Coastal Services Center

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Geologic Characteristics of Borrow Areas and Sediments

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Introduction

Probably the most important aspect of a nourishment project is locating appropriate sediment to place on the beach. More often than not this can be the most difficult undertaking of a nourishment project. This discussion will consider the depositional nature of environments that are candidates as potential borrow sites, and the types of sediment that each of these environments accumulate to assist decision-makers in understanding the nature of the sand sources available for a project. Many of these depositional environments are typically found on the seaward (as opposed to landward) side of a beach being nourished; however, a wide range of possible sources exists. The sediment types that are suitable for a borrow site are generally dominated by sand, but they may also contain small amounts of mud, and/or a large percentage of crushed or broken shell.

Two primary considerations in identifying a beach compatible borrow site include the sediment grain size and sediment composition. Ideally the borrowed sediment should be the same as the sediment found on the beach being nourished. The grain size should approximate that of the eroding beach, and ideally would be slightly coarser. Other factors to be considered are the color, sorting, and durability of the borrowed material, although the primary objective is to find that sediment which most closely matches the beach being nourished.

Offshore Linear Sand Bodies

Lying along the inner shelf areas of the Gulf and Atlantic Coasts are numerous large sand bodies containing millions of cubic meters of potential borrow material. These areas represent a variety of origins and positions relative to the beach where nourishment may be required. Four distinct types of offshore linear sand bodies include: drowned barrier islands; oblique sand bodies; longshore sand bars; and trough sand bodies. Each has the potential to provide sufficient high-quality material for beach nourishment.

Drowned Barrier Islands
The rise in sea level of the past 20,000 years was one in which numerous changes occurred in the ebb and flow of the oceans of the world. That is, sea level rose and fell at varying rates throughout this period. On average, these changes produced rapid rises of sea level. Periods of slow (or no) sea level rise provided the opportunity for the formation of barrier islands. It is quite likely that such islands would have been small, would have had low elevations in relation to sea level when formed, and would have been developed over decades or possibly even centuries. These barrier islands were probably very similar to those found on the Gulf Coast today.

A rapid increase in sea level eventually overtopped these barrier islands and drowned them as the shoreline encroached towards the interior of the continent. The remnants of this process are underwater islands of varying shapes and sizes. These ancient offshore relict barriers can be found at distances ranging from hundreds of yards to several miles offshore, and in depths approaching 30 meters. The sediment these remnant islands contain is dominated by clean, well-sorted sand that was originally deposited in the beach, dune, or washover environments of the barrier island. Mud is scarce or absent.

Typically, wave action will rework barrier islands during periods of sea level rise. Only a unique set of circumstances would permit an island to be preserved as originally formed. As a result, these elongate sediment bodies are uncommon, but do occur on the mid-Atlantic inner shelf. The Texas and Florida Gulf Coasts also have numerous such sand bodies that have been attributed to this origin.

Figure 1. Bathymetric map showing oblique sand ridges off the coast of New Jersey.

Oblique Sand Ridges
The shallow continental shelf environment found along the north and central Atlantic and along the west Florida Gulf Coast has numerous elongate sand bodies, referred to as oblique sand ridges. These ridges are generally oriented obliquely (at an angle) to the coastline. Such sand bodies can be kilometers long, up to ten meters thick, and hundreds of meters wide. They are comprised of sorted sand, and are typically separated by low swales that may contain some mud. The origin and age of these sand bodies is the subject of debate, with no clear consensus. Were they formed, for example, as sea level rose over these areas, or did they develop after sea level approached or reached its present position during the past 3,000 years? The difficulty of ascertaining the origin of such sand bodies lies with their orientation to the shoreline. What natural processes would produce such large sediment accumulations at an oblique angle to the coast?

Regardless of origin, these structures can provide enormous quantities of potential borrow sediment. Although the nearest point of such sand bodies often lies within a few kilometers or less from the coastline, they may extend to depths in excess of 30 meters, thereby affecting their economic viability as a source for beach nourishment.

Longshore Sand Bars
Found along most coastal areas of the Atlantic and Gulf regions are longshore sand bars that are, essentially, shore-parallel features. These longshore bars characterize all nearshore areas where the slope of the beach is relatively mild, and there is abundant sediment. Such bar features are generally shallow, wave-formed, and tend to migrate seasonally. They can also be modified by severe storms. These typically sorted sand bodies can lie within several hundred feet from the shoreline, covered by 6-10 feet (2-3 meters) of water, but can also be found further offshore with crests in depths of 10 to 25 feet (5 to 7 meters).

