Abstract
Some probabilistic characteristics of rainfall were analysed to make clear the relations among rainfall, drainage capacity and allowable flooding for the drainage of converted upland field. The results of the analysis were as follows.
(1) Relations between drainage capacity and maximum flooding under certain rainfalls were represented using simple water budget based on hourly rainfall and constant drainage capacity under the condition of no time lag and no water losses.
(2) Drainage capacity that was needed for a certain maximum flooding proved to be of log-normal distribution through the application of annual maximum 24-hour rainfalls.
(3) Relations between drainage capacity and maximum flooding were formulated to be Qd=c*exp (-kSm), where Qd: drainage capacity, Sm: maximum flooding, and c and k: coefficients.
(4) Relations between drainage capacity and maximum flooding were also formulated based on the Talbot and the Sherman formulae of the relations between intensity and duration of rainfall.
(5) Infiltration or runoff coefficients were taken into account in simple manner for the relations between drainage capacity and maximum flooding.
(6) Several examples of Qds are shown with Sm=30 and 50 mm for Mito, Utsunomiya, Maebashi, Yokohama, and Choushi.
