There are three outfall canals in New Orleans, Louisiana – the 17th Street, Orleans Avenue and London Avenue canals. These canals are a critical element of New Orleans’ flood control system, serving as drainage conduits for much of the city. There are of levees and floodwalls that line the sides of the canals. The 17th Street Canal is the largest and most important drainage canal and is capable of conveying more water than the Orleans Avenue and London Avenue Canals combined.
The 17th Street Canal extends north from Pump Station 6 to Lake Pontchartrain along the boundary of Orleans and Jefferson parishes. The Orleans Avenue Canal, between the 17th Street and London Avenue canals, runs approximately from Pump Station 7 to Lake Pontchartrain. The London Avenue Canal extends north from Pump Station 3 to Lake Pontchartrain about halfway between the Orleans Avenue Canal and the Inner Harbor Navigation Canal (also known locally as the Industrial Canal).
Outfall canal flood wall failures during Hurricane Katrina
When Hurricane Katrina made landfall on August 29 along the Louisiana and Mississippi coasts, storm surge from the Gulf of Mexico flowed into Lake Pontchartrain. The levees along the south shore of Lake Pontchartrain (including both Orleans and Jefferson Parish) withstood the elevated lake levels as designed. However, the floodwall structures along the three outfall canals failed in multiple locations.
The floodwall along the 17th Street Canal’s east bank breached just south of the Old Hammond Highway Bridge. In addition, two major breaches occurred on both sides of the London Avenue Canal – one on the west side near Robert E. Lee Boulevard and another on the east side near Mirabeau Avenue. The Orleans Avenue Canal’s floodwalls did not breach.
On September 1, 2005, helicopters began dropping sandbags into the 17th Street Canal breach, and sheet piling was driven across the canal at the Old Hammond Highway Bridge. Sand bags were brought in and a sheet pile closure structure was built across the London Avenue Canal on September 3, 2005. Temporary pumps were later brought in to remove the water and drain the city, and the Orleans Metro sub-basin was officially declared dry on September 20, 2005. In total, the corps removed more than of water."Sheet Pile Catalogy" In addition, the Corps replaced of floodwalls and of levees and repaired of scour damage following Hurricane Katrina.
According to the American Society of Civil Engineers External Peer Review released June 1, 2007, the engineers responsible for the design of the outfall canal levees overestimated the soil strength, meaning that the soil strength used in the design calculations was greater than what actually existed under and near the levees during Hurricane Katrina. They made an unconservative (i.e., erring toward unsafe) interpretations of the data: the soil below the levee was actually weaker than that used in the I-wall design.
A more recent study corroborates this findings and adds that the main reason the corps' engineers erred is because they misinterpreted the results of a large scale study they conducted in the Atchafalaya Basin in the mid-1980 in an attempt to limit project costs. The engineers wrongly concluded that sheet piles needed to be driven to depths of only 17 feet (1 foot 1⁄4 0.3048 meters) instead of between 31 and 46 feet. That decision saved approximately US$100 million, but significantly reduced overall engineering reliability.
The Orleans Avenue Canal outfall canal did not breach because it had an accidental spillway which relieved pressure and allowed water to flow out of the canal.
In 2007, the United States Army Corps of Engineers published results from a year-long study intended primarily to determine the canal's "safe water level" for the 2007 hurricane season. The Corps of Engineers divided the of walls and levee into 36 sections to analyze just how much storm surge each can withstand. It found that only two sections, those closest to Pump Station No. 6 and on the high ground of Metairie Ridge, can hold more than of water. Many other sections of walls and levees can't be counted on to contain more than of water.
New Orleans hydrology New Orleans is situated between the Mississippi River to the south and Lake Pontchartrain to the north and is approximately upstream from the mouth of the Mississippi River.
Over the years, humans have altered the hydrology of New Orleans to keep floodwaters out of the city, remove floodwater from within the city, improve navigation and / or shore up land.
Keeping floodwaters out of New Orleans motivated the region’s most influential landscape manipulation: the erection of artificial levees on the crown of natural levees to prevent overbank flooding. These levees were first built along the Mississippi River and then later along Lake Pontchartrain to prevent inundation of the city from the rear.
As navigation and drainage canals were dug throughout the city and as more levees were constructed, the natural hydrological basin was subdivided into several smaller drainage sub-basins. One of these sub-basins, referred to as “Orleans Metro” by the Corps of Engineers, today includes the most densely populated portion of New Orleans and is bound by Lake Pontchartrain to the north, the Mississippi River to the south, the Inner Harbor Navigation Canal to the east and the 17th Street Outfall Canal / Jefferson Parish line to the west (although a small portion of Jefferson Parish along the Mississippi River, known as Hoey’s Basin, is also included in this sub-basin). This sub-basin includes such well-known neighborhoods as the French Quarter, the Garden District, Uptown, Lakeview and Gentilly.
