Introduction
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. For example, the peak discharge of the standard SCS dimensionless unit hydrograph described in Chapter 16, NEH‑4, is inversely related to 0.67 Tc.
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.
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.