Houston’s in Harris County, IIRC. Harris County has a flood control district. What in bloody blue blazes do those folks do if they don’t mitigate flood risk?
Part of what they do is torture small builders who don’t have enough money to buy influence about the size of their detention pond or in-kind flood remediation. The other part is mow the bayous and drainage and such.
Basically the Galveston Bay Basin is a big flat bottomed bowl and if scaled down to the size of a pool table would probably be flatter than a pool table. If this much rain as expected falls on such an area and drains from the slightly higher surrounding area to the north, ne and nw there is simply no way to get it out fast enough There will be floods. It is unavoidable. Only the local high spots are saved.
Compound the rain with storm surge in Galveston Bay where all the water has to go before it exits the few passes from the Bay and you have even worse flooding.
Houston has about as much freeboard above the sea as you would have by placing a piece of cardboard in a puddle.