WHITMAN, Dean, Department of Geology, Florida International University, Miami FL 33199, whitmand@fiu.edu;
GUBBELS, Tim L., Hughes Information Technology Systems, Landover MD 20774;
POWELL, Linda A., Florida International University, Miami FL 33199.
Presented at the Geological Society of America, Annual Meeting, Salt Lake City, Utah, Oct 19-23, 1997
In this paper, we present initial results of a larger project funded through the NASA Topography and Surface Change Program. In this project we are studying karst processes and landform evolution on the Florida Peninsula at scales ranging from 10s of cm to 100s of km. A particular goal is application to forecasting sinkhole hazard in Central Florida.
We are taking a broad based approach, which relies heavily on computer visualization, remote sensing and GIS software. In this study, we examine some of the regional geologic phenomena that influence sinkhole hazard. In particular, the, spatial interrelationships between bake elevations, water tables and sinkhole occurrence in Central Florida.
Florida does have interesting topography! This
image is a color shaded relief map of the Florida Peninsula. It was produced
from USGS 3" digital elevation models. The present day physiography
of the Florida Peninsula is the result of Late Cenozoic marine and shoreline
deposition with later erosional modification. Several identified Plio-Pleistocene
marine terraces and shorelines are well expressed in the topographic "signal"
and give Florida's topography its step-like appearance. Most of the marine
shorelines and terraces are distinguished by prominent peaks in the hypsometric
curve. These include the Pamlico, Talbot, Penholoway, Wicomico, Sunderland,
and Coharie terraces.
In central Florida, the most prominent topography occurs in the Central
Highlands. This region is the erosional remnant of a once larger depositional
feature which has been modified by shoreline and karst processes. Prominent
features include the elongate system of sandy upland ridges exemplified
by the Lake Wales, Orlando, and Mt Dora ridges.
Cultural features shown in this image include Cape Canaveral and the NASA Kennedy Space Center, the Orlando metropolitan area, and Disney World. The large lake in the lower left corner is Lake Apopka. Land cover / land use in this region includes urban, orange groves, grazing lands, and wetlands.
Central and Northern Florida are known for their Karst topography, a suite of landforms which develop in response to dissolution of carbonate beadrock. Because the rocks of the Florida platform are almost entirely undeformed and regional topographic gradients are low, karst landforms form the primary topographic signal in the region. Many of the physiographic features are related to karst processes that have been occurring for last 5 million years. These include the numerous circular lakes in this region which occupy karst related depressions.
This topographic image was produced from a USGS
7.5' DEM to the west of Lake Apopka. Few other landscapes on Earth exhibit
the degree of process dominance as the central Florida region. Considered
on a gross scale, the topography resembles that of "Swiss cheese".
Surface fluvial drainage is poorly developed and most surface precipitation
percolates downward into the sandy soils of the many closed depressions.
Many of the closed depressions are occupied by shallow lakes.
This photograph was taken from a height of ~ 1500
feet over Lake Co, Florida. View is towards the east. Shallow circular depressions
are common in this region. They occur at a variety of scales and are often
occupied by shallow ponds.
Shallow closed depressions as shown in this ground level photograph are common in Central Florida. While they typically represent inactive ancient sinks, they can be reactivated by human activities. Often however, new sinkholes can form in areas that did not previously show any topographic expression.
Sinkholes are ubiquitous features of karst terrain and result from the subsidence or collapse of surficial material into subsurface cavities. They constitute a major hazard in central and northern Florida. This study focuses on an area of C. Florida in Lake and Orange Co (Green Box).
The white dots on the map indicate the location of reported sinkholes that have occurred between 1960 and 1993 which were compiled by the Florida Sinkhole Research Institute. Three general types of sinkholes occur in Central Florida. In areas where Eocene-age limestone is exposed or thinly covered (light blue), solution sinkholes form from gradual solution and subsidence. In regions where the limestone is covered Miocene and younger clastic cover (Red and Yellow) sinholes typically occur from gradual cover subsidence or catastrophic cover collapse.
