Expanding the Modified Rational Method for Dissimilar Drainage Areas

Abstract

The Rational Method (RM) and Modified Ration Method (MRM) are some of the most used approaches to determining peak runoff flows in urban hydrology designs. The RM gained popularity for its simplicity, requiring only easy-to-acquire hydrological properties to provide quick estimates for drainage design. Yet, in highly urbanized areas, the land use in a design area is often dissimilar, limiting the user to average the runoff coefficients and use the design rainfall duration that matches the entire design area’s time of concentration (Tc). The results from such limitations often fall short compared to more sophisticated, physically based approaches and tend to provide underestimated peak flow hydrographs. This work introduces the Discretized Modified Rational Method (DMRM), an approach that provides better estimate of runoff hydrograph for dissimilar design areas while retaining the simplicity of RM-based methods. The method is a variation of the MRM that avoids combining different drainage sub-area’s runoff coefficient and Tc and instead considers individual MRM-like runoff hydrographs from each sub-area. The rainfall duration is systematically varied yielding different rainfall intensities at each minute, and the sub-areas hydrographs are assumed to drain independently toward an outfall of the drainage area. Through this heuristic search, the individual hydrographs from each sub-area are combined and the one with the peak dicharge is selected for the analysis. The DMRM peak flow hydrograph is then compared to MRM, and for the tested results DMRM have presented higher peak flows. While a comparison between RM-based results and hydrological modeling results is difficult due to the very different nature of these tools, the hydrographs from the DMRM are compared with the ones yielded by EPA SWMM 5. To perform this comparison, the sub-areas parameters used in the DMRM examples, including CN values, were also used in the SWMM input file. In addition, the DMRM hyetograph, i.e., fixed intensity short duration rain event, was also input in SWMM. The resulting peak flow comparison between these two averages is at 6.6%, with a maximum being 18% and a minimum of 1%, with DMRM peak flows generally similar to the ones yielded by SWMM 5, which is considered a superior tool for hydrological analysis. This can indicate the importance of considering the effects of different land uses and their abstraction and time of concentration values when applying RM-based approaches.