John McQuaid

Not good: water has begun to overtop floodwalls in the New Orleans Industrial Canal. This is happening in the Upper 9th Ward – i.e., on the other side of the canal from the Lower 9th. As the picture and the name of the canal indicate, this is an industrial area of town. The floodwalls in question have been fortified post-Katrina. Corps officials are saying that this is not true overtopping – i.e., it’s mainly waves being pushed over by wind.

However, indications are the water is still rising. In Shell Beach, which lies to the east, water levels have reached 10 feet:

The Industrial Canal connects both to Lake Pontchartrain and to the eastern marshes where Shell Beach is, so the water level has the potential to keep rising as the storm moves through. Also, once water starts coming over the top of a levee or floodwall, its capacities are obviously exceeded; structural problems begin to mount and the risk of a breach increases. Fortunately, these floodwalls are equipped with concrete splashpads on the dry side so that water flowing over them won’t erode the foundations.

Update: Corps officials in Baton Rouge say they believe the wall will hold. Water levels in the canal will likely drop because the water will flow into Lake Pontchartrain, where the water level has not risen appreciably (which is why they have not had to shut the new floodgates on the drainage canals that lead into city neighborhoods). Gustav’s winds have been pushing water into New Orleans from the south and east. But to get into the lake, the Gulf storm surge has to travel both farther north and then a lot farther inland. That would require powerful, sustained easterly winds. That’s why the lake isn’t filling up – Gustav is too far to the west, and moving away. Also, as you can see from the Shell Beach graphic, the water level to the east of New Orleans appears to have topped out at 10 feet. As Gustav moves on, the winds will shift to the west and begin pushing that water back out to sea.

Later update: The water level in the Industrial Canal (AKA the Inner Harbor Navigation Canal – that is the official name, but in NO it’s universally known as the Industrial Canal) is dropping, though the TP reports there is still some wave overtopping occurring.

Hurricane Gustav weakened somewhat overnight, but is expected to strengthen rapidly again – thanks to the warm waters of the Gulf’s loop current – before striking the coastline midday Monday. Meanwhile, the forecast track again shifted incrementally eastward. Each increment increases the likelihood of hurricane-force winds and levee-topping storm surges in the New Orleans area, especially in suburbs on the west bank of the Mississippi River (i.e., south of the city proper) which abut marshes and where the levee system is incomplete. This is bad.

Hurricane Gustav strengthened very rapidly today into a Category 4 storm, and forecasters believe it may reach Category 5 for a time Sunday, then weaken before it makes landfall, probably on the central Louisiana coast. The current forecast track has changed little over the past three days. Relatively speaking, this is good news for New Orleans. There will be miles of wetlands, suburbs, and the river (and its levees) interposed between the city and the center of the storm. Areas to the south and west of the city would bear the brunt of the storm surge. Most of this is sparsely-populate bayou country, but there is one substantial urban area, Houma-Thibodaux, population 32,000.

The Saffir-Simpson scale is somewhat misleading, especially where Louisiana is concerned, because it’s based on wind speed. What really matters in Louisiana is storm surge. A big storm pushes an incredible amount of water over the coastline’s flat, marshy topography. Channels and decaying marshes offer avenues for water to get further inland, where it can easily engulf levees and inhabited areas.

Gustav looks especially ominous for several reasons. The longer the hurricane remains at Category 4 and 5 before making landfall, the bigger its storm surge will be. This is what made Katrina so devastating – it was a large Category 4 and 5 storm for nearly two days. Wind speeds fell as it came ashore, but the huge wave it generated didn’t dissipate with them. Gustav is also coming in diagonally, which will increase the time it is pushing water inland.

My friend and colleague Mark Schleifstein has some reporting up on this today:

Hurricane Gustav will be at Category 4 strength with winds of 145 mph only 12 hours before it hits the central Louisiana coastline Monday afternoon, according to a 2 p.m. National Hurricane Center forecast.

On that track, a Louisiana State University coastal scientist says, storm surge could reach the top or overtop levees on the West Bank, could raise water to 8 feet — plus waves — along levees on the Mississippi River-Gulf Outlet, and combined with a water-swollen Tchefuncte River, could push surge into Madisonville on the Northshore.

Water also could rise as high as 8 feet in the Industrial Canal, he said.

In a nutshell: areas around New Orleans will get swamped. The city itself will see some high water, but not high enough to top the levees (of course, this didn’t help last time) but may avoid the worst.

Update: Mayor Nagin has ordered a mandatory evacuation of New Orleans starting Sunday morning. The 11 p.m. forecast has shifted the track slightly eastward, increasing the risk to New Orleans.