New Orleans Outfall Canals

The primary outfall canals in New Orleans are the 17th Street, Orleans Avenue and London Avenue canals. They serve as the major draining conduits for a major portion of the metro area.

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 13 miles (21 km) of levees and floodwalls that line the sides of the canals.

The 17th Street Canal extends 13,500 feet (4,100 m) 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 11,000 feet (3,400 m) from Pump Station 7 to Lake Pontchartrain. The London Avenue Canal extends 15,000 feet (4,600 m) 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). ^[1]

Role of Outfall Canal Failures After Hurricane Katrina

Before August 29, 2005, the three outfall canal levees languished in relative obscurity. The larger and longer Mississippi River system, which faces annual flood stages of ten to twenty feet lasting weeks, garnered significantly more engineering and maintenance attention.

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 and floodwalls along the lake faced elevated levels but none of the levees or floodwalls were overtopped, contrary to initial reports from government officials.^[2] The levees along the entire south shore of Lake Pontchartrain (including both Orleans and Jefferson Parish) withstood the elevated lake levels as designed, but the floodwall structures along the outfall canals failed in multiple locations.^[3] 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 along 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 were not breached.^[4][5]

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 250 billion US gallons (950,000,000 m³) of water.^[6][7] In addition, the Corps replaced 2.3 miles (3.7 km) of floodwalls and 22.7 miles (36.5 km) of levees and repaired 195.3 miles (314.3 km) of scour damage following Hurricane Katrina.

Louisiana Historic Plaque at 17th Street Canal Breach Site

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 over-estimated 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 Another critical engineering oversight that led to the failure of the 17th Street Canal and London Avenue canals involveed not taking into account the possibility of a water-filled gap which turned out to be a very important aspect of the failures of the I-walls around New Orleans. No later expert studies disputed these findings.^[8]

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 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 divided the 4.8 miles (7.7 km) 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 13 feet (4.0 m) of water. Many other sections of walls and levees can't be counted on to contain more than 7 feet (2.1 m) of water.^[9]

New Orleans Hydrology

New Orleans is situated between the Mississippi River to the south and Lake Pontchartrain to the north and is approximately 100 miles (160 km) upstream from the mouth of the Mississippi River.^[10]

The Orleans Metro drainage sub-basin. Nearly all water in this sub-basin is eventually drained into the outfall canals.

Over the years, humans have altered the hydrology of New Orleans to keep 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.^[11]

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 United States Army 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.^[12]

As much of New Orleans lies below sea level, 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 sub-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.^[13]

History of the Outfall Canals

Early History

An historical look at the City of New Orleans in 1878. The outfall canals were already constructed and included, from west to east, the 17th Street Canal, the Orleans Avenue Canal and the London Avenue Canal.

New Orleans was founded in 1718 by Jean-Baptiste Le Moyne, Sieur de Bienville, along the high ground adjacent to the Mississippi River (about 17 feet (5.2 m) above sea level). The city struggled early on with rainfall drainage because of the topography of the region.^[14]

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 1800s. 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 36 miles (58 km) of drainage canals feeding into Lake Pontchartrain to remove rainwater from populated areas. Today there are 90 miles (140 km) of covered drainage canals, 82 miles (132 km) of open channel canals and several thousand miles of storm sewer lines that feed into the system.^[15]

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.

A hurricane in 1947 overwhelmed the existing levees along the outfall canals. Here, the 17th Street Canal is breached by hurricane storm surge.

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 10 feet (3.0 m) 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.^[16]

The Lake Pontchartrain & Vicinity system, which includes much of southeast Louisiana.

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.

This report’s recommended protection plan for Lake Pontchartrain and Vicinity consisted of a barrier at the east end of Lake Pontchartrain. The barrier would include locks in the Rigolets and Chef Menteur Pass, as well as a lock in the Inner Harbor Navigation Canal at its confluence with Lake Pontchartrain (in the Seabrook area). The locks would limit hurricane storm surges from entering into Lake Pontchartrain and reduce high-salinity flows into the lake from the Gulf of Mexico via the Mississippi River-Gulf Outlet (MRGO), which was under construction at the time. As part of this original “Barrier Plan,” the existing levees along all three outfall canals were deemed adequate for hurricane protection purposes.

