Tuesday, October 23, 2007
Some information on Lake Ponchartrain, the second largest salt water lake in the U.S., and some of the bridges of New Orleans:
I was terrified as I tried to find my way out of New Orleans Saturday night late, because I got lost, and ended up crossing the bridges in the dark, silent, surreal night landscape. The fear I experienced crossing these bridges stems from long standing dreams experienced by the women in our family of driving off of high bridges and going down under the water. How wierd is that? New Orleans is somewhat deserted and being alone with an occasional, I thought, sinister looking vehicle, ok, ok, too much sci fi channel, passing by like a silent dark ghost in the night, gave me the spooks unequal to anything I'd experienced for a long, long time. It was like a living dark dream.
Even given the wierd phobias, bridges have always fascinated me for some unknown reason. On my list of books I want is a book on the building of the Brooklyn Bridge. From the building to the standing finished structures, I'm fascinated. I found this information in Wikipedia, on New Orleans bridges, down to the murders on one of the bridges.
But first a bit on Lake Ponchartrain:
Map Coordinates 30.206055° N 90.102132° W
Lake type: salt-water lake
Basin countries: United States
Surface area: 630 sq mi (1630 km²)
Settlements around lake: New Orleans, Metairie, Kenner, Mandeville, Slidell, Madisonville, etc.
Lake Pontchartrain (local English pronunciation [leɪk ˈpʰɑntʃətʰɹeɪn]) (French: Lac Pontchartrain, pronounced [lak pɔ̃ʃaʀtʀɛ̃]) is a brackish lake located in southeastern Louisiana. It is the second largest salt-water lake in the United States, after the Great Salt Lake in Utah, and the largest lake in Louisiana. It covers an area of 630 square miles (1630 square km) with an average depth of 12 to 14 feet (about 4 meters). Some shipping channels are kept deeper through dredging. It is roughly oval in shape, about 40 miles (64 km) wide and 24 miles (39 km) from south to north.
The south shore forms the northern boundary of the city of New Orleans, plus its two largest suburbs Metairie and Kenner. On the north shore are the cities of Mandeville, Covington, and Madisonville. To the northeast is the city of Slidell.
Another Bridge: The Crescent City Connection, abbreviated as CCC, (formerly the Greater New Orleans Bridge) refers to twin cantilever bridges that carry U.S. Route 90 Business over the Mississippi River in New Orleans, Louisiana. Today, they are the tied as the 5th longest cantilever bridges in the world. Each span carries four general-use automobile lanes; additionally the westbound span has two reversible HOV lanes across the river. The westbound span, which carries motorists into the city's Central Business District on the Eastbank, requires a toll of $1 ($0.40 with an electronic toll tag). It is the most downstream bridge on the Mississippi River
Sir Isaac Newton did a number on the Interstate 10 bridges in New Orleans, according to a team of researchers at the University of Missouri-Rolla that helped document some of the damage caused by Hurricane Katrina.
The team believes Newton’s theory of gravity can explain why the 65-foot concrete slabs dropped off their supports into Lake Pontchartrain.
“The bridge’s design created structures in the shape of an upside-down rice bowl in each span, with the opening faced downward to the water,” explains Dr. Genda Chen, associate professor civil, architectural and environmental engineering at UMR. “Our preliminary analysis shows that the main culprit for the collapse or displacement of spans is the significant reduction of effective gravity loads. Air was trapped underneath the bridge decks in these ‘rice bowls,’ allowing the bridge decks to partially float and tilt.”
The reduced gravity load weakened the ability of the bridge spans to resist the water current.
“The main span over the navigation channel is a much taller structure than the approach spans; it experienced no damage during Hurricane Katrina,” Chen says. “All the approach spans experiencing excessive horizontal displacements were moved toward the ocean, which indicates that the displacement was caused by the storm surge and wave-structure interaction rather than the wind gusts associated with the hurricane.”
(Hee hee – this must have been an analysis by the insurance companies.) (My comment)
After the water level was lowered or air escaped from underneath the bridge decks, the bridge structures landed again on piers -- but at wrong positions -- or dropped off their supports altogether.
The rice bowl theory helps explain why Highway 11 and railroad bridges – located near the twin bridges – suffered minor or virtually no damage, Chen says.
“The Highway 11 bridge consists of shorter spans and shallower girders, reducing the volume of air that could be trapped underneath,” Chen explains. “The railroad bridge is a solid deck structure, which means no air can be trapped underneath the bridge.”
The U.S. Geological Survey (USGS) in Rolla sponsored the team’s trip to New Orleans and provided researchers with satellite imagery, and various forms of mapping information and scientific assessments of the damaged areas.
The team is preparing its final report and intends to share its findings on the Internet by early next spring, using a tool developed by the USGS and the Department of Defense.
The Danziger Bridge is a vertical lift bridge which carries seven vehicular lanes of U.S. Route 90 (Chef Menteur Highway) across the Industrial Canal in New Orleans, Louisiana. When this bridge was completed in 1988, replacing a previous bridge of the same name, it was the widest lift bridge in the world. Most marine traffic is accommodated in the down position.
A deadly police shooting occurred on the bridge in the aftermath of Hurricane Katrina in 2005. On Sunday, September 4, seven police officers responded to a police dispatch reporting an officer down. According to the police, at least four people were firing weapons at the officers who then returned fire. Nineteen-year-old James Brisette and forty-year-old Ronald Madison were killed in the gunfire, and four other civilians were wounded. All claim to have been unarmed. The police officers involved in the shooting were taken into custody on January 2, 2007 and have been indicted for murder.