Storm Drain Capacity Simulator

Storm Conditions

Rainfall intensity 40 mm/hr

Catchment imperviousness 60%

Storm Drain Pipe

Pipe diameter 450 mm

Pipe slope 1.0%

Live Readout

Pipe flowing 0% full

Inflow Q_in

0 L/s

Pipe capacity Q_cap

0 L/s

Pipe fill depth

Street ponding

0 mm

How it works

Inflow — the Rational Method. A fixed 1.2-hectare street catchment turns rainfall into runoff:

Q_in = C · i · A / 360
  C = runoff coefficient (imperviousness)
  i = rainfall intensity [mm/hr]
  A = catchment area [ha]  → Q_in in m³/s

Capacity — Manning's equation for a circular pipe running full, using concrete roughness n = 0.013:

Q_full = (1/n) · A_p · R^(2/3) · S^(1/2)
  A_p = πD²/4,  R = D/4 (full-flow hydraulic radius)
  S = pipe slope [m/m]

Partial flow. When Q_in is below capacity, the actual flow depth y inside the pipe is solved from the partial-full Manning relation (flow area and wetted perimeter depend on the water's subtended angle θ):

θ = 2·acos(1 − 2y/D)
A(y) = (D²/8)(θ − sinθ),  P(y) = (D/2)θ,  R(y) = A(y)/P(y)
Q(y) = (1/n) · A(y) · R(y)^(2/3) · S^(1/2)

Real pipes actually reach their true peak capacity fractionally before running 100% full (around 94% depth) because the wetted perimeter keeps growing faster than the flow area near the crown — this sim uses the simpler full-pipe capacity as the surcharge threshold, the standard first-order design check used in stormwater engineering.

Surcharge. Once Q_in exceeds Q_full, the inlet can no longer swallow the runoff. The excess (Q_in − Q_full) accumulates on the street as ponding depth; when rainfall eases, the pipe drains the backlog at its spare capacity until the street clears. This is exactly why undersized storm drains flood streets in short, intense downpours even when total rainfall is modest.

Pipe flow / fill gauge Street ponding Rainfall

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