Land elevation studied
New La. numbers help storm-surge researchers
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A new way to measure land elevations in the state doesn’t dramatically affect the expected potential storm surge in Louisiana, researchers from the National Hurricane Center said.
“The pattern (of flooding) in general is not that different than the one we’ve previously seen,” Will Shaffer, with the National Weather Service, said during a presentation in Baton Rouge Monday.
The difference is in the detail of what gets put into the computer storm surge model that shows flooding potential for south Louisiana, he said.
Roy Dokka, executive director of the Center for GeoInformatics at LSU, explained that for years it’s been difficult to get accurate ground elevation information for Louisiana mainly because the ground is sinking through subsidence.
The traditional method of surveying and placing “benchmarks” in the ground and using these as reference points doesn’t work over time because the ground changes.
To solve that problem, Dokka’s group has developed a different system that uses Global Positioning System technology with 42 stations that update the ground elevations every second.
“Think of these stations as smart benchmarks,” Dokka said.
The system can be used to measure levee elevations by installing a device in a vehicle and driving the levees. That’s just what Dokka and his group did in southeast Louisiana where they drove 1,000 miles of hurricane protection system measuring 50,000 points and got results that were accurate to plus or minus three-tenths of a foot, he said.
Other areas such as Lake Charles are being measured in the same way.
It’s these new elevations that have been entered into the National Oceanic and Atmospheric Administration National Hurricane Center’s storm surge model.
“The flow of water during a hurricane and other tropical storms is really controlled in many ways by the surface topography,” Dokka said. That includes the height of levees, roads, railroads and many other features, he said. How and where hurricane protection gets built makes a difference in where the water will collect and pile up during a storm, Dokka said.
“The highest water in a hurricane is where water has nowhere else to go so it goes up (in elevation),” Dokka said.
In the past, hurricane storm models haven’t been as accurate as they could be because the elevations of structures were based on what they were built to, not where they currently are because of subsidence, he said.
That’s what happened during Hurricane Katrina in St. Bernard Parish, he said. Using the old elevations of levees along the Mississippi River Gulf Outlet, the computer models show a “funnel effect” that pushed water into the Industrial Canal in New Orleans.
However, using elevations of what was likely in place at the time of the storm — an elevation several feet lower than the official 17 feet — the computer model doesn’t show a “funnel effect,” he said.
“There is no hurricane funnel. It did not have that effect,” he said.
In addition, the loss of coastal land after the 2005 hurricane season — measured at more than 200 square miles — doesn’t have much of an impact on the storm surge they’re seeing, Shaffer said.
“What we’ve found is as you get higher and higher flooding levels it doesn’t really matter what’s underneath (the water),” Shaffer said.
Although there is a widely cited figure that every 2.7 miles of marshland can reduce one foot of storm surge, that doesn’t seem to be the case, he said.
“It just doesn’t seem to be true,” Shaffer said.
“The pattern (of flooding) in general is not that different than the one we’ve previously seen,” Will Shaffer, with the National Weather Service, said during a presentation in Baton Rouge Monday.
The difference is in the detail of what gets put into the computer storm surge model that shows flooding potential for south Louisiana, he said.
Roy Dokka, executive director of the Center for GeoInformatics at LSU, explained that for years it’s been difficult to get accurate ground elevation information for Louisiana mainly because the ground is sinking through subsidence.
The traditional method of surveying and placing “benchmarks” in the ground and using these as reference points doesn’t work over time because the ground changes.
To solve that problem, Dokka’s group has developed a different system that uses Global Positioning System technology with 42 stations that update the ground elevations every second.
“Think of these stations as smart benchmarks,” Dokka said.
The system can be used to measure levee elevations by installing a device in a vehicle and driving the levees. That’s just what Dokka and his group did in southeast Louisiana where they drove 1,000 miles of hurricane protection system measuring 50,000 points and got results that were accurate to plus or minus three-tenths of a foot, he said.
Other areas such as Lake Charles are being measured in the same way.
It’s these new elevations that have been entered into the National Oceanic and Atmospheric Administration National Hurricane Center’s storm surge model.
“The flow of water during a hurricane and other tropical storms is really controlled in many ways by the surface topography,” Dokka said. That includes the height of levees, roads, railroads and many other features, he said. How and where hurricane protection gets built makes a difference in where the water will collect and pile up during a storm, Dokka said.
“The highest water in a hurricane is where water has nowhere else to go so it goes up (in elevation),” Dokka said.
In the past, hurricane storm models haven’t been as accurate as they could be because the elevations of structures were based on what they were built to, not where they currently are because of subsidence, he said.
That’s what happened during Hurricane Katrina in St. Bernard Parish, he said. Using the old elevations of levees along the Mississippi River Gulf Outlet, the computer models show a “funnel effect” that pushed water into the Industrial Canal in New Orleans.
However, using elevations of what was likely in place at the time of the storm — an elevation several feet lower than the official 17 feet — the computer model doesn’t show a “funnel effect,” he said.
“There is no hurricane funnel. It did not have that effect,” he said.
In addition, the loss of coastal land after the 2005 hurricane season — measured at more than 200 square miles — doesn’t have much of an impact on the storm surge they’re seeing, Shaffer said.
“What we’ve found is as you get higher and higher flooding levels it doesn’t really matter what’s underneath (the water),” Shaffer said.
Although there is a widely cited figure that every 2.7 miles of marshland can reduce one foot of storm surge, that doesn’t seem to be the case, he said.
“It just doesn’t seem to be true,” Shaffer said.
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