Simulations with the Chicago and SCS 24-hr design storms were compared with a series of real storms to determine if the flow frequency results they produce are reliable. The design storms and a historical storm series were tested on rural, urban and mixed land use watersheds using the OTTHYMO model. For the historical storms antecedent moisture conditions were accounted for using the modified curve number CN*. According to the IMPSWM methodology the antecedent precipitation index, API, is determined at the beginning of a storm event. The modified curve number is then found by using the API vs. CN* relationship for the watershed. In addition, the SWMM model was used with the Chicago design storm on an urban area. The conclusions of the study are listed below.

  1. In rural areas antecedent moisture conditions were shown to be critical in determining peak flows.
  2. A modified curve number was calibrated which in conjunction with the SCS 24-hr. design storm gives adequate results on rural watersheds. This “design curve number” is proposed to be part of the design storm concept.
  3. Using the “calibrated design curve number” on large rural watersheds it was found that Chicago design storms underestimate the flow. The SCS 24-hr design storm gave good flow predictions on both large and small rural watersheds. For a small rural watershed, the difference between the Chicago and SC-S 24-hr peak flow was found to minimal.
  4. For urban areas the SCS 24-hr design storm underestimated the peak flow. The Chicago storm gave consistent peak flow results. It was also demonstrated that the peak intensity is an important factor in determining he peak flows in urban areas. For this reason, design storm peak intensities should be selected on the basis of a study of peak intensities of critical real storm events.
  5. On the large mixed land use watershed antecedent moisture conditions must be considered if the contribution of the rural watershed is important. For flood control purposes in large mixed areas it is preferable to use a series of real storms. One cannot tell prior to analysis of the watershed which design storm will produce acceptable results.
  6. Results found for the homogeneous watershed indicate that for routine SWM studies design storms can be used. It is however desirable to compare the two types of design storms and eventually a critical historic storm.