|
|
|
Sinkhole
Lake Evolution and
Effects of Urbanization
|
|
Progressive
Sequence of
Lake Evolution
|
|
Pre-sinkhole
development (no visible expression)-
the process begins with subsurface dissolution of limestone
below the unconsolidated overburden. Since there is no
surficial expression of the dissolution process, predicting
areas of sinkhle development is difficult. The process
continues, undermining the structural integrity of the
overburden, until collapse occurs.
|
Active
subsidence or collapse phase
(young) - At the initial surface appearance of a sinkhole,
the basin is steep-sided and potentially deep. As surface
material is removed by erosion and/or slumping from the
expanding perimeter to the center of the sink, the basin
walls decrease in angle and the lake basin becomes more
extensive. Examples of this phase include sinkholes at
Orange Lake, Crescent
Beach Spring and Red Snapper Sink (Table
1, Fig.
1).
|
Transitional
phase (middle age) - When the
sinkhole becomes partially or completely plugged, the
lake begins to develop a shallower and flatter bottom.
During this phase the plug may flush through the subsurface
conduit system, allowing the sinkhole to reactivate and
revert to an active subsidence phase. This may occur several
times until sediment accumulates faster than dissolution
of the underlying limestone. Many of the lakes in the
Interlachen Karst Highlands
are in a transitional phase.
|
|
Baselevel
phase (mature) - Once a transitional
phase sink becomes plugged, its growth is limited and the
lake becomes shallower. Continual erosion of material into
the basin over time will then eventually fill the basin
if no reactivation of the sinkhole occurs. The level to
which the sinkhole basin erodes is also related to the water
table elevation and the potentiometric surface of the Floridan
aquifer. Many lakes in the east central study area are in
a base-level phase.
|
|
Polje
(drowned prairie) -The lake floor is cut entirely across
karst rock (sometimes covered with unconsolidated alluvium)
but is located in the epiphreatic zone and is inundated
at high stages of the water table. These lakes may have
one or all phases of sinkhole development and many karst
features. Orange Lake,
for example, is a polje and includes active subsidence and
transitional features.
|
|
The
term polje arises from a lowland or depression is
flooded by a rising groundwater table -poljes. The Croatian
word ‘polje’ means “field”. Gams
(1978) identified three criteria for a lake to be classified
as a polje: (a) flat floor in rock (which can also be terraced
or occur in unconsolidated sediments such as alluvium);
(b) a closed basin with a steeply rising marginal slope
at least on one side; and (c) karstic drainage. There are
three basic poljes -border, structural, and base-level.
All poljes have a common hydrologic factor: their development
occurred close to the local water table, even though the
lake may be perched in some cases (Ford
and Williams, 1992). Of the basic types of polje, only
the base-level polje (to date) has been identified in north-central
Florida and locally described as drowned prairie.
|
|
|
|
