Evaluation Of Calculated Peak Runoff Flow Rates At The Limitations of Different Hydrologic Methods in Knox County, Tennessee

Determining the anticipated peak flow for a drainage crossing is critical for infrastructure design. Improper design can lead to degradation of downstream water quality, flooding of adjacent neighbors, or failure of critical infrastructure. There are many applicable hydrologic methods that may be used for a location, but the requirements for these methods may vary depending on the governing authority. For example, the Stormwater Management Manual for Knox County, Tennessee has a restriction on the applicable drainage area of the Rational method of 5 acres, which is not consistent with surrounding local, state, and federal entities. Urban Regression equations (Urban Regression) by the US Geological Survey (USGS), the Rational method, and the Natural Resources Conservation Service (NRCS) Technical Release 55 (TR-55) are common methods for determining peak flows for different rainfall events. Urban Regression equations are based upon statistical analysis of observed flow data for a specific region. The Rational method and TR-55 method are widely accepted, but the input parameters are dictated by the requirements from the respective governing authorities. USGS StreamStats is a web-based application that provides peak flow results for ungauged locations based on the USGS regression equations. This study calculated the peak flows for the 2-, 10-, and 100-year storm events for 29 drainage areas within Knox County using Urban Regression, Rational method, and TR-55 to compare the results between the different methods at a size range where the applicability of the different methods overlap between different entity requirements (20-600 acres). USGS results were used as the baseline to normalize the results from the three methods since the results can be obtained by all design engineers. The study yields similar results between the Rational method and Urban Regression for drainage areas up to 150 acres. The Rational method had similar trends to TR-55, but the TR-55 results were 1 to 1.5 times larger for the 10-year and 100-year storm events. From the results of the study, the Rational method results are consistent with other methods up to the drainage area size above 100 acres. It appears the limit of 100 acres for the Rational method is reasonable.