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Riverine flooding occurs when the flow of rainwater runoff <br />exceeds the carrying capacities of the natural drainage <br />systems. During extended periods of heavy rainfall, certain <br />low-lying neighborhoods within the County are subject to <br />considerable flood damage and isolation caused by the <br />inability of natural and mechanical drainage systems to <br />effectively remove the water. Heavy rainfalls can cause <br />considerable damage to County infrastructure including <br />roadbeds, bridges, drainage systems, and the water supply. <br />The buildup of uncontrolled sediment contributes to the <br />problem of inadequate drainage in natural and mechanical <br />drainage systems. When a storm produces an overwhelming <br />amount of stormwater runoff, the accumulation of loose <br />sediment causes flooding by clogging the drainage systems. <br />This buildup of sediment in Indian River County waterways has <br />led to the degradation of the national estuary. The County is <br />currently working to address this issue by replacing bottom <br />opening radial gates with tilting gates at four water control <br />structures. <br />Long-term climate monitoring stations indicate that rainfall in <br />Indian River County averages about 51.5 inches annually, with <br />about half of this volume occurring during the 4 months from <br />June through September. Only about 20% of the total annual <br />volume of precipitation occurs during the four driest months, <br />December through March. For the 20 year period from 1969 to <br />1988, rainfall averaged 54.4 inches per year. The highest <br />rainfall recorded during this period was 81.7 inches in 1982 <br />(Indian River County 2020 Comprehensive Plan, Stormwater <br />Management Sub -Element). <br />In comparison to riverine flooding, coastal flooding is usually <br />the result of a severe weather system such as a tropical storm <br />or hurricane. The damaging effects of coastal floods are <br />caused by a combination of storm surge, wind, rain, erosion, <br />and battering by debris. All coastal property and inhabitants <br />are subject to severe damage and loss of life resulting from <br />floods caused by hurricane -associated storm surge. Some <br />coastal property, road arteries, and bridge approaches are <br />subject to severe flooding caused by rare astronomical tides as <br />well. <br />Historical Flooding Events. Data on previous occurrences of <br />flooding events in municipalities are limited; therefore, the <br />Indian River County Comprehensive Emergency Management Plan Basic Page 10 <br />