Abstract

Almost twenty-five years after the first generation computer models were made available in the 1970s, we have to recognize that the question of appropriate rainfall data to use for standard urban drainage design is still unresolved in a completely satisfactory manner. The first attempts to derive a design storm and specific synthetic time distributions were inevitably based on the intensity-duration frequency (IDF) curves, which are used with the rational method, e.g. the Chicago design storm (Keifer and Chu, 1957). Recognizing the limitations of this approach, many researchers at the end of the 1970s and beginning of the 1980s proposed different alternatives based on a more realistic analysis of the rainfall data (Pilgrim and Cordery, 1975; Walesh et al., 1979; Hogg, 1981; Hogg, 1982); these approaches were not however used widely as they implied tedious calculations and expensive computer time. Practitioners therefore continued using synthetic design storms, as they provided a simple and apparently appropriate tool for routine designs.

The basic argument developed here is that with the powerful micro-computers available today, the costs and complexity of a more thorough analysis are no longer an argument for simplistic design storms applied uniformly for any type of design problems. A methodology is therefore proposed to define a series of design storm events using actual rainfall data and computer simulations. The analysis has a definite practical orientation in order to obtain a set of design storms that can be used in different design situations, specifically for new urban developments and considering only the quantitative aspects. The important parameters taken into account are the duration of the rainfall event, the land use and whether there is retention or not. After a brief examination of the design storm concept with an historical perspective, the proposed conceptual framework is given and discussed. A case study for the Montréal metropolitan area is presented, based on actual rainfall data for the Dorval International Airport in Montréal. Comparison with synthetic design storms commonly used in the area is provided.

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