Conventional asphalt is nearly impervious, so almost all rain becomes surface runoff. Permeable pavement replaces the dense surface with an open-graded aggregate structure that lets water pass through and be held temporarily in the void space of its base course before it slowly exfiltrates into the native soil.
Runoff intensity: q = max(0, i − f)
Rational coeff.: C = q / i
Storage balance: dS/dt = f_used·(1 − S/S_max) − f_exfil
Volume infiltrated: V = ∫ f_used · A dt (mm·m² = litres)
Here i is rainfall intensity, f is the surface's infiltration capacity (≈0.5 mm/hr for dense asphalt; the slider value for permeable pavement, reduced by sediment clogging), S is water stored in the aggregate reservoir, S_max its void-space capacity, and f_exfil the constant rate at which the native subgrade soil absorbs water out of the reservoir. When the reservoir approaches capacity its effective intake throttles down — exactly like a real storage-infiltration structure backing up during an intense storm.
- Rainfall intensity — the design storm's depth of rain per hour.
- Infiltration capacity — how fast clean permeable pavement can pass water through its surface and base.
- Clogging — sediment accumulation over years of service life reduces the usable capacity; this is why permeable pavements need periodic vacuum-sweeping maintenance.
- Reservoir bar — the fraction of the aggregate base's void space currently holding water, draining to the subgrade at a fixed exfiltration rate.
Real-world relevance: this mass-balance model is the same logic behind permeable-pavement and bioretention sizing guidance used in nature-based stormwater infrastructure (NbS) — undersized capacity or heavy clogging both show up here as a rising runoff coefficient, exactly as they would in an under-maintained real installation.