Florida Sinkholes: Science and Policy

3. Geologic Setting and Urban Sinkholes

3 Geologic Setting and Urban Sinkholes When examining the hydrology of Florida, one is struck by its vast number of lakes and wetlands, particularly in the peninsula. The distribution, size, and orientation of these bodies of water reveal a great deal about the geologic history of Florida and the formation of the state’s karst terrain. Most of these lakes and wetlands are sinkholes and have their origins in the dissolution of limestone and subsequent subsidence. This chapter discusses the distribution of sinkholes in the state of Florida and highlights some particular sinkholes that are interesting because of the way they formed or because the research that has been conducted on them in recent years reveals something interesting about the geology of Florida. Descriptions of spectacular geologic wonders in other parts of the world, such as the Grand Canyon or Mount Lassen, can include ground-level images that give us a sense of the topographic variations in the landscape. However, when it comes to discussing sinkholes in Florida, aerial photos and maps are usually the best choice for giving readers a visual sense of the landscape. Therefore, in this chapter, I liberally employ maps and aerial photos to give readers a visual context through which to understand the discussion. Many maps inadequately show the true distribution of sinkholes in the state because they lack detailed contour intervals that delineate subtle depression features. Thus aerial photos are perhaps the best way to demonstrate the extent of the distribution of sinkholes in the state. Another issue at stake in examining sinkholes is determining the size at which to begin mapping them. Is there a particular size at which sinkholes are defined as such? Is there a specific depth at which a karst depression is defined as a sinkhole? Our science has not tackled these questions yet, and the size at which we should begin to map sinkholes is open for debate. Certainly, regional studies only include the largest sinkholes that show up in aerial photos or on topographic maps, but in local studies, particularly Florida Sinkholes: Science and Policy 32 in studies of foundation failures or other property damage, much smaller sinkholes can be mapped. Some of these smaller sinkholes can continue to grow or they can end up as ephemeral features on the Florida landscape by filling with sediment through slope wash. There are a variety of scales for examining sinkholes in Florida. Patterns exist at the state, regional, and local levels. Many of these patterns are visually fractal in the sense that patterns observable on the larger state level can be seen repeating at the regional and local levels as well. Yet what controls these patterns? Clearly subsurface geology is the dominant factor in the distribution of depressions across the landscape, although existing geomorphology and surface geology also impact the distribution of sinkholes. The size, shape, and depth of depressions also vary considerably across the state. The distribution of variation in these characteristics again reveals that subsurface geology is crucial to the size, shape, and depth of sinkholes. However , surface geology and geomorphology also are important in the formation of these attributes. Our land uses, too, have had a major impact on sinkholes, particularly in agricultural and urban areas. We have filled sinkholes in and developed housing projects on top of them in the urban areas, and we have filled them in and created agricultural fields on top of others in the rural areas of the state. As a result, many current aerial photos provide only a glimpse of the surviving sinkholes in the state. As places such as St. Petersburg, Tampa, Ocala, and Orlando grew in the twentieth century, housing developments sprang up around the larger sinkholes while more troublesome depressions such as wetlands or seasonally wet lowlands were filled to allow for development . Nevertheless, the existing record of sinkholes as identified by large lakes and depressions is quite helpful in determining the significance of karst processes in the development of the geology and geomorphology of the state. Geomorphology of Florida Before discussing the sinkhole regions of the state, it is important to place this work in the context of the broader geomorphology of the region. On a continental scale, Florida is wholly within the coastal plains physiographic province (figure 3.1), a region that in the eastern half of the United States extends in a broad swath from Texas to Florida on the Gulf Coast and from Florida to New England on the Atlantic Coast. This region is...