| Recharge Bed
The recharge bed in an integral part of the porous pavement system because it holds the stormwater until it can slowly infiltrate into the subgrade and into the ground water system. The bed is constructed of a uniform, open graded aggregate, usually 1.5-2.5 inches, with no fines. This creates approximately 40% air voids in the bed.
The depth of the recharge bed is calculated based on the regional hydrographical data. A conservative design approach will consider a 100-year 24-hour storm event to determine the maximum amount of stormwater that will need to be retained. Local regulations will dictate the hydrologic data required for the design. The design volume used for this project was a 2 year, 24-hour event (Dowl, 2007). This data coupled with the percolation rate of the subgrade will determine the storage capacity needed in the recharge bed. The design should drain the system no faster than 12 hours and no slower than 72 hours, with an ideally designed system draining in 24 hours. The percolation rate of the subgrade is imperative in the design of a porous pavement system. The lower the percolation rate the thicker the recharge bed will need to be to create the storage capacity needed.
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