A comparative study of the time of concentration methods for designing urban drainage infrastructure

The time of concentration (ToC) is an important parameter in rainfall−runoff simulation for designing and evaluating an urban drainage system (UDS). There are several lumped and distributed methods available in the literature for estimating the ToC. However, these methods lead to significantly varied values. Therefore, it is imperative to choose an appropriate and best-suited method for estimating the ToC. This study analyses eight lumped approach-based and two distributed approach-based methods for estimating the ToC in an urban area of Gurugram, a satellite city in the National Capital Region (NCR) of Delhi in India. Considering the ToC obtained by the Natural Resource Conservation Service (NRCS) method as the ‘true’ value, the Carter method among lumped methods and the SWDM method between the distributed methods results in ToC values in agreement with the NRCS method. Furthermore, to study the impact of the underestimation or overestimation of ToC on drainage, the system is evaluated in terms of variation in flood volume, duration, peak discharge, and the time to peak for different ToC values. The simulations were carried out by setting the model in SWMM, and it was found that flood volume increases by 4.25 times and the duration increases by 7.25 times if the ToC is increased from 0.1 to 6.14 h. The results infer that ToC estimation methods significantly impact the design and performance of an urban drainage infrastructure.