| Example | Method | Inputs | Expected Tc |
|---|---|---|---|
| EX-1 | Kirpich | Length = 950 m, Slope = 0.006 | ≈ 27.39 min |
| EX-2 | TR-55 | P2 = 3.6 in; Sheet: n=0.24, L=100 ft, s=0.01. Shallow: unpaved, L=1400 ft, s=0.01. Channel: n=0.05, a=27 ft², pw=28.2 ft, s=0.005, L=7300 ft. | ≈ 91.65 min |
| EX-3 | Bransby–Williams | Length = 4.2 km, Area = 18 km², Slope = 0.010 | ≈ 114.31 min |
- Select a method matching your catchment and design standard.
- Choose unit system, then enter lengths, slopes, and roughness.
- For TR-55, split the flow path into sheet, shallow, and channel segments.
- Press Calculate to see Tc above the form.
- Use the CSV or PDF buttons to document assumptions and share results.
Selecting the right Tc method for a project
Time of concentration is method-sensitive, so start by matching the equation to site conditions. Kirpich fits smaller natural basins with a clear hydraulic flow path and measurable slope. TR‑55 is preferred when flow transitions through sheet, shallow concentrated, and channel segments. Bransby–Williams supports broader catchments when only map-based length, area, and average slope are available.
Key inputs that drive results
Length and slope dominate Tc because they represent travel distance and energy grade. For TR‑55, Manning n in the sheet segment can strongly increase early travel time, while channel geometry and n govern velocity in later segments. Keep slope dimensionless (m/m or ft/ft) and confirm it follows the hydraulic path, not a straight-line map shortcut.
Segmentation and defensible assumptions
TR‑55 is most defensible when the flow path is segmented at realistic transitions: vegetated sheet flow near the divide, then shallow concentrated flow in swales or roadside conveyance, and channel flow where a defined section exists. Sheet flow is typically limited to short distances; if your sheet length is long, revisit the segmentation.
Design checks and sensitivity
Use quick sensitivity checks to understand risk: increasing slope lowers Tc, while higher roughness increases Tc. Compare at least two methods when regulations allow, and record why one was selected. If Tc appears unusually small or large, verify flow path length, segment definitions, and units.
Example data for quick benchmarking
The sample sets below illustrate how Tc changes with length, slope, and roughness. Values are representative only and should be replaced with surveyed or as-built inputs.
| Set | Method | Representative inputs | Indicative Tc |
|---|---|---|---|
| D‑1 | Kirpich | L=650 m, S=0.012 | ≈ 17–20 min |
| D‑2 | TR‑55 | P2=3.0 in; Sheet: n=0.20, L=80 ft, s=0.02; Shallow: paved, L=900 ft, s=0.02; Channel: n=0.04, a=18 ft², pw=22 ft, s=0.006, L=4200 ft | ≈ 35–55 min |
| D‑3 | Bransby–Williams | L=6.0 km, A=30 km², S=0.008 | ≈ 140–170 min |
1) What is time of concentration used for?
It helps select critical rainfall duration for peak-flow methods and supports hydrograph timing in drainage design, detention sizing, and inlet capacity checks.
2) Which method should I choose first?
Use TR‑55 when you can segment the path and have roughness and channel data. Use Kirpich for small natural basins. Use Bransby–Williams for broad screening when inputs are limited.
3) Why does slope reduce Tc?
Higher slope increases velocity and reduces travel time along the flow path, so runoff reaches the outlet sooner and the summed travel time decreases.
4) What is a reasonable sheet flow length?
Sheet flow is usually short before concentrating into rills or swales. If the sheet length is large, convert the remainder to shallow concentrated flow or add a channel segment.
5) Can I override shallow concentrated velocity?
Yes. If you have observed or modeled velocities, enter the override so travel time is computed directly from length and velocity instead of using the slope-based estimate.
6) How do I document results for review?
Calculate Tc, then export CSV or PDF to capture inputs, chosen method, and segment travel times. Attach the export to your design notes or submission package.
7) Why do different methods give different Tc values?
Each method represents different flow regimes and calibrations. Differences are expected when roughness, segmentation, and catchment descriptors change, so align the selection with site conditions and local guidance.