While these longshore bars potentially contain sediment that is well suited for beach nourishment, most are simply too close to the shoreline to be excavated without undesirable consequences. If utilized for nourishment, and especially if a trench were to be produced by excavation, it is highly likely that there would be a change in the wave climate at the shoreline. A prime example can be found along Treasure Island on the Gulf Coast of Florida. The excavation of a longshore bar to re-nourish adjacent beaches caused the development of an erosional "hot spot" (area of extremely high rate of erosion) along the shoreline, thereby creating a greater problem than the nourishment activity temporarily resolved.

Trough Sand Accumulation
The southeast coast of Florida presents an unusual phenomenon of both topography and nearshore geology. Between Miami and Palm Beach in particular, there are numerous elongate, shore-parallel reefs that lie within 1.5-5.0 kilometers of the shoreline. These reefs rise typically some two to five meters from the ocean floor. Large amounts of sediment have accumulated between these reef troughs, referred to as trough sand accumulation. This sediment is comprised mainly of carbonate reef debris, but also contains siliciclastic sand as well, both of which are suitable for beach nourishment. The volume of sediment in these inter-reef troughs can be millions of cubic meters. Several of the large nourishment projects along the southeast portion of Florida, including projects located in or near Fort Lauderdale and Miami Beach, have relied heavily on these sediment accumulations for borrow material.

Tidal Deltas

A large sediment body that accumulates on either the landward or seaward end of tidal inlets is referred to as a tidal delta (also called a tidal shoal) because these bodies mimic river deltas, and contain large amounts of sediment deposited at an inlet mouth. The presence and the size and shape of a particular tidal delta are determined by a number of factors. These include sediment availability, the interaction of wave and/or tidal processes, and the tidal flux occurring during a tidal cycle. The accumulation of sediment on the seaward side is referred to as an ebb-tidal delta (or ebb shoal). In contrast, the flood-tidal delta (or flood shoal) is found on the landward side of an inlet. These accumulations are among the largest sand bodies found along the Atlantic and Gulf Coasts, and can provide a major source for beach nourishment material.

Flood Tidal-Deltas
Severe storms commonly breach or overrun barrier islands and carry large amounts of sediment into the environment behind these islands, forming flood tidal-deltas. Flood tidal-deltas can also be produced when the pathways between adjacent barrier islands become constricted as the barriers extend toward the pathway or inlet. These sediment bodies are generally dominated by sand, and commonly have two quite distinctive shapes: fan-shaped deltas having multiple lobes, and horseshoe crab shaped deltas facing landward.

Fan-shaped flood deltas are typically found in areas where the tidal range is less than 1.5 meters in depth. The sand body is typically subtidal, and the tidal flux passes back and forth over the delta having little or no influence upon it. This type of tidal delta is commonly only a meter or two thick. Its sediment may contain a small percentage of shell debris and mud. The mud is typically a combination of mineral mud and fine particulate organic material.

Horseshoe crab-shaped flood deltas are typically found in areas where the tidal range exceeds 1.5 meters and where the sand body is intertidal. As a consequence of a relatively large tidal range, the sand body becomes essentially molded by the combination of flooding and ebbing tidal currents. The ebb currents are deflected around these sand accumulations as the tide falls and the tidal delta becomes emergent.

It is not uncommon for such tidal deltas to contain hundreds of thousands of cubic meters of sediment. Tidal deltas have not been a common source of nourishment borrow material, due largely to the relatively high percentage of mud and the presence of a substantial benthic community (animals that live on or in the bottom sediment that commonly include sea grasses). Because of cost or scarcity, it is quite likely, however, that such sediment bodies may be utilized as nourishment sources in the future.

Ebb Tidal-Deltas
The large sediment bodies that accumulate in the open water end of a tidal inlet are called ebb tidal-deltas. They are generally comprised of sand and fine shell gravel. This sediment characteristic makes ebb deltas prime sites for nourishment borrow material. Ebb tidal-deltas are susceptible to constant interaction of wave and tidal processes and often their shapes reflect this interaction. When waves dominate an environment, ebb tidal-deltas tend to be small and thinly spread along the barriers on either side of an inlet (see Figure 5a). Due to limited sediment volume, such deltas provide little potential for borrow material.

