10. mai 2019
By Morten Jødal, biologist, (translated from Norwegian by Tim Crome)
At the very end of August 2005, Hurricane Katrina struck New Orleans, and significant parts of the city were flooded. Parts of the town were completely destroyed, however, the French quarter was largely saved. The devastation was enormous, the cost of reconstruction equally. 14 years later there are still houses that have not been rebuilt. In our Western world, the results of hurricanes and floods are blamed on climate change. We still hear this in the climate debate. This explanation is far from reality, and is only politically correct. There are many other reasons for the flood damage in and around New Orleans.
New Orleans is located on the US Gulf Coast, in the state of Louisiana. The whole area is pancake flat, made up of fine-particle material that the Mississippi River has carried with it for millennia. There are many thousands of square kilometers of swamps and marshland (“bayou”), without any hills. When you drive in towards the city, the road is elevated on piles for many miles across the wetlands. The whole area is prone to flooding – for two reasons.
This river in the United States is one of the largest watercourses on the planet, with an enormous flow of water. When I visited New Orleans in May 2019, there was a spring flood caused both by snow melting further north, and large rainfall throughout the spring. The Mississippi had a water flow of about 28,000 cubic meters per second, and was several meters above it’s “normal level”. Nevertheless, it did not breach its banks.
Before the Europeans started populating the land along the Mississippi River, the high water flows gave annual flooding. The river flooded over the plain, not only in the low-lying area outside the coast, but also in the states inland in the country. Here, too, it is flat. It could flood all the way up to the American Great Lakes. These water masses carried large amounts of fertile sludge and fines over the land. It was an important part of the ecology of the plain, and in the river delta area.
When the newcomers came to this part of America, they built dikes along the river banks. Eventually these walls came to stretch continuously on both sides of the watercourse, hundreds of miles to the west. That meant two things. First, the flood-plain was given far less fine-particle material, which fertilized both the natural and the cultivated land. The flat flood plain, formed by river material deposited by floods, will slowly dry out and compress naturally, and therefore sink, if it no longer experiences regular flooding. The lack of water means that the pores in the soil are not filled, which causes the soil to contract and the land to sink, as it also does in Bangladesh due to the regulation of waterways. Secondly, it led to better control of the floods.
Flood dikes along the Mississippi. One finds these miles after miles up the river.
Over the centuries the flood control has been improved and the water masses in Mississippi were eventually tamed. However, when sand, gravel and fine-particle material cannot flow out over the plains and the floods are controlled, this is deposited on the river bed and the level of the river rises while the flood-plain around it drops. Therefore, the Mississippi River today, as well as the waters and bay outside the city (the “Pontchartrain Lake”) is above the level of the city. It goes without saying that this is a very dangerous situation for a large city, which has completely established its existence on the solid and high dikes. These must be constantly reinforced and built higher, both because the city is sinking and because the river bed is rising.
This was the situation during the hurricane and flood in 2005. Although the entire city of New Orleans is below the level of the river the high dikes kept the river water out of the town. In the same way they have done for many decades. The river was not to blame. Neither were the dikes built against it, they did their job. There problem for the city was that some of the dikes constructed along man-made canals inside the city burst.
The land areas along the coast of the Gulf of Mexico are exposed to hurricanes. The season begins in June and lasts until November. For thousands of years, hurricanes have come ashore in what is now the states of Texas, Louisiana, Mississippi, Alabama, Georgia and Florida. They do not reach land every year, and their strength varies. The one that made landfall in 2005 was listed on August 23rd as a category 5 hurricane. At this time, the storm was over the sea. When it hit the country on August 29th, it had been downgraded to category 3. The scale ranges from 1-5, of which 5 is the strongest. Hurricane Katrina was in other words originally very powerful, and had the potential to do great damage. And it did. But what was the cause?
Changed hydrology and ecology
The Mississippi River is, as described above, greatly altered, through the construction of flood barriers. In addition, the forest along the waterway has been cut down, and the wetlands drained and cultivated. Thus reduces the river’s ability to absorb flood water. However, the water masses in the river are no longer the major flood problem for New Orleans, there is another explanation.
The areas along the east coast of the United States have experienced a rapidly growing population, as well as increased industrialization and urbanization. With the expansion of the oil business in the Gulf of Mexico in the 1960s and 1980s, large canals were dug through the swamp areas into salt water. Ten of them have been particularly important. Other areas were dredged. All these changes were made to allow boat traffic, countless miles of pipelines were also constructed through the area. They brought oil and gas up to 50,000 installations.
