Abstract

Urbanization leads to the transformation of green pastures into impermeable surfaces, resulting in both advantages and challenges. Among the challenges, urban flooding emerges as a significant issue. The rapid urbanization in Thimphu has generated a substantial amount of runoff. Thimphu Thromde encompasses a total area of 26 sq. km, extending from Dechencholing in the north to Babesa in the south. However, this research focuses solely on the core Thromde area, which spans 8.5 sq. km and experiences drainage-related flooding.

This study aims to achieve several primary objectives. Firstly, it seeks to estimate the amount of surface runoff in the core Thimphu Thromde catchment by utilizing historical rainfall data. Secondly, it intends to investigate the relationship between flooding and drainage design elements, such as capacity, alignment, joints, and slope. Thirdly, the study aims to analyze the influence of drain blockage factors on flooding. Lastly, it aims to assess the magnitude of storm drainage flooding, considering both individual factors and combinations of factors, utilizing SWMM software.

A hydrological model was developed using SWMM to analyze the study areas. The study area was divided into 211 urban sub-catchments, and the maximum daily rainfall from each year was employed to assess the existing stormwater drainage flooding. In terms of drainage design factors, the simulation results revealed that alignment and slope were the primary factors influencing drainage flooding, accounting for 72.99% of the occurrences. Regarding joints, out of the total number of junctions experiencing drainage flooding, an average of 7.23% was attributed to capacity, resulting in four flooded junctions per simulation. Upon investigating blockage factors, 10 potential blockage points were identified, with seven out of ten experiencing flooding. The blockage factor contributed to 12.66% of the drainage flooding in the study area.