Permeable Pavement Stormwater Cutaway

Storm & Pavement Setup

Rainfall intensity 50 mm/hr

Surface clogging 10%

Reservoir course depth 300 mm

Subgrade infiltration k_sub 12 mm/hr

Live Water Balance

Reservoir storage0%

Infiltration in

0 mm/hr

Exfiltration out

0 mm/hr

Bypass runoff

0 mm/hr

Captured / total

100%

How it works

A permeable-pavement system is a layered infiltration reservoir: open-jointed pavers or porous asphalt sit over a choker course and a thick open-graded stone reservoir course (porosity n ≈ 0.30–0.40) that stores water while it slowly drains into the subgrade. Two independent limits control performance:

Surface entry:   f_surface = f0 · (1 − clog)      [Darcy-limited flow through pore throats]
Reservoir storage: capacity C = depth · n           [mm of water the voids can hold]
Bottom exit:     f_sub = k_sub                      [Darcy's law through native subgrade soil]

The reservoir's stored depth S evolves as a simple mass balance each step:

inflow  = min(i, f_surface)            (i = rainfall intensity)
outflow = min(k_sub, S / capacity · k_sub_max)
dS/dt   = inflow − outflow,   0 ≤ S ≤ C

Runoff (bypass) appears from two distinct failure modes you can trigger independently:

Surface-limited bypass — raise clogging or rainfall so i exceeds f_surface: excess water never enters the pavement at all and sheets straight to the drain.
Storage-limited bypass — once the reservoir is full (S = C), any inflow beyond what the subgrade can absorb (k_sub) has nowhere to go and overflows back to the surface, even on a perfectly unclogged pavement.

This is why real permeable-pavement design sizes the reservoir depth against both the local soil's infiltration rate and the design storm — a deep reservoir over slow clay soil still floods if the storm outlasts the subgrade's drainage capacity, while a well-drained sandy subgrade can support a much shallower system.

Rainfall Reservoir water Bypass runoff

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