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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 == Before August 29, 2005, the three outfall canal levees operated with little attention. The larger and longer Mississippi River system, which faces annual flood stages of ten to twenty feet lasting for four weeks at a time, garnered significantly more engineering 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 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.〔http://levees.org/2/wp-content/uploads/2010/06/American-Society-of-Civil-Engineers-What-Went-Wrong-ERPreport-1.pdf〕 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. 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「New Orleans Outfall Canals」の詳細全文を読む スポンサード リンク
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