Western Kentucky University TopSCHOLAR® Geography/Geology Faculty Publications Geography & Geology January 2007 Sinkhole distribution based on pre-development mapping in urbanized Pinellas County, Florida, USA Robert Brinkmann University of South Florida Kelly Wilson University of South Florida Nichole Elko Pinellas County Government Larry D. Seale University of South Florida Lee J. Florea Western Kentucky University, lfl[email protected]See next page for additional authors Follow this and additional works at: hp://digitalcommons.wku.edu/geog_fac_pub Part of the Environmental Monitoring Commons , Geophysics and Seismology Commons , and the Natural Resources and Conservation Commons is Contribution to Book is brought to you for free and open access by TopSCHOLAR®. It has been accepted for inclusion in Geography/Geology Faculty Publications by an authorized administrator of TopSCHOLAR®. For more information, please contact [email protected]. Recommended Repository Citation Brinkmann, Robert; Wilson, Kelly; Elko, Nichole; Seale, Larry D.; Florea, Lee J.; and Vacher, H L.. (2007). Sinkhole distribution based on pre-development mapping in urbanized Pinellas County, Florida, USA. Natural and Anthropogenic Hazards in Karst Areas: Recognition, Analysis and Mitigation, 5-11. Available at: hp://digitalcommons.wku.edu/geog_fac_pub/11
Follow this and additional works at: http://digitalcommons.wku.edu/geog_fac_pub
Part of the Environmental Monitoring Commons, Geophysics and Seismology Commons, andthe Natural Resources and Conservation Commons
This Contribution to Book is brought to you for free and open access by TopSCHOLAR®. It has been accepted for inclusion in Geography/GeologyFaculty Publications by an authorized administrator of TopSCHOLAR®. For more information, please contact [email protected].
Recommended Repository CitationBrinkmann, Robert; Wilson, Kelly; Elko, Nichole; Seale, Larry D.; Florea, Lee J.; and Vacher, H L.. (2007). Sinkhole distributionbased on pre-development mapping in urbanized Pinellas County, Florida, USA. Natural and Anthropogenic Hazards in Karst Areas:Recognition, Analysis and Mitigation, 5-11.Available at: http://digitalcommons.wku.edu/geog_fac_pub/11
Sinkhole distribution based on pre-development mapping in
urbanized Pinellas County, Florida, USA
R. BRINKMANN1, K. WILSON1, N. ELKO2, L. D. SEALE1,
L. FLOREA1 & H. L. VACHER1
1University of South Florida, Tampa, FL 33617, USA
(e-mail: [email protected])2Pinellas County Government, 513 S. Ft. Harrison Avenue,
Clearwater, FL 33756, USA
Abstract: Locating sinkholes in Pinellas County, Florida, is confounded by the presence of acover of Quaternary sediments that mute the surface appearance of these sinkholes. As a firststep in addressing the sinkhole hazard in the county, we analysed aerial photographs from 1926and 1995 that covered the entire county. We digitized all identifiable sinkholes in each set ofphotographs in a GIS (Geographical Information System) using a set of criteria establishedto differentiate between karst depressions and depressions resulting from other geologicalprocesses. The 1926 photographs, although of low quality, helped to establish a baseline priorto urbanization. The 1995 photographs provided a post-urbanization distribution of natural sink-holes and man-made depression features (e.g. retention ponds). From these two data sets, we areable to assess natural and anthropogenic changes in the karst landscape of the study area. In par-ticular, we discovered that 87% of the sinkhole features identified in the 1926 photographs are nolonger present in the photographs from 1995. Many of the lost depressions have been incorporatedinto retention ponds.
