Coastal Services Center

---

Methods of Investigation to Identify Sources of Suitable Sand for Nourishment

---

Introduction

Key to the performance of a beach nourishment project is the quality and quantity of sand used to nourish, or replenish, the eroding shoreline. In the planning of a project, locating an affordable high quality sand source is the most important aspect of designing a beach nourishment project. Therefore, it is important that decision-makers understand the costs and the nature of the studies to identify suitable sources of sand for a project. Sand source investigations are performed as part of the reconnaissance or feasibility studies in the initial planning stages of a project and again in the design development phase.

Borrow sites differ in terms of their geological origin and sediment characteristics, both physical and chemical, thereby affecting their suitability for beach placement purposes. Geophysical studies to locate potential borrow areas, identify sediment quantities, investigate sediment characteristics, and rank candidate sites are usually undertaken in multiple phases. Such investigations can be costly, requiring geophysical and geotechnical studies that typically include sub-bottom profiling, side-scan sonar, bathymetric mapping, core boring collection, sand testing, and analysis. Costs for such investigations range from $100,000 to more than $500,000.

Environmental suitability and permit considerations, construction cost, and project performance are principal factors that influence selection of a potential sand source. Therefore, initial planning for a project involves examination of several candidate sites for the purpose of identifying environmental issues and tentatively ranking the sites prior to developing the work plan for design level investigations. A project's performance is primarily dependent on the sediment grain size characteristics, sediment placement quantities (typically unit placement quantities range from 50 to 150 cubic yards per foot of shoreline), and length of the project area. Therefore, the borrow site investigation is critical to the success of the project.

Geologic Setting

Environments that are potential sources of sediment for beach nourishment are typically offshore (on the seaward side) of the beach placement area. Differing sediment characteristics arise depending on the location of the sand source. Thus, the type and location of a sand source will affect the procedures for investigation. Types of offshore sites can be described as linear sand bodies, including remnant shoal features, ebb or flood tidal shoals, drowned barrier islands, oblique sand ridges, longshore bars, trough sand accumulations, and migratory sand spits attached or unattached to tidal inlets. In addition, upland, or onshore, sand sources formed from relict sand dunes provide valuable sources of sand for small scale projects at less developed rural beaches or for purposes of emergency post-storm interim projects. A detailed discussion of the nature of these environments and the types of sediments contained within these sites can be found in "Geologic Characteristics of Borrow Areas and Sediments."

Reconnaissance Level Investigations

Figure 1. Hyrdographic surveying is performed to map the borrow site complex for further geotechnical investigations.

Reconnaissance or feasibility level sand source investigations are performed to locate and evaluate prospective candidate borrow areas for the planning phase of a beach nourishment project. As the economic viability of the project relies on the outcome of the benefit-to-cost ratio for the project, the process of locating a borrow site with beach quality sediment in close proximity to the beach placement area is critical to the federal, state, and local cost participation in a project. These initial feasibility studies will usually include hydrographic surveys and sub-bottom profiles and broadly spaced core borings taken to evaluate sediment composition and thickness of sub-bottom strata.

Hydrographic surveys establish the present bottom depths and provide contour orientation of the borrow area for shoal type sand bodies. Mapping of the borrow area complex for such above grade shoal sites requires depth contouring which is used to establish the locations for the "reconnaissance level" vibracores, as shown in Figure 1. Vibracores or vibratory corers are a type of sampling tool used to obtain soil samples of the seabed. They are generally operated from a supply vessel (less than 51.2 meters) and use a winch or a crane and an A-frame mounting. The available penetration of such types of pneumatic or mechanical samplers is generally 40 ft (12.2 meters). Vibratory corers consist of a steel pipe driven into the sea bed using vibrations at the top, where the vibrations are developed by a hammer that is connected to an umbilical cord which delivers power to the hammer. Upon retrieval and cataloging of the cores, core logs are prepared and marine soil samples are identified for removal and testing for grain size distribution (size classifications) and chemical composition. Other planning level soil testing may include jet-probes to identify sediment thickness at discreet locations when sub-bottom rock layers are known to exist, or when the qualitative information on the characteristics of the sediment is needed to determine locations for vibracores.

Design Level Investigations

Design level borrow site investigations are undertaken to further refine the geotechnical data for the sites identified in the reconnaissance/feasibility level borrow site investigations. Design level studies provide the design professional with sufficient data to develop permitting documents and project design specifications, including mapping spatial variations in subsurface sediment strata and sediment characteristics for a variety of geometric borrow site boundaries. Different borrow area configurations are evaluated in order to optimize sediment quality. Sub-area delineations of a site allow for sequencing excavation operations to improve sediment quality in the constructed project and to ensure efficiencies for multiple use of the site. Therefore, additional vibracores are typically recommended in the design phase in order to refine the site boundaries for initial construction and subsequent maintenance events.