Hurricane Betsy

In September 1965, Hurricane Betsy struck the Louisiana coast near New Orleans, causing massive property damage and loss of life. While Betsy had similar characteristics of the Standard Project Hurricane, Betsy’s wind field and associated wave action called into question the adequacy of the original design heights for project levees and floodwalls outlined in the 1962 Interim Survey Report. As a result, the New Orleans District requested and received permission from the Corps’ Mississippi Valley Division and Corps Headquarters to increase levee and floodwall heights by 1–2 feet across the project network.

In addition, a Design Memorandum issued in 1968 revealed that the outfall canals needed to be addressed because the existing canal levees did not meet the design heights required by federal design criteria adopted after Hurricane Betsy. However, no immediate guidance was given as to how to increase flood protection along the canals and it remained an unresolved issue for over two decades.^[17]

Congress authorized the post-Hurricane-Betsy-modified Barrier Plan when the Flood Control Act of 1965 was passed, positioning the federal government to assume 70 percent of construction costs. After authorization, the New Orleans District set out to develop detailed engineering plans, secure the required funding and real estate, and construct project features. The District initially estimated that the project would be complete by the mid to late 1970s.

State governmental elected officials, congressional representatives and various local citizen and interest groups met the Barrier Plan with opposition soon after it was authorized. Some opponents feared that the barrier would adversely affect navigation access to the lake, while others cited the possible flooding of the north shore of Lake Pontchartrain when the barriers were closed. Also a concern was potential operation and maintenance costs. It was the environmental effects of the barrier, however, that caused the most opposition.

As part of the National Environmental Policy Act (NEPA), the New Orleans District prepared an Environmental Impact Statement (EIS) to address environmental impacts associated with the project and how the Corps would mitigate those impacts. In 1975, a local environmental advocacy group challenged the adequacy of the EIS in U.S. District Court.

In 1977, the Court ruled that the EIS for the Barrier Plan did not meet NEPA requirements, and an injunction was issued on further construction until the deficiencies were resolved. In 1978, the injunction was lifted for all non-barrier elements of the project, including levee work in St. Bernard Parish and other parts of the Lake Pontchartrain and Vicinity area. Any work along the lakefront and outfall canal levees remained on hold because the design and construction of those features would be affected by the final resolution of the proposed barriers.

Hurricane protection floodwalls are installed along the Inner Harbor Navigation Canal in 1968.

After the injunction of the Barrier Plan in 1977, the Corps studied an alternative “High Level Plan,” which involved higher lakefront levees instead of barriers. In 1985, the Director of Civil Works approved replacing the barriers with increased levee heights along the lakefront. Construction of other non-barrier features continued as part of the original authorization. With the adoption of the High Level Plan, the New Orleans District could now move forward with providing hurricane and storm surge protection features along the lakefront and outfall canals.

Determining a Specific Hurricane Protection Plan for the Outfall Canals

Beginning in the late 1970s, the Corps’ New Orleans District began identifying and examining hurricane protection alternatives for the outfall canals. Through a series of Design Memorandums in the late 1980s and early 1990s, the Corps recommended building butterfly gates at the Orleans Avenue and London Avenue canals. Butterfly gates are hydraulically-operated gates that swing on a pivot and close if storm surge threatens to enter the canals.

For the 17th Street Canal, the cost of a butterfly gate structure was about the same as the cost of the locally-preferred plan of higher parallel levees along each side of the canal, so the Corps agreed to pursue “parallel protection” along that canal. There is no evidence in the project record that the Army Corps felt that there were differences between the approaches in providing reliable surge protection (Woolley Shabman, 2-48). The straight-line shape of the 17th Street Canal would require the construction of a breakwater out in the lake to reduce the wave load, while the S-shaped geometry of the Orleans and London Avenue canals would naturally reduce the wave loads.