Pinellas County, in west-central Florida (Fig. 1), inthe USA, is located in one of the most active karstregions of the world. Karst, a collection of surfaceand subsurface landforms produced from the dissol-ution of limestone and other soluble rocks, is presentin many regions throughout the world and is ofdistinct interest to individuals and organizationsassociated with these landscapes. This is due, inpart, to unique engineering difficulties and contami-nant transport properties of karst aquifers. PinellasCounty is a peninsula and is underlain by distinctlayers of Cenozoic limestone that are coveredin most areas by Quaternary sands. The county ismodified by coastal processes on the current shore-line, and marine terraces indicate the presenceof higher sea levels. In the interior portions of thecounty, numerous karst depressions are present.
The most identifiable surface features in the karstlandscape of west-central Florida are sinkholes.However, in contrast to classical karst landscapesof the Central Lowlands and Appalachian Moun-tains in the USA, the topographic expression ofthese depressions is partially masked by Quaternarysediments deposited by eolian processes or duringhigher sea levels. Within the past century, humanshave filled many depressions and modified othersfor storm-water retention as part of the rapid andintense urbanization of this coastal area. Mappingand analysis of sinkholes was not completed prior
to or after development to assess the distributionof karst features in Pinellas County or to documentchanges in topography as a result of urbanization.Pinellas County, like much of the Tampa Bayregion and Florida in general, is rapidly urbanizing.Construction projects significantly impact the karstlandscape and vice versa. Without a clear under-standing of where karst features are located it is dif-ficult to implement appropriate land-use strategiesand management decisions that are suitable for thesite geology and of benefit to the citizens of thecounty. As Pinellas County loses remaining greenspace, increases densities in existing developmentsand redevelops brownfields, it will be critical tounderstand the landscape that exists beneath theveneer of human development because the under-lying geological processes that created the originalsurface features will continue to modify the soil,bedrock and topography.
In Florida, sinkholes occur in clusters or‘sinkhole regions’ (Tihansky 1999). These clustersrepresent areas of past sinkhole formation andare locations where future sinkholes may develop,although the occurrence of topographic sink-holes does not predict the development of newcover collapses. Certainly, sinkhole regions maybe hidden by geological deposition or human devel-opment, and may extend into or exist withinurbanized regions. A useful method of identifying
From: PARISE, M. & GUNN, J. (eds) Natural and Anthropogenic Hazards in Karst Areas: Recognition, Analysis andMitigation. Geological Society, London, Special Publications, 279, 5–11. DOI: 10.1144/SP279.20305-8719/07/$15.00 # The Geological Society of London 2007.
masked (obliterated) areas affected by subsidence isto map topographic depressions and wetland fea-tures that are probably the result of karst processesusing historic air photographs and maps. Such anexercise will not only assist county planners, itwill aid our scientific understanding of karst pro-cesses in Florida and the extent of sinkholeregions that existed in the pre-urbanized past.Such an exercise has rarely been conducted oversuch a large area.
Pinellas County is underlain by the unconfinedFloridan Aquifer System and, as in most carbonateterrain, karst processes exert significant control onthe geomorphology (White 1970, 1988; Schmidt &Scott 1984; Lane 1986; Miller 1986). Aggressivegroundwater circulation and diagenetic alterationhas dissolved limestone and increased porositywithin the stratigraphic framework of the FloridanAquifer. This solution porosity dominates ground-water flow. The complex nature of conduit systemsin the unconfined Floridan Aquifer is becomingbetter understood (Miller 1997). However, the diffi-culty of locating and exploring submerged conduitsand the complex porosity makes analysis and model-ling difficult.
In Pinellas County, surficial expression of karstis limited to sinkholes, sinkhole-associated featuresand springs. Karst features provide unique problemsfor planners and environmental scientists. Landstability is perhaps the issue citizens are mostfamiliar with and are the most concerned about,although contaminant transport and water quantityare also important issues.
Sinkholes form from a variety of processes (e.g.White 1988; Upchurch & Randazzo 1997). The
bulk of sinkholes in Pinellas County are categorizedas cover-collapse sinkholes that form when Pleisto-cene sands above limestone ravel into void space inbedrock (Tihansky 1999). The spatial distributionof these sinkholes appears linear on many Floridamaps. Sinkholes form in linear trends becausethey are localized by joints and fractures that facili-tate groundwater flow through the soluble rock.