Cover collapse sinkholes are the most hazardous
of the three major types of sinkholes because of their tendency to collapse
catastrophically. Within the central Florida highlands west of Orlando,
these are the most common type of sinkhole encountered. The generally occur
in areas where unconsolididated semicohesive cover overlies a confining
unit above limestone strata. In the sequential evolutionary sequence diagramed
above, downward erosion of the clastic cover into underlying cavities serves
to create a void that enlarges progressively. Catastrophic collapse of the
cavity can be triggered by various mechanisms, including drawdown of the
confined aquifer (the Floridan), or acceleration of erosion rate caused
by accelerated surface recharge.
The karst processes of central Florida are profoundly influenced by the
region's complex hydrogeology. In this region, two distinct aquifer systems
exist. The principal aquifer system, the Floridan, is a regionally extensive,
confined aquifer which is situated within Eocene to Miocene age limestones
which have been subjected to extensive dissolution and cavity formation.
The Floridan aquifer provides the principle municipal and agricultural water
supply for much of central and northern Florida, and spatial and temporal
variations in its potentiometric surface are often related to groundwater
withdrawal. The Floridan aquifer is capped by a variable thickness upper
confining unit composed of Miocene clay rich clastic sediments. Near the
surface, a discontinuous unconfined aquifer system exists within the highly
permeable unconsolidated clastic cover. In contrast to the Floridan, the
water tables of these surficial aquifers are more directly influenced by
local changes in rainfall, runoff, and landuse. Cover collapse sinkhole
formation is integrally related to the interplay of these aquifer systems
and can be triggered by various forcing factors including rises in the surficial
water table increasing the load on the underlying units, or by a drops in
the potentiometric surface of the Floridan decreasing support from below.
Because the head differences between these two aquifer
systems are so important in driving sinkhole collapse, we will derive a
map of the head differences.
Landsat false color image bands 3, 4 and 5. Note
the many lakes. The big lake in the middle is Lake Apopka. The Floridan
potentiometric surface slopes gently down to the northeast. Wikeva springs
is the green area. Land use includes Orange groves. Urbanization encroaching
form the east. The subtle vertical alignment of lakes along the west side
of the image is related to ancient shorelines.
The high topography to the west of Lake Apopka defines the northern end
of the Lake Wales ridge, and the high topography to the east of Lake Apopka
is the Mount Dora ridge. The low elevations around Lake Apopka and to the
north are known as the central lowlands.
A water body mask was produced from the TM image by selecting pixels where the ratio of Band 2 to Band 5 was greater than 1. This mask was applied to the DEM to produce a map of lake elevations relative to sea level. Since lake levels usually lie near the surficial aquifer water table, this map is a first order approximation of the head level of the surficial aquifer system. Overlayed on this map are contours of the potentiometric surface of the Floridan aquifer.
A digital projected cross--section was then produced that "projected in" all lake elevation values within a 10 km wide swath. Also shown in black is the surface topography averaged over the +/- 10 km swath. The same techniques were then used to "project in" the reported sinkholes and the elevation of the Floridan aquifer on 9/85 (green) .
3 general observations can be made from these diagrams:
Raster subtraction of the potentiometric surface from the lake elevation map was used to produce a map and projected profile of the lake elevations relative to the Floridan aquifer potentiometric surface. This map is a first order expression of the head differences between the Floridan and surficial aquifers. On the projected profile, this adjustment causes the altitude of the Floridan to be represented by a horizontal line at zero altitude.
Lakes whose surface elevations are at levels significantly above the Floridan potentiometric surface are concentrated on the eastern half of the study area. They generally occur in elevated areas corresponding to Orlando and Mt Dora Ridges. Somewhat surprisingly, with the exception of artificial quarry lakes, elevated regions of the Lake Wales ridge do not tend to have lakes at elevations significantly above the Floridan PS.
All lakes lying more than 6 meters above the level
of the Floridan are shown with the locations of reported sinkholes. These
lakes delineate distinct regions where the head difference between the Floridan
and the surficial aquifer is large. These regions are spatially correlated
with the areas of highest reported sinkhole occurrence in the study area.
This exercise independently confirms previous studies which suggest that
head differences between the surficial aquifer system and the Floridan play
an important role in influencing sinkhole occurrence.