe to accommodate a greater flow rate.

#Increasing the size of the downstream pipe to accommodate a greater flow rate.