Florida Sinkholes Explained

By Eban Bean

Since Hurricane Irma, several sinkholes have developed around Alachua County and Florida, many in infiltration basins. An infiltration basin is sometimes also referred to as a dry retention pond or basin. You can find these in many residential and commercial areas where soils are sandy and the water table is not near the surface. Western Alachua County has hundreds of these.

Sinkholes are common in Florida, often forming after heavy rains. A popular video (see below) explains sinkhole formation, but there’s more to consider with development and stormwater management.

Karst Talk
Weak acids dissolve karst, CaCO3. Karst refers to topographic features where the subsurface is dissolved by surface or groundwater. This leaves large openings that allow water to move very quickly through the material. Karst is not unique to Florida and can be found in many parts of the US and around the world. Karst topography and sinkholes are naturally occurring.

Infiltrated rainfall leaches organic acids from surface that naturally dissolves Florida karst. Acid rain can accelerate this. Eventually, voids develop and overlying soil is not supported, collapsing at the surface. In well drained, undeveloped landscapes infiltration occurs across the entire area, uniformly except in low lying areas. When urbanized, runoff is conveyed from impervious areas commonly into dry infiltration basins. Several times more water is now infiltrating through the bottom of the basin, compared to before the area was developed. The acids in rainfall or from the landscape are focused in a much smaller area, accelerating dissolving CaCO3. Increased infiltration volumes also accelerate erosion of overlying soils as the karst void develops.

Sustainable Solutions

Sinkhole in Land O' Lakes, FL - Summer 2017 (image via NYT)

Sinkholes are often ‘fixed’ by filling them with concrete to stabilize the soil and geology below. Green infrastructure (GI) and low impact development (LID) distribute infiltration in developed landscape, using it more effectively. Examples of GI & LID: permeable pavement, bioretention, swales, cisterns, downspout disconnects, and infiltration trenches. Several local governments have incorporated LID practices into recent updates to stormwater programs. The water management districts are generally in support of it as well. The big hurdle is mainstreaming it into the engineering and design process. The first step in that direction is showing examples of these types of practices and projects where they not only perform well, but are cost effective, and easily maintained, compared to the conventional approach to land development.

Green infrastructure examples

We will be putting out a new series of EDIS documents and short videos that cover individual practices in the next few months. We are also working with developers to implement LID and green infrastructure into their projects, and evaluating the effectiveness of these practices. In the future we expect to offer continuing education for engineers and landscape architects on these subjects. UF/IFAS works with developers, government officials, and researchers on solutions for a more sustainable Florida future.