A framework for hazard assessment using an ecological indicator approach: Freshwater wetland impact, recovery and resilience in southwest Louisiana
Roussie, Suzanne M.
MetadataShow full item record
Analyzing salinity intrusion and vegetation dynamics inside a patchwork of freshwater and wetlands is a great challenge. They are difficult to access and measure. It is important to be able to understand the characteristics and ecosystem dynamics of the area affected before, during, and after an extreme event of salt water intrusion, in this case, the surge of Hurricane Rita of 2005 and the surge of Hurricanes Gustav and Ike in 2008. The study was carried out in an area of Vermillion Parish called Grophes Island, consisting mostly of wetlands, focusing specifically on freshwater vegetation. Development of a causal map is driven by specific parameters to assist in understanding the interactions of ecosystem dynamics. Ecosystem resilience to salinity intrusion can be measured by local salinity measurements in the field and by remotely sensed imagery for vegetation activity, here Gross Primary production (GPP). By using signal processing techniques from Moderate-Resolution Imaging Spectroradiometer (MODIS) sensors one can validate the local measurements. Using targeted points in the freshwater wetlands in the study area one can also use this technique and apply it to a larger scale. The first part of this research examines the history of hurricanes to determine if a correlation exists between the increase in water salinity levels and the occurrence of hurricanes or tropical storms. This requires a historical analysis of hurricanes and salinity measurements in southwest Louisiana. Times series analysis is used to study the variance of data in the time domain. The Mermentau Basin Salinity Monitoring sites, operated by the United States Army Corps of Engineers (USACE), operate over 50 gauges throughout southwest Louisiana collecting water temperature and salinity measurements. Salinity measurements are in parts per thousand (ppt). These sites are located from the Corps lock structure at Leland Bowman, along the Gulf Intracoastal Waterway (GIWW) to Grand Lake, and West to the Corps Calcasieu lock structure. The monitoring stations from Vermillion Parish located in Schooner Bayou will be utilized. Of the thirty-three gauges located within Schooner Bayou, only three gauges were found to have data on a daily basis for the time period 2001-2010: sites S2, S3, and B1a. While not 100% complete, these three sites were 80-95% complete while other were missing almost 80% of data. In Part II of this dissertation, a dynamic framework is established to show the relationships, causalities, influences, and interactions of some of the variables involved in understanding the dynamics of the freshwater ecosystem and salinity intrusion. Insights from the development of this conceptual framework will better enable future assessments of extreme impacts on salinity intrusion. Further, the framework will be used to demonstrate how the protection and restoration of wetlands is important to protecting inland areas from storm surge and salinity intrusion. Part III examines remote sensing as a tool for validating the results of the time series analysis. Remote sensing is an advantageous source for studying the temporal and spatial effects of storm surge and spatial patterns of vegetation. Remote sensing has the ability to be utilized in disaster recovery to monitor changes in flood duration and vegetation changes due to storm surge inundation. A database was built by integrating Terra MODIS GPP time series (2001-2010), a salinity-monitoring network, and precipitation data. This approach is an improvement to the study of freshwater wetland vegetation salinity resilience for larger inaccessible areas. This study reveals that MODIS GPP imagery validates local ground measurements and the GPP signal can be suited to monitor vegetation changes before, during, and after salinity intrusion from a hurricane event.