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Stormwater management practices increasingly are required to meet not only peak flowrate restrictions but also are required to include measures that can minimize the impacts on base flow, water quality in receiving waters and erosion in watercourses. The best management practices (BMPs) and low impact development (LID) techniques that are specifically designed to reduce these impacts generally consider small to moderate rainfall events and the hydrologic modeling of these events is in some respect different from the modeling for the larger storms that are typically used in control of peak flowrates.Small storm hydrology features have been defined and studied for the past 20 y (Pitt, 1987;1999a) to examine the specific elements that should be taken into account for the design of BMPs used for recharge, quality and erosion control. After a discussion of characteristics of rainfall events and design criteria, this chapter reviews small storm hydrology concepts and the main findings as they are currently applied in different stormwater management guides.The chapter subsequently describes how SWMM5 (Stormwater Management Model, V5-as implemented in PCSWMM.NET) could be used to reproduce results obtained empirically in small storm hydrology research, with a specific discussion of design storms. Finally, the use of SWMM5 for the analysis of filter strip, infiltration trench, porous pavement and bioretention is discussed.

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Almost twenty-five years after the first generation computer models were made available in the 1970s, we have to recognize that the question of the 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 ofthe 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 tmiformly 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

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