The value of wetlands in protecting southeast louisiana from hurricane storm surges

Abstract

The Indian Ocean tsunami in 2004 and Hurricanes Katrina and Rita in 2005 have spurred global interest in the role of coastal wetlands and vegetation in reducing storm surge and flood damages. Evidence that coastal wetlands reduce storm surge and attenuate waves is often cited in support of restoring Gulf Coast wetlands to protect coastal communities and property from hurricane damage. Yet interdisciplinary studies combining hydrodynamic and economic analysis to explore this relationship for temperate marshes in the Gulf are lacking. By combining hydrodynamic analysis of simulated hurricane storm surges and economic valuation of expected property damages, we show that the presence of coastal marshes and their vegetation has a demonstrable effect on reducing storm surge levels, thus generating significant values in terms of protecting property in southeast Louisiana. Simulations for four storms along a sea to land transect show that surge levels decline with wetland continuity and vegetation roughness. Regressions confirm that wetland continuity and vegetation along the transect are effective in reducing storm surge levels. A 0.1 increase in wetland continuity per meter reduces property damages for the average affected area analyzed in southeast Louisiana, which includes New Orleans, by $99-$133, and a 0.001 increase in vegetation roughness decreases damages by $24-$43. These reduced damages are equivalent to saving 3 to 5 and 1 to 2 properties per storm for the average area, respectively.

Figure 1. Location of the transect analysis for the Caernarvon Basin in southeast Louisiana. Panel A

indicates the topo-bathymetric view used to obtain wetland-water ratio (W_L) during the transect analysis. The color map shows depth below sea-level in meters, and the solid white line indicates the location of the transect where the analysis was performed. Panel B indicates habitat distribution used to obtain wetland roughness (W_R) during the transect analysis. The color map shows the value for the Manning’s n indicator of bottom friction, and the solid white line indicates the location of the transect where the analysis was performed.

Figure 2. Attenuation (A_S) of storm surge (S) as a function of wetland continuity (W_L) and roughness (W_R) along a storm track segment of distance (x) in m for four hurricanes in the Caernarvon Basin of southeast Louisiana.

Panel A shows maximum attenuation as influenced by wetland continuity, , where W_L is represented by the wetland/water ratio ranging from open water (W_L = 0) to solid marsh (W_L = 1). Panel B shows maximum attenuation as influenced by wetland roughness, , where W_R is represented by Manning’s n for bottom friction caused by degrees of wetland vegetation ranging from no vegetation (W_R = 0.02) to high dense vegetation (W_R = 0.045). Storm A = Central pressure of 96 kPa, radius to maximum winds (R_max) of 67 km, forward speed of 20.5 km/hr. Storm B = Central pressure of 93 kPa, radius to maximum winds (R_max) of 47 km, forward speed of 20.5 km/hr. Storm C = Central pressure of 96 kPa, radius to maximum winds (R_max) of 46 km, forward speed of 20.5 km/hr. Storm D = Central pressure of 93 kPa, radius to maximum winds (R_max) of 33 km, forward speed of 11.1 km/hr.