What geographic problem existed in New Orleans, Louisiana, that allowed Hurricane Katrina to become so devastating to the area?

One major problem was the New Orleans mostly sits below sea level.  When the 17th Street Canal, the city's largest drainage canal, had been breached , massive storm surges sent torrents of water over the levees along the Mississippi River-Gulf Outlet (MRGO) and into St. Bernard Parish, located just southeast of New Orleans.

In all, levees and floodwalls in New Orleans and surrounding areas fell in more than 50 locations during Hurricane Katrina, flooding 80 percent of the city and fully 95 percent of St. Bernard Parish.