At the turn of the tide, at near zero velocities, the colloidal silt and clay particles settle to the bottom along the edge of the salt water wedge. This occurs along the middle reaches of the harbor. The proposed deepening could possibly relocate the high shoal areas further upstream, but the annual volume change should be negligible. It is anticipated that the possible relocation of the shoals further upstream would have a negligible impact on maintenance dredging costs. Very minor increases in salinity could be encountered upstream. Salinity increases are discussed later in this document under the section on environmental impacts.

The effects of channel deepening on bank stability should be minor, except at isolated locations where the extended channel slope would intercept the existing bank at the soft spots on Hutchinson Island, as previously noted. This potential problem will be investigated with field borings at the precise points in question during the final design.

Side Slope Stability

General. The findings of this section of the report are considered preliminary and will probably require revision as additional investigations performed during PED better define existing conditions. The findings of this section are based on a review of the channel cross sections, which describe the relative position of the proposed deepening to the existing topography and to the adjacent structures. Slope stability analyses were not performed as part of this determination.

Impacts on Adjacent Lands and Structures. Cross sections of existing topography were plotted at selected stations along the length of the channel, using the results of the 1990 annual survey. The sections were selected based on the existence and proximity of structures and/or real estate to the channel limits at these stations. The dredging template, including advance maintenance where applicable and overdigging (although not paid for, it will most likely be performed), was added to the cross section plot along with the location of all known structures and utility crossings. Locations of these sections and a discussion of the possibility of sloughing is presented in the Engineering Appendix. Evaluation of Impacts and Design Considerations. Areas where the channel deepening may have potential adverse impacts on either property or adjacent structures have been identified and are summarized by channel stations in the discussion of real estate requirements. Adverse impacts can generally be described as loss of real lands due to dredging activities and subsequent sloughing of materials. The estimates presented are based on preliminary studies of subsurface soil conditions from drilling and sampling efforts, test results, and past experience throughout all reaches of the

Savannah River.

Additional analyses will be performed during the Preconstruction Engineering and Design (PED) phase in order to better define probable sloughing limits for incorporation into final plans and specifications.

Disposal Area Diking Plan

The operation, management, and stability of dikes for the disposal areas which are required for this project will be evaluated and closely coordinated with the local assurer during the PED phase of the work. The local assurer has been made aware of the quantity of dredged material and the disposal locations required by this project and is responsible for providing the necessary dikes and appurtenances to accommodate this work.

The dredged material disposed of during this project represents a significant future benefit to the local assurer as a source of high quality borrow material. It is estimated that approximately 4.4 million cubic yards of "new work" material would be deposited in the disposal areas. The value of this material as a source of borrow was not determined as a part of this study, but is expected to offset, at least in part, the costs which the local assurer may incur due to dike improvements made specifically for this project. Other Considerations

Identification and Impacts of Hard Materials. Pockets, or lenses, of apparently soft rock-like material have been identified at widely scattered locations in the upper reaches of the project, primarily between Stations 95+000 and 101+000 (mile 18.0-19.1). Additional subsurface investigations are planned during the PED phase of the project prior to preparation of plans and specifications. The extent of this soft rock (as well as stiff clays and cemented sands) will be identified sufficiently to determine their impact on final design and cost.

Miscellaneous. The following items will be developed in sufficient detail in the final engineering design phase prior to issuance of contract plans and specifications:

(1) Completion of the ship simulator model and incorporation of any channel modifications into the final project design.

(2) Evaluation of depths below the existing authorized project which have been disturbed or previously dredged.

(3) Need for repair, maintenance, or improvement of disposal areas, such as raise dikes or add weirs.

The above items will be coordinated with the local assurer, Georgia Ports Authority, U.S. Coast Guard, bar pilots, and docking pilots.

Channel Alignment Changes

Based on the similarities between the design vessel used for this study and that used for the widening project, no changes in centerline alignment of the ship channel are deemed necessary at the present time.

Obstructions

Fort Jackson and the wreck of the CSS GEORGIA are both historical relics from the Civil War Era. Fort Jackson is a masonry structure subject to undermining by erosion if the side slopes from a full width channel intercept its foundation on the south side of the harbor channel.

A similar situation exists on the north side of the channel caused by the sunken wreck of the CSS GEORGIA. The side slopes from a full width channel bottom could cause the wreck to slide into the channel. Present maintenance dredging procedures restrict dredging a 1,000-foot long reach of the channel between Stations 58+000 and 59+000 (mile 11.0-11.2) 100 feet off the north toe. This is to ensure that the wreck is not damaged, nor the side slopes impacted, by dredging and also to prevent disturbance of possible live ordnance that has been reported to exist adjacent to the wreck.

Maintenance dredging in the remaining 400-foot wide by 1,000-foot long section of this reach is performed when required. This reach has 4 feet of advance maintenance and is dredged to -42′ mlw. Further analysis of the stability of the channel side slopes and channel bottom in the vicinity of Fort Jackson and the CSS GEORGIA will be performed in the Preconstruction Engineering and Design (PED) phase of the project. The results of this analysis will be the basis for design of the channel geometry in this reach, if required. A 400-foot wide channel with approach transitions, for width and depth, 1,000 feet long, as shown on Figure 28, is the current design channel through this reach and included in the ship simulator model.

In order to minimize the potential for scouring of the channel bottom adjacent to the CSS GEORGIA, there will be minimum transition distance of 1,000 feet on each side of Stations 58+000 and 59+000, where the channel depth will gradually increase from 42 feet plus 4 feet advance maintenance to the 42-foot depth without advance maintenance between Stations 58+000 and 59+000. This transition zone will be increased if investigations during PED indicate it is necessary to prevent erosion in this sensitive area.