The dependence of the historic storm flows to the peak intensity and antecedent moisture conditions were examined on the urban catchments. The peak intensity was found to be an important factor in determining the peak flows. The correlation between the peak intensity and peak flows was close to 1.


URBAN AREA (50% IMP.) CORRELATION BETWEEN API AND FLOWS

Antecedent moisture conditions do not show any correlation with the peak flows. It was found that runoff from urban catchments was independent of CN*. The SCS 24-hour design storm flows were compared with the historic storm series flows on urban watersheds No.1 and No.2. The comparison of the flows is shown on the figures below. The Chicago design storm was tested on the three urban watersheds. The flow predictions given by this storm are slightly below the historical storm series on urban areas No.1 and No.2. The SCS 24-hour design storm underestimated the flow on both of the watersheds. On watershed No.1 the flow was underestimated by 11% to 22% while on watershed No.2 from 14% to 26%. On these urban watersheds the SCS 24-hour design storm has underestimated the peak flow.


FLOW FREQUENCY CURVES, URBAN WATERSHED NO. 1, 35% AND 50% IMP


FLOW FREQUENCY CURVES, URBAN WATERSHED NO. 2, 50% IMP

The flows were underestimated by approximately 6% on watershed No.1 and 4% on watershed No.2. On urban watershed No.3 the SWMM simulation program was used. The Chicago storm gave results that were similar to the real storm flows as shown in the figure below. From the tests conducted on these three urban areas the Chicago design storm gave peak flow predictions close to the flows from the historic storm series.


COMPARISON OF REAL STORM AND CHICAGO STORM FLOWS, URBAN CATCHMENT NO. 3

The effect of changing the catchment imperviousness was examined on urban watershed No.1. Flows were generated on this catchment using the design storms and the historic storms for impervious conditions of 35% and 50%. Increasing the catchment imperviousness resulted in the SCS storm giving lower flows with respect to the real storm series. The relative position of the Chicago storm flows did not change. The Chicago storm is less sensitive to changes in the catchment imperviousness than the SCS 24-hour storm.

The comparison of the design storms on the urban catchments has shown that the Chicago storm gives a good prediction of peak flow on urban catchments. Flows in the urban areas were found to be dependent on peak intensity and independent of antecedent moisture conditions. The sensitivity of peak flows to peak intensities showed the importance of having design storm peak intensities similar to real storm peak intensities.

The Chicago storm that was used obtained the peak intensities by having a 10-minute step size. Increasing the imperviousness of urban catchment No.1 demonstrated that the Chicago design storm continued to give a good prediction of the peak flow. For these reasons the Chicago design storm may be used with single event simulation models on urban watersheds.

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