The canals and dredging destroyed the natural hydrology of the delta areas. Open water areas were formed, partly as a result of erosion, and through large saline penetration. In the period 1956-1978, this activity led to the loss of 30-59 percent of the wetlands. The canals that were built in the 1960s, between the Gulf of Mexico and the New Orleans port areas, destroyed a total of 110 km2 of wetland. They also served as funnels for the water masses from Hurricane Katrina. It is also important to consider the large reduction of material and nutrient transport down the Mississippi. This was due to two conditions: ponds and reservoirs further up the river, and the establishment of flood barriers on both sides of the river – far inland. With greatly reduced material transport, the renewal process of the river delta stopped. Reduced material transport also led to the surrounding area sinking and the entire delta area decreasing, while the river was built up. Less nutrients in addition meant a change in the biological processes in the area.
The outermost islands against the gulf have always been treeless, but in the swamps (“bayou”), there have been forests. The most common tree type is the marsh cypresses (Taxodium distichum), that grows well in the brackish water. When the canals were dug, salt water penetrated further inland and destroyed some of the forests. Many dead trees still stand as ghosts in the landscape. In addition, a single biological factor has been important: the swamp beaver (Myocaster coypus). The fur industry in this part of the country introduced this species from South America in the 1930s, for farming. Some escaped and established large wild populations. The changing salinity in the area, as well as the introduction of the beavers, has destroyed parts of the forest in the swamp areas and in the marsh landscape towards the coast. It has also reduced the landscapes resistance to the water that follows when a hurricane crosses the coast.
Trees killed by salt water that have penetrated the bayou system.
In recent decades, significant parts of the islands along the coast of the Gulf of Mexico have disappeared. There is now much more open water. The vegetation has changed, and the canals act as funnels for the water that follows the hurricanes, pushing it up into the city of New Orleans.
Many have called Hurricane Katrina for a natural disaster. Others describe it as being solely man-made. In reality, while it was a manifestation of nature’s enormous and recurrent forces, the devastation was mostly the result of human causes. The systems and structures established to deal with the effects of hurricanes were improperly planned and constructed, and the preparations for hurricanes – as well as the emergency procedures – did not work satisfactorily.
Some of the reasons behind the tragedy extend decades back in time, while others may be linked to actions and decisions just before, during, and after the floods. The dikes along the river have changed the dynamics of the area so that what should rise – sinks, while what should sink – rises. The loss of buffering wetlands was crucial, but poverty and technical problems have also played an important role. As New Orleans is below sea level, the city must always pump rainwater out. Because it is the US’s most rainy city, it has established a network of very powerful pumps. During the hurricane, the power supply failed and the pumps stood idle. This had major consequences.
More powerful hurricanes earlier
Hurricane Katrina is said to be a testament to human influence on the climate, and it is claimed that since it was of outstanding strength, it did damage that could never have occurred before our modern times. However, the argument does not hold water. Former hurricanes have been at least as powerful, but since population and infrastructure were much smaller in the past, the devastation and costs were also lower. A research group has estimated that if the hurricane that hit Miami in 1926 had hit New Orleans today, it would do more harm than any previous storm in American history.
Nor is it true that hurricanes occur more frequently than before. For the United States, the period from 1974 to 1994 was characterized by low hurricane activity, while the following decade had a higher frequency. However, following 2005, there was a period of twelve years during which there was a complete absence of category 3, 4 or 5 hurricanes crossing the coast of the United States. Hurricane Harvey landed on August 26, 2017, after an “hurricane drought” of 142 months. There has not been such a long period without hurricane landfall since 1851.
Man-made climate changes did not cause the devastation of Hurricane Katrina in 2005. Hurricanes have always made land fall in the United States and done great harm. There has been no increase in either hurricane frequency or strength over the past 45 years. The destruction from Katrina was made worse by human intervention in the hydrology and ecology of the wetlands, where the Mississippi River is approaching the sea, as well as by technical problems associated with flood protection in the city. Future measures to safeguard the city against damage to the town involve restoring the ecosystems of the Bayou and better technical solutions for flood control in the city. A focus on smaller greenhouse gas emissions has nothing to do with it – it’s not the problem here.
The article was written based on the background information provided by the Hurricane Katrina exhibition at the Louisiana State Museum, New Orleans.