Sinkhole Lake Evolution and Effects of UrbanizationSinkhole Lake Evolution | | Urbanization & Sinkholes
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.
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.