Storm time 0:00 hRain 0 mm/hInflow to garden 0 L/sPonding 0 mmGravel water 0 mmOverflow so far 0 m³FPS 0

Drag to orbit, wheel or pinch to zoom. Blue overlay in the soil = water: the wetting front moves down the engineered soil, the gravel fills from the bottom, and arrows show seepage. The section is schematic (layer depths are drawn at fixed proportions).

Curbside Rain Garden: Curve Number Runoff, Green-Ampt Infiltration and Bioretention Storage

A storm falls on a paved catchment and the runoff is steered through a curb cut into a bioretention cell. How much stays in the ground, how much overflows, and how long until the garden is empty again?

Storm

Catchment (SCS curve number)

Garden design

Time

Event result (whole storm + 3 days)

Flows (L/s)

Storage

Experiments

Each button re-runs the full event for several values and tabulates the result.

Independent numerical checks

Controls

Drag / one finger: orbit; wheel / pinch: zoom. Camera buttons jump to the section, the street or a map view.
Presets set storm and garden together. Every slider recomputes the whole event at once; the replay then runs at the chosen hours per second and loops.
Underdrain switches the perforated pipe near the bottom of the gravel (outlet to the storm sewer) on or off; its invert height is a slider.
Quality is High by default (also on phones); Low or ?q=low halves rain and plant counts.

The science

Design storm. Depth P over duration D with a bell-shaped hyetograph: cumulative fraction F(x) = x³ (10 − 15x + 6x²), x = t/D (the Beta(3,3) law; peak intensity 1.875 P/D). It is a simple stand-in for the NRCS Type II distribution, not the published table.

SCS curve-number runoff. S = 25400/CN − 254 mm, initial abstraction I_a = 0.2 S, and for cumulative rain P > I_a: Q = (P − I_a)² / (P − I_a + S). The composite CN of the catchment is the area-weighted mean of 98 (pavement) and the pervious CN; this is an approximation (the curve-number method is nonlinear). Runoff then passes a linear reservoir with lag K = 10 min before it reaches the curb cut.

Infiltration into the engineered soil. Green–Ampt: f = K_s (1 + ψ Δθ / F) with suction head ψ = 0.11 m, Δθ = n − θ_i, cumulative infiltration F, saturated conductivity K_s (slider). The soil then drains to the gravel at q_d = K_s s³, s = relative saturation above field capacity (illustrative exponent), limited by free space in the gravel.

Gravel store and exits. Gravel (porosity 0.40) fills from the bottom. It loses water by exfiltration into the native soil at its conductivity K_n (unit gradient) and, if the underdrain is on and the water level is above the pipe invert, through an orifice Q = C_d a √(2 g h) (C_d = 0.6, a proportional to the garden area). A weir at the maximum ponding depth sends excess straight to the street (overflow). Evapotranspiration 3 mm/day.

What is conserved. Rain on the garden plus runoff in = ponding + soil water + gravel water + overflow + underdrain + exfiltration + ET; the check table verifies this to machine precision. Drawdown time is counted from the end of rain until ponding is below 1 mm (guidance is typically 24 to 48 h).

Simplifications. One-dimensional vertical flow, no preferential flow or clogging dynamics, uniform soil, no lateral flow, no frost, constant ET, schematic drawing. Typical parameters, not a design: check local standards before sizing a real facility. Assets built with Blender; scene by Three.js.