Central Texas and the TexasHill Country topography has a thin layer of top soil with rocks and boulders under. That area of Texas is normally pretty dry so the ground doesn't hold much water so all they have are rivers around. All them rivers drain to the Gulf of Mexico which are not close to Central Texas.
 
They have had way above normal rainfall this summer and this week they got hit again. 10 inches of rain in a couple hours will do damage anywhere, especially a place that doesn't get that type of rainfall.
You could have the best drainage system, but it wouldn't drain fast enough with a storm dropping heavy rainfall rates.