This Beach Erosion Study has been approved by the GPA as a detailed task statement for performance of a Beach Erosion Study as recommended by the Stakeholder Evaluation Group (SEG) on June 8, 1999.
The project area is composed of a complex barrier and sea island system. Since the project area is located in a mesotidal region, the inlets in the area are tidally dominated (as opposed to wave dominated inlets). Therefore, analysis of the system will include assessment of both waves and currents. The analysis of waves and currents will be accomplished through the application of wave and hydrodynamic models. Additionally, the Savannah River entrance does not resemble a typical inlet system; the flows from the Savannah River entrance interact with the flows from Calibogue Sound to create a large, complex inlet system. Therefore, the minimum extent of the modeled area will include the region extending from Hilton Head Island to Tybee Island (which includes Calibogue Sound and New River, Wright River and Savannah River Inlets). The overall study area will meet the USACE’s requirement that the study area extend 10 miles up and down the coastline from the Savannah River entrance.
In order to determine the potential impacts of the deepening project, the study will first establish an accurate description of the existing coastal processes in the study area. An analysis of the historic changes in the system will provide necessary information for establishing the existing conditions. Previous engineering modifications to the inlet system, which must be assessed, include a number of deepening projects, construction of entrance jetties, and construction of a submerged breakwater. Those modifications will be assessed together as a group to allow identification of the cumulative impacts of previous inlet modifications. After the existing (without project) conditions have been established, the study will proceed to assess the potential impacts of the deepening project. The study goals can be summarized as follows:
Qualitatively and (to the practical extent) quantitatively define the historical bathymetry and shoreline changes that have occurred in the study area. Include the development of the federal navigation project and the construction of the jetties at the mouth of the Savannah River. Qualitatively assess changes in mechanisms that control shoreline accretion/erosion: incident wave energy, currents, sediment supply, and shoreline hardening (e.g., seawalls, groin fields, etc.).
Use the computer models to describe the existing conditions that influence sediment transport in the area, including incident wave energy and currents. Use the modeling results and historic change analysis to develop a sediment budget for the study area.
Use the computer models to determine effects to the local wave and current conditions caused by the proposed channel deepening. Utilizing the projected effects to the wave and current conditions, quantify the potential impact of the proposed channel deepening. Any effects of the proposed deepening on the nearshore and inlet sediment budget will also be identified.
The dynamic nature of the coastal environment has resulted in changes in the position of ocean shorelines and in the volume of nearshore submerged shoals throughout history. The observed changes over the past 150 years are the net result of the cumulative effects of natural processes and man-made alterations on the Savannah Harbor inlet and surrounding areas.
Changes in erosion along the Tybee Island shoreline may be caused by changes in the following factors: incident wave energy, currents, and sand supply. Since the proposed deepening could potentially affect each of the aforementioned factors, it is important to study the relationship between the navigation project and the local wave and current conditions, and their impact on adjacent shorelines. This proposed scope of study will address the historic and existing coastal processes that affect coastal erosion, and determine the potential impacts to Tybee Island caused by the deepening and expansion of the navigation project.
A. Development of Hydrodynamic Model
The sediment transport properties along the shoreline of Tybee Island (adjacent to the mouth of the Savannah River) are highly influenced by the local ebb and flood currents. The currents are driven by a 6 to 10 foot tide range, with the tidal wave propagating nearly 40 miles inland under low flow conditions in the Savannah River. These currents impact the incident wave conditions, as well as the sand exchange between the Savannah River and the significant offshore shoals. The offshore bathymetric features, including the seaward portions of the Savannah Harbor Navigation Channel, create a complex circulation pattern that must be understood and quantified to properly assess the sediment pathways at this inlet.
ATM has applied the state-of-the-art WQMAP hydrodynamic model for the Lower Savannah River. This model application accounts for all areas that presently influence the tidal prism. This includes the flooding and drying of the extensive marsh system in the upper reaches, and the progression of the tidal wave almost 40 miles inland from the mouth. The ATM model has been calibrated and verified against extensive hydrodynamic data including multiple tidal, current, and flow measurement stations. The coverage area of this model will be expanded to include the areas offshore of Tybee Island and Hilton Head Island in order to quantify the complex circulation patterns in the study area. This model will allow for the quantification of the temporal and spatial circulation patterns that influence sediment transport.
The model will be calibrated to a minimum of 30 days of measured currents in the offshore area at a series of critical stations, as well as tidal stations. This will provide verification that the model is accurately simulating the phase and amplitude of the tides and currents. In addition, measurements of flows over a spring and neap tidal cycle will be taken within the Savannah River, and Calibogue Sound to verify that the model is accurately simulating the tidal prism passing into each of these systems. Once calibrated, the model will be utilized within the wave model to project the incident wave fields.
The WQMAP model will be a two-dimensional, vertically integrated application of the model. The weakness of this approach is that it will not provide a true three-dimensional representation of the current patterns in the study area. However, unlike the estuarine region in the Savannah River, the vertical current variations in the offshore region are not as important as the horizontal current patterns. Sacrificing vertical resolution of the modeled currents will allow greater model resolution in the horizontal plane. If necessary, bottom current velocities can be extrapolated from the two-dimensional model results.
