How much stormwater can these systems absorb?
The coefficient of runoff for permeable pavements at dead flat has been proven to be .15, where asphalt is .98 using the rational method. We would recommend a minimum 1% grade and then the c value becomes .25. We would suggest that the design value be at .40 to allow for 50% clogging in the first five years. After that time period, the bacteria system is on a 100% breakdown of first-flush pollutants, which takes 5 years to complete. At this point, the system will continue to absorb a minimum 2.5″ storm event. The void ratio in the sub-base aggregate is 40%. There is still another 9″ of system (Paver depth, setting bed and base depth) left to store runoff from rainstorms; however, it is not part of the design… we just wanted to point out that we do have a degree of error designed onto this system.
