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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 unique 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. Much 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, 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. ==History== New Orleans was originally built on natural levees along the Mississippi River that 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, however 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. In 1859 surveyor Louis H. Pilié improved the drainage canals, bricking in some portions. Four large steam "draining machines" were built to push water through the canals into the lake. In 1871, some of canals were built in the city for both improved drainage and small vessel shipping within town. However, despite earlier efforts, at the end of the 19th century it was still common for water to cover streets from curb to curb after rainstorms, sometimes for days. In 1893, the city government formed the Drainage Advisory Board to come up with better solutions to the city's drainage problems. Extensive topographical maps were made and some of the nation's top engineers were consulted. In 1899, a bond was floated, and a 2 mill per dollar property tax approved, which funded and founded the Sewerage & Water Board of New Orleans. The Sewerage & Water Board had the responsibility of draining the city along with constructing a modern sewage and tap water system for the city, which, at the time, still relied heavily on cisterns and outhouses. (A different entity, the Orleans Levee Board, is in charge of supervision of the city's levee and floodwall system.) The Sewerage & Water Board found A. Baldwin Wood, a young engineer who not only supervised the plans for improved drainage and pumping, but also invented a number of improvements in pumps and plumbing in the process. These improvements were not only used in New Orleans, but adopted all over the world. As the 20th century progressed, much of the land that had previously been swampland or considered fit for no other use than cow pasture (due to periodic flooding), was drained. The city then expanded back from the natural higher ground close to the river and the natural bayou formed ridges. On 15 April 1927 in what became known as the Great Mississippi Flood of 1927, the city was deluged by a downpour of some of rain within 19 hours. At the time, almost all of the city's pumps relied completely on the municipal electricity system, which went out early in the storm, thus knocking the pumps off line, which led to extensive flooding in the city. After this, back up diesel generators with sufficient fuel to run the pumps for at least a day if electricity failed were added to the pumping stations. The "Good Friday Flood", as it was known locally, happened when the Mississippi River levels were dangerously high along the levees at the city, but was not directly connected to the more wide-ranging flood. 1927 also saw the start of a project to build a more extensive system of levees on the shoreline of Lake Pontchartrain. After 1945, all land up to the lake had been developed. The city's system was effective when the 1947 Fort Lauderdale Hurricane directly hit the city. Wood's drainage pumps kept the city proper mostly dry, while the neighboring suburbs on the East Bank of Jefferson Parish (which at the time did not have a comparable system operational), flooded under up to of water. Most of the city weathered Hurricane Betsy in 1965 without severe flooding, with the major exception of the Lower Ninth Ward neighborhood. The Lower Ninth Ward is separated from the rest of the city by the Industrial Canal and Gulf Intracoastal Waterway. It was flooded not by rainfall, but by a breach in the Industrial Canal levee, resulting in catastrophic flooding and loss of life in the neighborhood. By the 1980s, the city boasted a system of 20 pumping stations with 89 pumps, with a combined capacity of per minute, equal to the flow of the Ohio River. In May 1995, torrential rains (up to in 12 hours in some places) overwhelmed pumping capacity, flooding substantial portions of the city. Slab houses in some low areas were flooded, and great numbers of automobiles on the city's flooded streets were declared insurance write-offs. This prompted projects increasing drainage capacity in the worst hit areas. By early 2005, the city had 148 drainage pumps. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Drainage in New Orleans」の詳細全文を読む スポンサード リンク
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