Table 17 shows the shoaling rates before and after construction of the sediment control works.

Sideslopes and Bank Stability. The experience of over 100 years of channel dredging in Savannah Harbor has established a ship channel sideslope of 1 vertical to 3 horizontal for the harbor channel and 1 vertical to 5 horizontal for the jetties channel and bar channel. Some isolated soft spots that have a natural slope of 1 vertical to 6 or 12 horizontal were discovered along Hutchinson Island during a previous improvement project. Also, some areas along Jones and Oysterbed Islands have near vertical banks and present erosion problems when attacked by large ship waves. These banks do not appear to impact vessel maneuverability and are well beyond the harbor channel sailing prism.

Groundings. There have been incidents of reported groundings in the harbor by both inbound and outbound vessels. Groundings have been promptly investigated by the Coast Guard. Most of the groundings have been the result of loss of power or steerage of the vessel. Investigations have not revealed that any accident was the result of an inadequate or deficient channel. Pilots concurred that groundings have been the result of loss of power or other failure and not the result of insufficient channel

mechanical

depths.

Widening. The harbor channel between Stations 97+750 and 69+310 (mile 18.5-13.1) is presently being widened by 100 feet. The channel between Stations 76+500 and 71+000 (mile 14.5-13.4) is also being realigned to remove several short ranges and will improve an inbound vessel's ability to align itself and transit under the high level Highway 17A bridge. Widening will be completed in 1992. Geotechnical Considerations

Site Geology and Soils. The project area is underlain by approximately 2,500 feet of unconsolidated and partly consolidated Atlantic Coastal Plain sediments. Subbottom profiles done by the Skidaway Institute of Oceanography indicate the city of Savannah is situated near the center of a local depression in the surface of the Oliocene sediments. This depression causes the overlying Miocene sediments to be thicker. The greatest thickness of these sediments within the depression is located approximately 10 miles northwest of Savannah, where it reaches thicknesses of approximately 180 feet.

Any dredging operations in the Savannah Harbor Deepening Project will be concerned with only the upper 50 to 60 feet of sediments between elevations -40 feet mlw and -46 feet mlw. This stratigraphic horizon is composed of Quaternary and Tertiary sediments, as generally described below, in descending order. Specific information on material classification and characteristics is generally based on borings that were constructed for the Savannah Harbor Widening Project, which is under construction at the time of this writing.

Recent to Pleistocene (Undifferentiated). These sediments within the project area are comprised of interbedded flood plain deposits of reworked alluvial and beach material, and reworked Miocene sediments, consisting of poorly graded sand (SP), silty sand (SM), poorly graded gravel (GP), organic silt (OH), inorganic low and high liquid limit silts (ML and MH), clayey sand (SC), and low and high liquid limit clays (CL and CH).

Standard penetration tests from borings indicate the consistency of the fine grained soils ranging from very soft (0 to 4 blows per foot) to very dense (50 or greater blows per foot). Two borings obtained in the vicinity of Station 76+000 to 76+500 (mile 14.414.5) indicated zones of partially cemented sand with significantly higher blow counts (80 to over 100).

Typically, these flood plain sediments are tan, gray, or greenish gray in color. Borings drilled on the north bank indicate maximum thickness of the flood plain deposits to be on the order of 65 feet. Borings drilled in the river indicated thicknesses up to 50 feet.

Miocene. The Miocene sediments consist of grayish green silty sand (SM), clayey sand (SC), low liquid limit clay (CL), high liquid limit silt (MH), and high liquid limit clay (CH). Borings indicate the consistency of the fine grained sediments range from stiff (8 to 15 blows per foot) to hard (greater than 30 blows per foot), while the coarse grained sediments range from dense (30 to 50 blows per foot) to very dense (50 or more blows per foot). Borings indicate the Miocene sediments, when encountered, are generally characterized by a significant increase in blow counts. Two core samples tested from a boring at Station 94+000 (mile 17.8) at approximate elevations -48 and -49 feet mlw, yielded unconfined compressive strengths of 125 and 135 psi, respectively. Thirty of the 55 widening borings encountered the Miocene material, although only six borings between approximate Stations 93+000 and 97+000 (mile 17.6-18.4) encountered it at or above -40 feet mlw. The highest elevation at which a boring encountered the Miocene was -38 feet mlw at approximate Station 97+500 (mile 18.5). Several borings between approximate stations 89+000 and 92+500 (mile 16.917.5) indicated the Miocene to be around -42 to -44 feet mlw. Lenses of moderately hard to hard limestone of thicknesses up to 2.5 feet have been found within the Miocene in borings outside of the limits of the borings taken for the widening project. However, its occurrence has been generally below the limits of dredging. Boring Locations and Material Summary. Several hundred borings have been drilled by the U.S. Army Corps of Engineers within and adjacent to Savannah Harbor over the past 30 years. However, most have been located in the upper and middle harbor, with a few located in the lower harbor. The majority of these borings were drilled outside the widened harbor channel and are of little use for characterization of the channel. Of the borings that were drilled within the harbor, the majority are along the right side of the channel. There is even less geotechnical information available in the jetties channel and bar channel. Furthermore, a wide variety of sampling devices were utilized during the previous drilling efforts, and a large portion of the borings will not be adequate for plans and specifications.

