A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Water Engineering and Management

Thesis (M. Sc.) - Asian Institute of Technology, 2022

Several studies have been conducted relating to the impact assessment of the future climate change on the urban drainage system and the studies have shown that the impact will be considered in the higher design and maintenance work in the future. Climate conditions are not stationary, consequently, undesirable flooding problems occur. It is very important to understand the changes in extreme precipitation at the time and spatial scales and the response of the large-scale drainage system to control the risk of flood sustainably. This study focuses to analyze the conditions of large scale urban drainage systems under changing climate conditions. The study area is Eastern Bangkok, a big city covering 930 km2 of the catchment area with 104 numbers of pumping stations. The characteristic for the time of concentration was calculated and it was more than one day, 1.34 days while that of secondary drainage is 3 hours. This implied different time scales for hydrological design and operation. Intensity Duration-Frequency curves for sub-daily scales developed using data of 3 CMIP6 GCMs under two shared socioeconomic pathways (SSP) showed a high rate of increments ranging from 24.15 to 61.88%. Analysis of future annual maximum precipitation in consecutive days (Rx1-5days) calculated using Climpact2 with NorESM2-MM SSP 2-4.5 scenarios and EC-Earth3 under both SSP scenarios showed significant increasing trends. By using the MIKE Hydro River model, the impact of climate change on the drainage system with two-time scales which are three design storms of 2 hr, 3 hr, and 6 hr duration and consecutive rainfall was simulated. Computed runoff increased with a range of 18.6% to 76.34%. Analysis of the impact of consecutive rainfall caused increased initial water levels higher than the controlled plan water level and it caused the longer recession times of each outlet at 0.8 days, 2.8 days, and 3.6 days for the Bangsue outlet, Phrakhanong outlet, and Samsen outlet respectively.