The Gulf LNG site is a permittee-responsible, on-site mitigation area (50 acres) that provides tidal salt marsh compensatory mitigation for the Gulf LNG Liquefaction Project located near Pascagoula. Mitigation at the site involves enclosing the marsh creation area with a riprap breakwater and filling it with material from the Bayou Casotte Dredged Material Management Site (BCDMMS). The fill will be added to a level that supports tidal marsh. A hydrologic connection is made through tidal inlets that provide access for fisheries and aquatic resources.
Thompson Engineering conducted a topographic and bathymetric survey of the marsh mitigation area, dredged material management facility, and surrounding waters. We performed a subsurface exploration to gather information on the subsurface soil conditions.
The BCDMMS was investigated to evaluate the quantity and suitability of the materials for the marsh creation area. Vibracore and standard penetration test samples were performed. Laboratory analysis was performed to classify the various subsurface soil strata encountered. Specifically, grain size analysis was performed to determine D50 and other grain size characteristics for sediment transport modeling and to determine the suitability of soils within the borrow area for use in marsh creation fill. Other tests included Atterberg limits, moisture content, shear strength, consolidation, and unit weight tests to support the geotechnical engineering and analyses for design.
Thompson Engineering designed the layout of the breakwater structure. Various design configurations modeled by CH2M were reviewed to determine optimal length, height, and distance offshore of the breakwater. Stability of the structure was evaluated with respect to crest height, storm stage, and wave height. Appropriate sizing of the armor stone was determined. Geotechnical findings were used to predict marsh elevation ranges throughout the design life. Initial and long-term settlement, subsidence, and sea level rise is taken into account when establishing marsh elevation.
Thompson Engineering assisted with coordination with the U.S. Army Corps of Engineers (USACE) throughout the design process to ensure that the final project meets with agency approval.