The Hydraulic Notebook

Wahl (2004) showed the empirical breach regressions carry factor-of-two-to-four uncertainty on peak Q and recommended Monte Carlo. Twenty years later every breach memo still reports a single number. Teton calibration: deterministic estimate misses the observed peak by ~80%; the 5–95% ensemble brackets it. Built on the EMS-submitted swmm-breach Python package.

SWMM gives you a discharge at every node and almost nothing about local turbulence at a bridge pier or outfall. The gap between Manning's-class screening and full RANS CFD is forty years old. Built on the JWMM-submitted Adaptive Lagrangian Refinement post-processor — a 200-particle VPM that recovers 99.8% of a 6,000-particle baseline at 20× the speed.

A worked example showing why a 15% swing in Manning's roughness coefficient produces a 20% swing in design discharge — and a practical way to bound the real uncertainty before you sign and seal.

The two most-used Tc formulas can disagree by 2× on the same watershed. Kirpich was calibrated on rural Tennessee in 1940; NRCS lag is calibrated for modern urban/suburban work. Pick the wrong one and your peak flow is off by 30%.

Q = C·I·A is the simplest peak-runoff equation in civil engineering. The C is also the dirtiest number — varying by 4× for the same land cover depending on whose table you read. Why the disagreement exists, and what to actually use.