B. Determine the Existing Conditions
The existing conditions will be determined by first examining the historic evolution of the study area. Computer models will be used to examine the pre-navigation project and existing channel conditions. The results of the historic analysis and the computer modeling will provide the basis for the development of a sediment budget. To determine the "without project" condition (for purposes of an incremental analysis), the study will include proposed actions that are reasonably certain to occur in the project area. These actions may include beach renourishment projects, construction of a bird island off Turtle Island and navigation channel modifications.
In determining the existing conditions, it would be beneficial to determine the cause of the ongoing erosion of Tybee Island. However, it is unlikely that the historic erosion of Tybee Island can be attributed to a single factor and described by a simple cause-and-effect relationship. Ortel et. al1 listed a number of factors to consider when evaluating the erosional problems at Tybee Island:
- Location and orientation of the island with respect to the dominant wave energy.
- Meteorological climate.
- Characteristics of the littoral materials.
- Impacts of human intervention, including shore protection devices and the navigation project.
- Rise in sea level.
- The history of shoreline and offshore changes.
- The effect of water levels and tidal currents.
Ortel et. al postulate that the geographic location and orientation of Tybee Island has probably been responsible for a portion of its shore erosion. The island is a headland extending into the Atlantic Ocean and does not have much protection for wave energy.
Man-made projects have also contributed to the erosion of Tybee Island. Human actions that have impacted shoreline changes along Tybee Island include:
- Beach nourishment projects.
- Construction of groins, seawalls, and breakwaters on and near the island.
- Construction of the Savannah River navigational jetties.
- Deepening of the Savannah River navigation channel.
- Maintenance dredging operations on the Savannah River navigation channel.
- Hydropower and flood control projects on the Savannah River.
- Changes in land usage along the Savannah River upstream of the harbor.
Considering that all of the above natural and anthropogenic factors have influenced the evolution of the Tybee Island shoreline to one degree or another, it is difficult to filter out the historic change due to only the federal navigation project. Any analysis that aims to determine the relationship between the navigation project and erosion on Tybee Island must consider the shoreline change that would have occurred in the absence of the project (i.e., the "background change"). While this study endeavors to establish the historic changes that have occurred to the study area and establish the existing conditions of the study area, the study is not designed to isolate the historic impact of the federal navigation project on the Tybee Island shoreline.
C. Analysis of historical shoreline and bathymetry changes
Analysis of long-term shoreline changes of Tybee Island will be reviewed. The island-wide average erosion rate of Tybee Island will be compared with the historical erosion rates of nearby barrier islands that have experienced a similar wind and wave environment. These islands will be within Georgia and South Carolina, but outside the influence of the Savannah Harbor Navigation Project, and have a geographic orientation similar to that of Tybee Island.
Analysis of long-term changes in area wide ebb tidal shoal bathymetry will be performed through map-differencing techniques based upon digitized historical boat sheet data. This will require the conversion of historic maps to a common horizontal and vertical datum. Additionally, depths will be adjusted to account for sea level rise. Color graphics depicting isolines of depth change over time will be formulated. The analysis will also be used to provide volumetric changes over time of the various shoals in the study area.
D. Model currents using a hydrodynamic model
The WQMAP hydrodynamic model will be utilized to model the nearshore currents in the study area for the pre-navigation and existing (without project) conditions. An 1855 hydrographic chart of the Savannah River pre-dates major changes to the harbor and will provide the pre-project bathymetry to input to the hydrodynamic model. Since the tidal cycle repeats every 19 years, the pre-navigation model will use the same tidal forcing that was used for the calibrated existing condition model. Additionally, the water levels in the pre-navigation model will be adjusted for sea level rise. This will provide an indication of likely current pathways that existed before development of the navigation project, as well as the current pathways in the existing condition.
E. Model incident wave energy
The pre-navigation project and existing project incident wave fields will be modeled using REF/DIF 1 Version 2.5 (Kirby and Dalrymple, 1994). REF/DIF 1 is a state-of-the-art weakly nonlinear combined refraction and diffraction wave model. The model will accurately propagate water waves over irregular bottom bathymetry and incorporate the processes of shoaling, refraction, energy dissipation and diffraction. Additionally, the REF/DIF1 incorporates the effects of wave breaking and wave-current interaction. However, REF/DIF1 will only provide steady-state results. That is, the results will show a "snap-shot" in time of the input conditions. Multiple model runs will be required to examine the wave field under various conditions.
Digitized historical boat sheet data will be used to develop the model grid over which the modeled waves will be refracted and diffracted. Input offshore wave conditions will utilize USACE's Wave Information Studies (WIS) hindcast data or historical wave data. Average and storm conditions (determined by their statistical significance, but at least the 10-, 50- and 100-year events) will be modeled. Since REF/DIF 1 is capable of modeling wave-current interaction, the current data developed by the WQMAP hydrodynamic model will be input to the wave model grid to assess combined wave-current interactions. Scenarios to include in the model runs will include high tide, mid ebb tide, mid flood tide, and low tide current fields for each incident wave field.
