Comparative Analysis of Hurricane Vulnerability in New Orleans and Baton Rouge. Dr. Marc Levitan LSU Hurricane Center. April 2003

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1 Comparative Analysis of Hurricane Vulnerability in New Orleans and Baton Rouge Dr. Marc Levitan LSU Hurricane Center April 2003 In order to compare hurricane vulnerability of facilities located in different locations, all of the major hazards associated with hurricanes must be considered. These major hazards are: Storm surge flooding Extreme rainfall and rainfall flooding Extreme winds and windborne debris Storm Surge and Associated Flooding Storm surge is the abnormal rise of sea level accompanying a hurricane, caused by the low atmospheric pressure in the storm and strong onshore winds. As the hurricane makes landfall, this surge creates significant flooding. NOAA s SLOSH (Sea, Lake, and Overland Surges from Hurricanes) Model, version 1.31 (release date 5/10/2002) was used to compare storm surge flooding vulnerability for New Orleans and Baton Rouge. Figures 1 and 2 show the maximum potential storm surge flood elevation for Category 2 and 3 Hurricanes respectively. These are plots are generated by combining the maximum predicted surge flooding for many hurricanes of a given intensity with varying tracks and forward speeds (called MOMs, for Maximums of Maximum Envelopes of Water). For example, any specific point on the map shown in Figure 1 represents the worst case surge flooding potential at that site, for any Category 2 storm from any track. No single storm Category 2 storm would flood all of the areas shown. The range of colors indicate the maximum flood elevation potential (referenced to NGVD). The actual depth of floodwaters over land would be determined by subtracting the land surface elevations. This would be a more helpful way to view the data for comparison of relative flooding risk, but the program does not provide outputs in that form. In New Orleans, much of the city is near or even below sea level, meaning that the actual depth of flood water over ground would be similar or in may cases worse than the elevation levels shown in Figures 1 and 2. The much higher land elevations in Baton Rouge and its distance inland makes it far safer from storm surge flooding than New Orleans. Examination of Figures 1 and 2 clearly demonstrates that New Orleans is at significant risk of flooding from Category 2 and 3 hurricanes. All locations on the West Bank and many points on the East Bank could flood even in Category 2 intensity storms from certain directions (the parts of the city shaded gray where the grid is clearly visible would not flood). Figure 2 shows that locations anywhere within Orleans and Jefferson Parishes can experience significant storm surge flooding in a Category 3 storm (again, no single Category 3 storm would flood all of the shaded areas). The situation deteriorates rapidly if Category 4 and 5 storms are considered. Any single storm can easily flood broad areas of both parishes to depths over land of 10 feet or more. 1

2 Fig. 1. Category 2 Storm Surge Maximum Flood Elevation. East Baton Rouge Parish would not experience storm surge flooding in any Category 2 hurricane except perhaps for minor flooding along the lower Amite River. Stronger hurricanes have the potential to cause some flooding in extreme southern parts of the parish, but storm surge flooding under even the most intense hurricanes would not extend into the city. Extreme Rainfall and Associated Flooding Unlike wind and storm surge, rainfall amounts are not well correlated with Hurricane intensity. This was clearly demonstrated in June 2001 by Tropical Storm Allison, which dumped record amounts of rain in areas across southeast Texas and Louisiana. From a climatological standpoint, tropical cyclones of all intensity are slightly wetter in the New Orleans area compared to Baton Rouge. However, rainfall flooding hazards are significantly greater in New Orleans. This is because the city is ringed with levees, creating a bowl. All of the rain that falls that doesn t evaporate or get absorbed or must be pumped out. Protection from rainfall flooding therefore depends on pumping stations remaining operational. New Orleans is also very urbanized, with less retention and faster runoff than the more suburban-like Baton Rouge. 2

3 Fig. 2. Category 3 Storm Surge Maximum Flood Elevation Extreme Winds and Windborne Debris Although flooding is the leading cause of fatalities, extreme winds and wind borne debris are often the greatest source of hurricane damage to buildings (e.g., the $30 billion of damage caused by Hurricane Andrew, the most expensive natural disaster in US history, was mainly due to winds). The extreme winds decay as the storm moves inland, with an average decrease of 10% per 30 miles distance inland. The maximum expected wind speeds in New Orleans and Baton Rouge for a 100-year storm (the worst storm on average that would occur during a 100-year period) are roughly 120 and 100 miles per hour, respectively. Although this may not seem like a huge difference, wind pressures and forces on a building increase with the square of the wind speed, and damage increases even faster. The expected wind forces on a building in New Orleans are nearly 50% greater than on a similar building in Baton Rouge. This variation of expected maximum speeds is taken into account in modern building codes, which makes construction more expensive in higher wind regions. However, when design wind speeds are equaled and exceeded (strong Category 2 storms and larger), expected damage would be more intense in New Orleans due to the higher base wind speed. 3

4 History of Hurricane Events A comparison of the history of significant hurricanes passing close to both cities was performed using the historical hurricane database maintained by the NOAA Coastal Services Center. This database contains track and intensity information for tropical weather systems of significance from The database was queried using place name searches for Baton Rouge and New Orleans, for storms that passed within 50 nautical miles of each city while they were Category 2 or stronger intensity. This intensity storm is the threshold at which significant damage to constructed facilities generally begins to be observed. The wind resistant design requirements of the latest edition of the national standard for wind loads (ASCE 7 Standard for Minimum Design Loads on Buildings and Structures) are roughly equivalent to a strong Category 2 storm. Older codes provide generally lower levels of protection. The results of the query of significant hurricanes for Baton Rouge and New Orleans are presented in Figures 3 and 4. Storm tracks that change from red to yellow indicate intensity reduction from hurricane to tropical storm strength. It is important to note that the tracks shown are the best estimates of the center of the eye of the hurricane. Extreme winds, surge and rainfall extend out a significant distance from the track, generally strongest in the right front quadrant of the advancing storm. For example, although the track of Hurricane Betsy passed well to the west of New Orleans (see Figure 4), it still damaged and destroyed a large number of buildings in the city. As seen in Figures 3 and 4, Baton Rouge experiences significantly fewer strong hurricanes. This is because it is located well inland. Hurricanes lose strength rapidly after making landfall, because they become cut off from the warm ocean waters that provide their energy source. Queries to the historical hurricane database that include Category 1 hurricanes show many more storms passing close to both cities. However, damage to engineered buildings and structures from extreme winds and storm surge flooding in these low intensity hurricanes is usually minimal. Conclusion New Orleans faces far greater hurricane hazards and is more vulnerable to significant damage from hurricanes than Baton Rouge. The Crescent City is exposed to significantly greater storm surge and extreme winds. Although rainfall rates and maximum rainfall amounts are not much larger in New Orleans compared to Baton Rouge, the more urban nature, bowl-like topography, and dependency on pumps make it more vulnerable to rainfall flooding as well. 4

5 Fig. 3. Category 2-5 Hurricanes Passing within 50 nautical miles of Baton Rouge Fig. 4. Category 2-5 Hurricanes Passing within 50 nautical miles of New Orleans 5