The Orleans Levee District and the Sewerage and Water Board, however, favored parallel protection for the Orleans Avenue and London Avenue outfall canals. The two agencies believed interior drainage would be inhibited when the gates were closed during a tropical event because water would not be able to escape from the canals and into Lake Pontchartrain. As interior drainage was not part of the federal authorization for hurricane protection, the Orleans Levee District and the Sewerage and Water Board would be responsible for any costs associated with pump installation at the mouths of the outfall canals. The Orleans Levee District and Sewerage and Water Board contended that parallel protection could provide increased hurricane storm surge protection while maintaining interior drainage.

The Corps argued that the butterfly gates plan for the Orleans Avenue and London Avenue canals was more cost-effective. The cost of parallel protection for those two canals was estimated to be $90 million, while the butterfly gate structures would cost approximately $25 million. There is nothing in the project record indicating that the gates-only plan was the superior option, only that it was cheaper.^[18]

Furthermore, the Corps argued that any incremental cost over the baseline least-cost plan of installing butterfly gates at the mouths of the outfall canals would constitute a project “betterment.” The project authorization stated that such betterments were the full responsibility of the local sponsor. If parallel protection were to be pursued at the Orleans Avenue and London Avenue canals, the federal government would limit its contribution to 70 percent of the total cost of the least-cost plan.

Floodwalls were built instead of levees to protect against flooding because many homes, pictured here, back up to the levees. Any widening of the levees would be nearly impossible given the limited right of way.

For several years the project was at a stalemate. The local sponsor successfully lobbied Congress to direct the Corps of Engineers to implement the much more expensive parallel protection plan for all of the outfall canals in the Water Resources Development Act of 1990. Two years later, Congress stipulated that the federal government would pay 70 percent of the cost, with the local sponsor picking up the other 30 percent, in the Energy and Water Development Appropriations Act of 1992.

With the dispute over hurricane protection solutions for the outfall canals seemingly resolved, the Corps began designing and constructing predominantly “I-wall”-type floodwalls on top of the levee crowns. I-walls met the project goals of providing increased embankment heights within the limited existing rights-of-way with minimal disruption to the adjacent residential neighborhoods. Construction began in 1993, and project work along the outfall canals was reported to be nearing completion in 2005 prior to Hurricane Katrina.

Closure Structures

Interim Closure Structures were built at the mouths of the three outfall canals following Hurricane Katrina to block storm surge from entering the canal. Pictured here is the Interim Closure Structure at the Orleans Avenue Canal. USACE Photo by Paul Floro.

Despite the dewatering and rapid pace of repairs, the Corps of Engineers determined that it did not have the time to complete repairs to all of the breaches along the outfall canals before the start of the 2006 hurricane season. As a result, the Corps built interim gated closure structures at the mouths of all three canals to prevent storm surge from entering the canals. The Corps also initially installed 34 temporary pumps near the closure structures to drain floodwaters from the sub-basin.^[19]

The gated structures stay open during normal, non-tropical conditions. When storm surge threatens to exceed the maximum operating water level of a canal, the Corps closes the gates and operates the pumps. Interior pumps operated by the Sewerage and Water Board pump rainwater out of the sub-basin and into the outfall canals. The interim pumps then pump rainwater out of the outfall canals, around the gates and into Lake Pontchartrain. The closed gates reduce the risk of storm surge entering the canals and threatening the city. When the surge recedes to a safe level, the gates are reopened and normal drainage resumes.

