KWRRI Research Reports


The Kentucky Department of Highways, as do most other agencies which build small drainage structures, estimates flood peaks as the product of a runoff coefficient, a rainfall intensity, and the drainage area, Available procedures were applied to 39 gaged watersheds in and near Kentucky and compared with the results of frequency analysis of historical stream gage records. The methods consistently underestimated the flood peak.

Therefore, a more intensive study (using the Stanford Watershed Model) of the runoff coefficient was undertaken by dividing it into overland flow and streamflow components. A set of curves was developed based on the 50-year event at Lexington, Kentucky, to estimate the peak rate of overland flow as a fraction of rainfall intensity from five measurable watershed characteristics (soil depth, permeability, overland slope, impervious area, and soil surface exposure). The streamflow component was not studied.

A study was made of the variation of the runoff coefficient with mean annual rainfall, rainfall intensity, and frequency. The investigation was based on applying the Stanford Watershed Model to six hourly-rainfall records. The result was a set of correction factors to apply to the overland flow values estimated from watershed characteristics.

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Funding Information

"Evaluation of Runoff Coefficients from Small Natural Drainage Areas" is the result of interaction between two research projects. One project, "Economic Analysis of Alternative Flood Control Measures," (OWRR Project No. A-001-KY) is sponsored by the University of Kentucky Water Resources Institute and supported in part by funds provided by the United States Department of Interior as authorized under the Water Resources Research Act of 1964, Public Law 88-379. The other project is sponsored by the Division of Research, Kentucky Department of Highways, and supported in part by funds provided by the United States Department of Transportation, Bureau of Public Roads (KYHPR-64-23).