Above: Carrollton Gage on the Mississippi River at New Orleans. Flood stage on the gage is at 17', which is 3' below the top of the city's levees. (USACE)
Tropical Storm Cristobal is predicted to make landfall as a tropical storm along the U.S. Gulf Coast on Sunday. Cristobal was upgraded from depression status at 2 pm EDT Friday. A tropical storm watch is in effect for the area from Intracoastal City, Louisiana, to the Alabama/Florida border, and storm surge watches are in effect for most of the southeast Louisiana coast as well as Florida's Big Bend. We'll have a full update on Cristobal later today.
Though the uncertainties are currently high, storm surge inundations of 2 - 4 feet are predicted for southeast Louisiana. New Orleans’ levee system should be able to handle such a surge with ease. However, this means that for the second consecutive year, Louisiana will be seeing an early-season storm surge move up the Mississippi River when the river is running high due to spring runoff—highlighting New Orleans’ vulnerability to storm surges moving up the river and potentially overtopping the city’s levees. In the unlikely event that Cristobal overachieves and makes landfall as a category 2 hurricane near New Orleans, the city may be at risk of a storm surge moving up the Mississippi River that overtops its levees. Update: The crest forecast issued late Friday morning called for a peak of 17 feet, up from the 16 feet in the illustration below.
The lesson of Hurricane Barry of 2019
Hurricane Barry made landfall on the western coast of Louisiana on July 13, 2019, as a category 1 hurricane with winds of 75 mph. Ordinarily, the storm surge from a category 1 hurricane would not be a threat to the New Orleans levee system, which is designed to withstand a category 3 hurricane. The city’s levees successfully held back a storm surge of approximately 10’ from category 2 Hurricane Gustav of 2008 and category 1 Hurricane Isaac of 2012.
But as detailed in a July 2019 Cat 6 post, New Orleans’ Achilles Heel: A Hurricane Storm Surge During a Mississippi River Flood?, the New Orleans has an Achilles Heel which makes it vulnerable to the storm surge from a category 1 hurricane—the levees along the Mississippi River. Although these levees lie more than 100 miles upriver from the Gulf of Mexico, five hurricanes over the past 50 years—Betsy (1965), Katrina (2005), Georges (1998), Isaac (2012), and Gustav (2008)—have pushed storm surges of seven feet or higher upriver to New Orleans. If a surge that high moves upriver when the Mississippi River is at flood stage, the levees could be overtopped.
The U.S. experienced its wettest 12-month period on record ending in June 2019, putting the Lower Mississippi River at flood stage for the longest period on record. At the time Hurricane Barry formed, the river was just four feet below the tops of the levees in New Orleans, putting the city at risk from a storm surge moving up the river from just a category 1 hurricane.
Indeed, the official forecast from NOAA predicted that scenario on July 10, three days before Barry made landfall. A storm surge of 4’ was predicted to move up the Mississippi River to New Orleans, which would have been high enough to overtop the lowest portions of the levees protecting the city. Fortunately, Barry intensified more slowly than predicted, and the winds of the storm were not strong enough to bring a significant storm surge to New Orleans. At the mouth of the Mississippi River, Barry brought a storm surge of 2.5’, and a surge of about 1’ was observed along the Mississippi River at New Orleans. Other locations were not as fortunate: Barry brought a damaging storm surge of 4 - 6’ to the central Louisiana coast.
Flow rates of the Mississippi River predicted to increase by 11 to 60% by 2100
New Orleans may be at increased risk of levee overtopping along the Mississippi River from climate change, due to increased river flows and stronger hurricanes driving storm surges up the river. There does not appear to have been a significant change in flood volumes on the Lower Mississippi River in recent decades: the biggest floods, with flow rates between 24,000 and 26,000 cubic meters per second, accounted for about 10% - 14% of all discharge events during 1986 - 2015, and 11 – 15% of all events during 1973 - 2015. However, there is the danger that climate change could increase flow rates on the river by up to 60% by the end of the century.
A warming climate has already caused an acceleration of the hydrological cycle—increased precipitation, evapotranspiration, runoff, and river flow. These changes are predicted to intensify further as the climate continues to warm. A 2014 study by Tao et al., Increasing Mississippi river discharge throughout the 21st century influenced by changes in climate, land use, and atmospheric CO2, used a high-resolution Dynamic Land Ecosystem Model (DLEM) to examine future changes in discharge from the Mississippi River and to evaluate the relative roles of future climate, atmospheric CO2, and land use during 2011 - 2099. The model predicted that the flow rate of the Mississippi River--in both drought and flood conditions--would increase by 11 - 60% by 2100, depending upon the amount of human-caused climate change and land use change assumed.
In the higher emissions scenario, the predicted increase in precipitation and temperature in the model accounted for 49% of the increased Mississippi River flow, while changes in land use (due to an increase in urban areas and farmland) contributed 15% of the increased flow; paved surfaces deprive the land of its ability to store and slowly release water, allowing more runoff into the river. The other 36% of the increase was attributed to increased atmospheric CO2, which affects plant growth and evapotranspiration and soil processes in a manner that increases river runoff.
