Systematic stormwater management: Getting the rainfall right

Mike Gregory

ABSTRACT

The field of stormwater management has been dynamically evolving in recent years, as is evident in the following ways:

  • Widespread acceptance of green infrastructure (e.g., vegetated facilities and rainwater harvesting systems) into the fold of urban drainage design, complementary to traditional grey infrastructure (e.g., pipes and ponds);
  • Increasingly demanding regulatory and permitting compliance requirements for infrastructure design, whether retrofit of existing systems, new development or infill/redevelopment projects;
  • Improved quantification of flood risk or damage potential (e.g., to assist floodplain regulation or the insurance industry);
  • Improved quantification of the water quantity and quality impacts to downstream properties and receiving watercourses/waterbodies; and
  • Promotion of asset management principles and the desire to extend the service life of stormwater infrastructure by optimizing system performance and efficiency over a range of operating conditions, rather than a single design flow rate or loading condition.

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.


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