Specific emphasis will be placed on assessing the combined wave-current effects on the Tybee Island shoreline. The pre-navigation and existing incident wave energy along the Tybee Island shoreline will be compared by calculating the alongshore transport potential at the shoreline.
F. Sand Supply
The historic maintenance dredging volumes along the entrance channel and the Jones/Oysterbed Island reach will be tabulated. The volumes of sands and fines will be determined, if the data is available from historic dredging records. Changes in maintenance dredging volumes after each deepening project will be determined. This task is dependent on the amount of data available from the USACE. Additionally, available sand grain size data collected for the GADNR/Skidaway sediment transport study, Tybee Island, Daufuskie Island and Hilton Head Island sand search projects, monitoring of the ODMDS site, and other studies will be used to map the sediment characteristics in the study area. Together with the historic analysis of the volumetric change of the various shoal features, these data will provide the basis to identify and assess changes to the sand sources and sand transport pathways in the study area.
Proper identification of the type of sediments removed from the entrance channel is critical to determining the effects of the entrance channel on the sediment budget. After review of the available sediment data, the necessity for additional sediment sampling along the entrance channel will be determined. If inadequate data is available, additional samples will be taken at a spacing of roughly 5,000 ft along the length of the entrance channel. This sampling would extend from Fields Cut (Station 27+000) to the outer end of the deepened entrance channel (Station -70+000B). The exact sampling locations would be determined upon examination of "Before" dredging surveys to ensure areas of typical shoaling are included.
G. Sediment Budget
A sediment budget is a sediment transport volume balance for a selected segment of the coast. It is based on quantification of sediment transportation, erosion, and deposition for a given control volume. Usually, the sediment quantities are listed according to the sources, sinks, and processes causing the additions and subtractions. In particular, the volumetric changes at the ODMDS will be used to determine the accumulation of sediment at the site and will be compared with volumes of dredged materials from the channel.
Based on the data collected in the tasks identified above, a sediment budget for the existing condition will be developed for the Savannah River Entrance (including Jones/Oysterbed Island reach). Both the methodology recommended by Rosati and Kraus as well as the "Family of Solutions" approach presented by Bodge will be utilized to formulate the sediment budget.
H. Incremental Impact of Deepening ProjectUsing the previously developed hydrodynamic model, the existing (without project) navigation channel and post-deepening project scenarios will be modeled to assess the incremental impact of each project depth alternative on the study area currents.1. Currents
2. Incident Wave Energy
Using the previously developed wave model, existing and post-deepening scenarios will be modeled to assess the incremental impact of each project depth alternative on the study area shorelines. The current data developed by the hydrodynamic model for the existing and post-deepening scenarios will be input to the wave model. Average and storm conditions will be modeled. The alongshore transport potential will be computed along the Tybee Island shoreline for existing and post-deepening scenarios.
3. Incremental Project Impact
An estimate of the incremental project impact will be determined based on the following: (1) changes to the incident wave energy, (2) changes to the inlet current patterns, and (3) changes to the critical sediment transport pathways. A sediment budget for the predicted post-deepening project condition will be developed for comparison purposes. The predicted net change in pre- and post-deepening sand budgets will be identified. This analysis will not include the effects of any placement of sediments excavated during construction of the Expansion Project. Analysis of those effects would be included in a separate task statement.
A technical review group will be formed to provide third party peer review for the Beach Erosion Study. This group will be made up of technical representatives as designated by GPA, with consideration for input from the BEC. They will review and provide comments regarding the technical adequacy of the implementation of this task by ATM. The review process will include a complete review of the study execution, results and conclusions. The reviewers will provide comments regarding the technical adequacy of the study and adherence to the objectives of the task statement. This will include a review of the data analysis, model inputs, evaluation methodologies, results and conclusions. This group will also review the study report in draft form and provide feedback to ATM for resolution. Four weeks is allocated for this review. The final report will be prepared with appropriate attention paid to the reviewer's comments.
The deliverables for this task will include five interim reports and a final
report:
Compilation of historic data and analysis of bathymetry changes.
The final report will summarize the work performed under Section III. The final report will include all assumptions, methodologies, and procedures used in the study. In addition, the final report will address the goals outlined in Section I, summarize the findings, and draw conclusions from this work task.
The following schedule is recommended for this project task.
SEG approval of task scope: June 8, 1999
While the primary objective of the study is to assess the incremental impact of the
deepening project, the secondary objective of the study is to provide information useful
to other studies and issues related to the Savannah River Inlet and adjacent islands. The
beneficiaries of the study will include:
Savannah District, US Army Corps of Engineers – The Savannah District is
performing a study under the authority of Section 111 of the Rivers and Harbors act of
1968. This evaluation will "investigate the relationship of the Federal Savannah
Harbor Navigation Project and beach erosion at Tybee Island." The Beach Erosion Study
will identify a sediment budget and a description of the processes at work in the inlet
system – both of which are critical to investigating the historical relationship
between the navigation project and beach erosion on Tybee Island.