Time of Concentration
Time of concentration (Tc) is one of the most fundamental parameters in applied hydrology. Whether you're designing stormwater infrastructure or using models like EPA SWMM, understanding Tc is essential for producing accurate and defensible results.
In this article, we’ll break down what time of concentration is, why it matters, and how it’s calculated in practice.
What Is Time of Concentration?
Time of concentration (Tc) is defined as:
The time required for runoff to travel from the hydraulically most distant point in a watershed to its outlet.
This definition is consistent with guidance from the:
USDA Natural Resources Conservation Service (NRCS)
Federal Highway Administration (FHWA)
It’s important to note that the hydraulically most distant point is not necessarily the farthest point geographically—it is the point with the longest travel time.
A Watershed Response Parameter — Not a Rainfall Parameter
Time of concentration is often misunderstood as a rainfall-related variable. It is not.
Tc is a watershed response parameter, meaning it depends on physical characteristics such as:
slope
surface roughness
land cover
flow path geometry
Because of this, Tc is considered a property of the drainage area, not a specific storm event.
Why Time of Concentration Matters
Time of concentration plays a critical role in peak discharge estimation, which directly impacts the design of:
culverts
storm drains
detention facilities
In the Rational Method
Rainfall intensity is selected from an IDF curve using a storm duration equal to Tc.
This is based on the assumption:
Peak discharge occurs when rainfall duration equals the time required for the entire watershed to contribute flow.
In the NRCS Method
Tc is used to compute lag time, which affects:
hydrograph shape
peak flow magnitude
Because of this, even small errors in Tc can significantly affect design outcomes.
The Concept Behind Time of Concentration
Conceptually, Tc represents the point in time when:
Runoff from all areas of the watershed is contributing to flow at the outlet.
Before Tc: only part of the watershed contributes
At Tc: the entire watershed contributes simultaneously
While real watershed behavior is more complex, this assumption is embedded in standard engineering methods like:
the Rational Method
NRCS procedures
How Time of Concentration Is Calculated
Tc is typically calculated by dividing the flow path into segments based on flow type, then summing travel times across each segment.
Common Flow Segments:
Sheet flow
Shallow concentrated flow
Channel (or pipe) flow
Each segment reflects a different flow regime with different velocities.
1. Sheet Flow
Sheet flow is shallow, broad overland flow that occurs before runoff becomes concentrated.
Key Characteristics:
Occurs over short distances
NRCS limits sheet flow to 100 feet
Strongly influenced by:
surface roughness (grass, pavement, forest)
slope
Why It Matters:
Velocities are typically low, so sheet flow can significantly contribute to total Tc—especially in small watersheds.
2. Shallow Concentrated Flow
After sheet flow, runoff begins to concentrate into small channels such as rills or swales.
Key Characteristics:
Higher velocity than sheet flow
Still not fully developed channel flow
Velocity estimated using empirical relationships (NRCS TR-55)
Influencing Factors:
slope
surface type (paved vs unpaved)
This is a transitional phase before flow enters a defined channel or system.
3. Channel (or Pipe) Flow
Channel flow occurs when runoff enters a defined conveyance system such as:
streams
ditches
storm drains
Key Characteristics:
Highest velocities
Often calculated using hydraulic equations (e.g., Manning’s equation)
Importance:
In larger watersheds, this segment often dominates total travel time.
The Impact of Urbanization
Urban development significantly affects time of concentration.
Key Effects:
Increased impervious surfaces → faster runoff
Storm sewer systems → more efficient conveyance
Reduced travel time
Result:
Shorter Tc values
Higher peak discharges (since shorter durations correspond to higher rainfall intensities)
Common Methods for Estimating Tc
NRCS TR-55 Method
The most widely used approach for small watersheds:
Breaks flow into segments
Uses standard procedures for each flow type
Rational Method Applications
Tc estimated using accepted formulas or jurisdictional standards
Other Approaches
Manning’s equation for channel segments
Region-specific empirical equations
Always follow the requirements of your governing agency or jurisdiction.
Limitations and Sources of Error
Time of concentration is not an exact physical constant—it is an engineering estimate.
Common Sources of Error:
Incorrect slope measurements
Poor roughness assumptions
Misidentifying the longest hydraulic path
Additionally, real watersheds involve:
spatial variability
dynamic processes
These are not fully captured in simplified Tc calculations.
Best Practices
To improve reliability:
Carefully identify the hydraulic flow path
Use appropriate segment definitions
Apply accepted methods (e.g., TR-55)
Follow local design standards
Document assumptions clearly
Final Thoughts
Time of concentration is a foundational concept in hydrology that directly influences peak discharge and infrastructure design.
To summarize:
Tc is the travel time from the hydraulically most distant point to the outlet
It is a watershed property—not a rainfall parameter
It determines rainfall duration in the Rational Method
It influences hydrograph timing in NRCS methods
It is calculated by summing travel times across flow segments
Because of its impact on design, Tc should always be estimated carefully using accepted procedures and sound engineering judgment.
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