Time of Concentration

Prior to use of TR-55 (Win TR-55, or the Citygreen implementation of TR-55) or the analysis of results from any of these models, an understanding of time of concentration is recommended. This paper is a technical discussion of the components that make up Tc for a watershed.

To accurately determine the Tc for a watershed, the hydraulics of each part of the flow path must be considered separately. This can be done by dividing the flow path into overland, channel (small), channel (large), and pipe flow segments. The travel time (Tc) can then be computed for each segment and totaled to obtain the Tc. Each of these will be discussed separately.

The time of concentration (Tc) is used in most procedures to develop runoff hydrographs or estimate peak discharges. The peak rate of runoff is very sensitive to Tc, particularly for small watersheds.

The procedures used to estimate Tc depend upon several factors including watershed characteristics (especially drainage area), climatic conditions, required accuracy, available data, and available time. For example, to design a small conservation practice such as grassed waterway, a shortcut procedure that assumes a certain generalized relationship between Tc and a few watershed characteristics but no relationship between Tc and rainfall intensity might be acceptable. However, for the development of a storm water management plan, an accurate estimate of the peak rate of runoff for a small watershed from at least two storm frequencies for the undeveloped, developing, and fully developed conditions would be needed. In this case, all available factors should be considered with particular attention given to the overland flow.

February 1983 (Revised June 1986)

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