Previous efforts have described and/or mappedkarst features in Pinellas County. For example,Sinclair et al. (1985) analysed sinkholes inwest-central Florida and described their formationbased on cover and bedrock type. They alsoreviewed data on recently formed sinkholes in theregion. However, they did not map them. Frank &Beck (1991) assessed the causes of subsidencedamage in a small area of the town of Dunedin innorthern Pinellas County. They examined themechanisms behind the formation of subsidencein the area in the years 1990–1991 and found thatalmost all of these damaging subsidences werecover-subsidence sinkholes, although this couldnot be guaranteed for 100% of the damage.
In 1991, Beck & Sayad (1991) published asummary of the sinkhole hazards in PinellasCounty. They utilized reported sinkhole events toassess the overall hazard from subsidence. Stewartet al. (1995) examined foundation failures inPinellas County to assess the causes of failures.More recently, the Florida Geological Surveymapped closed depressions on United StatesGeological Survey (USGS) 1:24 000 topographicmaps as part of their overall Florida AquiferVulnerability Assessment (FAVA) programme. Avariety of other attempts have been made to mapsinkholes in other parts of Florida, includingnearby Pasco County (Currin & Barfus 1989).
Although all of the past research is useful, todate, there has been no effort to map all sinkholesin Pinellas County, including those lost to urbaniz-ation. Therefore, the focus of this project is todevelop a county-wide map of sinkholes presenton historic and recent aerial photographs in orderto assess the number and spatial distributionof depressions lost through development, and inorder to understand how pre-existing sinkholesmay have been modified through urbanization.
One measure of the human impact on karst land-forms is to assess the number of obliterated depressionsand particularly those modified to create storm-waterretention ponds. Storm-water retention ponds areused to manage storm-water drainage to prevent flood-ing during common high-intensity storm events in therainy season and to allow time for polluted runoffto clear up before discharging into the aquifer orsurface streams. If natural depressions like sinkholesare used as sites for storm-water pond creation, thedegradation of groundwater quality may be aggravated
Fig. 1. Location of Pinellas County, Florida.
R. BRINKMANN ET AL.6
because polluted surface water can quickly enteraquifer systems through sinkhole-ravelling zones.The unfortunate use of the sinkholes may also resultin the reactivation of the subsidence features, withthe consequent rise in the sinkhole frequency. Storm-water retention ponds became the best managementtool for urban storm water in the 1970s when the com-bined effects of flooding and surface-water pollutioncaused environmental problems. Thus, many areas ofFlorida are dotted with these ponds.
Recent rules preclude the use of the sinkholes assites for pond construction. Yet, many sinkholeswere used prior to the approved establishment ofthese rules. Thus, we seek to understand the extentof storm-water construction in natural depressionsin Pinellas County in order to assess the potentialdetrimental effects on the karst aquifer. This is par-ticularly important because water-retention pondsare more frequently being used as sites of storm-water storage to prevent polluted water from enter-ing nearby surface-water bodies like the importantTampa Bay estuary or many of the lakes and smallstreams that flow in Pinellas County.
Methods
Karst features were mapped on aerial photographsfrom 1926 and photographs from 1995 importedinto a GIS (Geographical Information System).Four GIS layers were created using the 1926 blackand white photographs: sinkholes, possible sinkholes,exclusion areas (areas that were obscured or unclearin the photographs) and developed areas (thoseareas that showed notable development such asclusters of roads and/or structures). The 1995 aerialphotographs are full-colour high-resolution infraredimages. Using these images, four GIS layerswere constructed: sinkholes, possible sinkholes,water retention ponds and undeveloped and/or non-urbanized areas (mainly wetlands and forest).
