Time of Concentration: Kirpich vs. TR-55
Time of concentration (Tc) is the travel time for runoff to move from the hydraulically most remote point in a watershed to the design point of interest. It is not just an intermediate number — it directly sets the rainfall duration used to select intensity from an IDF curve, which makes an inaccurate Tc one of the most common sources of error in peak discharge estimates.
Kirpich Formula — When to Use It
The Kirpich formula, Tc = 0.0078 × L0.77 × S−0.385, was developed from data on small agricultural watersheds in Tennessee and remains best suited to similarly small, homogeneous rural or agricultural catchments where a single dominant channel or overland flow path controls travel time. It requires only channel length and slope, which makes it fast to apply but limited in its ability to represent mixed land covers or multi-segment flow paths.
TR-55 Three-Segment Method
The NRCS TR-55 method divides the flow path into up to three distinct segments — sheet flow (limited to 300 ft, governed by surface roughness and 2-year 24-hour rainfall), shallow concentrated flow (velocity from a surface-type coefficient and slope), and channel flow (Manning's equation) — and sums the travel time for each. This produces a more detailed and generally more defensible Tc for urban and mixed-cover watersheds where sheet flow across a lawn transitions to a swale and then a defined channel, each with markedly different velocities.
Why Tc Controls Rainfall Intensity Selection
The Rational Method assumes steady rainfall over a duration equal to Tc — so the calculated Tc is used to enter a local IDF curve and read off the matching intensity for the design return period. A Tc that is too short overstates rainfall intensity and peak flow; one that is too long understates it. Because of this direct link, many jurisdictions require a minimum Tc of 5 minutes for storm sewer design, since very short computed values often reflect an oversimplified flow path rather than a genuinely fast-responding watershed.
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