As Tropical Storm Barry Looms – 4 Surprising Facts About New Orleans And Flooding

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New Orleans, Louisiana like much of the Gulf Coast region is watching as a sloppy-looking Tropical Storm Barry slowly starts to drift northward. The storm will likely produce some windy conditions, minor storm surge, and tornadoes. However, the most significant threat from Tropical Storm Barry is likely to be the significant rainfall and flooding. The city of New Orleans is particularly vulnerable to flooding because roughly 50% of it is below sea level. I have traveled to New Orleans on numerous occasions and always find it interesting that in certain places I walk “up” to the Mississippi River. While at my daughter’s volleyball match this week, someone asked me how they get rid of water when it floods in New Orleans, and where does it go? Herein, I answer those questions and share a few more facts about flooding in New Orleans.

Projected rainfall totals from Tropical Storm Barry over the next 1 to 3 days.

NOAA WPC

Tropical Storm Barry could cause significant flooding in New Orleans and the Gulf Coast region. I have often referred to the city as a “worst case scenario for an excessive rainfall event.” Even earlier this week, a burst of rainfall from pre-Barry thunderstorms caused the city to flood.  Latest rainfall projections from NOAA suggest that Tropical Storm Barry could dump 10 to 20 inches of rainfall in parts of southeastern Louisiana. Some models suggest that even 2 feet or more is possible in some isolated spots. Additionally, there may minor storm surge though it does not appear as dire as river level projections earlier in the week.

New Orleans has been sinking (and still is): New Orleans was originally built above sea level but several things have changed.  According to a story on CNN’s website, Ryan Prior writes:

the original part of the city, the French Quarter, was built on higher ground beginning in the early 18th century.
Settlers who got the best land were able to build only about 10 feet above sea level. Even from the beginning, the city was fighting an uphill battle as it expanded. New Orleans is mostly flat, and areas around the French Quarter are just a little lower.

Pryor goes on to point out that the city is vulnerable to flooding because it was built on a marshy, loose soil landscape and relied on an early drainage system that eroded sediment. Such factors led to a sinking city. A NASA study using airborne observations found that certain parts of the city are sinking faster than others:

The highest rates of sinking were observed upriver along the Mississippi River around major industrial areas in Norco, and in Michoud, with up to 2 inches (50 millimeters) a year of sinking. The team also observed notable subsidence in New Orleans’ Upper and Lower 9th Ward, and in Metairie, where the measured ground movement could be related to water levels in the Mississippi. At the Bonnet Carré Spillway east of Norco — New Orleans’ last line of protection against springtime river floods overtopping the levees — research showed up to 1.6 inches (40 millimeters) a year of sinking behind the structure and up to 1.6 inches (40 millimeters) a year at nearby industrial facilities.

When you couple this with sea level rise and the occasional tropical storm or hurricane like Barry, you have a flood-vulnerable city.

NASA map showing sinking rates in New Orleans.

NASA JPL

The city probably has less than 5 years to figure things out. The city may not be able to withstand future hurricanes and tropical storms if information in a recent U.S. Government federal register of solicitations is accurate:

Southeast Louisiana, including the Greater New Orleans area, is generally characterized by weak soils, general subsidence, and the global incidence of sea level rise that will cause levees to require future lifts…. to sustain performance of the HSDRRS (Greater New Orleans Hurricane and Storm Damage Risk Reduction System). The HSDRRS project authority did not provide for future lifts. Engineering analysis indicates the HSDRRS will no longer provide 1% level of risk reduction as early as 2023. Absent future levee lifts to offset consolidation, settlement, subsidence, and sea level rise, risk to life and property in the Greater New Orleans area will progressively increase. USACE will notify FEMA once the system no longer provides the 1% level of risk reduction, which may result in the loss of accreditation required for participation in the National Flood Insurance Program.

That’s lest than five years away by my math.

New Orleans, USA – Nov 28, 2017: Afternoon view of the 17th Street Canal Pump Station as viewed from the road. This structure forms part of the city’s flood control mechanism.

Getty

Gravity and an elaborate system remove water. The city of New Orleans has an intricate system of catch basins, drainage canals and pumping systems to remove water during extreme flooding events. According to an online blog dedicated to the pump system,

there are twenty-two drainage pumping stations in New Orleans, and station personnel are always on duty 24-hours a day, seven days a week. The Sewerage and Water Board’s drainage network includes approximately ninety miles of open canals.

When it rains in New Orleans, the water has to be absorbed by the ground (infiltration if you think back to 4th grade water cycle lessons) or pumped somewhere. Catch basins divert initial rainfall, varying from 0.5 to 1 inch per hour, into a system of pipes. Once those pipes are full, water may pool into the streets according to NOLA.com. Jeff Adelson wrote about a past flooding event in New Orleans:

The pumps, both interim ones in place in recent years and the permanent ones being completed by the U.S. Army Corps of Engineers, are designed to pump water over gates at the ends of the canals that can be closed to prevent storm surge from pushing into the city from Lake Pontchartrain. But when those gates are open, as they were Saturday, gravity moves water out of them more quickly than the pumps would….

My colleague Dr. Calvin Mackie is an engineer and president of STEM NOLA. He is one of the best STEM advocates in the nation and a resident of New Orleans. His life, like many other residents of New Orleans, was changed by Hurricane Katrina so I will give him the last word. In a social media post in response to flooding earlier in the week, Mackie wrote:

Say it with me: If the amount of water entering the tank is greater than the amount of water leaving the tank, the tank will fill up. The rain out of the sky is filling a tank called New Orleans, and the drains and pumps can only handle so much. To fix the problem, we must reduce the amount of water coming from the sky, or install bigger pumps which require bigger pipes, which will require uprooting every pipe in the city. Another possible solution, is turning all green space to retention ponds and live with the water like the Dutch or with boats like in Venice.

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