Sinkholes were defined by vegetation changes orvariation, presence or absence of water, soil moisture
and shape. The criteria used to identify these featureson both the 1926 and 1995 photographs were moist-ure content and circular and/or combined circularshapes. The features in the possible sinkhole layeron both the 1926 and 1995 air photographs wereidentified as having some moisture and circularityconditions, but lacking clear indications of one ofthose factors (moisture condition or circularity).Lakes that appeared to be depressional on the photo-graphs were considered to be sinkholes. It must benoted that the 1926 air photographs were takenduring the dry season in Florida so many of thesinks that would normally contain water were dry.So, vegetation patterns and coloration from soilmoisture were used to identify the circular patterns.The 1926 air photographs are black and white, andof poor quality. Many of the areas of these photo-graphs are completely unusable for photographicinterpretation. Thus, an exclusion layer was createdto remove these areas when carrying out densityestimates and other analyses.
The features identified in the retention pondlayer in the 1995 air photographs were thosedepressions considered to be man-made with atleast one straight edge.
The sinkhole density in Pinellas County was cal-culated using a total land area of Pinellas County of725 km2 for 1926 and 1995. In addition, the totalarea and percentage of land area of sinkholes wascalculated for 1926 and 1995. From this we wereable to calculate the percentage loss of the featuresthat were identified. Then, a map with a layerconsisting of all sinks in 1926 and a layer of allsinks from 1995 was constructed. An intersectwas then performed in ArcGIS and the resultswere the sinks that are still present in 1995. Wealso attempted to determine which sinkholes thatwere identified in 1926 are now storm-water reten-tion ponds. To do this, we created a map that con-sisted of the intersect of the layer of total sinksidentified in 1926 and the retention pond layerconstructed in 1995.
Table 1. Total area, density and per cent land area of sinkholes using the 1926 and 1995 air photographs
We identified 1570 depressions using the 1926air photographs of Pinellas County. These probable,or likely, sinkholes account for nearly 20 km2
(2.7%) of land (Table 1, Fig. 2). In addition,we identified 1133 depressions that are possiblesinkholes. While the number of possible sinkholesis smaller than likely sinkholes, the area coveredis larger at nearly 25 km2 (3.4%). Collectively,
this accounts for a density of 3.8 sinkholes km22.It must be noted that several areas of PinellasCounty were already developed in 1926 (Fig. 3).These areas account for some of the earliesturbanized areas of the Florida peninsula in thevicinity of present-day downtown St Petersburgand the fishing community of Tarpon Springs.However, the developed portions account for only14.5% of the total land area. Certainly, some sink-hole modification occurred in these areas, but the
Fig. 2. Distribution of depressions in 1926 in PinellasCounty, Florida. Fig. 3. Developed areas of Pinellas County, Florida.
R. BRINKMANN ET AL.8
vast majority of the county remained undevelopedat the time the air photographs were collected.
A total of 261 depressions were identified on the1995 air photographs that are likely sinkholes(Table 1, Fig. 4). This number contrasts sharplywith the number of sinkholes identified on the 1926air photographs and accounts for only 0.2% of themodern land surface. Likewise, only 639 possiblesinks were identified in 1995. While this number isgreater than the number of likely sinkholes found in
1995, these depressions only cover 0.6% of theland surface. Collectively, the sinkhole density in1995 was only 1.2 sinkholes km22. A great deal ofurbanization has taken place over the 1926–1995period. The undeveloped land portions account foronly 2.7% of the county. It is evident that develop-ment significantly altered the karst landscape overthe 72 year period of study. Most of the undevelopedland areas were lowlying and adjacent to Tampa Bayor in terrestrial wetlands in the NE portion of thecounty. A total of 87% of the sinkholes and possiblesinkholes mapped in 1926 were buried or maskedby 1995.
It is striking to compare similar land areas in 1926and 1995 in order to see the loss of depressions andthe extent of urbanization that has taken place. Oneof the most obvious examples can be found incentral Pinellas County and clearly demonstratesthe dramatic loss of depressions through urbanization(Fig. 5). The dark areas in the older air graphphoto inFigure 5 largely represent depressional features prob-ably formed through karstification and subsidence
Fig. 4. Distribution of depressions in 1995 in PinellasCounty, Florida.
