Welcome to the Bonnet Carre’ Spillway.....the area designated by the co-ordinates is handicapped accessible and will give you a nice view of this marvelous structure....if you like you can drive into the spillway as there is a road behind the structure leading to the retaining wall on the opposite side.
Bonnet Carré Spillway
Earlier studies to determine the best location for a spillway along the lower river had identified one at the site of the 19th century Bonnet Carré Crevasse, about 33 river miles above New Orleans. Between 1849 and 1882, four major crevasses had occurred at this location. In fact, during the flood of 1849, a 7,000-foot-wide crevasse at Bonnet Carré flowed for more than six months.
Several problems confronted the Corps of Engineers in designing a controlled spillway to divert a portion of the Mississippi River's flood flows to Lake Pontchartrain. The most critical concern was the possibility that the river might cut a channel through the spill way and thereby divert its course. Also of concern were the poor foundation conditions at the site -- would they support the massive concrete structure?
A series of preliminary investigations were conducted to address the design concerns and ensure an effective and high quality construction project. Three separate studies were performed on-site. The first of these were site and foundation studies to determine the properties of the soil. Piling tests were undertaken to compare the qualities of wooden and concrete pilings.
The tests indicated that long timber piles, in the range of 70 feet, would be quite adequate if sufficient numbers were driven under the structure. Untreated piles were selected after examining the Robert E. Lee Monument foundation in New Orleans, which showed that such piles last indefinitely as long as they are kept below the water table.
A temporary hydraulics laboratory was constructed on-site to determine the best shape of the weirs and to test various designs for dissipating the energy of water coming over the weir. To accomplish these design objectives, two wooden flumes (channels) were built: the first, a 1/6th scale model of a single spillway gate opening; and the second, a flume of 1/20th scale permitting the modeling of 22 spillway gates. Tests conducted with these models not only established the spillway's design parameters, but also demonstrated the usefulness of modeling in designing large hydraulic structures.
Because of the large quantity of concrete involved in the structure (135,000 cubic yards), it was essential that the concrete mixture produce strong, durable and economic concrete. The Corps established a concrete laboratory to determine optimal mixes of sand, gravel, cement and water quality for the spillway.
Tests from the lab proved critical, resulting in a very high strength concrete with a compressive capacity of 5,000 pounds per square inch. This was a significant advancement for the time, as typical concrete in the 1930s provided only 2,500 to 3,000 pounds per square inch.
The design and construction of the Bonnet Carré Spillway was completed in just two and a half years. However, the urgency to complete the spillway, following the Great Flood of 1927, did not diminish its quality as an effective and enduring flood control structure. Today, the spillway remains as it was originally constructed; no significant modifications to the structure have been needed.
The Bonnet Carré Spillway is an excellent example of engineering design and construction in the era before "high-tech" solutions, and it is listed on the National Register of Historic Places.
Spillway Operational Effects
During operation of the spillway (about once every 10 years), materials suspended in the Mississippi River's water are deposited in the floodway and lakes Pontchartrain and Borgne. In addition, the vast input of fresh water into these brackish and saline lakes has an immediate, short-term, adverse environmental effect.
The long-range effect, however, is extremely favorable because it simulates the natural flooding cycle of the river and provides a replenishment of valuable nutrients to the ecosystem. Spillway openings are strongly associated with increased oyster, crab and other fisheries production in lakes Pontchartrain and Borgne for several years after flood events.
With each opening, the river deposits an average of 9 million cubic yards of sediment, mostly silts and sand, within the floodway. These deposits are removed by private contractors and local government agencies for use as fill material in residential and industrial developments. Because most of the surrounding region is near or below sea level, this sediment is a valuable local resource.
In addition to the infrequent operation of the spillway for flood control, about every other year a small portion of the Mississippi River leaks through the spaces between the timbers of the spillway. This minor diversion of fresh water normally occurs for a few weeks in the spring or early summer when the river is high enough to exceed the elevations of the spillway weir but not high enough to warrant project operation. These minor diversions are termed leakage events (less than 10,000 cfs in comparison to a spillway opening with its design flow of 250,000 cfs).
