Physical and Environmental Assessment of Sand Resources Sabine and Heald Banks Second Phase 1994-1995

Sand deposits on the continental shelf of the western Gulf of Mexico are potential sources of fill material to nourish recreational beaches in Texas. Demand for beach nourishment sand is increasing as the combined effects of relative sea-level rise and reduced sediment supply cause rapid erosion and accelerated loss of recreational beaches. Particularly promising for leasing and commercialization in the near term are offshore deposits of sand that form shoals on the inner continental shelf. Preliminary geological and engineering analyses indicate that these sand deposits are suitable for beach replenishment because sediment textures of the shoals are generally compatible with those of native beach sand. Also, offshore sand extraction may be economically feasible if onshore or nearshore sources of beach-quality sand are volumetrically limited.

In Texas, Sabine and Heald Banks are two offshore sand deposits that have the greatest economic potential for near-term exploitation because they are suitable for beach replenishment, the largest sand deposits located offshore of some of the most rapidly eroding developed shores, relatively close to potential markets in both southeastern Texas and western Louisiana, and relatively close to major ports that can support offshore mining activities.

A prior geological investigation demonstrated that large volumes of sand-rich sediments are associated with Sabine Bank and Heald Bank (Morton and Gibeaut, 1993). The total volume of sandy sediments, estimated at more than 1.8 billion m³, constitutes a large hard-mineral resource. Most of that material would be suitable for beach replenishment and other construction activities that can use well-sorted fine sand with some shell and some sediments finer than sand. The previous study also showed that the offshore sand deposits are located in water depths ranging from 4.5 m to about 16 m and the greatest thicknesses of beach-quality sand generally coincide with the shallowest water depths.

The second phase of this study was directed principally toward assessing the quality and volume of Sabine and Heald Bank sediments. To accomplish this, the banks were cored, sediment textures and mineralogy were determined, and sand volumes were estimated using bathymetry and lithologic information. Geographic locations and attributes of all the pertinent offshore data sources were incorporated into ARC/INFO, a widely used Geographic Information System (GIS). An additional task of the second phase evaluated the potential environmental impact of mining the sand deposits by examining the potential changes in wave refraction patterns if large volumes of sand were removed from Sabine and Heald Banks. Another task analyzed the wave heights and wind patterns near the Banks to estimate the maximum number of working days for shallow-draft dredges working in the Gulf, and we also conducted a preliminary investigation of dredging costs based on experience with the 1995 Galveston Beach replenishment project and two other beach nourishment projects planned for the western Gulf of Mexico.