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2.3.1 EROSION AND SCOUR Where sand dunes existed before Hurricane Opal, significant loss of dune height and width was observed. Many dunes were breached or flattened (see Figure 2-3). Those that remained after the storm were scarped and weakened. Dunefuce retreat of 75 feet to 100 feet was observed in several locations. Overwash of eroded dune sediments was common, sometimes extending over 500 feet inland and causing burial of roads and at-grade consuuction by 1 to 4 feet of sand. In some cases, an estimated 10 to 20 feet of vertical relief was lost at the seaward edge of high dune and bluff areas. Many suuctures atop high dunes or bluffs collapsed because of a loss of support, either from the undermining of slab foundations or from inadequate pile embedment (see Figure 2-4). Ground levels at many front-row elevated structures that survived Hurricane Opal were typically reduced 3 to 7 feet, or more. In addition, local scour depressions were observed at the bases of many piles, indicating that 6 to 12 inches of additional soil was lost immediately adjacent to the piles. Scour during the storm probably rendered greater than 6 to 12 inches of soil around the piles unsupporting. Large scour depressions were observed where large volumes of water flowed during the storm. Depressions measuring 10 to 40 feet in length and width and 2 to 4 feet in depth were observed around some pile-supported structures and near the corners of some at-grade consuuction. SUUctures that seemed particularly vulnerable included the following: . structures at the landward termination of roads and driveways that funneled floodwaters toward the sUUctures . suuctures between drainage basins or lakes and larger bodies of water . suucture5 near locations where floodwaters crossed or breached the barrier islands 2.3.2 DEBRIS Small debris was widespread, ranging from household items to construction materials. These items did not cause structural damage to buildings, foundations, or other building components. Evidence of much larger debris shifted by floodwaters was also observed, including pier piles and braces, concrete slabs, dumpsters, automobiles, boats, and collapsed houses (see Figure 2-5). Many of these objects washed into buildings, and some caused structural damage. 2.3.3 SLAB FOUNDATIONS Many slab failures were noted in all types of structures (see Figure 2-6). The m,!jor reason for these failures was the loss of support coupled with a lack of reinforcing in the slabs. Welded reinforcing wire fabric was observed in many slabs but did not prevent failure of the slabs once they were undermined. HURRICANE OPAL IN FLORIDA 2-3