Author(s): D. H. Yoo; T. H. Lee; M. H. Lee
Keywords: Flood forecasting models; Surface runoff flow; Roughness height; Bottom slope; Gravity; Surface tension; Time of concentration
Abstract: Most flood forecasting models assume that the surface runoff flow is gravity flow when they compute the time of concentration. But the laboratory results of Woo&Brater (1961) show that the friction factor of surface runoff flow is a function of bottom slope, which indicates the sheet flow might be strongly influenced by surface tension and hence bottom slope. New non-dimensional physical numbers are introduced by adopting bottom slope, water depth and roughness height as major variables and new empirical equations of concentration time are developed respectively for both conditions of gravity flow and surface tension flow. New three non-dimensional physical numbers are the products of combining Reynolds number, Froude number and Weber number which is strongly related to surface tension. All physical numbers appear as functions of bottom slope and water depth as well as roughness height. The friction factor of surface runoff flow is related to the new physical numbers and the time of concentration has been computed by using the new forms of friction factor. Employing the laboratory data of Butler (1977), the computation has been tested. When the surface runoff is related only to roughness height, the time of concentration is estimated smaller than the value of measurement, But when the surface runoff is related to both roughness height and bottom slope, the time of concentration is well estimated in comparison with the measurements. The fact indicates that the impermeable surface runoff flow is strongly influenced by surface tension as well as gravity force. Furthermore, an equation for computing the complex time of concentration, which can consider rainfall characteristics that change in accordance with local frequency when the rainfall intensity is given under the condition of surface laminar flow in general and when the return period or other conditions are given instead of the rain intensity, was developed.