Reliability of Design Storms used to Size Urban Stormwater System Elements
The objective of this study was to generate hydrographs using design storms and real rain data recorded over a 30 y time period. In the first step rainfall characteristics were investigated using Intensity-Duration-Frequency (IDF) relationships. IDF curves were developed, called UM IDF curves, and when compared to the official Hamilton IDF curves showed a striking similarity for events with low return periods, and some variation for 50 and 100 y return periods. From the UM IDF curves three new design storms (SCS type 2, AES Canadian and Chicago Storm), referred to as UM Design Storms, were developed for comparison with the Hamilton design storms. The differences in intensity-duration of the less frequent storms (observed in the IDF comparison) were also reflected in the design storm volume and peak discharge. All the different design distributions, for a wide range of return periods, were entered into the rainfall-runoff model, PCSWMM, for event simulation of design storms. PCSWMM was also run for continuous simulation of the coarse observed data. Keeping the geomorphic conditions and loss parameters constant, differences were quantified in terms of computed flow and storage capacities for two design applications. The designs derived from the coarse observed data were considered to be optimal and used to decide suitability or otherwise of the designs based on other synthetic distribution patterns, and it was found that almost all the application designs obtained from design storm hyetographs were over-designed. In other words, for every return period, the flow capacity and detention storage required to accommodate the design storm hydrographs was greater than what would be required if the same applications were designed for historic storms in continuous simulation. Given the coarse continuous data these results are to be expected, and should be reexamined when continuous at a fine time step become available.
Evidently design storms generate a hydrograph with greater volume and a higher peak, requiring more detention storage, greater flow capacity and larger pipe sizes, than using coarse continuous simulation. It is still not yet clear whether structures based on design storms are over-sized and costlier than necessary.
The study showed that SCS type 2 and the AES Canadian design storms yield flow and storage capacities relatively closer to the coarse continuous data while the Chicago design storms produce over-sized flow and storage capacities. The latter may be deemed to not provide representative rainfall distributions for Southern Ontario.
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