SHEP Sediment Basin Rock Weir and Fill Design

Objectives: The project, described below, is approximately at a 65% design level. Several challenges have been identified during the design process. The intent of providing this draft information is to get industry feedback to inform the 95% design for the project to be more manageable to bid and construct, in order to design a successful project. Project Location and Water Variations: The work is located within the Back River channel near the downstream of the existing sediment basin and approximately 1500’ upstream of the confluence with the Savannah River near Channel Station 59+000, as shown on the vicinity map. Access to the project site is by water only, with the exception to the tie-in on the South Carolina side which can be accessed by the DMCA 13A riverside dike. Bathymetry data is included in this package as a .xyz file. The work area is affected by tides. Mean tidal range at the site varies from approximately a minimum of -2.2 to a maximum of +10.5 MLLW. Anticipate presence of port activity, commercial boat traffic, recreational boaters, fishermen, and jet skiers within the waters adjacent to the project. Project Description: The project work is located entirely underwater, with the exception of the North and South tie-in locations. Only one end of the rock weir is accessible by land. The work consists of the construction of a rock weir structure across the Back River channel, approximately 1,500 feet wide to a crest elevation of -9.5 feet MLLW. The work includes stone protection tie-in on both sides of the river. The work also includes filling approximately 2.5 MCY of sand to fill an area that extends 2000 feet of the Back River channel directly behind the proposed rock weir to an elevation of -9.5 feet MLLW. The fill material must be greater than 75% sand. The proposed borrow area is located at Jones Oysterbed Island (JOI), approximately 6 miles downstream of the Savannah River from the proposed rock weir and fill location. It is the responsibility of the Contractor to determine the actual amount to fill the template. Existing Conditions: The Back River channel was dredged to a depth of approximately -40 feet MLLW, since 2005. Since then, the channel has not been maintained. The soil boring logs are included with this package. They show soft, plough mud like material from an elevation of approximately -22 feet MLLW to a hard channel depth of -45 feet MLLW. At this time, it is unknown if the rock will settle to the hard bottom or somewhere between the top of surface and hard bottom. Raw data is included for the Sediment Basin from hydrographic survey conducted in April 2021. Borrow Area: JOI has been identified as the borrow area for the fill material. The island is a dredged material containment area (DMCA) adjacent to the Savannah River navigation channel. A West and East area have been identified on JOI as borrow areas with 75% or greater sand content. The majority of borrow area is anticipated to come from JOI (West), with some additional borrow anticipated from JOI (EAST). The water around JOI is fairly shallow with sandbars located along most of the eastern side of the island. A loading/access area exists on the western end of the island, near Fields Cut (AIWW). Additional geotechnical information will be gathered and will have characterized the material prior to final design in addition to volume estimates for each borrow area. Environmental Construction Constraints/Considerations: Measures will be required to reduce impacts to the aquatic environment, including confining the material that is discharged into open water and avoiding discharge of dredged sediments into these open water areas during the striped bass spawning season, for an approximate two week window within the date range of 01 April to 15 May to allow for bass spawning. The Environmental Window for striped bass is approximate. The two-week window could possibly start in March and after May 15. Appropriate erosion and turbidity controls shall be utilized wherever necessary to limit sediments from entering the water. Prior to final design, coordination with NMFS will be completed to coordinate the time of year restrictions for in water work/placement activities to avoid impacts to sturgeon.

Division 31 -	Earthwork, Erosion and Sedimentation Controls.
Division 33 -	Utilities, Storm Drainage Utilities, Ponds and Reservoirs.
Division 35 -	Waterway and Marine Construction, Shoreline Protection, Waterway Construction and Equipment.