Core borings or vibracores (a type of vibratory extraction coring method used to penetrate the highly compacted seabottom sediments) are typically the most costly component of the geophysical and geotechnical investigations and yet most valuable information for designing the borrow site. For this reason, limiting the core sampling to reduce project costs may prove counter-productive given the value of this data in delineating a site's geometric configuration and resulting sediment quality at the beach placement site. Costs for vibracore investigations and tests, typically comprised of mob/demobilization at $10,000 to $20,000 and extraction of 30 cores at $1,500 for a 20 foot core, are $75,000 or more for a 100 acre site. These costs include core splitting, logging, archiving, and sediment grain size testing but do not include the professional services fees for the on-site engineer and/or geologist. Vibracore samples following extraction and cutting are shown in Figure 2.

In addition to supplemental coring at the candidate borrow site(s), rigorous studies of the environmental conditions are required to evaluate whether other factors will affect the site's viability from a regulatory permitting perspective. In general, the investigations that comprise the design level work include magnetometer, archaeological, side-scan sonar, and biological resource surveys and mapping. These field studies are undertaken to determine the presence of physical obstructions, significant historical artifacts, utilities such as optical or phone cables, hardbottom (reef) habitats, or sensitive biological resources either within or in close proximity to the borrow site.

Identifying Borrow Site Obstructions

Magnetic anomalies and submerged cables located within a borrow area require identification to determine radial buffer or exclusion areas, in addition to allowances for side-slope sloughing. Following mapping and field verification of such obstructions using side-scan sonar and magnetometer surveys of the borrow area, a re-configuration of the borrow site boundaries is usually required to exclude dredging where significant anomalies exist. When the borrow site dredge limits are changed, the computations of available sediment and sediment composition must be updated. Another consideration is archaeological artifacts that may be known to exist within, or in proximity to, a borrow area. If these features are identified, the plan view (horizontal) boundaries must be adjusted to reflect the actual borrow site geometry or final excavation volumes and to adjust the representative sediment composites that characterize the borrow site. Figure 3 provides a graphic depiction of varying geologic subsurface strata within a tidal shoal at Cape Island.

In addition to geometric impediments and limitations, the total available quantity of sediment in the borrow area must be reduced to account for the loss of "fines," or the fine fraction of the sand, during project construction. This is because the fine fraction is usually lost, due to the small particle sizes as suspended material in the surrounding waters, during dredging operations either at the dredge site or the beach placement site. A typical project will require a 25 to 50 percent surplus sediment quantity in the borrow site to compensate for the loss of fines and other factors that reduce the effective sediment volume during construction of the design beach fill template. A cross-section view through the borrow site for the Jupiter Carlin project on the southeast coast of Florida is shown in Figure 4 where the differing subsurface strata and vibracore locations are depicted for this offshore, deepwater site.

Importance of a Site's Sediment Size Characteristics

Figure 5. The borrow site shown is located near Lake Worth Inlet and provided the sand source for the 1995 Mid-Town Beach Nourishment Project.

A comparison of the composite grain size distribution curve for the borrow site and the native beach sediment is performed to evaluate the borrow sediment similarity and thus suitability for beach nourishment, and to predict the project's performance, or longevity. Compatibility analysis is critical to compare the overall qualities of the native beach and the borrow site sediments to determine the suitability of the borrow sediment for a project area. Sediment characteristics are evaluated using one of several techniques including a method based on the relative characteristics of the composite grain size distributions for the native and the borrow site sediments developed by James (1974) and a method based on equilibrium beach profile theory (Dean 1991). A compatibility analysis is performed to quantitatively compare the sediments for alternative borrow sites and for evaluating differing geometric configurations for a site. An overfill factor is commonly used to evaluate the compatibility of the sediments and to relate the volume of borrow site sediment required for a project to perform similarly or comparably to the native beach sand. Thus, an "overfill" factor of 1.0 indicates direct compatibility (that is, borrow and native sands are identical) and an "overfill" factor of 1.1 indicates that the borrow site material is finer and thus 10 percent additional material placement (coverage) is required to compensate for the incompatibility and expected loss of fine sediments. In other cases, the sediment size is predetermined because the sand is a by-product of an inlet channel maintenance project, and thus the design professional is evaluating only the expected longevity of the project.

As would be expected, the fine fraction of the sediment composition is sorted out by wave action during the adjustment of the project following initial construction. Littoral processes result in a "winnowing out" or cross-shore redistribution of the fine sediments seaward to increased depths along the profile during the evolution of the project. A discussion of the equilibration or evolution process that occurs after project construction can be found in "Cross-shore and Longshore Transport Models for Large Scale Geological Processes."

