SCS Urban Peak Flow Methods

The peak flow hydrologic methods of the U.S. Soil Conservation Service (SCS) (as contained in their Technical Release No. 55) are widely used for planning and design on small urban watersheds. The urbanization factors of the TR‐55 methods (i.e., the lag factors for the graphical method and the peak factors for the chart method) were apparently not directly based on analyses of measured data and have not previously been systematically tested. Therefore, the objective of this study was to test the TR‐55 methods and recalibrate the urbanization factors if the methods were found to be biased. A data base was compiled for 51 small urban watersheds under 4,000 acres (1,600 ha) in the United States. The annual maximum flood series were used to develop Log‐Pearson Type III frequency curves, from which peak discharge rates were estimated for the following return periods: 2‐yr, 5‐yr, 10‐yr, 25‐yr, 50‐yr, and 100‐yr. The bias and accuracy of the SCS methods were evaluated using the Log‐Pearson Type III estimate as the expected true value. The results indicate that no urbanization correction factors are needed for the graphical methods when time of concentration is determined using the velocity method. Modifications to the urban adjustment factors used in the chart method are proposed.