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.
Which method should I use if my watershed has both natural and developed portions?
TR-55's three-segment method is generally better suited to mixed land cover, since sheet flow, shallow concentrated flow, and channel flow can each be represented with different velocities, whereas Kirpich assumes one uniform flow path.
Why is sheet flow length capped at 300 feet in TR-55?
Beyond roughly 300 feet, unconcentrated overland sheet flow essentially always transitions into shallow concentrated flow (rills and swales) — the cap reflects observed field behavior, not an arbitrary limit.
What happens if my computed Tc comes out unrealistically short?
A very short Tc often signals an oversimplified flow path assumption rather than a genuinely fast-responding watershed — many jurisdictions require a 5-minute minimum for storm sewer design because an unrealistically short Tc overstates rainfall intensity and peak flow.
Does channel flow velocity use the same Manning's n approach as pipe design?
Yes — the channel-flow segment of TR-55 uses Manning's equation for open-channel flow, with roughness values selected for the specific channel type (grass swale, riprap, natural channel) rather than smooth-pipe values.
Should I use pre-development or post-development Tc for a site that's being developed?
Both, separately — pre-development Tc sets the allowable release rate many jurisdictions require detention to match, while post-development Tc (typically shorter, due to added impervious area) is used to compute the actual post-development peak that must be controlled.