A Note on the Time of Concentration

Abstract

Time of concentration is an important parameter in rainfall–runoff models, such as the rational method and the unit hydrograph method, used for drainage design. There are several equations for computing the time of concentration. Many of these equations are empirical and are mostly based on length and slope, and some equations depend also on drainage area. Equations have also been derived using the kinematic wave theory, which accounts for length, slope, roughness coefficient, quality of flow (laminar, transient or turbulent) and shape (rectangular, converging, diverging, or diverging-converging), and rainfall or rainfall excess intensity and duration. In all of the equations, it is assumed that the rainstorm is stationary, but rainstorms are often dynamic and move in a particular direction and for a particular period of time, which significantly affect the time of concentration. There seems to have been no attempt to evaluate the impact of storm direction and duration as well as watershed shape on the time of concentration. The objective of this paper therefore is to revisit the concept of time of concentration and empirically derived equations, present the time of concentration equations for rectangular, converging, and diverging planes under stationary storms reported in the literature, examine the impact of moving storms for a rectangular plane, and compare these equations with empirically derived equations. The objective here is not to test or validate these equations for different watersheds.