How much will future wetland loss increase storm surge?
Over the next century along the Gulf Coast, sea levels will continue to rise, coastal land will continue to subside, and human impacts due to shipping and the oil and gas industry will continue to cause erosion that will reduce wetland acreage. According to Harold Wanless of the University of Miami, global sea level is presently rising at 3 mm per year, and the land along the Louisiana Mississippi River delta is subsiding at 4-11 mm per year. Relative sea level is, thus, presently rising at 70-140 cm (2.3 - 4.6 feet) per century. Global warming may increase this sea relative sea level rise even further. Wetlands are being lost at an average rate of more than 23 square miles (60 square km) per year, with higher pulses during hurricanes. Water is about one meter deep across recently lost marshes and up to two meters deep in earlier lost marshes. This is of concern not only for the potential loss of hurricane storm surge protection, but because wetlands serve to increase fisheries production, filter pollutants out of water, and provide wave protection.
Wamsley et al. (2007) performed a surge surge simulation using the ADCIRC model of what would happen if the wetlands were allowed to continue to deteriorate with no restorative efforts over the next 50 years. Their results suggested that 50 years from now, storm surge heights would increase by 10-15% along Louisiana coastal areas to the east of New Orleans. These results held for both a severe Katrina-like hurricane, and a more modest hurricane (both making landfall at about 12 mph). However, the authors cautioned that "the impact of landscape features on surge propagation is a relatively new application for surge models and research is required". To underscore this lack of understanding, a White Paper put together by 25 coastal scientists and engineers held in July, 2007 found that adequate storm surge data do not exist for calibrating and verifying the models used to predict the impact of wetlands (or other features) on storm surge. Wamsley et al. are working on a field program in southern Louisiana to reduce these uncertainties. They intend to measure water level and wave attenuation across a wetland between Lake Borgne and the Mississippi River Gulf Outlet channel, using four non-directional water level/ wave gauges, an anemometer, and a periodic characterization of the wetland, including elevation, plant type, plant density, and plant height. The data collected will be analyzed to determine the surge and wave attenuation based on the vegetation type, density, and height.
