Small-watershed peak flow, Q = CiA, with a composite runoff coefficient built from sub-areas and time of concentration built from TR-55 flow segments. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: The Rational Method is for small, simple watersheds — most local criteria cap it around 100–200 acres and require the jurisdiction's own C values, Tc method, and IDF data. This tool implements the textbook/TR-55 forms; where the county hydrology manual differs (and in Southern California it usually does — several counties use their own Modified Rational procedures), the manual governs. Design storm intensity comes from your jurisdiction's IDF curves or NOAA Atlas 14, not from this tool.
Composite Runoff Coefficient
Time of Concentration (TR-55 segments)
Intensity
Results
Total area
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Composite C (area-weighted)
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Time of concentration
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Intensity used
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Peak flow Q = CiA
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How the Numbers Are Derived
The Rational Method
Q = C · i · A — Q in cfs, i in in/hr, A in acres. (The unit conversion factor is 1.008, universally taken as 1.0.) Composite C is the area-weighted average: ΣC·A / ΣA. The core assumption: peak flow occurs when the entire watershed contributes, i.e. when storm duration equals Tc — which is why intensity is read at Tc.
TR-55 travel time segments
Sheet flow (first ≤100 ft of the path): Tt = 0.007·(nL)^0.8 / (P₂^0.5 · s^0.4) hours — n is the sheet-flow roughness (smooth surfaces 0.011, short grass 0.15, dense grass 0.24, light woods 0.40), L in ft, s in ft/ft, P₂ the 2-yr 24-hr rainfall in inches. Current NRCS guidance limits sheet flow length to 100 ft; the tool warns beyond that.
Shallow concentrated flow:V = 20.328·√s (paved) or 16.135·√s (unpaved) ft/s, then Tt = L / (3600·V).
Channel flow:Tt = L / (3600·V) with V entered directly — compute it with the Manning's tool for the actual channel or pipe section.
Tc is the sum of segment travel times along the hydraulically longest path, floored at the entered minimum.
Typical C values (verify against the local table)
Roofs and pavement 0.85–0.95 · gravel 0.35–0.70 · lawns, flat sandy 0.05–0.10 · lawns, steep clayey 0.25–0.35 · undeveloped brush 0.20–0.40. Many manuals adjust C upward for rarer storms (e.g. ×1.1–1.25 for 25–100 yr) — apply that per the manual if required, it is not automatic here.
Field/reviewer notes
The single most common error in Rational Method submittals is a Tc path that isn't actually the hydraulically longest one — walking the site beats tracing the obvious swale on the plan.
An absurdly short Tc (all pipe, steep site) drives intensity — and Q — way up. That's the model working correctly; it's also why criteria floor Tc.
For pre- vs. post-development comparisons, keep the same method, storm, and IDF source on both sides or the comparison is meaningless.
Above the method's area limit, or with significant storage/attenuation in the watershed, switch to a hydrograph method (TR-55/TR-20, or the county's unit hydrograph).
References
NRCS TR-55 — Urban Hydrology for Small Watersheds (travel time methods). https://www.nrcs.usda.gov/
NOAA Atlas 14 — Precipitation-Frequency Data Server. https://hdsc.nws.noaa.gov/pfds/
The local county hydrology manual — governing C values, Tc method, IDF data, and method applicability limits.