Time of Concentration (T ): The T - Civil Engineering

1 time of Concentration (Tc): The Tc is most often defined as the time required for a particle of water to travel from the most hydrologically remote point in the watershed to the point of collection. Or The maximum or peak runoff rate from small areas usually results from a rain that covers the area uniformly and lasts long enough so that there is a Concentration of runoff from all parts of the watershed. The time required for this accumulation of runoff is called Tc. A key parameter in most methods for predicting runoff. Numerous methods exist for estimating time of Concentration There are various formulas to calculate the time of Concentration . Following is one simple formula. Kirpich Method:Kirpich developed a formula for predicting time of Concentration based on data from seven rural watersheds in Tennessee.

2 These watersheds had well defined channels and steep (3 to 10%) slopes. Tc = (L/H) Tc is the time of Concentration in minutes, L is the maximum hydraulic flow length in feet, and H is the difference in elevation in feet between the outlet of the watershed and the hydraulically most remote point in the watershed. Kirpich method yields very conservative or short times of Concentration that result in high peak runoff rates, especially from the Rational method. This method should only be used if the available data are limited to watershed length and slope (or) the method is specified. Practice Problem: Given a watershed with a maximum flow path of 6,000 feet and an average slope of 7%, estimate the time of Concentration ?

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