Efficacy of Wetlands for Storm Damage Mitigation

Linda Benedict, Westra, John  |  9/8/2014 8:32:45 PM

Figure 1. Average Wetland Protection Score. This figure shows the degree to which wetlands are observed to mitigate storm damages. A score of zero is the average for all parishes. Positive values indicate stronger damage reduction and negative values indicate weaker damage reduction.

Figure 2a. Hurricane Cindy, a category 1 hurricane, made landfall in 2005, causing $47.4 million in damage in Louisiana. Parishes impacted by this storm have wetland protection scores near or above the average for the sample, indicating a stronger negative relationship between wetlands and economic damage.

Figure 2b. Hurricane Rita, a category 3 hurricane, made landfall in 2005, causing approximately $8 billion in damages. The parishes impacted by Rita in this analysis have factor protection scores significantly below the average for the sample. Before landfall, Rita was the strongest hurricane ever recorded in the Gulf of Mexico with sustained winds over 180 mph.

J. Luke Boutwell and John V. Westra

Each year, tropical storms and hurricanes threaten the lives and livelihoods of Louisiana residents. In the past decade, powerful hurricanes have caused billions of dollars in damages to coastal communities. Even less-powerful tropical storms can cause widespread damage and interrupt economically important activities. The largest component of coastal storm damage is property loss from storm surge inundation.

Modern trends in coastal dynamics do not favor the alleviation of these damages. Coastal regions along the Gulf of Mexico are seeing relatively rapid population growth, which is expected to continue. Growing populations increase coastal vulnerability by placing more human and economic resources in areas that are known to be at-risk of storm damage. Additionally, hurricanes are expected to become more frequent and increasingly severe as sea temperatures, the fuel for hurricanes, rise.

Coastal wetlands are known to be an effective means of reducing storm surge energy. However, rapid wetland loss is reducing the capacity of natural coastal systems to mitigate damages. In recognition of these notions, Louisiana plans to allocate $50 billion through 2061 for wetland restoration and conservation projects, many of which are intended for storm impact reduction. The basis for these investments – that wetlands have the potential to reduce wave and storm surge energy – is well founded in physical science research. However, little is known about the context in which wetlands are most protective and the degree to which that protection is economically valuable.

Many factors influence the degree of damage inflicted by a coastal storm. Three factors are of interest in this research project. First, the amount of valuable property that is at risk of damage is expected to influence the severity of a storm impact. Second, because stronger storms generally produce a greater storm surge, the intensity of the storm can be expected to affect the resulting damages. Finally, the magnitude of damages will be influenced by the degree to which a region is protected from storm surge. In this analysis, the protection provided by wetlands is of particular interest.

An analytical method called factor analysis is used to explore the relationship between damages and the three factors of interest (economic risk, storm intensity and wetland protection). Through factor analysis, data describing each of these factors are combined in a manner that allows for the examination of the associations between each factor and economic damages. The factors are composed of two variables each, and the damage variable is included to observe how it is associated with the other factors.

The data used to describe economic risk are population and an estimate of the value of property and infrastructure that are vulnerable to storm surge flooding in each parish. For storm intensity, minimum barometric pressure and maximum sustained winds at landfall describe the factor. Finally, the presence of two wetland types, marine wetlands and estuarine wetlands, are used to represent the wetland protection factor.

The results of the factor analysis suggest that economic damages are most closely associated with the wetland protection factor. The economic damage variable and the wetland protection factor, which has a positive correlation with the wetland data, have a correlation of -0.361, suggesting that the presence of wetlands is associated with reduced storm damages. This value implies that the wetland protection factor embodies approximately 13 percent of the variance observed in tropical storm and hurricane damages. Considering the magnitude of storm damages, which are regularly in the hundreds of millions of dollars, this result suggests that wetlands are quite valuable for storm-damage mitigation. But from which storms do wetlands provide the greatest degree of protection?

To address this question, scores for the wetland protection factor are computed for each parish impacted by a storm used in the factor analysis. These wetland-protection scores describe relative to the sample average the degree to which wetlands are observed to have reduced economic damages for a parish that was impacted by a storm. A score of zero implies that that parish experienced a reduction in damages consistent with the relationship described in the wetland protection factor. Positive scores imply stronger damage reduction; negative scores suggest weaker damage reduction, although damage reduction is still observed.

The average scores for each category of coastal storm used in the analysis are shown in Figure 1, which shows a reduced capacity for wetlands to mitigate damages from stronger storms. This is thought to be the result of a physical threshold, beyond which the attenuating effects of wetlands for wave energy reduction are overcome by the static rise in water levels associated with storm surge. This result is important because it highlights the limitations of wetlands for use as damage mitigation measure for the most powerful storms. However, the findings also underscore the importance of wetlands in mitigating storm damage from more frequent, less-powerful tropical storms or weaker hurricanes

To illustrate this phenomenon, maps that show the degree to which damage reduction is observed are provided in figures 2a and 2b. “Warm” colors indicate positive wetland protection scores; “cool” colors indicate negative wetland protection scores.

These findings are important for providing context under which hurricane protection strategies can be generated. For strong hurricanes, additional measures (i.e., sea walls, levees, flood gates, etc.) will be necessary to protect coastal communities. However, for more common and less-intense tropical storms or weaker hurricanes, wetlands provide a valuable means of damage mitigation. Additionally, wetlands provide multiple benefits, such as fisheries habitat, recreational opportunities and water quality. These notions are central to the decisions faced by coastal resource managers. Understanding the appropriate application of mitigation strategies will be crucial for insuring cost effective damage mitigation in the future so that coastal communities can maximize the use of scarce resources.

J. Luke Boutwell is a graduate research fellow and John V. Westra is a professor in the Department of Agricultural Economics and Agribusiness.

This article was published in the summer 2014 issue of Louisiana Agriculture Magazine.

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