HYDROGEOLOGIC FACTORS AFFECTING NEW SINKHOLE DEVELOPMENT IN THE ORLANDO AREA, FLORIDA

Sinkhole development affects approximately 15 percent of the land area of the world. Development of new sinkholes is extremely site-specific, but general associations between new sinkhole development and hydrogeologic factors can be established for areas with adequate historical records of sinkhole development. In the Orlando area, east-central Florida, at least 140 sinkholes have developed between 1961 and 1986. The Orlando area is situated on a thickly mantled karst area, and sinkholes form by cover collapse or, less commonly, cover subsidence. On the average, 11 new sinkholes collapse each year. The mean diameter is 30.8 feet and the mean depth is 15.5 feet. Eighty-five percent of the new sinkholes occur in high recharge areas (10 to 15 inches/year) on slightly elevated, sandy ridges. Few or no sinkholes occur in discharge areas where net downward erosion of surficial sediment is very unlikely. Forty-two percent of all new sinkholes occur during April and May when ground-water levels are usually at seasonal low stands. When the potentiometric surface declines 5 feet below its mode, then more sinkholes than expected per unit time begin to occur. When the potentiometric surface declines 10 feet below its mode, then more than 10 times as many sinkholes as expected per unit time begin to occur. Management of ground-water withdrawal to avoid drawdowns in excess of 5 feet in sinkhole-prone areas will minimize new sinkhole development induced by man.