Updated 6/1/07, see belowAlso updated 7/28/07, see bottom of post
Synopsis:The Corps of Engineers knows the exact location of the weakest area of the levees and walls along the London Avenue Canal. Their analysis of that weakness shows that the canal depth cannot exceed four feet above sea level without risking a breach of the canal walls and levees. That depth is known as the Safe Water Level, and the four foot figure was arrived at in early September. The initial figure released earlier in summer, 2006 was five feet.
The Corps, when they spoke about the floodgates during 2006, said they would only be closed upon the approach of a major tropical storm surge which could exceed the safe water level. They also said that such conditions had only occurred three times in the last 45 years (Hurricane Juan in 1985, Tropical Storm Isidore in 2002, and Katrina in 2005). Never was simple pumping of rainfall mentioned as a possibility which could cause the canal depth to exceed the safe water level.
However, that possibility has become reality, with the canal depth reaching 3.6 feet at the weak section of the canal during the December 21, 2006 storm, as well as other unspecified instances during the fall and winter when the four foot safe water level has possibly been exceeded in other sections of the canal. Further investigation has uncovered the fact that the Corps is
extremely concerned about normal rainfall pumping - combined with only a minor increase in the level of Lake Pontchartrain from high tides or a stiff north wind - causing the London Avenue canal safe water level to be exceeded, bringing about the strong possibility of a canal breach
without the presence of a tropical storm.
Besides the very serious consequences of a canal breach, there are also problems which arise should the Corps try to prevent the topping of the safe water level during a rainstorm. They can do so through one of two means: a) close the floodgates and activate the temporary pumps installed at the gates, or b) ask the Sewerage & Water Board to turn off pumps in the permanent pump stations which feed the London Avenue canal.
In the case of option (a), pumping capacity would drop approximately 64% because there are not enough pumps at the floodgates to keep up with the pumps in the stations. This would cause flooding in the neighborhoods served by the pump stations. In the case of option (b), neighborhood flooding would also result, because pumps draining those neighborhoods would be turned off.
Since September, the Corps has not moved to make emergency repairs to the weak section of the walls and levees on the London Avenue canal, despite knowing the exact location and the obvious consequences to leaving the weakened section exposed to repeated flows of water. Doing so would involve driving a 40 or 60 foot long sheet pile coffer dam around the weak section, identical to what was done around the breached areas of the canals after Katrina. Other, less critical sections of hurricane protection levee around the New Orleans area have been shored up using emergency funding in the last 16 months.
Instead, the Corps is stationing senior engineering personnel in the pump stations during every rainstorm to monitor the rise in canal depth. This has happened as recently as the night of January 4, 2007. Presumably, they are also preparing to close the gates during every rainstorm. The engineers are acting in an ad hoc "partnership" with Sewerage & Water Board drainage department personnel to keep the canal depth from rising above the safe water level.
There are no written procedures for this ad hoc partnership outside of hurricane season. There is no written legal framework either. The only procedure, the
Floodgates Operating Manual, lays out rigorous, extensive plans which anticipate five days of tropical storm warnings before the time when the gates must be closed. Use of the Floodgates Operating Manual depends on the extra time inherent in the behavior of tropical storms. It is rare that normal rainstorms can be accurately predicted five days in advance. And during summer in New Orleans, severe, heavy rainstorms can bubble up with no warning at all. Closure of the floodgates and full startup and operation of the floodgate pumps would take a number of hours, with no guarantee that the pumps will work.
Much of the same reasoning applies to the 17th Street canal, except that the safe water level is six feet there. In general, it is less likely for 17th St. to reach six feet than for London Ave. to reach four feet.
In sum, the area around the London Avenue canal, as well as those areas drained by the canal, are at a much higher risk for flooding outside of hurricane season than has been previously discussed by various public officials. And while Corps officials express deep concern about the current problems at the canal, and have expressed strong intentions to get the safe water level higher, they have yet to move to actually solve the problems. Their previous solution - the floodgates and temporary pumps - did not account for rainfall pumping without storm surge exceeding the safe water level. The walls of the canal remain a very serious problem outside of hurricane season.
The facts:The Safe Water Level (SWL) on the London Avenue Canal is 4 feet above sea level. This is the level above which the Corps of Engineers says the walls and levees are in danger of imminent breaching. The SWL applies along the entire length of the canal. The Corps first announced the four foot level in an
interview with Col. Richard Wagenaar on the WLAE program "Road to Recovery" on September 7, 2006. It was also mentioned in a September 12, 2006 Times-Picayune
article. I have independently confirmed it currently sits at 4 feet. This is a very low water level, in some cases not even touching the base of the canal walls.
There are five or six new electronic level sensors installed along the canal, which are connected via a
SCADA (Supervisory Control And Data Acquisition) system to allow for real-time remote monitoring of the canal depth. The readouts from these level sensors (there are also six sensors on the 17th St canal and four on the Orleans Avenue canal) are available to the Corps at their Emergency Operations Center, as well as in a computer terminal in Drainage Pump Station 3 at the southern end of the canal. Terminals are also placed in Station 7 (Orleans Avenue canal) and Station 6 (17th St canal).
