Development of Guidance for Runoff Coefficient Selection and Modified Rational Unit Hydrograph Method for Hydrologic Design

Abstract

The rational method is the most widely used method by hydraulic and drainage engineers to estimate design discharges. The runoff coefficient (C) is a key parameter for the rational method. Literature-based C values (Clit) are listed for different land-use/land-cover conditions in various design manuals and textbooks, but Clit appear not to be derived from any observed data. In this study, Clit values were derived for 90 watersheds in Texas from two sets of land-cover data for 1992 and 2001. C values were also estimated using observed rainfall and runoff data for more than 1,600 events in the study watersheds using two different approaches (1) the volumetric approach (Cv) (2) the rate-based approach (Crate).When compared with the Cv values, about 80 percent of Clit values were greater than Cv values. This result might indicate that literature-based C overestimate peak discharge for drainage design when used with the rational method. Similarly, when compared with the Crate values, about 75 percent of Clit values were greater than Crate values, however, for developed watersheds with more impervious cover, Clit values were greater than Crate values. Rate-based C were also developed as function of return period for 36 undeveloped watersheds in Texas using peak discharge frequency from previously published regional regression equations and rainfall intensity frequency for return periods of 2, 5, 10, 25, 50, and 100 years. The C values of this study increased with return period more rapidly than the increase suggested in prior literature.

To use the rational method for hydraulic structures involving storage, the modified rational method (MRM) was developed. The hydrograph developed using the MRM can be considered application of a special unit hydrograph (UH) that is termed the modified rational unit hydrograph (MRUH) in this study. Being a UH, the MRUH can be applied to nonuniform rainfall distributions and for watersheds with drainage areas greater than typically used for the rational method (a few hundred acres). The MRUH was applied to 90 watersheds in Texas using 1,600 rainfall-runoff events. The MRUH performed as well as other three UH methods (Gamma, Clark-HEC-1, and NRCS) when the same rainfall loss model was used.