Assessing Flood Risk in Thimphu: Analyzing Contributing Factors through Urban Flood Modeling

Thimphu City is grappling with a growing problem of urban flooding due to rapid development. The increase in impervious surfaces and resulting runoff has led to overflowing storm drainage systems that have been in place for the past two decades. The objective of this study is to determine the factors that contribute to urban flooding in Thimphu City and identify the specific sites that are most affected. To accomplish this, a hydrological model of the study area was created using SWMM software. The study area was divided into 211 urban sub-catchments and the daily maximum rainfall from each year was used to assess the current state of the stormwater drainage system. The study focused on locations that had previously experienced urban flooding and were chosen as major sites for investigation. The factors contributing to urban flooding were analyzed, with a focus on the drainage capacity, alignment or slope of the drainage, and blockages in the system. It was determined that the alignment and slope of the drainage system were the primary factors contributing to urban flooding, accounting for 72.29% of incidents. Blockages in the drainage system were found to be a factor in 12.66% of cases. This study highlights the importance of considering the alignment and slope of storm drainage systems in urban planning to mitigate the risk of urban flooding. It is recommended that drainage systems be designed with a slope that facilitates water flow and that regular maintenance is performed to prevent blockages. These measures will go a long way in reducing the impact of urban flooding in Thimphu City.