How Is Permeable Pavement Designed for Stormwater Management?

I design the section from the bottom up. The subgrade gets scarified — not over-compacted — and tested for infiltration at the proposed elevation. The aggregate reservoir depth comes from the storage volume the stormwater report requires: design storm depth over the contributing area, divided by the void ratio of the open-graded stone. A choker course levels the top of the reservoir for the bedding layer, and the surface — porous asphalt, pervious concrete, or permeable pavers — is selected for the traffic and the owner's maintenance capability. Pavers cost more but repair in kind; porous asphalt is cheaper but patches are visible. Edge restraint and overflow complete the section. An underdrain at the reservoir bottom, daylit or tied to the storm system, handles storms beyond the design event and sites where infiltration is partial. I set the overflow elevation so the reservoir fills before anything bypasses — otherwise the system short-circuits during the storms that matter. On sloped sites, I terrace the reservoir with check dams inside the stone so storage does not all migrate to the downhill end.

Permeable pavement is destroyed more often during construction than in service. I write explicit protection into the specs: no stockpiling on the subgrade, no construction traffic across the excavated area, silt fence between the work zone and the pavement footprint until the site is stabilized, and the permeable area paved as late in the sequence as possible. The subgrade gets re-tested if it rains on it before stone placement — a sealed subgrade under a permeable section is a very expensive conventional pavement. Testing during construction is straightforward and I require it: infiltration testing of the finished surface to confirm it meets the design rate, and density testing of the aggregate lifts. For pervious concrete, I require a test panel because placement technique — strike-off, compaction, curing — determines whether the slab infiltrates or not. A contractor who has never placed pervious concrete learns on the test panel, not on the owner's parking lot.

A permeable pavement design is ready when it clears this checklist. The system is an infiltration facility, so every item protects the infiltration path. • Field infiltration rate at subgrade elevation with groundwater separation verified • Reservoir depth sized from the design storm volume and the stone void ratio • Subgrade protected from over-compaction, rain sealing, and construction traffic • Recorded maintenance agreement with vacuum sweeping frequency and no-sand winter rules • Overflow or underdrain for storms beyond the design event