How Floods Work

A great piece here on how floods work.    It's about upstream, downstream and development.    Go read the whole thing for a full explanation, but I'll excerpt some of it.

Texas can be divided into roughly three major drainage areas. Those places that eventually flow into the Rio Grande, those that feed into the Brazos and Trinity, and those that supply the Red. Many of those streams drain south and east into the plains around what is now Houston – Port Arthur.

This is a huge file.  Click for larger.

Major cities are normally built near ports.  Houston is no exception.

So Houston was developed on wetlands, or specifically on higher areas within wetlands and streams called bayous. Buffalo Bayou is one of the best known, but there are scads of them, or were. Houston also sits on sediment that is slowly sliding down into the Gulf of Mexico and has been since the end of the last ice age. To compound the problem, in the ’50s and ’60s, groundwater pumping was tried to augment Houston and some of the surrounding towns’ water supplies, and that lowered a few areas almost below sea level before pumping stopped.

We all know that grass, trees and open spaces absorb rain.  Conversely, buildings, parking lots and  roads to not absorb rain.  That rain has to go somewhere and that's what we call "drainage".

Buildings, parking lots, and streets do not absorb rain. So the water must be directed somewhere, through storm sewers usually, and sent away. To make a long story really short and simple, a lot of urban areas have so much pavement (impermeable surface) as compared to absorbent ground that hard, fast rainfall causes street flooding. Denver, Amarillo, Garden City in KS, LA, anywhere you get a lot of rain in a short period of time can have minor flooding.

Major cities tend to grow, and developers like to build on flat areas.  Houses, roads, shopping malls, all contribute to moving water.  When all the bayous, creeks, and sloughs fill up, the water quits moving.  And, the folks upstream start seeing their water quit moving.    Which contributes to flooding in places that don't normally see flooding.    When you have a catastrophic storm like Harvey, or Katrina, or Sandy, we see flooding in areas that didn't flood 50 years ago.

The problem with Houston and the surrounding area is that over the past 150 years or so, more and more has been built on the bayous and wetlands along the Texas Gulf Coast, and upstream. So not only has there been far too much rainfall for the ground to absorb (30″ in 36 hours is too much even on absorbent ground), but it is falling upstream and filling the rivers that drain into the lower areas. And the development in that part of the state has covered over the soil and streams, reducing the amount of vegetation, and so it flooded

Go read the whole thing, It's worth the time,