Is it time to rethink Louisiana’s coastal restoration strategies?

Along the coast, a storm surge can be the greatest threat to life from a hurricane. This abnormal rise of water generated by a storm can cause extreme flooding. In 2005, at least 1,500 people lost their lives during Hurricane Katrina directly and indirectly because of the storm surge. Another imminent danger of living along the coast is the fact that Louisiana’s coast is deteriorating. It has been estimated that in Louisiana alone,  the protection and restoration of coastal areas is projected to cost $50 billion. Coastal wetlands provide storm protection for coastal communities. They act as a buffer, stabilizing shorelines and reducing wave energy.

Storm surges, though infrequent and destructive, result in the deposition of inorganic sediments into the soil profile. However, the frequency, magnitude, spatial distribution, and influence on wetland soil development of this sedimentation is not well understood in Louisiana or other coastal systems. What if coastal restoration strategies could be improved with more knowledge of this deposition?

Storm surge damage, Photo credit: https://eo.ucar.edu/kids/dangerwx/hurricane4.htm

Storm surge damage, Photo credit: https://eo.ucar.edu/kids/dangerwx/hurricane4.htm

Scientists Andrew Tweel and Eugene Turner of Louisiana State University’s School of the Coast and Environment investigated the long-term contribution of hurricane events to soil inorganic matter content in Louisiana coastal wetlands in their research article Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA, published in July in Landscape Ecology. To better understand how the mineral sediment in coastal Louisiana wetland soils got there to better inform coastal restoration, scientists studied all wetlands within the coastal zone from the Louisiana/Mississippi border west to Galveston Bay. The sediment budget impact from three hurricanes, Rita, Katrina and Gustav, were studied. A database from the National Oceanic and Atmospheric Administration isolated hurricanes, tropical storms and tropical depressions that entered this area between 1851 and 2008. These three storms were used to estimate the average deposition on the deteriorating Louisiana coast during this 157-year period.

“Basically, we went out to the wetlands after hurricane events and measured the amount of sediment that had been deposited on the marsh surface,” said Tweel. “It was clear what sediment was from the hurricane, and what was the existing sediment because they were so different.”

Prediction plot showing model performance at study scale for Hurricanes Katrina, Rita, and Gustav. The outer bands mark the 95% prediction interval, the inner bands mark the 95% confidence interval, and the open shapes indicate the most under-predicted values. Photo credit: Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA

Prediction plot showing model performance at study scale for Hurricanes Katrina, Rita, and Gustav. The outer bands mark the 95% prediction interval, the inner bands mark the 95% confidence interval, and the open shapes indicate the most under-predicted values. Photo credit: Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA

Surprising similarities between different events were discovered, allowing the scientists to make predictions about other historical events. All the predictions from 1851 to 2014 were layered on top of each other to determine the average rate of sedimentation along the coast for different areas at different times.

This figure depicts spatial distribution of mean annual hurricane sedimentation for coastal Louisiana wetlands. The data are based on estimated deposition from Category One and greater events from 1851 to 2008. Image from Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA.

This figure depicts spatial distribution of mean annual hurricane sedimentation for coastal Louisiana wetlands. The data are based on estimated deposition from Category One and greater events from 1851 to 2008. Image from Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA.

The long-term and coast-wide effect of these events on the distribution of inorganic sediment is difficult to quantify because of the irregular and infrequent occurrence of these storms, and the logistical challenges of sampling across such a vast area in a timely manner after each storm.

This data suggests most of the inorganic material in soils along the coast beyond active deltas directly corresponds to hurricane events. This could positively impact Louisiana’s deteriorating coast by improving coastal restoration strategies. Currently, the majority of coastal restoration strategies are based on the assumption that leveeing the river will restore the coast. However, this research shows this is not the case for most of the coast beyond active deltas.

“The general restoration strategy should be revised to address the organic processes that are mostly responsible for maintaining the elevation of wetlands on the Louisiana coast,” said Tweel. “There is always the excitement of making a new discovery that helps us better understand how coastal systems work. One of the coolest aspects of my work is getting to go out in the marsh and to see the coast from small airplanes.” Prior to this model, there was too little data on how hurricane sedimentation varied across the coast.

The research article Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA can be found here. (http://link.springer.com/article/10.1007/s10980-014-0047-6)

Maddie Duhon

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One thought on “Is it time to rethink Louisiana’s coastal restoration strategies?

  1. Good job with this! So hurricanes actually add more sediment/land to the coast?
    1 question: You say “Currently, the majority of coastal restoration strategies are based on the assumption that leveeing the river will restore the coast.” Do you mean un-leveeing the river, or creating diversions?

    Like

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