Drainage in New Orleans, Louisiana, has been a major concern since the founding of the city in the early 18th century, remaining an important factor in the history of New Orleans today. The central portion of metropolitan New Orleans (New Orleans/Metairie/Kenner) is fairly unusual in that it is almost completely surrounded by water: Lake Pontchartrain to the north, Lake Borgne to the east, wetlands to the east and west, and the Mississippi River to the south. Half of the land area between these bodies of water is at or below sea level, and no longer has a natural outlet for flowing surface water. As such, virtually all rainfall occurring within this area must be removed through either evapotranspiration or pumping. Thus, flood threats to metropolitan New Orleans include the Mississippi River, Lake Pontchartrain, canals throughout the city, and natural rainfall. Artificial levees have been built to keep out rising river and lake waters but have had the negative effect of keeping rainfall in, and have failed on numerous occasions.
As of 2017, the New Orleans pumping system - operated by the Sewerage and Water Board - can pump water out of the city at a rate of more than per second. The capacity is also frequently described as in the first hour of rainfall followed by per hour afterward. The scope and scale of the Mississippi River levees, built along either side of the river to keep it on its course, has often been compared to the Great Wall of China.
Geography
The city of New Orleans is located in the Mississippi River Delta on the east and west banks of the Mississippi River and south of Lake Pontchartrain. The city was originally settled on the natural levees or high ground, along the river. In the 1960s, floodwalls and man-made levees were constructed around a much larger geographic footprint that included previous marshland and swamp. Over time, pumping of nearby marshland allowed for development into lower elevation areas. Today, a large portion of New Orleans is at or below local mean sea level and evidence suggests that portions of the city may be dropping in elevation due to subsidence.
A 2007 study by Tulane and Xavier University suggested that "51% of the contiguous urbanized portions of Orleans, Jefferson, and St. Bernard parishes lie at or above sea level," with the more densely populated areas generally on higher ground. A more recent study published by the ASCE Journal of Hydrologic Engineering in 2016, however, stated, "...most of New Orleans proper - about 65% - is at or below mean sea level, as defined by the average elevation of Lake Pontchartrain.
Because of the low flat terrain of the New Orleans area, a complex system of levees, canals, and pumps are required to reduce the risk of flooding.
History
The natural levees along the Mississippi River were a result of soil deposits left from the river's annual floods. The site chosen for New Orleans had many advantages. Because it sits where distance between the river and Lake Pontchartrain is shortest, Louisiana Indians had long used the area as a depot and market for goods carried between the two waterways. The narrow strip of land also aided rapid troop movements, and the river's crescent shape slowed ships approaching from downriver and exposed them to gunfire. Despite the advantages, flooding was always a hazard.
The first artificial levees and canals were built in early colonial times. They were erected to protect New Orleans against routine flooding from the Mississippi River. The "back of town" away from the river originally drained down into the swamps running toward Lake Pontchartrain. Flooding from the lake side was rare and less severe as most of the old town had been built on high ground along the riverfront.
As the city grew, demand for more land encouraged expansion into lower areas more prone to periodic flooding. For most of the 19th century most residential buildings were raised up at least a foot above street level (often several feet), since periodic flooding of the streets was a certainty at the time.
In the 1830s state engineer George T. Dunbar proposed an ambitious system of underground drainage canals beneath the streets. The goal was to drain water by gravity into the low lying swamps, supplementing this with canals and mechanical pumps. The first of the city's steam engine powered drainage pumps, adapted from a ship's paddle wheel and used to push water along the Orleans Canal out to Bayou St. John, was constructed in this decade. However, only a few of Dunbar's plans were actually implemented as the panic of 1837 largely ended major systematic improvements for a generation.