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Fact Sheet - Limited Sand Resources for Eroding Beaches
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A recent beach nourishment project at Sand Key is an increasingly common countermeasure to
beach erosion in highly developed coastal regions.
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Beach erosion is a major concern on the highly developed west-central Florida coast. The region between Anclote Key
in southern Pasco County and Venice Inlet in central Sarasota County contains 18 barrier islands and inlets spanning
130 km of coastline, much of which has undergone significant change within the past century.
Under natural conditions, barrier islands shift in response to storms, sediment supply, and changes in sea-level.
Manmade structures such as sea walls, jettys, and even buildings and roads can alter the influence of these natural
processes. Consequently, some areas have experienced rapid beach erosion, resulting in narrow beaches that are
insufficient for recreational use or protection of upland properties.
Study area map showing locations of barrier islands.
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Beach erosion and loss of protective dunes have left the coast susceptible to damage from storms and hurricanes.
A 1992 report issued by the Florida Department of Natural Resources designated 65% of west Florida beaches as
"critical erosion areas." In response, several sections of the coast have undergone nourishment projects
to restore beach widths to previous dimensions. This method of beach maintenance is expensive, typically costing well
over a million dollars for each mile of beach, and needs to be repeated as often as every 4 years. While beach
nourishment is becoming the most common means of addressing beach erosion, it may not provide a cost- effective
long-term solution, as sand resources for nourishment are limited along much of the west Florida coast.
In 1992, Congress directed the U.S. Geological Survey (USGS) to initiate a 5-year study of this region in order to
better understand the recent geologic history of the barrier island system and the processes that control the distribution
and transport of sediment on the inner shelf. The West-Central Florida Coastal Studies Project is a cooperative effort
between the USGS, the University of South Florida, and Eckerd College.
Geologic Setting
The west-central Florida coast barrier-island chain sits near the center of a broad, gently sloping carbonate
platform. The continental shelf is underlain by limestone bedrock with a thin, discontinuous cover of sand deposits
of both quartz and carbonate origin. Prior to the initiation of this project, it was generally believed that sand resources
were evenly distributed on the continental shelf. Early results, however, have demonstrated that sand is concentrated
in specific, nearshore areas and is of limited thickness.
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Researchers take vibracore samples to examine sediments lying below the sea-floor.
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USGS scientists and collaborators have systematically mapped the thickness of sand deposits near the coast through
seismic surveys and jet-probing. The first phase was a regional mapping with widely spaced geophysical and sample
data that found that most beach quality sand is concentrated in active ebb-tidal deltas located just off tidal inlets and
in long, linear ridges found on the inner shelf. Across the entire study area, bottom samples and core samples have
been collected and are being analyzed to develop a map of sea-floor sediment types. This research indicates that
although there is coarse, biogenic carbonate sand being produced in the modern environment, there is little or no
modern source of beach-quality quartz sediment in this region.
In addition to the reconnaissance mapping, USGS scientists and collaborators have completed detailed studies in two
localities, one north and one south of the entrance to Tampa Bay. Detailed sidescan-sonar maps were made of localized
areas which are being used to understand the variability in sediment distribution of beach-quality sand, carbonate shell
material, and hardgrounds and to shed new light on the processes that shape the inner shelf.
Sidescan-sonar imagery in an area off Sarasota, Florida reveals that fine sand (dark gray)
is concentrated in long linear ridges, and in ebb-tidal deltas located off tidal inlets. Light gray areas consist of coarse,
shelly material or hardgrounds. |
Coastal Processes and Morphodynamics
The west-central Florida barrier island and inlet system is among the most varied in the world. The combination
of low wave energy and microtidal conditions produces a wide range of barrier and inlet morphologies. Barrier islands
and inlets in this region are affected by a complex suite of processes including sediment availability, impacts of storms
and hurricanes, circulation, and underlying geologic control. For example, analysis of core samples and seismic data
indicate that, in some areas along the coast, the underlying limestone bedrock exerts a significant control on the
location of the barrier islands. Processes affecting the barrier island and inlet system have been active throughout
recent geologic history, evidenced by the range in ages of barrier islands, from greater than 4,600 years at Siesta
Key in the southern part of the study area to less than a decade at 3-Rooker Bar in the north. There is also evidence
that the location of some islands have shifted considerably landward over thousands of years.
Researchers are also examining dominant oceanographic processes, how they influence sediment transport, and
how they affect sand distribution in this region. Several methods are being utilized to examine circulation patterns on
the inner shelf including deploying instruments that measure winds, currents, temperature, and salinity. Early results
have demonstrated the importance of seasonal variations in the mean circulation and in sea level and have identified
coastal upwelling of colder, deeper water following storms.
Data collected by current meters and satellite imagery are being utilized in the development of three-dimensional
numerical circulation models. These models help to describe the responses of the entire west Florida continental shelf
circulation and sea level to storms and seasonally varying conditions.
Research Applications
The west-central Florida coast is a highly populated region with an economy closely tied to its natural resources.
The results of this project will allow planners and managers to better understand the effects of land-use decisions based
upon a scientific understanding of the natural coastal processes.
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