ANALYSIS OF RAINFALL SERIES IN THE DESIGN OF URBAN DRAINAGE CONTROL SYSTEMS

Abstract

Increased urbanization has caused increased flash flooding after sudden rain in Delhi. As the area of vegetation are replaced by concrete, tarmac or roofed areas, the area loses its ability to absorb rainwater. The rain is directed to drainage systems and causes urban flooding. The idea behind urban drainage control systems is to drain away dirty and surface runoff through collection,storage and cleaning before allowing it to released into water courses. Past efforts were mainly directed towards flood control of urban area with the pollution control being largely ignored. Currently, it is important to develop design methodologies for both flood and pollution control in the city of Delhi because there is heavy pollutional load in River Yamuna. The current criteria used in determining water quantity volumes in urban drainage control systems design neglect predecessor conditions resulting from preceding rainfall. During the dry period between rainfall events,treatment of stormwater proceeds and is considered complete after a specific time period. The inter-event-dry period refers to that time period which occurs between rainfall events. The minimum inter event dry period used for urban drainage control system design should be consistent with the time required for infiltration,chemical precipitation,residue removal and biological assimilation,the time required for the transport system( pipes, open channels and other structural controls) to return to their design elevations. Spreadsheet programming was used for calculation using latest 15 years of data from eight rainfall station in state of Delhi. A minimum inter-event-dry period was specified and all rainfall volume was cumulated before the minimum inter-event-dry period. Exceedence probability distributions were calculated for rainfall precipitation volume(p) given interevent- dry period(I) of 24 , 48, 72, 96, 120 hours. The precipitation volume for each inter event dry period and specified frequency (F) or return period was calculated and results are presented in graphical form are called PIF curves. Statistical data on rainfall volumes and duration for each minimum inter-even-dry period were also developed. These statistics are useful when probability distribution function are used for hydrologic and storm water designs.