The Corps received authorization to build the interim closure structures under emergency flood control legislation passed in September 2005, specifically Public Law 109-61 and Public Law 109-62. It cost about $400 million to build the interim closure structures.^[20][21]

The Corps did not have the authority, however, to build permanent canal closures and pumps under this legislation. It was not until emergency appropriations in the years after Hurricane Katrina that the Corps received authorization to build the permanent gates and pumps. This authority superseded any previous legislation prior to Hurricane Katrina that prohibited the Corps from building closure structures at the mouths of the outfall canals. The appropriations for this project total about $804 million.^[22]

In the years since Hurricane Katrina, the Corps of Engineers has been constructing and bolstering 350 miles (560 km) of levees, floodwalls, barriers, gates and other structures as part of the Greater New Orleans Hurricane and Storm Damage Risk Reduction System (HSDRRS), which stretches across five parishes and includes much of the original Lake Pontchartrain and Vicinity Basin, as well as the West Bank and Vicinity Basin on the West Bank of the Mississippi River. When complete, the HSDRRS will reduce the risk from a storm that has a one percent chance of occurring in any given year, or a 100-year storm.^[23]

The interim closure structures already provide a temporary 100-year-level of risk reduction, and the permanent canal closures and pumps will continue to provide that same level of risk reduction. The contract for the permanent canal closures and pumps will be awarded in 2011 and construction will be complete in the fall of 2014.

Outfall Canal Wall Remediation

Although interim closure structures (and eventually the permanent canal closures and pumps) prevent storm surge from entering the canals and provide the 100-year-level of risk reduction, several portions of the outfall canal floodwalls are being remediated, or strengthened, to meet the more stringent post-Katrina design requirements. When remediation is complete, all canals will be able to operate under a maximum operating water level of +8.0 NAVD 88. All remediation work along the outfall canals will occur within the existing rights-of-way and is scheduled to be completed in June 2011.^[24]

References

1. ^ Individual Environmental Report #5, pages 69-72
2. ^ "Experts Say Faulty Levees Caused Much of Flooding".
3. ^ "Multiple Failures: New Orleans Times-Picayune"
4. ^ "Geotechnical Reconnaissance of the Mississippi River Delta Flood-Protection System After Hurricane Katrina"
5. ^ "Breach of Faith: Hurricane Katrina and the Near Death of a Great American City"
6. ^ "U.S. Army Corps of Engineers Response to Hurricanes Katrina and Rita in Louisiana, Environmental Assessment, EA #433"
7. ^ "Journal of Contemporary Water Research & Education"
8. ^ http://levees.org/2/wp-content/uploads/2010/06/American-Society-of-Civil-Engineers-What-Went-Wrong-ERPreport-1.pdf
9. ^ http://blog.nola.com/times-picayune/2007/08/corps_analysis_shows_17th_st_c.html
10. ^ "How Stuff Works"
11. ^ "Bienville's Dilemma: A Historical Geography of New Orleans" page 82
12. ^ "U.S. Army Corps of Engineers, Lake Pontchartrain and Vicinity Hurricane Protection - Orleans Parish"
13. ^ "U.S. Army Corps of Engineers, Outfall Canals and Closure Structures"
14. ^ [1]
15. ^ "Independent Levee Investigation Team"
16. ^ [Interim Survey Report, Lake Pontchartrain, Louisiana and Vicinity. New Orleans: U.S. Army Corps of Engineers, 1962. Print]
17. ^ "Decision-Making Chronology for the Lake Pontchartrain and Vicinity Hurricane Protection Project"
18. ^ biotech.law.lsu.edu/katrina/hpdc/Final_HPDC_Apr3_2008.pdf
19. ^ "Hurricane Katrina: Strategic Planning Needed to Guide Future Enhancements Beyond Interim Levee Repairs"
20. ^ "Public Law 109-61, 109th Congress"
21. ^ "Public Law 109-62, 109th Congress"
22. ^ "Permanent Protection System for Outfall Canals - Report to Congress"
23. ^ "U.S. Army Corps of Engineers Engineering and Construction Bulletin"
24. ^ "The Times-Picayune"

External links

• Levees.Org (non-profit flood protection group in New Orleans)

See also

• Flood Control Act of 1965
• U.S. Army Corps of Engineers civil works controversies