Design Hyetographs for Small Drainage Structures

Improved methods for design and pollution control of storm drainage facilities require not only the average rainfall intensity but also the time distribution of the rainfall. Since the temporal pattern of rainfall varies with storms and the patterns of future storms cannot be predicted exactly, the design hyetograph can only be determined probabilistically using statistical values of past events. The nondimensional triangular hyetograph method is based on the statistical mean of the first time moment of recorded rainstorms and triangular representation of the hyetographs, which are nondimensionalized using the rainfall duration and depth. For a given season the expected nondimensional triangular hyetographs for heavy rainstorms are nearly identical, being insignificantly affected by the duration of rainfall. The established nondimensional triangular hyetograph can then be used to produce the design hyetographs by simple procedures. Results of an analysis of 7,484 rainstorms at three locations indicate that such a method is feasible.