United States Department of Transportation
i
Estimating design discharges for drainage structures in western Kansas.
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2013-06-01
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[PDF-2.41 MB]
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NTL Classification:NTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and Structures;NTL-HIGHWAY/ROAD TRANSPORTATION-Design;
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Abstract:KDOT engineers have expressed concern that the hydrologic methods in the current KDOT Design Manual (Volume I,
Part C, 2011) may lead to over-sizing of drainage structures in Western Kansas. Some new structures designed by the current
methods are much larger than the previous structures at these locations or existing structures directly upstream or downstream,
where the older structures had no known history of overtopping. There are reasons to suspect that current methods may not be well
suited to small watersheds in Western Kansas, particularly for areas with high soil permeability.
This report examines the applicability of KDOT’s current hydrologic methods to Western Kansas and develops new
Rational C values and flood-frequency regression equations for this region. In addition, KDOT’s current hydrologic methods are
compared with those of nearby state DOTs.
In order to develop new flood-frequency regression equations and recommendations for Rational C values for Western
Kansas, we assembled a data set of all USGS gaging stations that met the following criteria: (1) at least 10 years of peak flow
records, (2) watershed area less than 100 mi2, (3) unregulated watersheds (no major lakes or reservoirs), and (4) watersheds within
100 miles of the Kansas border and west of 97.5° longitude. The resulting data set contains 156 stations, 62 of which are in
Kansas.
Regional flood frequency analyses were performed on this data set using Generalize Least Squares regression in WREG
1.0. Soil permeability was found not to be a significant predictor variable. Regression equations were developed for Western
Kansas, but our comparisons show that these equations are not a substantial improvement over existing regression equations.
Based on an evaluation of available methods, we recommend the Extended Rational method for watershed areas > 640 ac and < 30
mi2 and the USGS four-parameter regression equation for watersheds ≥ 30 mi2 in both Western and Eastern Kansas.
An analysis of Rational C values indicates that C values currently used for design in Western Kansas are too high for
recurrence intervals below 100 years. New Rational C values for Western Kansas were developed and checked against regression
methods for consistency. Our proposed C values for Western Kansas are lower than the current values for all recurrence intervals
below 100 years. We also propose certain adjustments to the Rational C values for Eastern Kansas. We recommend that urban open
spaces and pervious surfaces within the right-of-way be considered equivalent to pasture/range rather than cropland in both
Western and Eastern Kansas.
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