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People walk in floodwaters during a flash flood near Banks Street in New Orleans, La. Saturday, Aug. 5, 2017. 

Advocate staff photo by LESLIE GAMBONI

New Orleans, in consultation with state and federal officials, is developing a plan to evacuate the city if unusually heavy rainfall is expected while repairs are being made to the pumps and power turbines that drive its drainage system.

The exact threshold at which an evacuation would be called depends on a variety of factors, but one person familiar with the plans said that while the Sewerage & Water Board’s equipment is at diminished capacity over the next few weeks, a forecast of a rare storm that would drop 12 inches of rain over a 24-hour period could be the trigger.

Experts say the chance of a storm that intense occurring in any given year is between 1 percent and 2 percent.

While New Orleans is no stranger to evacuations during hurricanes, the possibility that one might be necessary during a strong storm that threatened only lots of rain – and not the devastating storm surge that a hurricane typically threatens – would be unprecedented.

It is not clear whether officials are also considering lower thresholds for ordering evacuations in case of a tropical storm or hurricane, such as for a Category 1 or 2 hurricane.

One problem is that while the track and strength of a tropical storm can be predicted with some degree of certainty, it’s far more difficult to forecast how much rain a regular storm will produce in a specific area and over what period.

The Aug. 5 storm -- which revealed the problems with the city's drainage system as cars, homes and businesses flooded in areas of the New Orleans including Mid-City and Lakeview -- would not have met the threshold being considered.

The intensity of that storm, however, had not been predicted ahead of time. While forecasters said there was a strong chance of rain that day, they had no idea the storm would stall over New Orleans and drop as much rain as it did within a few hours.

To call for an evacuation, officials would need to know at least a day in advance that torrential rain is expected. 

It's not clear whether such an evacuation call would be considered voluntary or mandatory.

City officials would not directly answer questions about what would trigger a call to evacuate the city during what Mayor Mitch Landrieu and others have described as a heightened state of vulnerability.

As of Saturday, 14 of the city’s 120 pumps were offline for repairs, including seven of the 100 large drainage pumps that operate during heavy storms. In addition, city officials are still installing and testing generators to provide backup power while three of the five turbines that power the drainage system remain offline.

“There is no specific estimate available on the amount of rain that would trigger certain decisions because several variables such as pump status change daily with repairs,” Landrieu spokeswoman Erin Burns said in an email. “At this time, we urge residents to revisit their emergency plans as we get into the height of hurricane season.”

However, a knowledgeable source said the current plan would involve calling an evacuation if a three-day forecast includes 12 inches of rain over a 24-hour period. The New Orleans Advocate is not naming the source, who has not been authorized to speak to the media about the plans.

The last time such a heavy storm hit the area was in May 1995, when rainfall led to severe flooding throughout the city. That flooding prompted the design of the massive and ongoing work by the U.S. Army Corps of Engineers to build the Southeast Louisiana Urban Flood Control Project in New Orleans and surrounding parishes.

Only a handful of storms have dumped that much rain on New Orleans in such a short time over the course of the last century.

The chance of a storm dropping 11.5 inches of rain on New Orleans in 24 hours is about 2 percent each year, said Robert Ricks, a forecaster with the National Weather Service in Slidell. Rainfall totals of 13 inches over 24 hours have about a 1 percent chance of occurring each year, he said.

The most recent case was the 1995 storm, where some gauges in the region registered nearly 12.25 inches, Ricks said.

Before that, the most recent storms that topped the thresholds were in 1927, 1929 and 1937. There were about 13 inches of rain in a 24-hour period during each of those storms, Ricks said.

The highest total recorded in the storm two weeks ago came from a S&WB station that reported about 9.4 inches of rain in four hours.

The city is continuing to discuss options for how to handle any severe weather with a variety of state and federal agencies, including FEMA, the Governor’s Office of Homeland Security and Emergency Preparedness, the Louisiana National Guard and the Corps of Engineers.

“We regularly conduct exercises for a variety of scenarios to test our plans and assumptions,” Burns said. “This week we held a table top exercise to discuss and test our plans in light of the recent events, identifying areas for improvement. The planning teams are assessing a variety of triggers and will continue to work.”

Follow Jeff Adelson on Twitter, @jadelson.​