The lack of borings in the lower harbor channel and the jetties channel and bar channel will necessitate additional drilling in those areas. Additional drilling will be needed throughout the channel along the left side and centerline. A boring plan for additional investigations is presented in the Engineering Appendix. This additional effort has been initiated. Boring logs obtained through January 1992 are also presented in the Engineering Appendix.

Possible Impacts of the Project on Groundwater. In the past, any proposed deepening of Savannah Harbor has raised questions about the possible effects this action would have on the confining layer of the principal artesian aquifer, known as the Floridan Aquifer. The breaching of the confining layer; if possible, could have severe consequences as the aquifer is the primary supply of fresh water for the city of Savannah, neighboring areas in South Carolina (including Hilton Head Island), and virtually the entire coastal area of Georgia.

Previous investigations have indicated that the surfaces of the Early Miocene and Late Oligocene Age aquifers in the Savannah River channel appear to be sufficiently deep to be spared from damage caused by channel construction to -50 feet, or less, mlw. Additionally, these studies show that the sediments comprising the confining layer above these aquifers are of adequate thickness and low permeability to allow deepening of the present harbor channel without causing damage to the aquifers.

An offshore geologic structural feature, known commonly as the Beaufort Arch, created an uplift to the Tertiary Age sediments in the vicinity of the Savannah Light Tower (some 12 miles offshore). This resulted in the Parachucla formation (of early Miocene Age) surfacing at about -90 feet mlw. The Parachucla is the uppermost confined aquifer in the study area and possibly the most vulnerable, though certainly not the most valuable confined aquifer. By far, the most valuable confined aquifer in the study area is the Late Eocene aged Ocala Limestone (also known as the upper Floridan). The highest elevation this Upper Eocene (principal) artesian aquifer can be expected to occur is at, or about, -190 feet mlw.

In every case, the uppermost aquifer (early Miocene Age) is confined by the highly impermeable middle Miocene clays of 40 to 70 feet in thickness. These clays are further overlain by clayey sands and soft limestones of the Upper Miocene, which also contribute to confinement. Above the Upper Miocene are soft granular Pliocene and Pleistocene Age deposits (sands, silts, clays, gravels, shells, limestones, etc.), and soft deposits of Holocene Age (Recent) in which most of the dredging will take place.

The dredging of Port Royal Sound, SC, has resulted in the removal of Oligocene Age limestone pieces, where the sediments are uplifted and the Miocene cap is absent, exposing the late Oligocene Age limestone. But, even in this case, any salt water intrusion of aquifers has never been proven to be caused by this dredging and does continue to be linked to heavy drawdown (unwatering) of confined aquifers in the coastal areas.

Subsurface investigations of the confining layer eastward of Tybee Island to the offshore light tower at the Savannah River entrance is represented by two borings. A boring made in 1962 to aid in the foundation design of the tower indicates the top of the aquifer is a approximate elevation -105 feet mlw, and the top of the confining layer at about -62 feet mlw. Georgia Geologic Survey boring 772 at the north end of Tybee Island shows the aquifer at approximate elevation -150 feet mlw.

As with any subsurface excavation or investigation, a degree of uncertainty always exists. However, the available geologic information contributes to the belief that uncovering the principal confined artesian aquifer in Savannah Harbor by construction of a channel to an elevation of −50 feet mlw, or less, is very unlikely.

ECONOMIC CONSIDERATIONS

Overview

Savannah Harbor is one of the East Coast's major deep draft ports. It is a first tier port for containerized traffic, as well as a major bulk cargo handling port. Both imports and exports represent major portions of the overall commerce of the port. Local port interests, both private and public, have invested significant sums of capital in cargo handling facilities to support local industries as well as foreign and inland markets.

Many of the world fleet's most modern vessels call at Savannah, although the present 38 foot channel depth is an increasingly severe constraint to the larger and more efficient vessels presently under construction. Many local interests believe that Savannah will find it increasingly difficult to compete if the channel is not deepened to accommodate these new vessels.

Assumptions

The annual cost of transporting the commerce moving through the Port of Savannah can be changed considerably through the economies of scale provided by larger, more efficient vessels. If significant cost reductions can be realized, they may offset the cost of channel improvements. This is the basis for the economic analysis, which is described in detail in Appendix A. Because an economic analysis is predicated on a set of assumptions as a baseline, the District explicitly assumed the following in order to determine the beneficial contributions from channel improvements: