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M. Seismic Hazards (Sinkholes and Subsidence) — Hazard <br />Identification. Sinkholes are a common feature of Florida's <br />landscape. They are only one of many kinds of karst <br />landforms, which include caves, disappearing streams, <br />springs, and underground drainage systems, all of which occur <br />in Florida. Karst is a generic term that refers to the <br />characteristic terrain produced by erosional processes <br />associated with the chemical weathering and dissolution of <br />limestone or dolomite, the two most common carbonate rocks <br />in Florida. Dissolution of carbonate rocks begins when they <br />are exposed to acidic water. Most rainwater is slightly acidic <br />and usually becomes more acidic as it moves through <br />decaying plant debris. Limestones in Florida are porous, <br />allowing the acidic water to percolate through them, dissolving <br />some limestone and carrying it away in solution. Over time, <br />this persistent erosion process has created extensive <br />underground voids and drainage systems in much of the <br />carbonate rocks throughout the state. Collapse of overlying <br />sediments into the underground cavities produces sinkholes <br />(Florida Geological Survey, 1998). <br />The Florida Center for Instructional Technology (2008) has <br />recorded six sinkholes in Indian River County. The first three <br />sinkholes developed in 1981; two were located in Fellsmere <br />and the other in Vero Beach. All three sinkholes are less than <br />10 feet in width. The forth and largest developed in 1985, in <br />Fellsmere and ranges from 31-80 feet in width. The fifth and <br />sixth developed in 2002 and 2005, in Vero Beach and both are <br />less than 10 feet in width. The Florida Center for Instructional <br />Technology database does not document any sinkholes in <br />Indian River Shores, Orchid, or Sebastian. (Maps, Etc., 2014) <br />Vulnerability Assessment. There are areas in western <br />Indian River County where canal bank failures could cause or <br />exacerbate flooding during heavy rain events or storms. This <br />problem is, however, more related to soil erosion than to actual <br />levee failure. There has never been any seismic activity, soil <br />failures, and few sinkholes in Indian River County. While these <br />hazards may exist, County vulnerability to them at this time is <br />considered very low. <br />Seismic events can have the following potential impacts within <br />a community: <br />Indian River County Comprehensive Emergency Management Plan Basic Page 51 <br />