Failure of the Levees

 

 

The hurricane itself did not flood the city. Rather, a series of failures in  the designed levees and floodwalls allowed water from the Gulf of Mexico and Lake Pontchartrain to flow into the city.

The Industrial Canal was overwhelmed when storm surge, funneled in by the Mississippi River Gulf Outlet, overflowed and breached levees and floodwalls in several locations, flooding not only the Lower Ninth Ward, but also Eastern New Orleans and portions of the Upper Ninth Ward west of the Canal.

Meanwhile, waters from storm-swollen Lake Pontchartrain poured into the city, first from a breach in the 17th Street Canal,  and then from a pair of breaches in both sides of the London Avenue Canal. These canals were among those used to channel water pumped from city streets into the lake.

The storm caused the flow to reverse, and as water levels rose the entire drainage system failed. Examinations afterwards showed that water levels in these locations never topped the floodwalls, but instead the levees failed with a water level supposedly within their safe tolerance.

In much of town west of the Industrial Canal, residents who did not evacuate before the storm reported that after the storm they were relieved to see their streets dry and the precipitation from the storm successfully pumped out. However, disaster was already spreading from the series of levee breaches. In areas of town far from the breaches, flood water came not in through the streets, but up from the storm drains beneath the street, in some places changing streets from dry to under 3 feet (0.91 m) of water within half an hour.
By that evening 80% of the city was under water.

Most of the city's pumping stations were submerged. The few above the water line had no power and the emergency diesel fuel had run out.

Knabb, R., Rhome, J., & Brown, D. (2005). Tropical Cyclone Report: Hurricane Katrina. National Hurricane Center, 1-79.