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 20 L/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, 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>

<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: