The Situation
You are designing a drainage system for a small watershed (a 3-acre residential area) and need to determine the time of concentration (Tc). This is the time it takes water to travel from the most hydraulically remote point in the watershed to the outlet. Tc is a critical input to the Rational Method — it determines which rainfall intensity to use for your design.
The watershed has three distinct flow segments:
- Sheet flow across a grassy backyard (150 feet)
- Shallow concentrated flow down a paved swale (300 feet)
- Channel flow in a roadside ditch to the outlet (500 feet)
Which Calculator to Use
The Time of Concentration Calculator offers multiple methods for estimating Tc. For this example, we will use the Kirpich method, which is one of the most widely used for small watersheds.
Walking Through the Inputs
Method: Kirpich
The Kirpich method estimates Tc from the flow path length and the elevation difference between the highest point and the outlet. It was developed for small agricultural watersheds but is widely used for residential sites.
Flow Length (950 feet)
The total flow path length from the most remote point to the outlet:
- Sheet flow across backyard: 150 ft
- Shallow concentrated flow in swale: 300 ft
- Channel flow in roadside ditch: 500 ft
- Total: 950 feet
For the Kirpich method, we use the total hydraulic flow path length, not the straight-line distance.
Elevation Difference (19 feet)
The elevation at the most remote point (the far corner of the highest lot) is 119 feet. The elevation at the outlet (where the roadside ditch enters the storm drain) is 100 feet.
Elevation difference: 119 - 100 = 19 feet
This gives an average slope of 19/950 = 0.020 = 2.0%.
The Results
The Kirpich equation gives:
Tc = 0.0078 x L^0.77 x S^(-0.385)
Where:
- L = flow length in feet = 950
- S = average slope in ft/ft = 0.020
Tc = 0.0078 x 950^0.77 x 0.020^(-0.385) = approximately 18 minutes
What This Means for Your Project
Using Tc in the Rational Method
With Tc = 18 minutes, you now know which rainfall intensity to use for the Rational Method. Look up the intensity for your design storm at an 18-minute duration:
| Design Storm | Intensity at 18 min (typical) | Peak Flow (C=0.45, A=3 ac) |
|---|---|---|
| 2-year | 3.2 in/hr | 4.3 cfs |
| 10-year | 4.8 in/hr | 6.5 cfs |
| 25-year | 5.8 in/hr | 7.8 cfs |
| 100-year | 7.5 in/hr | 10.1 cfs |
Why Tc Matters
If you used a 30-minute Tc instead of 18 minutes, you would get a lower rainfall intensity and undersize your drainage system. If you used a 10-minute Tc, you would oversize it and waste money. Getting Tc right is the foundation of accurate drainage design.
Cross-Checking with Other Methods
It is good practice to check Tc using at least two methods:
| Method | Tc Estimate | Best For |
|---|---|---|
| Kirpich | 18 min | Small rural/suburban watersheds |
| FAA | 15-20 min | Airport and flat area drainage |
| SCS Lag | 16-22 min | Watersheds with known curve number |
| Kerby-Hathaway | 12-15 min (sheet flow segment only) | Overland flow component |
If the methods give significantly different results, use engineering judgment to select the most appropriate value for your specific conditions. For this watershed, 18 minutes from the Kirpich method is reasonable.
Try This Scenario
Try this scenario in the Time of Concentration CalculatorNext Steps
- Plug the Tc value into the Rational Method Calculator to determine the peak flow
- Look up your local IDF curves (Intensity-Duration-Frequency) from NOAA Atlas 14 for the exact rainfall intensity at your Tc
- Read Time of Concentration Explained for a deeper understanding of the different methods