The field of stormwater management has been dynamically evolving in recent years, as is evident in the following ways:
Collectively, these motivating factors reinforce the need to apply a systematic approach to stormwater management. The complementary nature of green and grey infrastructure as well as their multi-objective design functions demand that both event-based and continuous hydrologic simulation be applied in the analysis, design, and control of urban drainage systems. For example, the design of green infrastructure is intended to capture and retain a specified volume that corresponds to a common occurrence of rainfall (e.g., an X-percentile rainfall depth). Likewise, water quality treatment or control operations are focused on average annual or seasonal performance measures. However, the design of grey infrastructure is intended to achieve a specified level of protection that corresponds to a rare occurrence of rainfall (e.g., an X-year design storm event).
This paper presents the case that the proper characterization of rainfall is the first and most critical step in the context of systematic stormwater management. The timing aspects of infrastructure analysis, design, and control (i.e., in terms of rainfall frequency of occurrence) can be addressed through the careful analysis of long-term local rainfall data, in the form of recorded time series or a statistical representation (e.g., IDF curve coefficients). Using real rainfall data, this paper demonstrates that the results of the rainfall analysis are very sensitive to: the type of analysis (i.e., simple statistics, percentiles, histogram analysis, or other types of frequency analyses); the definition of minimum interevent time period; the rainfall recording interval (e.g., minutely, hourly, daily); and the runoff response time of the catchment that the rain falls on.