ASSESSMENT OF KINEMATIC WAVE TIME OF CONCENTRATION

There is a need for an accurate limit for the practical use of the kin ematic wave form of the St. Venant equations for computing the time of concentration for sheet flow. Such a limit will help ensure accurate designs for small watersheds. The current practice of limiting the flo w length in the kinematic-wave time-of-concentration equation is probl ematic. This paper presents and analyzes limiting criteria, including L, iL, and nL square-root S, where L is the overland flow length, i is the rainfall intensity, and S is the slope of the surface. Data repor ted in the literature from 59 actual and experimental watersheds are e mpirically analyzed. The criterion nL/square-root S appears to be the best criterion, and it results in minimum standard errors of estimate when the upper limit of nL/square-root S is 100 (English units). Theor etical routing using a range of basin slopes, roughness coefficients, and flow lengths gives an optimum limit on nL square-root S of 80 (Eng lish system). Since the empirical analysis has a more direct relations hip to the application of the kinematic wave model, the upper limit of 100 on the nL/square-root S criterion is recommended for design work. When the kinematic equation is employed to compute times of concentra tion for given values of n and S, the length of sheet flow should be c hosen such that the nL square-root S criterion does not exceed the rec ommended limit of 100.