USGS Sand Sources

Issue

B e a ch erosion is a major concern on the highly deve l o p e dwest-central Florida coast. The region between Anclote Keyin southern Pasco County and Venice Inlet in central SarasotaCounty contains 18 barrier islands and inlets spanning 130 kmof coastline, much of which has undergone significant changewithin the past century.

Under natural conditions, barrier islands shift in response tostorms, sediment supply and changes in sea-level. Man-mades t ru c t u res such as sea wa l l s , j e t t y s , and even buildings androads can alter the influence of these nat u ral pro c e s s e s .C o n s e q u e n t ly, some areas have ex p e rienced rapid beache ro s i o n , resulting in narrow beaches that are insufficient fo rre c re ational use or protection of upland pro p e rties. Beacherosion and loss of protective dunes has left the coast suscepti-ble to damage from storms and hurricanes. A 1992 rep o rtissued by the Florida Department of Natural Resources desig-nated 65% of west Florida beaches as “critical erosion areas”.In response, several sections of the coast have undergone nour-ishment projects to re s t o re beach widths to prev i o u sdimensions. This method of beach maintenance is expensive,typically costing well over a million dollars for each mile ofbeach, and needs to be repeated as often as every four years.While beach nourishment is becoming the most commonmeans of addressing beach erosion, it may not provide a costeffective long-term solution,as sand resources for nourishmentare limited along much of the west Florida coast.

In 1992, C o n gress directed the U. S. Geological Survey(USGS) to initiate a five-year study of this region in order tobetter understand the recent ge o l ogic history of the barri e risland system and the processes that control the distri bu t i o nand transport of sediment on the inner shelf. The West-CentralFlorida Coastal Studies Project is a cooperative effort betweenthe USGS, the Unive rsity of South Flori d a , and Ecke rdCollege.

Geologic Setting

The west-central Florida coast barrier-island chain sits near thecenter of a broad, gently sloping carbonate platform. The con-tinental shelf is underlain by limestone bedro ck with a thin,d i s c o n t i nuous cover of sand deposits of both quartz andcarbonate origin. Prior to the initiation of this project, it wasgenerally believed that sand resources were evenly distributedon the continental shelf. Early results, however, have demon-strated that sand is concentrated in specific, near-shore areasand is of limited thickness.

USGS scientists and collaborators have systematically mappedthe thickness of sand deposits near the coast through seismics u rveys and jet-pro b i n g. The fi rst phase was a regi o n a lm apping with widely spaced ge o p hysical and sample dat awhich found that most beach quality sand is concentrated inactive ebb-tidal deltas located just off tidal inlets and in long,linear ridges found on the inner shelf. Across the entire studya re a , bottom samples and core samples have been collected,and are being analyzed to develop a map of sea-floor sedimenttypes. This re s e a rch indicates that although there is coars ebiogenic carbonate sand being produced in the modern envi-ronment, there is little or no modern source of beach qualityquartz sediment in this region.

A rec e nt b e ach no ur i sh m e nt p roj e ct a t San d K ey i s a ni n c r e a s i n g ly c ommon c oun ter-meas ure t o be ach ero s i o nin hi gh ly devel o ped coa stal reg io ns .

U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY

February 1997

28° 00’

27° 30’

83° 00’ 82° 30’

M ap of s tu dy area showin g locat io n of b ar r ier isl ands.

barrier islands. Processes affecting the barrier island and inletsystem have been active throughout recent ge o l ogic history,evidenced by the range in ages of barrier islands, from greaterthan 4,600 years at Siesta Key in the southern part of the studyarea, to less than a decade at 3-Rooker Bar in the north. Thereis also evidence that the location of some islands have shiftedconsiderably landward over thousands of years.

R e s e a rch e rs are also examining dominant oceanograp h i cp ro c e s s e s , h ow they influence sediment tra n s p o rt , and howthey affect sand distribution in this region. Several methods arebeing utilized to examine circulation patterns on the inner shelfincluding deploying instruments that measure winds, currents,temperature, and salinity. Early results have demonstrated theimportance of seasonal variations in the mean circulation andin sea level, and have identified coastal upwelling of colder,deeper water following storms.

D ata collected by current meters and satellite image ry arebeing utilized in the development of 3-D numerical circulationmodels. These models help to describe the responses of theentire west Florida continental shelf circulation and sea level tostorms and seasonally varying conditions.

Research Applications

The we s t - c e n t ral Florida coast is a highly populated regi o nwith an economy cl o s e ly tied to its nat u ral re s o u rces. Th eresults of this project will allow planners and manage rs tobetter understand the effects of land-use decisions based upona scientific understanding of the nat u ral coastal processes.

In addition to the reconnaissance map p i n g, USGS scientistsand collaborators have completed detailed studies in two local-ities, one north and one south of the entrance to Tampa Bay.Detailed side-scan sonar maps we re made of localized are a swhich are being used to understand the variability in sedimentdistribution of beach quality sand, carbonate shell material,andhardgrounds and shed new light on the processes that shape theinner shelf.

Coastal Processes and Morphodynamics

The we s t - c e n t ral Florida barrier island and inlet system isamong the most varied in the world. The combination of lowwave energy and microtidal conditions produces a wide rangein barrier and inlet morphologies. Barrier islands and inlets inthis region are affected by a complex suite of pro c e s s e si n cluding sediment ava i l ab i l i t y, impacts of storms and hurri-c a n e s , c i rc u l at i o n , and underlying ge o l ogic control. Fo rex a m p l e, a n a lysis of core samples and seismic data indicat et h at in some areas along the coast the underlying limestoneb e d ro ck exe rts a significant control on the location of the

For more information please contact:Guy GelfenbaumU. S. Geological Survey 600 4th St. S.St. Petersburg, FL 33701(813) 893-3100

internet: ”[email protected]”

Homepage location:http://coastal.er.usgs.gov/wfla

S ide- scan s onar image r y i n an a rea off S arasot a, F Lr evea ls t hat f i ne s an d ( dark grey ) is co nce ntr at ed inl ong l inea r r idge s , an d in ebb t ida l d eltas loca ted offt id a l i n le ts . L ig h t g r ey ar ea s c on s is t o f c o ars e,s h e l ly ma te rial o r hard g ro u n d s .

R e s e a rch e rs take vi br acore sample s to ex a m i n ese di men ts lying be low t he se a-f loor.