Storm Duration and Antecedent Moisture Conditions for Flood Discharge Estimation
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Storm Duration and Antecedent Moisture Conditions for Flood Discharge Estimation

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    Final Report July 2001 – March 2003
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    Design flows estimated by flood hydrograph simulation can be reasonably accurate or greatly in error, depending upon the modeling procedures and inputs selected. The objectives of this research project were (1) to determine which combinations of modeling procedures and inputs yield the best discharge estimates under various conditions and (2) to develop specific guidelines for flood hydrograph simulation for possible inclusion in the KDOT Design Manual. Many different combinations of modeling procedures and inputs were tested in flood simulations for 66 gaged watersheds in Kansas. The test watersheds were all primarily rural with unregulated streams, drainage areas under 50 km2 and gaging records of 20 years or longer. The simulations were performed with the HEC-1 computer program of the U. S. Army Corps of Engineers. The key inputs were the duration of the hypothetical frequency-based storm and the antecedent moisture condition (AMC) in the NRCS loss model. Floods with six different recurrence intervals (2, 5, 10, 25, 50 and 100 years) were simulated using four different storm durations (3, 6, 12 and 24 hours), five different antecedent moisture conditions (AMC 2, 2<, 2=, 2> and 3) and the two different unit-hydrograph models (NRCS and Snyder). The results for the watersheds in eastern and western Kansas were analyzed separately. We computed the bias and standard error of the simulated flows, relative to the gage-based estimates, for each combination of recurrence interval, storm duration, AMC and unit-hydrograph model in each region. From these results, we identified combinations of storm durations and AMCs that yield unbiased discharge estimates for each set of conditions. Longer storm durations and/or higher AMCs are needed for higher recurrence intervals. The storm durations are shorter and the AMCs are lower in the western region than in the eastern region. Flood hydrograph simulation with the recommended inputs is approximately as accurate as the USGS regression equations in eastern Kansas and more accurate than the USGS regression equations in western Kansas. The standard errors of the simulated flows are larger in western Kansas than in eastern Kansas. We recommend simplified guidelines for flood hydrograph simulation for inclusion in the KDOT Design Manual, Volume I, Part C. These guidelines specify the NRCS UH model and a single combination of storm duration and AMC for each recurrence interval and region. 50 p.
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