LacPontchartrain.ogg) is a brackish lake located in southeastern Louisiana. It is the second largest salt-water lake in the United States, after the Great Salt Lake in Utah, and the largest lake in Louisiana. It covers an area of 630 square miles (1630 square km) with an average depth of 12 to 14 feet (about 4 meters). Some shipping channels are kept deeper through dredging. It is roughly oval in shape, about 40 miles (64 km) wide and 24 miles (39 km) from south to north.

The south shore forms the northern boundary of the cities of New Orleans, and its two largest suburbs Metairie and Kenner. On the north shore are the cities of Mandeville and Madisonville. To the northeast is the city of Slidell.

Description

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Lake Pontchartrain is an estuary which connects with the Gulf of Mexico via Rigolets strait (known locally as "the Rigolets") and Chef Menteur Pass into Lake Borgne, and therefore experiences small tidal changes. It receives fresh water from the Tangipahoa, Tchefuncte, Tickfaw, Amite, and Bogue Falaya Rivers, and from Bayou Lacombe.

Salinity varies from negligible at the northern cusp west of Mandeville up to nearly half seawater level at the eastern bulge past Interstate 10 (or I-10). Lake Maurepas connects with Lake Pontchartrain on the west via Pass Manchac. The Industrial Canal connects the Mississippi River with the lake at New Orleans. Bonnet Carré Spillway diverts water from the Mississippi into the lake during times of river flooding.

History

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The lake was created 2,600 to 4,000 years ago as the evolving Mississippi River Delta formed its southern and eastern shorelines with alluvial deposits. Its Native American name was Okwata ("Wide Water"). In 1699, French explorer Pierre Le Moyne d'Iberville, renamed it Pontchartrain after Louis Phélypeaux, comte de Pontchartrain, the French Minister of the Marine, chancellor of France and minister of finance during the reign of France's "Sun King," Louis XIV, for whom Louisiana is named.

Human habitation of the region began at least 3,500 years ago, but increased rapidly with the arrival of Europeans about 300 years ago. The current population is over 1.5 million. The United States Geological Survey is monitoring the environmental effects of shoreline erosion, loss of wetlands, pollution from urban areas and agriculture, saltwater intrusion from artificial waterways, dredging, basin subsidence and faulting, storms and sea-level rise, and freshwater diversion from the Mississippi and other rivers.

New Orleans

New Orleans was established at a Native American portage between the Mississippi River and Lake Pontchartrain. In the 1920s the Industrial Canal in the eastern part of the city opened, providing a direct navigable water connection, with locks, between the Mississippi River and the lake. In the same decade, a project dredging new land from the lake shore behind a new concrete floodwall began; this would result in an expansion of the city into the former swamp between Metarie/Gentilly Ridges and the lakefront. The Lake Pontchartrain Causeway was constructed in the 1950s and 1960s, connecting New Orleans (by way of Metairie) with Mandeville and bisecting the lake in a north-northeast line. At 24 miles (39 km), the causeway is the longest bridge in the world.

Hurricanes

During hurricanes a storm surge can build up in Lake Pontchartrain, just as with Florida's Lake Okeechobee. Wind pushes water into the lake from the Gulf of Mexico as a hurricane approaches from the south, and from there it can spill into New Orleans.

A hurricane in September, 1947 flooded the city, most of which is below sea level (and sinking). After the storm, hurricane-protection levees were built along Lake Pontchartrain's south shore to protect the city. When a storm surge of 10 feet (3 meters) from Hurricane Betsy left much of the city under water in 1965, the levees encircling the city and outlying parishes were raised to heights of 14 to 23 feet (4-7 meters). Due to cost concerns, the levees were built to protect against only a Category 3 hurricane.

Experts using computer modeling at Louisiana State University subsequent to Hurricane Katrina have concluded that the levees were never topped but rather faulty design, inadequation construction, or some combination of the two were responsible for the flooding of most of New Orleans.

Funding

Congress failed to fully fund an upgrade requested during the 1990s by the Army Corps of Engineers, and funding was cut in 2003-04 despite a 2001 study by the Federal Emergency Management Agency warning that a hurricane in New Orleans was one of the country’s 3 most likely disasters. Raising and reinforcing the levees to resist a Category 5 hurricane might take 25 years to complete. Some estimates place the cost at $25 billion.

Hurricane Katrina

When Hurricane Katrina reached Category 5 in 2005, some experts predicted that the levee system might fail completely if the storm passed close to the city. Although Katrina weakened to a Category 4 before making landfall on August 29 (with only Category 3 strength in New Orleans on the weaker side of the eye of the hurricane), the levees designed to withstand Category 3 storms suffered multiple breaks the following day (see Effect of Hurricane Katrina on New Orleans) flooding 80% of the city.

The walls of the Industrial Canal were breached by storm surge via the Mississippi River Gulf Outlet, while the 17th Street Canal and London Avenue Canal experienced catastrophic breaches, even though water levels never topped their flood walls. Louisiana State University experts presented evidence that some of these structures might have had design flaws or faulty construction.

There are indications that the soft earth and peat underlying canal walls may have given way. In the weeks before Katrina, tests of salinity in seepage pools near canals showed them to be lake water, not fresh water from broken mains. The 5.5 mile (9 km) long I-10 Twin Span bridge heading northeast between New Orleans and Slidell was destroyed. Apparently, a bit farther east, the shorter Fort Pike Bridge crossing the outlet to Lake Borgne remained intact. By mid-October, one side of the Twin Span had been repaired and was ready to reopen to two-way traffic. But there are doubts whether the levees can even be restored to pre-Katrina condition by next summer (2006).

On September 5, 2005, the Army Corps of Engineers started to fix levee breaches by dropping huge sandbags from Chinook helicopters. The London Avenue Canal and Industrial Canal were blocked at the lake as permanent repairs started. On September 6, the Corps began pumping flood water back into the lake after seven days in the streets of New Orleans, even though it was fouled with human and animal corpses, sewage, heavy metals, petrochemicals, and other dangerous substances. Filters on the pumps kept out large debris.

Aerial photography suggests that 25 billion gallons (95 bn liters) of water covered New Orleans as of September 2, which equals about 2% of Lake Pontchartrain's volume. Due to a lack of electricity, the city was unable to treat the water before pumping it into the lake. It is unclear how long the pollution will persist and what its environmental damage to the lake will be, or the hazards from the mold and contaminated mud remaining in the city.

On September 24, 2005, Hurricane Rita again overtopped levees and breached walls, allowing water back into the nearly-dry city. The Corps of Engineers had most of it pumped out by the second week in October.

References

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