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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. The maximum annual rainfall that <br /> has been recorded for the Vero Beach climatological station is <br /> 81.74 inches, (Indian River County Public Works, 2002). <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 /> Indian River County Comprehensive Emergency Management Plan Basic Page 9 <br />