Analyze runoff timing with proven hydrology methods. Switch units, review formulas, and export result sheets. Built for accurate watershed response checks and drainage planning.
Use Kirpich for quick watershed checks or segmented TR-55 for sheet, shallow, and channel travel components.
The chart updates after calculation. Kirpich plots sensitivity to flow length, while segmented routing shows the travel-time share of each component.
Use these sample values to test the calculator and compare quick-check and segmented watershed routing scenarios.
| Method | Key Inputs | Estimated Tc |
|---|---|---|
| Kirpich (SI) | L = 1200 m, S = 0.03 | 17.67 min |
| Kirpich (US) | L = 3500 ft, S = 0.015 | 21.05 min |
| NRCS / TR-55 | Sheet 100 ft, shallow 1400 ft, channel 7300 ft | 1.53 hr |
Choose Kirpich for a quick empirical estimate or use the segmented method when sheet, shallow, and channel travel all matter.
Use the longest hydraulic path, a realistic slope, and field-based roughness or channel data from surveys and design drawings.
Check segment times, hydraulic radius, and velocities so assumptions are visible before using Tc in later runoff work.
After calculation, download a CSV summary for spreadsheets or a PDF summary for design notes and reviews.
Local drainage manuals may set minimum Tc values, preferred methods, or maximum sheet-flow lengths for permitting.
Changes in grading, channel lining, pavement, or watershed routing can significantly shift travel time and peak discharge timing.
Time of concentration is the travel time from the distant point to the outlet overall. Designers use it to position the runoff peak, select rainfall duration, and size conveyance elements. A site with a fifteen minute response produces a sharper hydrograph than one with a sixty minute response, so changes can affect pipes, ditches, and inlets.
Flow length and slope control how quickly water moves through a watershed. Under the Kirpich approach, travel time rises with increasing length and falls as slope becomes steeper. Doubling flow length does not double Tc, yet it creates a noticeable increase. Reducing slope from 0.03 to 0.01 can slow response enough to change detention assumptions and discharge estimates.
Segmented analysis is useful when runoff passes through sheet flow, shallow concentrated flow, and channel flow before reaching the outlet. Each segment behaves differently. Sheet flow is roughness sensitive, shallow flow responds to surface type and slope, and channel flow depends on hydraulic radius and roughness. Summing segment travel times represents drainage behavior better than forcing one equation across the full path.
The output should be reviewed as more than one number. Segment times, velocity values, and hydraulic radius help verify whether the selected method is behaving reasonably. If one segment dominates total travel time, it deserves a field check. Engineers should compare the raw result against agency minimum values, sheet flow limits, and adopted standards before using Tc in hydrograph or storage calculations.
Small parameter adjustments can create meaningful timing shifts. Increasing surface roughness from turf to dense cover raises sheet flow time. Regrading a channel to improve slope can reduce Tc and increase peak runoff coincidence at the outlet. Lining a ditch may increase velocity, while extending flow length through site expansion can slow response. Sensitivity checks show whether design is robust or overly dependent on one uncertain estimate.
Good practice is to store inputs, assumptions, and exported results with the drainage package. Record the selected method, unit basis, survey source, rainfall parameter, and any design minimum applied during review. Where possible, compare the calculated value with observed site behavior, previous studies, or guidance. Clear documentation improves peer review, supports permit responses, and reduces rework when plans or channel details change later.
Use Kirpich for quick screening on small, steeper watersheds. Use segmented routing when sheet, shallow concentrated, and channel flow each meaningfully affect the drainage path.
Because it adds separate travel times for rough sheet flow, shallow flow, and channel flow. Long shallow or channel segments can materially extend the total watershed response.
The calculator flags an advisory. In many workflows, overly long sheet flow should be split into sheet and shallow concentrated segments for a more realistic estimate.
Slope in these methods is dimensionless. Enter vertical drop divided by horizontal run, not a percent value unless you convert it first.
Not automatically. Always compare the result with local drainage criteria, minimum travel times, and method limitations before finalizing peak flow or storage calculations.
Yes. After calculation, the page can generate both CSV and PDF summaries so your assumptions and reported output can be filed with design notes.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.