This is the 2D-native counterpart to the 3D street-and-pipe scene. Instead of orbiting a rendered street, it draws the three things that actually generate the physics: the pipe's true circular cross-section filling to its solved flow depth, a scrolling hydrograph strip chart racing inflow against capacity over time, and a crowned street gutter profile that ponds the same accumulated backlog volume according to real street camber. Same equations underneath:
Q_in = C · i · A / 360 (Rational Method, A = 1.2 ha)
Q_full = (1/n) · (πD²/4) · (D/4)^(2/3) · S^(1/2) (Manning, full pipe)
θ = 2·acos(1 − 2y/D), A(y) = (D²/8)(θ − sinθ), P(y) = (D/2)θ
Q(y) = (1/n) · A(y) · (A(y)/P(y))^(2/3) · S^(1/2) (Manning, partial flow)
Pipe cross-section panel. The blue chord filling the circle from the invert upward is the actual solved depth y from the partial-flow Manning relation — the same geometry the θ/A(y)/P(y) formulas describe, drawn directly rather than inferred from a fill bar.
Hydrograph panel. A live strip chart plots Q_in(t) against the fixed Q_cap line over the last 60 seconds — the moment the inflow trace crosses the capacity line is the moment the pipe starts surcharging, visible as a crossing event rather than a threshold color change.
Gutter profile panel. Real streets aren't flat — they crown at the centerline and slope ~2% down to each curb so rain sheds off the pavement. This panel distributes the same conserved ponded volume into that crowned V-shaped cross-section: for curb depth h below the 50 mm crown height, wedge area = h²/slope (per side); once both gutters reach the crown, extra volume forms a uniform layer across the full width — a more realistic ponding footprint than a flat rectangular sheet, built from the identical mass-conservation volume as the surcharge model.
Real-world relevance: this is exactly why undersized or aging storm drains flood streets in short, intense downpours even when total rainfall is modest — capacity is a hard ceiling set by pipe geometry and slope, and every liter above it has nowhere to go but the street.