The borrow site shown in Figure 5 is located immediately south of Lake Worth Inlet and provided a high quality sediment source for the 1995 Mid-Town Beach Nourishment Project at the Town of Palm Beach, Florida. Initially, the 1988 "permitted" borrow site for this project contained sediments that were finer than the native sediments owing to the limited depths and limited distribution of cores within the site; however, after additional vibracores were obtained to refine the depths and areas of excavation, the borrow site configuration was optimized to locate the coarser sediments within the site.

Factors that Affect Project Costs

The primary factors that affect sand placement costs include distance from the borrow site to the placement site, excavation depths, depth of water in the area surrounding the excavation site, sediment composition, and temporal dredging windows that result from moratoriums for sea turtle nesting at the beaches from North Carolina to Texas. The first consideration in developing cost estimates for a project is to determine the type of dredge equipment required, that is, a hopper dredge or a hydraulic dredge. Total pumping distance and sediment characteristics usually dictate whether a hydraulic dredge is capable of pumping the material from the designated borrow site. If the material is predominantly uniform, fine to medium grained sediment, hydraulic dredges are capable of pumping sediments to distances of five to seven miles, and a hydraulic dredge including required booster pumps will often be the least costly option. Figure 6 presents a typical example of a borrow site located at a remnant shoal that allows the use of a hydraulic dredge. This example in Martin County provided a low cost, high quality sand source. More often borrow sites contain a sediment composite that is comprised of a significant coarse fraction that reduces the distance that a hydraulic dredge can cost-effectively pump the material. Site-specific conditions where a hopper dredge is required include deep-water excavation and long haul distances. Use of a hopper dredge results in a higher unit sand cost and reduced production rates.

Environmental Factors that Affect Sand Quality Acceptability

Regulation of sediment excavation and placement for a project is the responsibility of many federal and state agencies. During permit review, each agency focuses on specific environmental concerns within its own jurisdictional responsibilities. Thereby, one agency's concerns and opinions as to the environmental viability of a sediment source may counter another agency's opinion. As an example, a borrow site characterized by a large fraction of coarse sediment would normally be the preferable borrow site when evaluating project longevity and performance. However, due to anticipated adverse impacts to sea turtle nesting, the preferred borrow site is deemed unacceptable to a particular regulatory agency, and the site must be excluded from consideration. Alternatively, fine sand can be added to the borrow material to offset the coarse sediment fraction and minimize unacceptable adverse impacts to turtle nesting habitat.

In addition to sediment size characteristics, sediment mineralogy (that is, silica and calcium carbonate content) and color also influence a regulatory agency's or local government's opinion in determining the ranking or acceptability of a candidate borrow site.

Offshore Territorial Boundaries

When an offshore sand resource is located beyond a state's jurisdictional boundary, generally a three-mile limit, the site is in federal waters and thus is controlled by the U.S. Department of the Interior, Minerals Management Service (MMS). Public Law 103-426 (October 1994) gave the MMS the authority to lease, on a noncompetitive basis, the rights to the Outer Continental Shelf sand and shell resources for beach nourishment funded or authorized by the Federal Government. Thus, use of offshore sand located in federal waters for beach nourishment will be reviewed by MMS to ensure that it is used in an environmentally sound manner.

In addition, international sources of sediment, such as aragonite sand located throughout the Bahamas and Turks and Caicos Islands, have been investigated as a source of high quality, coarse sand for projects. To date, the cost of conveying sand over long distances and the political sensitivity of this issue with local governments have prevented the use of these sand sources on US beaches, with the exception of two small projects recently constructed in southeast Florida. However, these sources may become economically viable as local sand reserves become depleted, and costs increase as sediments must be extracted from deep, offshore sites.

Summary

The decision-making process that determines the ranking and final selection of a borrow source for beach nourishment includes consideration of costs, sand quality and quantity, and environmental implications. Adequate geotechnical investigations are necessary to develop a three-dimensional analysis of potential borrow sites. This detailed analysis provides critical information as to the grain size characteristics of each site, giving decision-makers the necessary information to cost the project and rank the borrow sites. This will assist in streamlining the regulatory review process and thus reduce the time required to bring a project from planning to construction.

References

Dean, R.G. 1991. "Equilibrium Beach Profiles: Principles and Applications." Journal of Coastal Research. Volume 7, Number 1. Pages 53 to 84.

U.S. Army Corps of Engineers, Waterways Experiment Station, 1984. Shore Protection Manual, 4th ed. U.S. Government Printing Office. Washington, DC.