Fifty percent of New Orleans lies below sea level, so the city relies on manually operated pumps to remove rainwater from the land. The 17th Street, Orleans Avenue and London Avenue outfall canals serve as the main water expulsion routes for the Orleans Metro basin. During heavy rain and tropical weather events, drainage pumps operated by the Sewerage and Water Board of New Orleans pump rainwater out of the Orleans Metro sub-basin through a system of covered and open-channel drainage canals and into the three outfall canals and Lake Pontchartrain."U.S. Army Corps of Engineers, Outfall Canals and Closure Structures"
History of the outfall canals
Early history
New Orleans was founded in 1718 by Jean-Baptiste Le Moyne, Sieur de Bienville, along the high ground adjacent to the Mississippi River (about above sea level). The city struggled early on with rainfall drainage because of the topography of the region.
The Metairie and Gentilly ridges, both about 3–4 feet above sea level and located between the Mississippi River and Lake Pontchartrain, made it difficult for rainwater to move out of the city since the water would have to flow over these ridges in order to be drained northward into Lake Pontchartrain. Bayou St. John, a natural bayou and old navigation channel that ran from the northern edge of the French Quarter north to Lake Pontchartrain (today it runs from the Mid-City neighborhood to Lake Pontchartrain), was not enough to drain the often heavy rainfall that occurred.
With drainage and rainfall-related flood protection a huge concern, man-made canals were constructed by the mid-19th century. Drainage machines and pumps were built to lift the drained water over the high ridges and into the outfall canals. By 1878, there were approximately of drainage canals feeding into Lake Pontchartrain to remove rainwater from populated areas. Today there are of covered drainage canals, of open channel canals and several thousand miles of storm sewer lines that feed into the system.
These early primary drainage canals included (from west to east): 17th Street, Orleans Avenue and London Avenue. As the population expanded northward toward the lake, low-lying swamps were reclaimed by constructing shallow drainage ditches that fed into the newly created system of drainage canals. However, the reduction in the groundwater table, as a result of the drainage and reclamation of swamps and marshland, produced significant land subsidence in the drained area. That land subsidence continues today.
The construction of the outfall canals did have another major consequence. By digging canals through the Metairie and Gentilly ridges, the canals opened up storm surge avenues into the heart of New Orleans via Lake Pontchartrain.
The New Orleans Sewerage and Water Board, which was established in 1899, became the primary agency that tackled the tough drainage problems facing New Orleans. Flood protection levee upkeep, including those along the outfall canals, was done locally by the Sewerage and Water Board and the Orleans Levee District, the latter of which is a state agency established in 1890 to handle the operation and maintenance duties associated with levees, floodwalls, and other hurricane and flood protection structures surrounding the city of New Orleans.
In 1915 and again in 1947, devastating hurricanes struck the New Orleans area, causing millions of dollars in property damage and killing hundreds of residents. Water overtopped and breached the levees along the outfall canals and the Sewerage and Water Board and the Orleans Levee District raised the levees an estimated three feet after those hurricanes. However, some of these levees had subsided by as much as during their nearly 100-year existence.
Early federal involvement
The first federal involvement with hurricane protection in New Orleans began in 1955 with Public Law 71 of the 84th Congress, 1st Session, when Congress authorized the Secretary of the Army to examine and survey the eastern and southern seaboards of the United States, with an emphasis on those areas where severe hurricane damages had occurred in the past. This authorization was granted after several hurricanes in 1954 severely damaged portions of the eastern and southern U.S. Although this authorization marked the first federal involvement with hurricane protection in the city, it authorized only a feasibility study and did not authorize or fund any construction-related activities.Interim Survey Report, Lake Pontchartrain, Louisiana and Vicinity. New Orleans: U.S. Army Corps of Engineers, 1962. Print
As part of the authorization, the New Orleans District of the U.S. Army Corps of Engineers examined the Lake Pontchartrain and Vicinity area of coastal Louisiana, which includes most of St. Bernard Parish, Orleans Parish east of the Mississippi River (including most of the City of New Orleans), Jefferson Parish east of the Mississippi River and a small portion of St. Charles Parish east of the Mississippi River. The Lake Pontchartrain and Vicinity area encompasses one large natural drainage basin that today includes the Orleans Metro sub-basin.
After seven years of planning, the New Orleans District produced an Interim Survey Report, which was transmitted by the Secretary of the Army to the U.S. Congress in November 1962. This report outlined a comprehensive plan for preventing flooding in the greater New Orleans area from a Standard Project Hurricane, which is a hypothetical hurricane representing the most severe combination of hurricane parameters that is reasonably characteristic of the area.