Enter the sheet flow length, slope, Manning’s n value, and P2 (2-year, 24-hour rainfall depth) into the calculator to estimate sheet flow travel time using the NRCS TR-55 sheet flow equation (which uses Manning’s n as a roughness parameter). You can also estimate the average sheet flow velocity as length divided by travel time.

Sheet Flow Calculator

TR-55 sheet flow travel time for the first ≤100 ft of a drainage path.

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Custom Manning’s n
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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:

t_{hr} = 0.007 \frac{(nL)^{0.8}}{P_2^{0.5} S^{0.4}}

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

  • thr is the travel time (hours)
  • n is Manning's n roughness coefficient for sheet flow (dimensionless)
  • 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. To estimate an average sheet flow velocity over the segment, use V = L / t (be sure your length and time units are consistent).

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. Understanding sheet flow is important for estimating travel time (time of concentration) and managing stormwater runoff in urban and rural areas.

How to Calculate Sheet Flow Travel Time?

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

  1. Determine the sheet flow length (L) in feet (TR-55 sheet flow is typically limited to about 300 ft before flow becomes concentrated).
  2. Determine the slope (S) as a percentage and convert it to a decimal for the calculation (e.g., 2% = 0.02).
  3. Select or determine Manning's n for the overland surface.
  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 ≈ 0.169 hr ≈ 10.1 minutes (and the average velocity over the segment is ≈ 0.165 ft/s).