Postcards IV: Katrina's storm surge, and the Rita evacuation
I'm in Orlando this week for the 28th Conference on Hurricanes and Tropical Meteorology, sponsored by the American Meteorological Society. The conference, held once every two years, brings together the world's experts on hurricane science. Two postcards from today's talks:
Hurricane Rita evacuation survey
The evacuation effort for Hurricane Rita as it approached the Texas/Louisiana coast was one of the largest evacuation efforts of all time. Rebecca Mouss of the National Center for Atmospheric Research reported on the results of a survey of 120 residents of Houston, Galveston, and Port Arthur, Texas that evacuated from Rita. Since Rita came so soon after Hurricane Katrina's record devastation, 55% of the people surveyed reported that Katrina influenced their decision to evacuate, and 10% said it was their primary reason for evacuating. For future storms, a majority of the people said they would evacuate if evacuation orders were given for a Category 3 hurricane, but not for a weaker storm. Five percent said they would not evacuate ever, even for a Category 5 hurricane.
Figure 1. Hurricane Rita bears down on the Texas/Louisiana coast.
Katrina's storm surge
Pat Fitzpatrick of Mississippi State University studied the effect of the levee system along the Mississippi River on Hurricane Katrina's storm surge. Katrina had the highest storm surge on record in the Atlantic--an astonishing 27.8 feet along the Mississippi coast. Fitzpatrick showed that the levees along the Mississippi River acted to dam up the storm surge along the east side of the Mississippi River, increasing the storm surge by 2-3 feet within 15 miles of the levees. Calculations from a storm surge model showed that inundation of Chalmette and the Ninth Ward of New Orleans was accelerated by 1-5 hours, thanks to the presence of the levees, with a higher surge of 3-7 feet. In contrast, the levees had little impact on the timing or height of the surge on the Mississippi coast. It is possible that, without the river levees, hard-hit Chalmette and the 9th Ward may have experienced significantly less flooding.
Fitzpatrick also modeled the effect of the loss of wetlands due to erosion on Katrina's storm surge. The general rule of thumb developed for work in the 1960's credited wetlands with reducing storm surge by one foot for every 2.7 miles the storm surge had to pass over a wetland. The SLOSH storm surge model found that wetlands in Louisiana were actually twice as effective in reducing storm surge--each three miles of wetland the surge passed over reduced Katrina's storm surge by two feet. Note that this effect varied with the depth of the surge--an eight foot high surge was knocked down about 13% by wetlands, while a one foot high surge was reduced 59%.