Drainage design is required for roofs, patios, roadways, parking lots, and generally any exterior surface where accumulation of storm water would decrease usability. The classical approach used is the “rational method,” which utilizes for each area a permeability factor, rainfall intensity, and square footage. The lower the permeability of the surface, the higher the runoff and therefore the more water that must be managed. Rainfall intensity is taken from officially gathered weather data compiled over many years.
Once a reliable estimate of runoff is determined, area drains must be placed and sized. Drainage pipes corresponding to these drains must also be designed to ensure adequate flow to a storm sewer, combined sewer, or stream. Where none of these is available or where local ordinances limit additional runoff to an existing system, the design of one or more drywells may be necessary.
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