RunoffLab
A stormwater design tool that puts EPA SWMM behind an interactive site plan. Draw rain gardens, permeable pavement or green roofs on a site, then see what they do to runoff, peak flow and water quality — before anything is built.



- 200designs
- explored in about 5 seconds by the Design Explorer
- −30%
- runoff for the sample design in a 50 mm storm
- 128tests
- 76 pytest, 39 Vitest and 13 Playwright
- 0API keys
- everything runs locally, including the engine
Sample numbers come from the bundled fictional 2.4 ha campus with illustrative unit costs. They show what the tool does; they are not a claim about a real site.
The problem
Paved sites send most of their rain straight into storm drains, carrying sediment into creeks and overloading pipes in heavy storms. Green infrastructure helps, but every square metre costs money, and the standard engineering model, EPA’s Storm Water Management Model, is driven by text input files and long reports. RunoffLab asks a practical question of it: which features, where, and how big, give the most benefit for the budget?
What it does
- Draw the site
An SVG editor for roofs, parking, lawns, drains and pipes, plus five kinds of green infrastructure. Constraint checks run as you edit.
- Simulate with SWMM
Each design becomes a SWMM 5.2 model, run locally through PySWMM: every surface a subcatchment, every feature a native LID control.
- Compare outcomes
Baseline against design: runoff volume, peak outfall flow, the water balance, flooding, and sediment and phosphorus leaving the site.
- Search for trade-offs
The Design Explorer runs hundreds of layouts through SWMM in parallel and highlights the cost-versus-runoff Pareto front.
- Look beyond one storm
Year-long continuous runs, what-if storms and clogged soils, a 3D storm replay and PDF reports with every assumption listed.




