Sediment transport by medium to large drops impacting flows at subterminal velocity

Dripper-type rainfall simulators producing water drops from drop formers (metal or plastic tubes) have been widely used in laboratory experiments. These rainfall simulators produce medium (2.0-3.5 mm) to large (> 5.0 mm) raindrops and, in many cases, because of restrictions in the height of fall from the drop formers to the target, rain with drop impact velocities that are considerably less than terminal velocity. To answer the question "Can the results for drops impacting at low velocity be extrapolated to the terminal velocity situation using rainfall kinetic energy as an independent factor?", the results of rainfall simulator experiments with medium (2.7 mm) and large (5.1 mm) pendant drops impacting water flowing over surfaces of 0.2-mm sand and heights of fall 3 m and less are presented and compared with the results obtained when the drops fell 11.2 m. A linear relationship between sediment discharge and the expenditure of the kinetic energy of the rain in unit time was obtained with 2.7-mm drops but not with 5.1-mm drops. The results indicate that the departure from natural rainfall in terms of drop size and velocity is sufficient for data produced by dripper-type simulators using large drops to be not as useful in a practical sense as data produced by rainfall simulators that produce rain drop sizes and velocities that are closer to those observed in natural rainfall.