What Is the Watershed Time of Concentration (Tc) Calculator?
The Watershed Time of Concentration (Tc) Calculator determines hydrologic response timing for storm drainage design and detention pond routing.
How Does the Watershed Time of Concentration (Tc) Calculator Work?
It computes the potential soil moisture retention (S) from Curve Number (CN) and applies the empirical NRCS lag relationship.
Watershed Time of Concentration (Tc) Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
T_{lag} = \frac{L^{0.8} (S + 1)^{0.7}}{1900 \sqrt{Y}}, \quad T_c = \frac{T_{lag}}{0.6}, \quad S = \frac{1000}{CN} - 10
NRCS National Engineering Handbook watershed lag formulation relating hydraulic length, land slope, and soil retention.
How to Use the Watershed Time of Concentration (Tc) Calculator
- Input the longest hydraulic flow path length in meters.
- Specify average watershed slope percentage.
- Enter the area-weighted NRCS Curve Number.
Step-by-Step Example Calculation
800-Meter Suburban Watershed
Input Values:
hydraulicFlowLengthM:800
watershedSlopePercent:2.5
nrcsCurveNumberCN:75
Worked Steps: Produces an 0.81-hour basin lag time and 1.35-hour (80.8 minute) time of concentration.
Understanding Your Result
Time of Concentration (Tc): Total travel time from watershed boundary to catchment outlet.
Lag Time (T_lag): Delay from center of rainfall excess mass to peak runoff rate.
Factors That Affect the Result
- Urbanization and channelization increase CN and velocity, dramatically shortening Tc and spiking peak flood stages.
When Should You Use This Calculator?
- Storm sewer master planning, retention basin design, bridge culvert sizing, and regional flood risk studies.
Assumptions & Limitations
- Applicable to natural and agricultural/rural basins up to 800 hectares (2,000 acres).
Frequently Asked Questions
Calculation Accuracy & Reference Note
Based on NRCS National Engineering Handbook Section 4.