Local mitigation strategy (LMS) - Volusia County Government

SECTION 5: HAZARD PROFILESGEOLOGIC HAZARDS5.12 SINKHOLE5.12.1 BackgroundSinkholes are a natural and common geologic feature in areas with underlying limestone and other rocktypes that are soluble in natural water. Most limestone is porous, allowing the acidic water of rain topercolate through their strata, dissolving some limestone and carrying it away in solution. Over time,this persistent erosional process can create extensive underground voids and drainage systems in muchof the carbonate rocks. Collapse of overlying sediments into the underground cavities producessinkholes.The three general types of sinkholes are: subsidence, solution, and collapse. Subsidence sinkholes formgradually where the overburden (the sediments and water that rest on the limestone) is thin and only aveneer of sediments is overlying the limestone. Solution sinkholes form where no overburden is presentand the limestone is exposed at land surface. Collapse sinkholes are most common in areas where theoverburden is thick, but the confining layer is breached or absent. Collapse sinkholes can form with littlewarning and leave behind a deep, steep-sided hole.Sinkholes occur in many shapes, from steep-walled holes to bowl or cone shaped depressions. Sinkholesare dramatic because the land generally stays intact for a while until the underground spaces get toobig. If there is not enough support for the land above the spaces, then a sudden collapse of the landsurface can occur. Under natural conditions, sinkholes form slowly and expand gradually. However,human activities such as dredging, constructing reservoirs, diverting surface water and pumpinggroundwater can accelerate the rate of sinkhole expansions, resulting in the abrupt formation ofcollapse sinkholes.Although a sinkhole can form without warning, specific signs can signal potential development: Slumping or falling fenceposts, trees or foundations; Sudden formation of small ponds; Wilting vegetation; Discolored well water; and/or Structural cracks in walls, floors.Sinkhole formation is exacerbated by urbanization. Development increases water usage, alters drainagepathways, overloads the ground surface and redistributes soil. According to FEMA, the number ofhuman-induced sinkholes has doubled since 1930 and insurance claims for damages as a result ofsinkholes has increased 1,200 percent from 1987 to 1991, costing nearly $100 million.Volusia County Multi-jurisdictional Local Mitigation StrategyFebruary 20105:56