Fig. 5. Comparison of 1926 and 1995 air photographs.The photograph at the top is an image of an undevelopedportion of Pinellas County. The image below shows thesame area in 1995. Note the loss of depressionsthroughout the area.
SINKHOLE IN URBANIZED PINELLAS COUNTY 9
processes. Many of these features had gone by 1995,a time when urbanization and the associated regulargrid system significantly modified the landscape,destroying many depressions. Sinkholes were mostprobably filled or otherwise modified as develop-ment progressed in the region.
Of particular interest is the presence of waterretention ponds in Pinellas County present in the2000 air photographs and how the distribution ofthese features coincides with depressions mappedon the 1926 air photographs. We identified 1646water retention ponds on the 2000 air photographs.Approximately 21.8 km2 of retention pond areaoverlapped with depressions mapped on the 1926air photographs (Fig. 6). Of the 2703 depressionsmapped in 1926, 499 of them have been convertedor altered in some way into areas of water reten-tion ponds. This figure, approximately 18% of allnatural depressions in the county, does not necess-arily mean that these water retention ponds posea threat to groundwater. Indeed, we did not attemptto evaluate whether or not the retention pondswere lined or in some other way mitigated toprevent harmful effects of drainage through ravellingzones. However, it does demonstrate that there is apotential hazard to the subsurface system.
Although we did not intend to conduct anygeomorphic description of sinkholes in the county,it is important to note that there are distinct differ-ences in the karst landscape in the region (Fig. 2).The northern portion of the county had the mostcomplex karst depressions. Many of these featureswere uvalas that certainly interconnected duringhigh water flow. There may have been somefluvial modification of these features that occurredwhen subsurface drainage could not keep up withheavy rainfalls common during summer or duringtropical storm events. Further south, the depressionsare more rounded and exist in a less dense patternthan those in the north. It is not particularly surpris-ing that the northern portion of Pinellas County isthe last portion to undergo urbanization. Much ofthe area is in the form of low karst depressionsand is thus difficult to develop.
Conclusions and recommendations
The results of this study provides new informationon the distribution of depressions in PinellasCounty, Florida, as well on the effects of urbaniz-ation on Florida karst landscape. Specifically, atotal of 2703 depressions were identified on the1926 air photographs. These likely sinkholesaccounted for 43.9 km2 and 6.1% of the totalland area. By 1995, only 900 of these depressionsremained. These features totalled only 5.6 km2
and 1% of the total land area of the Pinellas
County. From these data we measured a loss of87% of depressions between 1926 and 2000. It islikely that this loss is owing to the rapid urbaniz-ation that occurred since 1926. Many of the existingdepressions were filled or modified into storm-waterretention ponds.
We mapped 1646 storm-water retention ponds inPinellas County. A total of 499 of these ponds arelocated in areas that were mapped as depressionsin 1926. It is clear that there is a potential threat of
Fig. 6. Distribution of water retention ponds in PinellasCounty, Florida in 1995.
R. BRINKMANN ET AL.10
subsurface pollution from leakage in the retentionponds owing to the overlap of these structureswith ravelling zones of sinkholes. It must be notedthat we did not evaluate individual water retentionponds to determine if they are lined or in someother way managed to prevent subsurface pollution.This would be a valuable future project.
The presence of so many buried or otherwisemodified depressions in an urbanized area is ratherstriking. We are very interested in the impact ofthe modified depressions on overall ground stability,and surface-water and groundwater flow. Floridalaw mandates that insurance carriers cover damageto homes from subsidence caused by sinkholeactivity (Eastman et al. 1995). There are dozens ofclaims filed each year in Pinellas County by prop-erty owners who have some form of structuraldamage to buildings caused by subsidence. Whilethe focus of our research did not address the corre-lation between lost depressions and structuraldamage, we do believe that a follow-up study thataddresses this issue is warranted.
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