Incidence of Rainfall Intensity in the Time of Concentration of Small Watersheds in Uruguay

Abstract

The existing methods of hydrological design intend to represent the response of a basin to a storm by determining the flood hydrograph. Time of concentration (tc) is a parameter that reflects the timing of the discharge hydrograph and characteristic values such as the maximum flow required for the design of hydraulic works, e.g, small dams and reservoirs. Design accuracy is sensitive to the precision with which the time of concentration is determined. However, the equations used in Uruguay to estimate the tc have not been tested against observations in domestic basins. In order to evaluate the fit of different empirical equations, the observed tc, computed from the analysis of measured hydrographs and hyetographs were compared, and their relationship with rainfall intensity and recurrence was analyzed. Three equations were compared in 68 events in four small watersheds of contrasting uses, with areas between 69 and 212 ha. The observed tc values varied with: i) the rainfall intensity for return periods shorter than five years, unlike commonly used equations which state a constant value, and ii) coverage for rainfall intensities below 60 mm h-1, limit from which no differences were observed between coverages. Equations to compute the time of concentration are proposed based on the tc values that were observed in domestic watersheds, noticing that the associated return periods are not high enough to hydraulic works design.