I resided in Las Vegas, and the city, due to so many casino hotels springing up almost overnight, and the mountains surrounding it, was subject to waist-high floods that cost millions in repairs. The building boom took place on desert land and there were no storm sewers under the high-rises, nor could they be put in after the buildings were already in place.
They devised a system, maybe 25 years ago, that effectively ended flooding. I do not know the details of what they did nor if it would be practical to copy for Bangkok and some other parts of Thailand but I am suggesting that the Thai water people should at least review their system
Don Aleman