The calculation of runoff is a complex endeavor, influenced by numerous factors including ground permeability, rainfall duration and pattern, and catchment characteristics. The Rational Method, a simplified approach, aims to determine the maximum discharge for design purposes. It operates under the assumption that rainfall duration matches the time of concentration, and the return period of rainfall intensity aligns with the peak runoff.
Time of concentration refers to the time it takes for stormwater from the farthest point within the catchment area to reach the outlet. When this time equals the rainfall duration, peak discharge occurs, with all rainfall collected within the catchment converging at the outlet. The Rational Method, however, only provides the peak discharge value and cannot generate a comprehensive hydrograph. For a more detailed runoff pattern, alternative methods like the unit hydrograph must be employed.
The accuracy of the Rational Method hinges on the correct selection of the runoff coefficient and precise delineation of the catchment area. This method tends to be conservative, and one of its fundamental assumptions is that rainfall intensity remains constant for an interval at least equal to the time of concentration. For prolonged rainfall events, this assumption might not hold true.
Furthermore, accurately determining the runoff coefficient in the Rational Method is challenging, as it depends on various factors like soil moisture content, rainfall intensity and duration, degree of soil compaction, and vegetation cover. Moreover, the Rational Method assumes the runoff coefficient is independent of rainfall intensity, which doesn’t reflect real-world scenarios where higher intensity often leads to increased runoff due to saturation and reduced infiltration.
Despite its limitations, the Rational Method remains a valuable tool in hydrology, offering a relatively simple way to estimate peak discharge for design purposes. However, it’s crucial to acknowledge its underlying assumptions and inherent uncertainties, especially when dealing with complex or large-scale catchments.