Calculate sheet flow travel time with the NRCS TR-55 equation from flow length, slope, 2-year rainfall and surface roughness, or solve for length or slope.

Enter the 2-year, 24-hour rainfall depth (e.g. from NOAA Atlas 14), not rainfall intensity.

Sheet flow is shallow (about 0.1 ft deep or less) and usually no longer than 100 ft (30.48 m).

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Sheet Flow Formula (NRCS TR-55 Travel Time)

The following formula is used in NRCS TR-55 to estimate sheet flow travel time:

tโ‚•แตฃ = 0.007 ((nL)0.8) / (Pโ‚‚0.5 S0.4)

Variables (US customary units as used in TR-55):

  • thr is the travel time (hours)
  • n is the empirical Manning-type roughness coefficient for shallow sheet flow; use the reference sheet-flow values with the equationโ€™s stated unit convention, rather than substituting a channel roughness value
  • L is the sheet flow length (feet)
  • S is the slope (ft/ft, expressed as a decimal; e.g., 2% = 0.02)
  • P2 is the 2-year, 24-hour rainfall depth (inches)

If you prefer minutes, multiply thr by 60, giving a customary coefficient of 0.42. Converting L to metres and P2 to millimetres gives a minute coefficient of approximately 5.4758832311; FHWA HDS-2 (2024), equation 3.7, commonly rounds this to 5.5. This calculator converts inputs to feet and inches and retains the 0.007-hour equation in either unit system. Average segment velocity is V = L/t with consistent units.

What is Sheet Flow?

Sheet flow is the overland flow of water that occurs when excess stormwater spreads out over the ground surface in a thin, even layer. This type of flow is common in the early stages of runoff and occurs before the water collects into a defined channel. A sheet-flow travel time is one segment of a flow path, not automatically the total watershed time of concentration. Add separately calculated shallow-concentrated and channel travel times where appropriate.

How to Calculate Sheet Flow Travel Time?

The following steps outline how to calculate sheet flow travel time using TR-55:

  1. Identify the actual sheet-flow reach. NRCS NEH 630 Subpart F (June 2025) describes typical sheet flow of no more than 100 ft (30.48 m) and discusses the McCuenโ€“Spiess criterion L = 100โˆšS/n ft. The historical 1986 TR-55 300-ft recommendation quoted by FHWA is not a blanket current NRCS allowance; field conditions and the governing local method determine the transition.
  2. Determine the slope (S) as a percentage and convert it to a decimal for the calculation (e.g., 2% = 0.02).
  3. Select a sheet-flow n for the actual near-surface cover at depths about 0.1 ft or less. The simplified equation assumes shallow steady uniform flow, constant rainfall excess in time and space, standard NRCS rainfall intensity-duration relations and minor infiltration effects on travel time.
  4. Find P2, the 2-year, 24-hour rainfall depth for your location (inches).
  5. Use the formula thr = 0.007 (nL)0.8 / (P20.5 S0.4) to calculate travel time.
  6. Optionally compute average velocity as V = L / t.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Length of sheet flow (L) = 100 ft

Slope (S) = 2% (0.02 ft/ft)

Manning’s n = 0.15 (short grass)

P2 = 3.0 in

Using TR-55: t โ‰ˆ 0.169 hr โ‰ˆ 10.1 minutes (and the average velocity over the segment is โ‰ˆ 0.165 ft/s).

These are correct rounded equation results: the unrounded travel time is about 10.119281 minutes. At these inputs, the McCuenโ€“Spiess length criterion is approximately 94.28 ft, so the 100-ft arithmetic example does not itself establish that sheet flow persists over the full reach. The calculator reports length screening separately and never silently caps an entered or solved length. Sensitivity results hold other inputs fixed and are not uncertainty bounds.