J.F. McLarens Ltd. (1978) has conducted studies on catchments in Edmonton and Winnipeg. They found that the ratio of the Chicago storm peak flow to the flows from an historic storm series ranged between 1.0 and 1.2. It was recommended that the Chicago design storm be used for urban drainage design.

Marsalek (1979) developed a Chicago design storm for the Burlington area. He found that the peak flows produced from the Chicago design storm are 80% larger than those produced from historical storm events. He also found that the peak flow was attenuated as the catchment size increased. The peak flow increased as the catchment imperviousness increased but the peak flow overestimation remains at approximately 80%. These results were analyzed in the IMPSWM pro-gram by Wisner and Gupta (1980). They concluded that discrepancies can be reduced if the peak intensity of the design storms are reduced to values in agreement with measured peak intensities.

Watson (1980) compared the peak flows obtained from the Chicago design storm and historical storm events. A 2-hr. duration and a non-dimensional time to peak of 0.28 is used to develop the Chicago storm. The rainfall data is discretized at 5 min. intervals.

Watson found that on the Pinetown catchment the peak flows from the Chicago storm agreed closely with those from the historical storms. The agreement for the Kew catchment was not quite as good. The peak flow is slightly underestimated. It is within 95% confidence interval bands of the historical storms, though. The Kew catchment is less impervious than the Pinetown catchment; therefore, it is more sensitive to antecedent moisture conditions.

Arnell (1982) used a Chicago design storm with a 4-hr duration. The non-dimensional time to peak, r, is 0.43 if the return period is less than 1 year. If the return period is greater than 1 year, r is 0.35. The Chicago storm is developed with a step size of one minute.

Arnell found that the Chicago design storm overestimated the peak flow by approximately 5%. On the Bergsjon catchment, the peak flow is underestimated by 2.2%. On the Linkoping 1 and Linkoping 2 catchments the flow is overestimated by 10.3% and 6% respectively. The Bergsjon catchment was the smallest of the three catchments. The Chicago storm produces peak flows al-most identical to the historical storms on this catchment.

With the exception of Marsalek (1979), the estimation of peak flow produced by the Chicago storm gave acceptable results compared with that produced by historical storms. Differences range from a 2% underestimation to an 10% overestimation of peak flow.

Watson recommended that the Chicago design storm be used for peak flow design. Arnell also found that the deviation of the Chicago Storm peak flow values from the historical storm peak flow values are not large. He concludes that the Chicago Storm should overestimate peak flows be-cause of the way it is developed. He does not recommend the use of the Chicago design storm because of the large overestimation of peak flow Marsalek found.


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