Hurricane Katrina
An example of the interaction of fluvial and coastal processes
- Figure 1 Hurricane Katrina over the Gulf of Mexico, on August 28, 2005.
- Figure 2 Failure of a floodwall along a canal in New Orleans.
- Figure 3 Flooding of New Orleans.
- Figure 4 Cross section from the Mississippi River to Lake Pontchartrain, through New Orleans.
On August 29, 2005, Hurricane Katrina (Figure 1) struck the city of New Orleans, Louisiana, USA. The water level in Lake Pontchartrain, north of the city, rose approximately 6 m. This caused the failure of floodwalls and dikes at three locations (Figure 2). As a result, 80 % of the city flooded (Figure 3), and over a thousand people drowned. Bridges collapsed (Figure 5), and power was out for several weeks. The damage and pollution (mainly as a result of damaged petrochemical plants) was beyond imagination.
- Figure 5 Collapsed bridge as a result of floodwaters raised by Hurricane Katrina.
- Figure 6 Seawater temperatures in the Gulf of Mexico and surrounding areas.
The old town, built on a natural levee of the Mississippi River, was not inundated, because it is at a level approximately 3-4 m above sea level, while the northern part of the city is below sealevel (Figure 4). This low-lying part of the city is a floodbasin. Over the past century, extraction of water, oil and gas led to compaction of peat and clay. This has increased the elevation difference between the natural levee and the flood basin. In this case, the flooding was not caused by a flood of the Mississippi River, but by a storm surge, caused by Hurricane Katrina. However, flooding by the river could potentially lead to a similar disaster.
- Figure 7 Location of Lake Pontchartrain and New Orleans.
A disaster like this had been predicted by the Corps of Engineers and Louisiana State University years before. Factors that contributed to the disaster, are:
- The coast of Louisiana is very vulnerable to hurricanes, that gain strength over the warm Gulf of Mexico (Figure 6).
- The category-3 hurricane took a path, that exposed New Orleans to northern winds, which raised floodwaters in Lake Pontchartrain to overflow the dikes and floodwalls. This caused erosion, leading to a collapse of a few crucial floodwalls, and this led to the inundation of the city. The total damage by the storm and the flooding is estimated at approximately 200 billion US$.
- Sealevel rose approximately 15 cm over the past century, as a result of global warming. Because Lake Pontchartrain (Figure 7) is only a few decimeters above sealevel, and the lake is connected to the Gulf of Mexico, the lake level is directly influenced by sorm surges.
- Subsidence. As a result of compaction, related to the extraction of oil, gas, and water, especially the floodbasins have subsided, and are now below sealevel (Figure 4).
- A decrease of the wetland area (Figure 8), generally called wetland loss. Wetlands significantly decrease the force of hurricanes. Because of wetland loss, New Orleans is now closer to the sea. This makes the city more vulnerable to hurricanes, so that weaker hurricanes now have a stronger inpact. Factors that contribute to wetland loss are: barrier island degradation, storms, salt water intrusion, the digging of canals, oil and gas development (Figure 9), the levee system leading to reduced sedimentation, sealevel rise and land subsidence.
- Figure 8 Wetland loss (light blue colors) in the Mississippi delta.
- Figure 9 Oil and gas development in Louisiana.
- Embankment of the Mississippi has stopped sedimentation in the floodbasins, thereby increasing the difference between levees and floodbasins.
- Dike elevations were insufficient, and not enough money was allocated in recent years to dike improvement. In 1965 hurricane Betsy flooded part of the city, and dikes had been raised over a length of 830 km after that event. The last improvements were completed only shortly before hurricane Katrina struck, and, ironically, it were those dikes that failed. However, it was known, that the city would get flooded by a category-3 hurricane.
- Underestimation of the chances for flooding. Chances for a 'direct hit' of a category-3 hurricane were calculated to be 1 in 500 years, but the power of hurricanes has increased over the last few decades.
- After the dikes failed, the emergency plans proved to be inadequate. Evacuation was too slow, partly due to horrendous traffic jams. To mention just a few problems: the Superdome, a huge stadium, acted as a refuge, but the surrounding area became inundated, and the number of people taking refuge there ran out of hand. Over 120 school buses, meant to be used for evacuation of people that got stranded, were parked in an area that got flooded.
All these factors, with the exception of the last one, are studied by physical geographers. Read more about the study of Geosciences at Utrecht University.
Literature
- Berendsen, H.J.A. (2005), New Orleans: Gone with the wind and the water. Geografie, november 2005. Download this article (in Dutch).
- Bourne, J.K. jr. (2004), Gone with the Water. National Geographic.
- Brouwer, G. (2003), The Creeping Storm, Civil Engineering Magazine. June 2003.
- Fischetti, M. (2001), Drowning New Orleans. Scientific American, October 2001.
- Laska, S. (2004), Disasters waiting to happen: What if hurricane Ivan had not missed New Orleans? National Hazards Observer, Vol XXIX (2), November.