Bonnet Carré Spillway

Bonnet Carré Spillway is a flood control structure in St. Charles Parish, Louisiana, operated by the United States Army Corps of Engineers. Its primary function is to divert water from the Mississippi River into Lake Pontchartrain to prevent flooding in New Orleans and surrounding areas. The structure is a critical component of the larger Mississippi River and Tributaries Project and represents a key engineering solution for managing the river's immense flow.

History and purpose

The need for the structure became tragically apparent following the Great Mississippi Flood of 1927, which inundated vast areas of the Mississippi Delta and prompted a federal overhaul of flood control policy. Authorized by the Flood Control Act of 1928, construction began in 1929 and was completed in 1931. Its fundamental purpose is to protect the Greater New Orleans region, particularly the east bank levee system, by providing a controlled outlet when the river's stage at the Carrollton Gauge in New Orleans threatens to exceed a designated safety threshold. This operational strategy is a cornerstone of the comprehensive flood control system developed by the United States Army Corps of Engineers for the lower Mississippi River.

Design and operation

The structure consists of a 7,000-foot-long concrete weir, or sill, paralleling the river, behind which are 350 individual bays, each closed by a series of wooden "needles" or timbers. These timbers are manually removed by crews using specialized equipment, allowing water to flow from the Mississippi River into a 5.7-mile-long guide levee that channels it into Lake Pontchartrain. The design capacity is to divert up to 250,000 cubic feet per second of water. Operation is triggered when the river stage at the Carrollton Gauge reaches 17 feet, a level determined to be the safe maximum for the New Orleans levee system. The decision to open any number of the bays is made by the United States Army Corps of Engineers in consultation with the National Weather Service and other agencies.

Environmental impact

Operations have significant and complex ecological consequences for Lake Pontchartrain and the northern Gulf of Mexico. The influx of massive volumes of fresh, sediment-laden, and nutrient-rich Mississippi River water drastically lowers the salinity of the brackish estuary, causing widespread mortality of oysters, dolphins, and marine species. The nutrient load can fuel algal blooms and contribute to hypoxic conditions. Furthermore, the freshwater plume can transport agricultural pollutants and contaminants into the lake and beyond. These events have been studied extensively by institutions like Louisiana State University and the National Oceanic and Atmospheric Administration, highlighting a persistent conflict between essential flood protection and estuarine ecosystem health.

Notable openings

Prior to 1950, it was used only three times, but the frequency of operation has increased in recent decades, reflecting changing hydrological patterns. A historic opening occurred in 1973 during a major flood on the Mississippi River that also affected the Ohio River basin. It was opened in 1975, 1979, 1983, 1997, 2008, 2011, and 2016. Unprecedented consecutive-year openings took place in 2018 and 2019, with the 2019 event lasting for a record 123 days and marking the first time it was opened twice in the same year. The 2020 opening further emphasized this trend of frequent activation, which has been linked to increased precipitation in the Midwestern United States and the Mississippi River watershed.

Management and future

Management is a continuous challenge for the United States Army Corps of Engineers, balancing flood risk with escalating environmental costs. Ongoing research focuses on understanding the long-term effects of repeated openings on the Lake Pontchartrain ecosystem and the fisheries of the Gulf of Mexico. Proposed alternatives and complements include the Mid-Barataria Sediment Diversion and other projects within Louisiana's Coastal Master Plan, which aim to more strategically use river water and sediment for wetland restoration. Future operations will likely be informed by advanced modeling from agencies like the United States Geological Survey and must adapt to scenarios influenced by climate change and subsidence in the Mississippi River Delta.

Category:Flood control in the United States Category:Buildings and structures in Louisiana Category:United States Army Corps of Engineers