human
And soil. Simply put Houston has paved too much of its land mass, such that the water is concentrated into the flood mitigation/drainage areas thus overwhelming them.

Better would be tearing up unnecessary roads and parking lots and replacing with trees, plants and porous surfaces. All those malls and vast outdoor parking lots should be designed to absorb water not concentrate it.

Also Houston has clay soil which doesn't help.....

Basically a once in a centenary(millenia?) natural disaster meets manmade hubris and overconsumption/building.
gpt
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