Wetlands Restoration: An Experimental Method to Increase Sediment Supply in Subsidence-Dominated Environments
This project examines the environmental conditions which led to the loss of 90% of the natural salt marsh in Galveston Island State Park since 1930 and analyzes one potential method to reduce future loss. Available data and recent studies suggest that the primary factor responsible for the historic loss was the lack of sufficient sediment supply to keep up with relative sea level rise. The average rate of sediment accretion for the period from 1963 to 2006 was estimated to be approximately 0.25 cm/year based on bomb fallout nuclides. According to data published by the National Oceanic and Atmospheric Administration (2006), this rate is about 0.4 cm/year less than relative sea level rise of approximately 0.65 cm/year during the same period. GPS observations collected by the Harris-Galveston Coastal Subsidence District from 2002 to 2005 support the common theory that rates of relative sea level rise over the last century will continue in the future. The marsh restoration project constructed in 1999 at the Galveston Island State Park focused on reduction of wave induced erosion and direct replacement of marsh substrate through terracing, but did not address the potential for marsh lost to submergence. As an alternative to geotextile tubes or other more permanent breakwater methods, a submerged, sacrificial earthen berm constructed around the marsh is a possible approach to address ongoing submergence by increasing the available sediment supply. In addition, the berm could be designed to limit wave height in the marsh to reduce wave induced erosion while allowing partial transmission of waves to create a net transport of sediment from the berm into the marsh. The proposed design method involves iteratively adjusting the berm top width and elevation to maximize sediment transport from the berm into the marsh. A sediment transport model can then be developed to quantify the increased transport into the marsh and estimate a nourishment interval. The Galveston Island State Park marsh was used for demonstration purposes; however, the restoration concept and method of analysis is applicable to other marshes in Galveston Bay.