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→Design Assumptions
Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:
Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:
* Maximum impervious drainage area to permeable surface area (I:P area) ratio of 20:1.
* Native soil infiltration rates for Full, Partial and No Infiltration Design scenarios were assumed to be 20 mm/h, 10 mm/h and 2 mm/h, respectively, and a safety factor of 2.5 was applied to calculate the design infiltration rate.
* Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, removal of sediment from pretreatment structures annually, and removal of weeds twice a year (where applicable). Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ surface infiltration rate testing (where applicable)
*Maximum impervious drainage area to permeable surface area (I:P area) ratio of 20:1.
* Length to width ratio of bioretention area is 10:1.
* Length to width ratio of bioretention area is 10:1.
* Default filter media depth of 1.0 metres to accommodate trees in planting plan.
* Default filter media depth of 1.0 metres to accommodate trees in planting plan.