On December 21, 2006, the level at one location in the canal reached 3.6 feet, 5 inches less than the safe water level (I had earlier reported it as 3.4 feet, but that was incorrect.).
That one location is the one the Corps has identified as the most vulnerable point, and is governing the setting of the safe water level at four feet. It is located approximately 4000 feet from Drainage Pumping Station 3, though I don't know which side of the canal it is on. 4000 feet from Station 3 is located behind the tennis courts at Dillard University and the forested area south of Gregory Junior High, as shown on
this map. The actual point may be slightly north or south of there.
[
Update, 6/1/06While it is still possible this is the location of the weakest point on the canal, it is not where the Corps will be conducting their testing to determine the wall's strength. That location, as noted in
this post, is just south of the Robert E. Lee bridge, a little over a mile north of the Dillard University location.]
The Corps has been dispatching senior engineering staff to New Orleans pump stations during every rainstorm, including the one this past Thursday night in which we received between one and two inches. They are doing so to ensure that canals do not exceed their SWL's. The safe water level at 17th Street is six feet and at Orleans Avenue it is eight feet. Orleans Avenue is extremely unlikely to reach eight feet. 17th Street could reach six feet with a lot of rainfall being pumped out and a somewhat larger than average tide on Lake Pontchartrain, though that is less likely than London Avenue reaching four feet.
A depth of four feet in London Avenue can easly be reached with only a moderate rainstorm and a stiff northern wind, or even just high tide in Lake Pontchartrain. There is no need for storm surge from a tropical storm. This means the threat for exceeding the safe water level is one that exists throughout the year, not just during hurricane season.
The safe water level at London Avenue has possibly already been exceeded in past rainstorms this fall and winter, as Corps personnel have authorized S&WB pump station personnel to continue pumping despite exceeding the level.
There is no specific written procedure dealing with non-tropical storms and the threat of exceeding the safe water level simply by pumping out rainfall. However, the
Floodgates Operating Manual, which is being used in lieu of a written procedure, certainly makes clear that the floodgates are to be closed when the safe water level is expected to be exceeded. Closure of the floodgates will constrain the stormwater drainage capacity to that of the pumps at the floodgates.
The total capacity of S&WB pump stations 3 and 4, which both discharge to the London Avenue canal, is
7740 cubic feet per second (cfs). Station 3 has a capacity of 4100 cfs (pumps A & B = 550 cfs each, pumps C, D, & E = 1000 cfs each); Station 4 has a capacity of 3640 cfs (pumps 1 & 2 = 320 cfs each, pumps C, D, & E = 1000 cfs each).
The total capacity of the pumps at the London Avenue floodgates is
2800 cfs, though it may be even less when one considers they may be run at slower speeds to prevent vibration problems. That is 36% of the current pumping capacity with the gates up.
There is no legal framework for what is taking place currently, i.e. the Corps being in the pump stations during rainstorms outside of hurricane season and absent a tropical storm threat. In other words, the delineation of responsibilities between the Sewerage & Water Board and the Corps has not been put down in print. Were a breach to develop in the London Avenue canal walls due to exceeding the safe water level without the gates dropping, it is unclear who would be responsible. On the flip side, were flooding to happen in the city because S&WB pumps were turned off to keep the canal level below the SWL, it is also unclear who would be responsible.
When the Corps released their
inundation maps July 26, 2006, showing the effect of varying degrees of rainfall when the floodgates are closed, the following quote was included in the accompanying
press release:
Dan Hitchings, director of the Corps’ Task Force Hope, points out that outfall gates will be closed only during a tropical storm event that would cause a five-foot or more storm surge in Lake Pontchartrain. Since 1959, storm surges exceeding five feet have been recorded three times: in 1985 during Hurricane Juan; in 2002 during Hurricane Isidore, and in 2005 during Hurricane Katrina. "One should not assume that the gates will be closed at all during this hurricane season," Hitchings said. "Certainly London Avenue and 17th Street Canals will be closed if we have a storm headed at us that will cause a five-foot or more storm surge."
At the time of that quote, the SWL was five feet. Since then the SWL's have been adjusted to four feet at London Avenue and six feet at 17th Street. However, no similar statements have been issued by the Corps in the intervening time to clarify to the public that the chances of gate closure on London Avenue (and accompanying drop in pumpng capacity) are far, far higher than the public was initially led to believe.
The Corps really does want to raise the SWL on the London Avenue canal, because it is so low. To that end, there are plans (or perhaps just discussions) to drive a sheet piling coffer dam around the vulnerable location, pump the area inside the dam dry, and then pump in varying heights of water to determine the exact safe water level. The wall and levee would be closely monitored for movement or damage. Hopefully, this will be done very soon, even though the aim would appear to be further analysis rather than repair.
[
Update, 6/1/07See the 6/1/07 note above regarding testing of the walls]
Regular meetings between the Corps, the S&WB, and Jefferson Parish officials, at which issues such as those above are discussed, do not have representation by the Orleans Levee District, the agency responsible for the maintenance of the canal walls.
[Update 7/28/07]See my June 11, 2007 post, "
Testing, testing," for the most up to date information on the London Avenue canal testing. The Engineering News-Record articles linked there are far better than the Times-Picayune's pathetic coverage. [end update]
