About the HEC-HMS Shallow Concentrated Flow Calculator
This tool estimates shallow concentrated flow velocity and travel time for a single watershed flow segment. It is useful for hydrology students, modelers, and engineers checking a component of time of concentration calculations for HEC-HMS or similar TR-55 based workflows.
How to use this calculator
- Enter the flow path length in feet.
- Enter the average watercourse slope as a percent.
- Select the surface condition: Unpaved or Paved.
- Click Calculate to compute velocity and travel time.
- Click Reset to restore the default 500 ft, 2.0%, unpaved inputs.
How it works
The calculator uses flow path length, average slope, and surface condition to estimate shallow concentrated flow velocity. The entered slope is converted from percent to ft/ft by dividing by 100.
Velocity is calculated with the NRCS TR-55 shallow concentrated flow relationship. For unpaved surfaces, V = 16.1345 × √S; for paved surfaces, V = 20.3282 × √S, where V is velocity in ft/s and S is slope in ft/ft.
Travel time is then computed as Tt = L / (3600 × V), where L is length in feet and Tt is in hours. The calculator also reports the same travel time in minutes and seconds.
Results are educational estimates for preliminary hydrologic checks. Confirm applicability of the TR-55 method, flow regime assumptions, units, and local design criteria before using results for engineering decisions.
Example calculation
For a 500 ft unpaved segment with a 2.0% slope, the slope used is 0.0200 ft/ft. Velocity = 16.1345 × √0.0200 = about 2.28 ft/s. Travel time = 500 / (3600 × 2.28) = 0.0609 hr, or about 3.65 minutes.
Frequently asked questions
What slope should I enter?
Enter the average segment slope as a percent, calculated as fall divided by length times 100. For example, 10 ft of fall over 500 ft is 2%.
What is the difference between paved and unpaved in this calculator?
The surface condition changes the TR-55 velocity coefficient. Paved surfaces use a higher coefficient, producing a higher velocity and shorter travel time for the same slope and length.
Does this calculate the full watershed time of concentration?
No. It calculates travel time for only the shallow concentrated flow segment. Total time of concentration may also include sheet flow, channel flow, or other flow path components.
Why must length and slope be greater than zero?
A positive length and slope are required because velocity is based on the square root of slope and travel time is based on distance divided by velocity.
Are the results valid for all flow conditions?
No. The TR-55 shallow concentrated flow equations are empirical and should be used only where the shallow concentrated flow assumption is appropriate.