Time of Concentration (Tc) is the time required for stormwater runoff to travel from the most hydraulically distant point in a drainage area to a specific point of interest, typically the outlet of the watershed, a culvert, or a storm sewer inlet. It represents the time at which the entire contributing drainage area is actively contributing flow to that point, and it is a fundamental parameter in hydrologic analysis and stormwater design.
In practical terms, Time of Concentration defines the critical storm duration used to estimate peak runoff. When rainfall duration equals the time of concentration, the resulting discharge is generally at or near its maximum because all parts of the watershed are contributing simultaneously. If the rainfall duration is shorter than Tc, only a portion of the watershed contributes to peak flow. If it is longer, peak flow may not increase further, though total runoff volume will.
Time of Concentration is not a single physical measurement but rather a calculated value that reflects how water moves across a landscape. It is influenced by several factors, including flow path length, slope, surface roughness, land cover, and flow type. Urbanization typically reduces Tc by increasing impervious surfaces and improving drainage efficiency, which leads to faster runoff and higher peak discharges.
Tc is commonly conceptualized as the sum of travel times through different flow regimes along the longest hydraulic path:
Sheet Flow (Overland Flow):
This occurs as shallow, diffuse flow over surfaces such as lawns, pavement, or bare soil, typically in the uppermost portion of the watershed. It is usually limited to short distances and is relatively slow due to surface friction.
Shallow Concentrated Flow:
As runoff begins to collect, it forms small channels or rivulets, increasing velocity. This type of flow occurs in swales, ditches, or along pavement edges.
Channel or Pipe Flow:
Further downstream, water enters defined channels, storm sewers, or culverts, where flow is more efficient and velocities are higher.
The total Time of Concentration is the sum of travel times through each of these segments along the hydraulically most distant flow path, not necessarily the longest straight-line distance.
In design applications, Tc is a critical input for methods such as the Rational Method, where rainfall intensity is selected from Intensity-Duration-Frequency (IDF) curves based on a duration equal to Tc. It is also used in hydrograph-based methods, such as unit hydrograph techniques, to shape the timing and peak of runoff responses.
Accurate estimation of Time of Concentration is essential because it directly affects calculated peak discharge. Underestimating Tc can lead to overestimating rainfall intensity and potentially oversizing infrastructure, while overestimating Tc can result in undersized systems that may fail during storm events.
In summary, Time of Concentration is the travel time for runoff from the most distant point in a watershed to its outlet, marking the point at which the entire drainage area contributes to flow. It is a key parameter that links watershed characteristics to peak runoff and plays a central role in the design and analysis of stormwater management systems.