Reformulating the Rational Method Considering Dissimilar Land Use Types

Abstract

The rational method (RM) is one of the most used approaches in engineering practice to determine peak runoff flows. Despite its limitations and the increasing availability of more sophisticated hydrological models, the RM is intuitive, simple, and easy to use, providing rapid estimates frequently used in urban drainage design. The modified rational method (MRM) is an RM-based model that assumes a linear concentration of flows so that there is a linear increase of the runoff off peak flow until the basin time of concentration is reached. In both methods, the design practice averages runoff coefficients when the land use in the drainage areas is dissimilar and assumes that the design rainfall duration should match the whole area’s time of concentration (Tc). These methods may provide underestimated peak flows in areas with varied land use in case of very different Tc values. This work introduces the peak flow search approach (PFSA), which generalizes the method for peak flow computation from drainage subareas with different land use types, resulting in the critical rainfall duration not matching the drainage area Tc. The PFSA proposes a heuristic search to determine the critical rainfall duration associated with the peak flow from the combined dissimilar areas. For the tested cases, and depending on the adopted formulation for Tc, the discrepancy in peak flows yielded by RM and the PFSA reached 158%. This easily implementable approach can potentially improve engineering practice through more representative estimates of peak runoff flows.