Time of Concentration: A Modification of the TR-55 Methodology

A new numerical procedure is presented for computing the time of concentration. Time of concentration, which is the time after commencement of rainfall excess when all portions of a contributing watershed simultaneously discharge to a point of concentration, can be a very significant hydrologic variable in the design of hydraulic structures. The proposed methodology combines the procedures and guidelines presented in the Soil Conservation Service (now the Natural Resource Conservation Service, NRCS) Technical Release 55 (TR-55) with a new computational algorithm. Using a unit width approach, the algorithm iteratively defines the average channel flow parameters to determine the channel element travel time and ultimately the time of concentration for the contributing watershed. The proposed methodology should only be applied to extreme flood events (i.e., the Probable Maximum Flood) where the natural watercourse does not have the capacity to contain the flood flows and wide shallow flooding would occur. Examples are provided for urban and rural watersheds to demonstrate the capability of this numerical algorithm. These examples are located in the north central part of the Buckhorn-Mesa watershed in Maricopa County, about 10 miles northeast of downtown Mesa, Arizona. 10 pp., 9 references.