The Value of Wetlands in Protecting Southeast Louisiana from Hurricane Storm Surges

by Andrew Walnum

Wetlands are recognized as important habitats not only for their benefits of maintaining biodiversity, water purification, erosion control, and carbon sequestration, but also their ability to reduce the impacts of storm surges. Hurricanes pose a particular threat coastal areas as can be seen during Katrina and other devastating hurricanes. Wetland restoration in areas along the Gulf Coast seems to be a logical way to help reduce the devastating impacts of surges and floods from ocean storms. However, there has never been a full analysis combining the hydrological and economic impacts of increasing wetland areas along the Gulf Coast. The authors of this study used models to look at the effects of increasing wetlands on property damage in Southeast Louisiana, near New Orleans. Their study finds that an increase in 10% vegetation cover per square meter saves $99-$133 in property damage per unit area and only a 1% increase saves $24-$43.

Barbier and colleagues used storm simulations across a transect along with estimates of analysis of the economic impact of a storm surge. The transect was chosen using numerical models and the (ADCIRC) unstructured grid hydrodynamic model to predict the direction, intensity, and duration of storm surges. Twelve locations along the transect were collected using ADCIRC and were sub-sampled to create 100 points from sea to land. Next, the wetland-water ratio and bottom roughness along the transect was collected. The wetland-water ratio (WL) was based on a scale of 0-1 with 0 being open water and 1 representing solid marsh. Bottom roughness (WR) is the value of friction caused by vegetation with 0.002 being no vegetation and 0.045 being dense vegetation. Reducing surge power was then measured as the maximum amount of attenuation over each of the 11 transects between the 12 locations. Each one of the 11 transects was 6,000 meters long and the WL and WR along each transect was averaged. The authors were next able to change the WL and WR values to observe changes in storm surge attenuation. Changes in storm surge frequency and duration can vary greatly but the authors used expected damage function approach to find the marginal values of WL and WR on damage to surrounding human-inhabited areas.

The authors found a direct correlation between both increasing wetland-water continuity and vegetative roughness on storm surge attenuation. More and wetlands and vegetation decrease the intensity of incoming waves from storms. Increasing the wetland-water ration by only 1% reduced storm surge intensity by 8.4% to 11.2% and 1% increase in wetland roughness decreased storm surge by 15.4% to 28.1%. This reduction in storm surge also has an effect on the amount of money saved from damage reduction. A 10% increase in wetland-water continuity saves $99-$133 dollars per unit area and a 1% increase saves $24-$43 per unit area. If an increase in wetland continuity is expanded along the transect, the results are even more positive. An increase in wetland-water along the full length of a 6,000 meter transect results in saving $592,000 to $792,100 for the average sub-planning units in local parishes surrounding the wetlands. An increase in bottom roughness from vegetation accounts for $141,000 to $258,000 saved for the average sub-planning unit.

Although used for only one transect, the study helps illustrate the need for wetland protection in the future. Wetlands provide a large array of environmental services but there most important benefit may be the protection of coastal property. However, restoration is expensive and even with a large scale project along the Gulf Coast, there would continue to be a decrease in the number of wetlands over time. As more information on the economic benefits of maintaining wetlands comes out, it may prove to be more beneficial in the long-run to spend money on restoration to protect damage by storm surges.

Barbier EB, Georgiou IY, Enchelmeyer B, Reed DJ (2013) The Value of Wetlands in Protecting Southeast Louisiana from Hurricane Storm Surges. PLoS ONE 8(3)

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