Larger ebb deltas develop along coastlines where tides are significant, causing a mixed-energy or tide-dominated sediment body that can extend a kilometer or more beyond the barriers. Deltas formed as the result of mixed energy tend to have a smooth outer portion, with sediment built up on either side of a main channel (see Figure 5b), straight and offset. Tide-dominated ebb deltas have long sand bodies that parallel the main channel, along with smaller shoals on either side (See Figure 5c).

Ebb deltas can contain millions of cubic meters of sediment suitable for beach nourishment. Several such deltas, in fact, have already been used for this purpose, including projects located at Redfish Pass, Pass-a-Grille and Longboat Key on the Gulf Coast of Florida. There is, however, a growing concern as to the effect of removing large quantities of sediment from the nearshore bottom. Such removal, it is argued, may actually alter the wave climate and focus energy at specific points on the shoreline, thereby creating a hot spot.

Upland Sources

There is considerable sand accumulated along the upland portion of many coastal areas. Generally these accumulations are found in the form of relict sand dunes (the remnants of high sea level stands), or in the landward portion of prograding shoreline areas. Coastal sand deposits that are substantially inland from the present coast are the result of high stands of sea level occurring over millions of years of extensive glacial advancement and melting. Along those coastal plains having little relief or elevation, such deposits can be found kilometers from the coast.

While there is some sentiment toward leaving sand dune accumulations undisturbed, there are circumstances under which it may be desirable to use the sediment found in such dunes in a beach nourishment project. For example, the lack of a proximate offshore borrow site, or the cost prohibition associated with removal from a more remote site may dictate consideration of the use of an upland sand source. The primary potential as nourishment material can be found within the makeup of dunes.

Because of their wind-blown origin, sand dunes that may rise 10 meters or more above the adjacent coastal plain tend to be comprised of relatively fine grained sediment as compared to the adjacent beach. This wind-blown origin also contributes to their well-sorted nature. Drawbacks to the use of dune sand include the possibility that such sand may actually be too fine to be stable on a beach, or that such sand commonly contains iron stain, thereby creating a yellow-brown beach which is aesthetically unacceptable to beach-goers/tourists. In addition, dune sand generally contains neither shells nor shell debris, making the nourished beach appear sterile for a time.

Despite the drawbacks, upland sand sources have potential to provide borrow material, and may fall within the budget guidelines of a particular nourishment project. There are, in fact, several large sand excavation pits located some 80 kilometers inland from the Gulf Coast of Florida. The primary use for this sand is in construction. These pits were considered, however, as a nourishment source for the Sand Key beach nourishment project. In that project, it was proposed that sand would be transported by rail to a loading area where it would be transferred to large trucks and then taken to the beach construction site. Although such an operation may prove to be economically feasible in the future, there are both practical and logistic problems associated with this type of project. First, trucks carrying material generally can only operate at night due to the volume of traffic during the day that would slow delivery. Second, the roads connecting the sand source to the beach site experience major degradation from the large number of heavy loads. Third, night construction disrupts the entire beach community with lights and noise. Because of these potential drawbacks, this upland source is often rejected as a borrow source as was the case with the Sand Key nourishment project.

Another project of modest size, at Honeymoon Island, Florida, appeared at first to be one of both opportunity and mutual benefit. A large development was under construction within several miles of a beach in need of nourishment. Some 140,000 cubic meters of beach quality dune sand was offered by the construction project owner, at no cost, to be used for the nourishment of the beach project. Sand was trucked to the nourishment site across a causeway from the construction site. Although relatively inexpensive, this concept ultimately proved unsuccessful. The causeway road became damaged by the heavy trucks delivering sand, and the color of the sand used did not match the native beach material and was therefore not well received by beach-goers. Over the following year, most of this sand was lost from the nourishment site.

Summary

As has been indicated, there are numerous possibilities for appropriate borrow material which can be used for beach nourishment. Some coastal areas are quite well positioned to make use of indigenous low-cost nourishment material, while other coastal areas have little or no readily available sediment. In evaluating a beach nourishment project, the cost and appropriateness of the nourishment sediment are generally the principal considerations. Cost becomes a function of distance of transport. Locally derived sediment sources are generally most appropriate, because such composition is typically dominated by quartz, feldspar, and carbonate shell debris commonly found on the beach to be nourished, although, at beaches along the Gulf of Mexico and the Florida peninsula, shell debris is particularly important in nourishment projects.