Hilda Voisin Buquet
August 25, 2009
Mary Little McFarland
August 27, 2009
Nearly all the tropical storms and hurricanes that have made landfall in Terrebonne Parish have approached from the southeast, moving past Plaquemines, Jefferson and Lafourche parishes.
Using the latest ADCIRC technology, Maurice Wolcott, a Geographic Information System specialist with the LSU AgCenter, showed the depth of flooding Terrebonne could face if a hurricane were to swing into the parish from the southwest.
Wolcott was in Houma last week speaking at a meeting held by the AgCenter about hurricane storm surge, the abnormal rise of seawater that accompanies storms.
Advanced circulation modeling simulates storm surge and provides more accurate flood-depth measurements.
As with any hurricane, the amount of flooding following a storm coming from the southwest would depend on the speed of the hurricane’s winds and on the speed that the entire hurricane is moving.
Faster-moving storms tend to do more severe damage to coastal areas, whereas the slower-moving ones do comparatively more damage further inland, Wolcott said. Because the elevation is higher inland, storm surge can sometimes rise higher there than at the coast.
Wolcott used three imaginary hurricanes with varying wind and movement speeds to show how Terrebonne would be affected by storms arriving from the southwest: hurricane number one, moving fast at 17 miles per hour with wind speeds of 120 mph; hurricane number two, moving slow at 7 mph with 105 mph wind speed; and the Hurricane Rita-like storm number 3, moving at 13 mph with 128 mph winds.
The ADCIRC modeling shows hurricane number 3 would be the worst scenario for both Terrebonne and Lafourche parishes coming from the southwest, Wolcott said. Flooding would occur from the coast to north of Louisiana Highway 182 (New Orleans Boulevard) in Houma, while flooding would happen deep into Lafourche as well.
Because of its slow movement, hurricane number two could push water even further inland.
Wolcott showed how different sites around Houma and Terrebonne would look under the three scenarios using aerial mapping from ADCIRC:
Hurricanes number two and three would push in floodwater three to eight feet deep around the area of the Wal-Mart on Martin Luther King Boulevard in West Houma.
Hurricane one would leave the Houma-Terrebonne Civic Center dry but the water would be close; with hurricanes two and three, the floodwater at the civic center would be four to five feet deep and three to eight feet across the rest of Houma.
In East Houma, hurricane one would place two to three feet of water in the area of the Houma Terrebonne Airport; hurricanes two and three would deposit five feet of water at the airport and four to five feet at Chabert Medical Center.
In south Terrebonne, hurricane one would inundate the area around Montegut Middle School with 11 feet of floodwater, while hurricane two would pour 16 feet of water on the school and 15 feet on Pointe aux Chenes.
Wolcott said in the past, Terrebonne Parish has been struck only by tropical storms coming from the southwest. The only hurricane, besides Gustav, to directly impact Terrebonne was Hurricane Betsy in 1965; both came from the southeast.
“Rita and Ike were misses for Terrebonne and Lafourche,” he said.
Pre-Hurricane Rita, Wolcott said ADCIRC modeling accurately predicted the amount of flooding that subsequently occurred. The system also forecasted accurately for Hurricane Ike and correctly predicted that little flooding would result from Hurricane Gustav.
ADCIRC could not predict the flooding occurring in south Terrebonne following Hurricane Ike from levee breaches there, and the system could not take into account the flooding that was prevented by sandbagging in Lafourche.
“Nothing can predict flooding from levee breaches,” Wolcott said.
And he cautioned that forecasters can still only predict a hurricane’s landfall no closer than 50 miles.
Wolcott said the National Hurricane Center beginning this year will stop linking storm surge levels to hurricane intensity on the Saffir-Simpson Hurricane Scale.
“It’s not an ideal relationship between storm category and storm surge,” he said.
For example, Category 2 Hurricane Ike produced a storm surge at least the size predicted for a Category 4 hurricane on the Saffir-Simpson scale. Katrina’s 27-foot storm surge was nowhere near what would be predicted for a Category 3 hurricane.
Wolcott also reminded attendees that larger hurricanes affect a wider area.
“Size is important,” he said.
Though Hurricane Camille in 1969 was devastating for the Mississippi Gulf Coast, the storm was compact. Katrina and Ike were huge, measuring across. Katrina’s hurricane force winds spread out 105 miles, Ike’s 120 miles.
Terrebonne was in Gustav’s wind field for 30 hours. The area was in Ike’s wind field for 60 hours.
The Terrebonne Parish Main Library had computers set up at last week’s presentation on storm surge for attendees to look up ground elevations in the parish. * Staff photo by MIKE BROSSETTE
