Other channel

reaches, including the jetties channel and bar channel, have no advance maintenance authorized at this time, and none is planned for the near future. In February 1992, New Cut was closed and the tide gate structure taken out of operation. Under these conditions, the sediment basin will only be able to trap about 2,000,000 cy per year as opposed to 4,000,000 cy annually with New Cut open and the tide gate structure operating.

Table 41 shows the predicted shoaling rates and the projected advance maintenance requirements which are the result of closing New Cut.

It is proposed to maintain the current advance maintenance practices with the new deepened harbor since deepening would not impact shoaling quantities or locations, except in the area of the CSS GEORGIA between Stations 57+000 and 60+000 (mile 10.8-11.4). No advance maintenance dredging would be conducted between these stations. Shoaling rates by harbor reaches would be monitored after completion of dredging for deepening. Initial studies indicate that 4 feet of advance maintenance would be required from Stations 24+000 to 80+000 (mile 4.5-15.2) and from Stations 96+000 to 100+000 (mile 18.2-18.9). Two feet of advance maintenance may need to be continued from Stations 100+000 to 112+500 (mile 18.921.3) and Stations 0+000 to 24+000 (mile 0.0-4.5).

The exact positions of reaches for future advance maintenance would be adjusted as indicated by ongoing monitoring of shoaling rates and the need to maintain controlling depths as authorized.

* Based upon deepening to -42′ mlw and closing New Cut and taking the tide gate structure out of operation.

777,000

407,000

1,184,000

Effects on Existing Turning Basins

The proposed deepening project would not affect the existing turning basins, except the Kings Island Turning Basin. All other turning basins are dimensionally too small to accommodate the design ship. The prototype vessel is a large container ship, which would load and off-load at the Georgia Ports Authority's Garden City Terminal facilities, which are located directly across the harbor channel from the Kings Island Turning Basin.

Kings Island Turning Basin basin would be deepened to the harbor channel design depth of 42 feet below mean low water to allow for the design vessel to turn around and head back to sea, fully loaded, at all tides. The basin length and width are adequate for the design vessel, and no changes are foreseen in turning times or tug operations.

Evaluation of Need for Ship Simulator Model

A ship simulator model performed at the Waterways Experiment Station in 1987 was used to evaluate the widened channel between Stations 69+000 and 97+750 (mile 13.1-18.5) during the design of the Savannah Harbor Widening Project. The model consisted of a 500-foot wide channel with a design depth of 39.5 feet. The design vessel used in the model was a U.S. Lines 950-foot New York Class containership. This project included channel realignment along City Front, Stations 71+000 to 79+000 (mile 13.4-15.0).

One of the recommendations of the ship simulator for widening was to include a channel modification in one area upstream of the Savannah River bridge where ships veered to the north side of the channel. Recent discussions with the river pilots indicated this condition does not exist in actual practice and they do not experience problems upstream of the bridge.

Another area where a maneuvering problem was identified is in the vicinity of Station 76+000 (mile 14.4). The pilots stated this problem will be eliminated with the new bridge and realignment of the widened channel presently under construction.

A ship simulator model is being conducted of the deepened channel between Stations 26+000 and 105+000 (mile 4.9-19.9) to determine if channel modifications are required in the Bight Channel to alleviate shearing problems previously identified by the pilots. In addition, the reach in the vicinity of Fort Jackson and the CSS GEORGIA will be included in the simulator. This is an area where no deepening would be performed. The purpose of including it in the model is to determine if this condition would impact safe operation of the larger ships.

The pilots stated all ships could safely transit the reach between Stations 58+000 and 59+000 (mile 11.0-11.2) with a 400-foot wide channel, with 100-foot standoff from the north toe. However, any further restrictions would adversely impact their ability to transit this reach.

The ship simulator model includes the Kings Island Turning Basin. The docking pilots do not have problems turning vessels under any conditions. However, it was decided this should be included in the model since this was a mid-channel turning basin where a vessel would be subjected to channel velocities at points all along the length of the hull during the turning maneuver. This will demonstrate that the design vessel can be turned in the existing basin without risk. The docking pilots have stated that they use the ebb tide to their benefit in turning vessels. at Kings Island Turning Basin.

It is not anticipated that major channel modifications or realignments will be identified in the simulator. However, minor changes to the channel geometry may result in more efficient or safer ship handling. The simulator is not scheduled for completion until April 1992, and any recommended changes will be addressed in the appropriate Preconstruction Engineering and Design (PED) or plans and specifications phase of the project.

At the request of the Master Pilot, two vessels will be used in the ship simulator. They are the 950-foot Econo-Class ship and the 961-foot Hanjin 'New Build'. The pilots were confident that these vessels would represent the best and worst handling characteristics of all vessels calling on the harbor.

Predicted Shoaling and Bank Stability

The planned 4-foot deepening of the navigation channel will have no any appreciable impact on shoaling. Changes in harbor geometry have had no overall effect on the average annual shoaling rate in Savannah Harbor since 1954, when shoaling leveled off at about 7.2 million cubic yards per year.

The concentration of shoal locations has shifted, with no appreciable change in the average annual shoaling as described in Technical Bulletin No. 8, "Channel Depth as a Factor in Estuarine Sedimentation," by Henry B. Simmons, Committee on Tidal Hydraulics, March 1965. In this report, Simmons concluded that "This trend indicates that by the time the third channel condition [30'-32'on following table] had been attained, density effects in the harbor had developed to the point that essentially all of the potential shoaling material from upland sources was being trapped somewhere within the system".

Savannah District records for study periods 1953-1962 and 1972-1982 show annual shoaling rates varying from 7.15 to 7.28 million cubic yards per year. This trend has continued to the present, being slightly less during drought periods and slightly more during flood periods. Future shoaling rates are estimated to be between 7.25 and 7.50 million cubic yards per year with any foreseeable deepening. Shoaling locations or concentrations may change, but total annual volume will remain constant for the long term average. Table 42 summarizes shoaling rate data from the Simmons report and District shoaling records. The channel depths refer to the low and high value of actual depths in the harbor channel, which indicate the gradual deepening of the navigation project over time. Figure 27 is a plot of the average annual shoaling rate for the midpoint of each time period.

(1) Data for 1923 to 1954 based on Figure 5, page 10 of the Committee on Tidal Hydraulics, Technical Bulletin No. 8, "Channel Depth as a Factor in Estuarine Sedimentation", by Henry B. Simmons, March 1965.

(2) Data for 1972 to 1982 based on conditions with the tide gate operating with sediment basin, New Cut, and advance maintenance.

The shoal material in Savannah Harbor is primarily from shifting sand in the form of "sand waves" (bed load) at the extreme upper and lower ends of the harbor. In the middle harbor, silt shoaling is primarily from electrochemical reactions at the tidal/fresh water interface. The silt material is borne in saline solution from the vast surrounding marshes and tidal flats.