Figure 2‑15. East Timbalier Marsh Conveyance Corridor

Temporary access corridors have been designed for construction access to both East Timbalier and West Belle areas.

The access channel to the East Timbalier Restoration Area (see Figure 2-2) will approach from the northwest within Timbalier Bay. The location and dimensions of the access channel was designed based on the existing water depths identified during the design survey (JCLS 2015), to avoid existing infrastructure, minimize pipeline crossings, and to connect to the existing channel directly behind East Timbalier Island. This existing channel is currently utilized and maintained by the local oil & gas operator and utilizing it to the maximum extent possible for construction access on this project, will reduce risk associated with removing sediment over buried pipelines.

Access channel segments requiring dredging would be cut to a maximum elevation of -9 feet NAVD88 and a maximum bottom width of 150 feet. Dredged material would be placed in temporary storage areas a minimum of 30 feet from the top of cut on both sides of the channel and have a maximum footprint width of 140 feet. For the temporary storage areas, the maximum height was set at +2.0 feet NAVD88, maximum crest width at 45 feet, and side slopes at 1V:6H. The total length of the access channel inclusive of the existing channel maintained by the oil and gas operations is approximately 8,500 feet. Once access to East Timbalier Island is no longer required via the dredged channel, backfilling operation shall commence and post-construction elevations shall be no higher than 0.5 feet above the pre-construction elevations.

Two access channels were designed for the West Belle Headland (Figure 2-4). The first option begins northwest of the Restoration Area and extends southeast approximately 10,800 feet to align with the exterior marsh containment dike borrow channel. Approximately 360,000 cubic yards (CY) of material may have to be dredged for this option. The second option begins in the GOM south of the Restoration Area, loops around the western end of the restoration template, connects to the first option, and then extends south to connect to the beach and dune fill at the western end of the containment dike. The length of this access option is approximately 13,400 feet. Approximately 448,000 CY of material may have to be dredged for this option. For both options, the access channel segments requiring dredging would be cut to a maximum elevation of -9.0 feet NAVD88 and a maximum bottom width of 150 feet. Dredged material would be placed in temporary storage areas a minimum of 30 feet from the top of cut on both sides of the channel and have a maximum footprint width of 140 feet. For the temporary storage areas, the maximum height was set at +2.0 feet NAVD88, maximum crest width at 45 feet, and side slopes at 1V:6H. Once access to the island is no longer required via the dredged channels, backfilling operation shall commence and post-construction elevations shall be no higher than 0.5 feet above the pre-construction elevations.

Sediment Mining and Conveyance to the Project Areas

Two (2) Borrow Areas containing beach compatible sand, the Ship Shoal and South Pelto Borrow Areas, are proposed for utilization for constructing the beach and dune fill templates. It is anticipated the preferred method of mining these two Borrow Areas and conveying the sand to the East Timbalier Island and West Belle Headland Restoration Areas will be either of the following two methods or both: trailing suction hopper dredges (hoppers) and/or hydraulic cutterhead dredges. The first method is by hopper dredge. The Hopper dredges are self-propelled ships that use suction to excavated sand through one or two drag arms that extend to the seafloor while underway. Sand is pumped into the hull (the “hopper) of the ship and when capacity is reached the dredge leaves the borrow area, heads to the pump-out location, and is hydraulically unloaded. Sea turtle relocation trawling (See Section 4.5.3) will be required for the hopper dredge method.

The second method involves the use of a cutterhead dredge. Cutterhead dredges utilize a rotary excavating bit to loosen the sediment. The bit or cutter is located on the end of an arm (the ladder) that is hinged off the forward end of the dredge. The ladder can be maneuvered vertically to control its depth and the dredge and ladder maneuvered laterally and fore/aft using anchors deployed off the forward quarters and stern. The loosened slurry is pumped up the ladder to a large suction pump in the dredge hull, which also pumps it ashore through a submerged pipeline, often aided by the booster pump. The cutterhead will excavate the sand and transfer it through a through a floating pipeline and barge mounted distribution system into scow barges within the borrow area. The full scows will be towed to the designated pump-out areas and sand hydraulically unloaded and pumped in slurry through a pipeline to the Restoration Areas. Table 2-1 below provides detail dredging considerations for various dredging operation scenarios.

Table 2‑1. Dredging Equipment Operation Considerations

Two (2) Borrow Areas containing sediment for marsh creation, the East Timbalier Marsh and West Belle Marsh Borrow Areas, are proposed for utilization for constructing the marsh platform. It is anticipated the preferred method of mining these marsh borrow areas and conveying the mixed sediments to the East Timbalier Island and West Belle Headland Restoration Areas for marsh creation is a hydraulic cutterhead dredge and booster pumps situated along a continuous pipeline that extends from the dredge to the Restoration Area. Sediment delivery pipeline conveyance corridors have been delineated to connect the marsh borrow areas to the Restoration Areas. The dredged material will be discharged into the Restoration Areas.