The mixed landuse watershed tested combines both urban and rural areas. The previous sections have shown that the SCS storm can be used on larger rural areas and the Chicago storm on urban areas. It would not be satisfactory to use the SCS storm on the rural segments and the Chicago storm on the urban areas of the mixed watershed. Both the Chicago and the SCS 24 hr. storm were tested on this watershed.

The flow frequency curves for the design storms and the historic storms are shown in the figure below. The SCS design storm underestimated the flow by 11% for a 5-yr. return period and overestimated the flow by 18% to 39.9% for return periods of 10 to 100 years. The Chicago storm was found to give better estimates of the peak flow; for a 5-year return period the flow was, underestimated by 5% and for return periods from 10 to 100 years the flow was overestimated by 0.3% to 28.0%.

The flow predictions made on this watershed with the design storms were not the same as the historical storm series estimates. This shows that the peak flow cannot be obtained by using an arbitrarily selected single design event. An analysis using a historical storm series should be conducted before a design storm is selected. In general, it is more desirable to use a historical storm series on this type of watershed.