The devastation did not occur because of poor planning, although that didn't help. However, poor planning (except failing to plan for climate change) didn't drop 51.88 inches of rain on the area.  It's not possible. Poor planning didn't cause the storm to stall and remain in place as it did.   Poor planning didn't cause the size of the storm. Climate change did all that.   

There is an increase of 7% water vapor for ever degree centigrade that the atmosphere warms, and the atmosphere is 1 - 1.2 C .  The gulf waters are 2 - 7 degrees warmer than historic temps, which also strengthens the storms. The gulf of Mexico sea level is one foot higher than it was 20 to 30 years ago.  

Higher sea level, warmer ocean, warmer air, bigger storms, and flooding that affects millions of people.  Climate change is responsible.   People who deny climate change, especially the O&G industry, are responsible.   Republicans who deny climate change are responsible.