The effect of rainfall movement on peak flow in sewers

Abstract

Kinematic and areal rainfall properties are physical phenomena of the rain event in the nature. Normally, these phenomena are not taken into consideration when designing urban sewer systems. This paper deals with the pure rainfall kinematics and focuses especially on the effect on peak flow in urban sewers. With the introduction of the computer technology the pure movement of the rainfall may be taken into consideration very easily, if found to be significant. In the study, computer calculations have been made to evaluate the effect of rainfall movement. A number of 25 extreme events have been selected from a historical rain series. The rain events have been simulated to move by the observed wind speed and direction across an artificial catchment rotated into eight different directions. The calculations were made with the mouse-program, modelling runoff using the dynamic wave approach. An additional program was developed to simulate the moving rain input. Peak discharge in the sewer system with and without rainfall movement was treated statistically in terms of return period and compared. The results show that the pure movement of the rain may be significant on an individual basis (at least up to 19% difference in discharge), while it is not important to the peak discharge from a statistical point of view (up to 5%). That applies to any of the catchments. The artificial catchment with flow direction towards NE gives the greatest increase in the maximum discharge, and vice versa with the catchment orientated towards SW. This is due to wind directions from the SW being the most frequent in Denmark.