The introduction of fresh water during leakage events simulates the natural cycle of overbank flooding and provides numerous ecosystem benefits to the aquatic and terrestrial resources in the spillway. These benefits include improved water circulation in the spillway's water bodies, nutrient introduction and restocking of fishery resources. Recreational crawfishing, for example, increases significantly due to the optimal conditions produced by these events. These frequent, small-scale diversions of Mississippi River water are also beneficial to the Lake Pontchartrain estuary.
The Bonnet Carré Spillway Values
Natural Resource
The diversity and abundance of productive habitats in the floodway support a wide variety of wildlife including game species, commercially important furbearers and alligators, endangered species, and numerous nongame species that are important to the ecology.
Game mammals include the gray squirrel, fox squirrel, swamp rabbit and feral hog. Common furbearers include otter, mink, nutria, muskrat, raccoon, opossum and beaver. Common nongame mammals include nine-banded armadillo, southern flying squirrel and marsh rice rat.
The forested wetlands and shallow margins of permanent water bodies provide excellent feeding and resting areas for American coot and dabbling ducks, such as the wood duck, mallard and the mottled duck. Diving ducks, such as the lesser scaup, are most common in Lake Pontchartrain and adjacent open water areas of the floodway. Other game birds occasionally found in the floodway include the American woodcock and common snipe.
A great diversity of nongame birds seen in the spillway includes sea birds, shorebirds, wading birds, songbirds and raptors. Numerous species of reptiles and amphibians are also found in the area.
The various water bodies in the floodway support a wide range of finfish and shellfish. Sport fishing and crawfishing are popular in the fresh water and brackish water habitats.
Environmental
Habitats in the floodway include bottomland hardwood forests, cypress swamps, canals and ponds, and disturbed areas. Bottomland hardwoods are located near the river and grade into cypress-tupelogum swamps closer to Lake Pontchartrain. The forested areas were logged in the past and second-growth timber covers the wooded areas.
The floodway acts as a catch basin for non-indigenous plants when flood waters are released from the Mississippi River. The introduction of seeds, rhizomes and other plant propagates permits establishment of new species and an ever-changing.
Cultural Resource
A cultural resource inventory of project lands at Bonnet Carre' was completed in 1991. The result was the listing of two properties on the National Register of Historic Places. One is the Kenner and Kugler Cemeteries Archeological District, located near the structure. This district consists of two African-American cemetery plots which date to the early 19th century and received interments until federal purchase of the property in 1928.
The Corps has established buffer zones to protect these important historic sites from project operations. The other National Register property in the project area is the spillway structure itself. The structure is significant as an engineering landmark and for its important historical association with flood control efforts on the Lower Mississippi River.
History of spillway opening Year Days Bays Opened (%) Opened Ideal flow capacity 1937 48 285 81.4% 203,571 cu ft/s 1945 57 350 100% 250,000 cu ft/s 1950 38 350 100% 250,000 cu ft/s 1973 75 350 100% 250,000 cu ft/s 1975 13 225 64.3% 160,714 cu ft/s 1979 45 350 100% 250,000 cu ft/s 1983 35 350 100% 250,000 cu ft/s 1997 31 298 85.1% 212,857 cu ft/s 2008 31 160 45.7% 114,286 cu ft/s 2011 42 330 94.3% 235,714 cu ft/s 2016 22 210 60.0% 203,000 cu ft/s Project Statistics
Distance above New Orleans 32.8 river miles
Length of Weir Opening 7,000 feet
Number of Bays 350
Width of Bays 20 feet
Creosote Timbers 20 per bay
Floodway Design Capacity 250,000 cfs (cubic feet per second)
Length 5.7 miles
Width at River 7,700 feet
Width at Lake 12,400 feet
U.S. Lands 7,623 acres
Frequency of Operation (est.) 10 years
To get credit for this Earthcache you must complete the following requirements.
PLEASE SEND AN EMAIL TO BAMBOOZLE AND ANSWER THE FOLLOWING:
1. How much concrete was involved in the structure ?
2. How many bay openings does the structure have ?
3. What was the area of the spillway called in the 19 th century ?
4. What percentage of the total flow of the Mississippi River do you think can flow through the spillway when completely opened ?
5. Lastly, when you log your visit please post a photo of something you find interesting with the spillway in the background. Even you if you like, but that is not a requirement.
Do not post your answers to these questions when you log your visit. Any log entry with the answers, even if they are encrypted, will be deleted.
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