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
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| 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.
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| 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.