Map shows potential for flooding
By Sheila Grissett East Jefferson bureau
If the kind of tropical rain produced by Hurricane Katrina were to occur this summer, with floodgates closed against a storm surge in Lake Pontchartrain, a 6,650-acre area of New Orleans and East Jefferson could get anywhere from a few inches to several feet of water, according to computer generated maps released Wednesday by the Army Corps of Engineers.
That would be enough to swamp some vehicles and invade homes in lower-lying areas, though corps officials have said history shows the chances are slim that a storm wouldproduce enough surge this hurricane season to warrant closing floodgates at the 17th Street, London and Orleans Avenue canals. Based on storms from the past 46 years, the gates would have been closed only three times, and only one of those hurricanes — Katrina — produced heavy rain.
The maps depict three rainfall events, including the 9 inches that Katrina dumped in 6 hours, and simulates what flooding could be expected from another Katrina-like storm with no overtopping or breaching of floodwalls. And in that scenario, the Broadmoor area would be among the hardest hit, with extensive flooding north of Claiborne, as well as areas of Hollygrove and Lakeview.
Corps engineers said additional work is being done to get a better picture of how such a storm would impact the Old Metairie-Airline area, a particularly complex drainage area. At present, the new maps show from two to four feet of water in the lowest-lying sections of that area, which is known as Hoey’s Basin.
The 9 inches in six hours is the worse-case scenario mapped by the corps to show potential rainfall depths if new floodgates must be shut shut to protect drainage canals from a hurricane-driven surge that could breach floodwalls.
The gates themselves would greatly limit the ability of city pumps to move water fast enough to minimize flooding in 22,000 acres of property that drain into the three canals.
Since 1959, only two other storms would have packed enough surge to close the floodg gates, Hurricane Juan in 1985 and Hurricane Isidore in 2002, , say corps officials who base their findings on research and computer modeling done by a team of government engineers and independent contractors.
And according to that research, each of those storms produced, on average, about three inches of rain in six hours. In those events, thecorps maps indicate that closed gates would have had very little impact on flooding if closed during those events.
“One should not assume that the gates will be closed at all during this hurricane season,” said Dan Hitchings, director of a corps task force working to make the levee system safer.
U.S Sen. David Vitter, who requested the inundation maps, was livid that the almost two months into the new hurricane season that the corps has provided only a minimum of pumping capacity to move storm water out of the 17th Street and London Avenue canals in case the gates must be closed.
“Tragically, the (corps) confirmed today what we all knew in our gut — its delay in providing adequate pumping capacity at the 17th Street and other canals could well lead to the reflooding of thousands of homes in the event of a significant storm this hurricane season.”
The nine maps released Wednesday show the potential for flooding during 3- and 6-inch rain events, as well as the 9-inch worse-case scenario, during a six-hour period when the rise in Lake Pontchartrain requires closing the gates against surge.
The gates would never be closed during a non-tropical rain event such as the watershed May 1995 that dropped as much as 19 inches in six hours, corps officials vow.
Rather, the gates would fall in response to a hurricane-driven storm surge that threatened to add more water to the canals than engineers think the floodwalls can handle without breaching — five feet at the 17th Street and London Avenue Canals and nine feet at Orleans.
Maps released by the corps several weeks ago showed only what areas could expect to take on water when the gates were closed - but not the depth of that flooding.
But the depth maps demanded by Vitter and released Wednesday show that a nine-inch, six-hour rain before Katrina could have flooded 4,500 acres, or almost 17 percent of the land that drains into the three canals.
By comparison, some 6,650 acres, or almost 25 percent of the area drained by the three canals, could flood if the gates were closed this summer while pumping capacity is so severely limited - especially in the 17th Street and London Avenue canals.
During a six-inch rainfall, some 4,000 acres could be expected to flood, versus the 2,250 acres that would take water if there were no gates. And during a three-inch rainfall, which corps engineers compare to the precipitation generated by Hurricanes Juan and Isidore, some flooding would be expected to occur over 850 acres — just 100 acres more than would flood if the gates were open.
The conditions created by Juan and Isidore are important because, according to corps figures, gates would have been closed during those storms as well because Lake Pontchartrain rose above the five-foot “safe water” level now in effect for the 17th and London canals.
Floodgates on the Orleans Avenue Canal, with a safe level of nine feet, would not be closed as early.
The computer-generated maps, which present only a snapshot of potential conditions under specific circumstances, are for information and planing only and should not be used in comparison or relationship to the Federal Emergency Managment Agency’s base flood elevations, corps engineers caution.
Each set of maps also represent a specific point in time, including: the pre-Katrina years before there were gates and limitations on pumping and canal levels; the different months of this hurricane season in which pumping capacity changes because of progress on installation of pumps at the three canals; and the June 1 start of next hurricane season, when corps engineers say they will have installed enough capacity to pump all the storm water that can be physically moved out of the canals against a storm-swollen Lake Pontchartrain.
Corps critics, including Vitter and a host of local elected officials, have pleaded for restoration of lost capacity by the height of the hurricane season, which traditionally stretches from mid-August to late September.
“Corps delays threaten to reflood our metro area and kill our recovery in the event of a signficant storm this hurricane season,” Vitter fired off in a press release after reviewing the maps Wednesday.
“Once again, I urge the corps to bring more capacity on line this season — to work with Jefferson Parish and the city of New Orleans on innovative ponding and other measures - before this happens.”
The corps schedule to incrementally increase capacity at the London Avenue and 17th Street Canals by adding additional pumps has been delayed several times. This season, at London Avenue, there will be only 2,800 cfs, a loss of more than 6,000 cfs.
But the corps and its contractors say they have experienced the most complications and greatest delays at the 17th Street Canal, where it looks as though may be no more than 1,400 cubic feet per second, or cfs, of capacity until late September.
By late September, the corps hopes to have pushed that number to 4,000 cfs, and when the 2007 season opens next year, the latest agency plan calls for increasing capacity to 7,300 cfs, of the 10,200 cfs ideal.
The maps provide only a snapshot of rainfall that could occur under certain conditions, and don’t make allowance for variables such as the randomness of rainfall or the failure of pumps.
The modeling is based on the assumption that rainfall would fall evenly over the basins, which engineers say never occurs.
(Sheila Grissett may be reached at email@example.com or (504) 717-7700.)