Detention Storage over 2D Laboratory Watersheds at Concentration Time

Abstract

Water balances, for two-dimensional (2D) laboratory experiments with long-duration rainfalls over planar aluminum surfaces, indicate that at time of concentration the detention storage comprises about 70% of the accumulating inflow. Under the applied experimental conditions, its spatially averaged depth is 1 to 6 mm. This depth increases with rainfall intensity and decreases with main watershed slope. The time of concentration is defined here from rainfall commencement until the outflow begins a gradual approach towards the equilibrium state. It was shown, in an earlier paper, that the ratio of peak runoff discharge to rate of supply attributable to rainfall of a shorter duration is linearly related to the ratio of rainfall duration to time of concentration. Such a relation led to the formulation of the 120-year-old Rational Formula, commonly used for hydrologic design of populous districts. The similarity between laboratory and field results allows considering the detention depths in the laboratory as fairly representing the depths over outdoor impervious surfaces.